Evaluation of Natural Spring Freeze Tolerance of Five Pecan Scion–Rootstock Combinations

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Amandeep Kaur Department of Horticulture and Landscape Architecture, Oklahoma State University, Stillwater, OK 74078, USA

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Lu Zhang Department of Horticulture and Landscape Architecture, Oklahoma State University, Stillwater, OK 74078, USA

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Ming Yang Department of Plant Biology, Ecology, and Evolution, Oklahoma State University, Stillwater, OK 74078, USA

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Niels Maness Department of Horticulture and Landscape Architecture, Oklahoma State University, Stillwater, OK 74078, USA

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Charles J. Graham Noble Research Institute, Ardmore, OK, 73401, USA

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Rashmi Kumari Department of Horticulture and Landscape Architecture, Oklahoma State University, Stillwater, OK 74078, USA

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Yanwei Sun Department of Horticulture and Landscape Architecture, Oklahoma State University, Stillwater, OK 74078, USA

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Srijana Panta Department of Horticulture and Landscape Architecture, Oklahoma State University, Stillwater, OK 74078, USA

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Louise Ferguson Department of Plant Sciences, University of California Davis, Davis, CA 95616, USA

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Abstract

Pecan [Carya illinoinensis (Wangenh.) K. Koch] is a member of the Juglandaceae family. During spring, pecan trees break their bud dormancy and produce new leaves and flowers. Carbohydrates stored in roots and shoots are thought to support the bloom and early vegetative growth during this time until new leaves start the full photosynthetic activity. Spring freeze is known for its damaging effects on pecan bud and flower growth and development. Pecan shoots with leaves and flowers from five scion–rootstock combinations were collected hours before and after a recent spring freeze (below 0 °C for 6 hours, 21 Apr 2021, Perkins, OK, USA). Morphologies of the leaf, bud, and catkin were visually observed, and the morphologies of the anther and pollen in paraffin sections were investigated by light microscopy. Soluble sugar and starch from bark and wood were analyzed using the anthrone reagent method. The Kanza–Mount showed the maximum damage to terminal leaves, buds, and catkins, whereas Maramec–Colby had the minimum damage only to leaves. Pollen grains were shrunk and reduced in number in the anthers in the protandrous Pawnee scions, whereas no pollen damage was observed in the protogynous Kanza scion. Furthermore, bark soluble sugar levels increased in all the scion–rootstock combinations after the freeze, which may indicate a physiological response to the cold stress. Overall, the extent of spring freeze damage of pecans is affected by the growth stage, types of scion and rootstock, and the scion–rootstock interactions. Furthermore, in addition to low temperature, scion–rootstock interactions also affected the starch and soluble sugar contents in wood and bark tissues.

Pecan [Carya illinoinensis (Wangenh.) K. Koch], a member of the Juglandaceae family, is native to the United States and North America (Thompson 2017). The United States is the world’s largest pecan producer, with an average yearly production of 250 to 300 million pounds. Among the total pecan nuts (in shell) produced, ∼95% (288 million pounds) were from “improved” cultivars and 5% (14.2 million pounds) were from “native” and “seedling” (USDA and National Agricultural Statistics Service 2021). The “natives” and “seedlings” are pecans that have not been grafted and are produced from open-pollinated seeds. In contrast, “improved” pecan cultivars are propagated by grafting or budding onto a rootstock (Grauke and Starr 2014). Each of the improved cultivars consists of two genetically different parts in a graft union. The rootstock forms the root structure, and the scion forms the aboveground part and canopy (Wells 2014). Selection of compatible scion and rootstock types is important for constructing healthy pecan orchards (Vahdati et al. 2021).

Cold temperatures, especially freezing, is an abiotic factor that can critically affect pecan tree growth at various growth stages. Freeze injuries to pecan trees can occur in the autumn before they have acclimated to the cold (Cade 2001; Smith et al. 1993), during winter dormancy (Wood 1986), or during the spring (Malstrom et al. 1982) during and after bud differentiation. An early freeze in Apr 2007 in the southeastern United States severely damaged fruit trees (including hickory, peach, apple, and pear) during budbreaking and bloom (Gu et al. 2008). This freeze caused an estimated $117.7 million in agricultural losses in North Carolina, Tennessee, and Georgia (Gu et al. 2008). Cold damage to pecan trees is a frequent problem in Oklahoma. Damage occurs during approximately 3 of the 5 years of pecan bud differentiation in Oklahoma (Mesonet 2023). A freeze during 15 Apr and 16 Apr 2018 (range, −0.3  to −2.6 °C) in Oklahoma damaged ∼70% of the pecan crop (estimate from pecan grower’s meetings). A spring freeze on 21 Apr 2021 (with a temperature less than 0 °C for 6 h) recently occurred in Perkins, OK, USA.

Spring low-temperature injury is usually confined to the developing shoots, buds (including primary, secondary, and sometimes tertiary buds), and leaves (Smith et al. 1993). The severity of cold damage to pecan trees in the spring was related to the bud stage of development; rootstocks that resulted in earlier scion budbreak were more susceptible to damage (Grauke and Pratt 1992), leading to specific rootstock recommendations to avoid spring cold injury in northern parts of the United States pecan belt (Carroll and Smith 2017; Kaur et al. 2020). Rootstocks from southern cultivars, such as Riverside, Elliott, or Moore, produce trees that break bud dormancy earlier in the spring and are less cold-hardy than rootstocks from northern cultivars, such as Giles, Peruque or Colby, which break bud dormancy later in the spring (Carroll and Smith 2017).

Carbohydrates such as starch, sucrose, and glucose play an important role in the floral induction signaling process and provide energy throughout flowering and reproductive development (Chen et al. 2018). In the spring, when floral induction begins, stored carbohydrates, particularly those from roots, serve as a carbon-energy source for floral development (Lockwood and Sparks 1978). Carbohydrate contents in leaves, stems, inflorescences, and roots of various fruit and nut tree species, including pistachio, pear, orange, chestnut, peach, apple, olive, pecan, and walnut, have been shown to have a close relationship with the timing of bloom and subsequent fruit set (Smith and Waugh 1938; Spann et al. 2008; Zwieniecki and Lampinen 2015). Starch and soluble sugar dynamics reflect tree physiological states that may be used to forecast the performance of the trees and their ability to tolerate environmental stresses (Wong et al. 2003). The variations in carbohydrate contents in pecan leaves and shoots from budbreak to defoliation have been studied (Kim and Wetzstein 2005). Recently, carbohydrate budgets have been investigated in pecan wood and bark during the fruiting season (Panta 2021). The two major parts of the vascular system in plants that play a crucial role in carbohydrate transport and storage are the xylem and phloem. During spring, soluble carbohydrates increase in the xylem sap of walnut, maple, grapevine, willow, and pear (Ito et al. 2012; Wong et al. 2003), suggesting that xylem plays a role in the translocation of these metabolites during budbreak and subsequent development, in addition to the role of the phloem in sugar transport. Phloem sieve tubes in J. regia have been reported to remain active at temperatures below −10 °C (Fisher 1983; Tixier et al. 2017). Increased frost hardiness was observed with increased carbohydrate content (glucose, fructose, sucrose) in walnut (Charrier et al. 2013). However, there is little information about how the carbohydrate content varies in pecans after exposure to a spring freeze, potentially playing a role in reducing the extent of damage. We hypothesize that there will be variations in carbohydrate contents of buds during the spring season because of differential budbreak timing influenced by different pecan scion–rootstock combinations.

This work describes the effects of a natural spring freeze on reproductive tissues and carbohydrate (soluble sugar and starch) contents of five pecan scion–rootstock combinations. Our objectives were to document visible damage to male and female flowers and correlate wood carbohydrate contents with degrees of freeze damage for different scion–rootstock combinations of pecans grown in Oklahoma.

Materials and Methods

Five pecan scion–rootstock combinations (27-year-old) used during the experiment were Pawnee–Peruque (PP), Kanza–Giles (KG), Kanza–Mount (KM), Kanza–Colby (KC), and Maramec–Colby (MC) grown at the Cimarron Valley Research Station (97°02′13″ W, 35°58′55″ N), in Perkins, OK, USA. KC and KM trees are planted ∼356.5 m and 528.8 m, respectively, from the MC, PP, and KG trees in the experimental orchard so that there is a 2- to 3-m visual difference in altitude.

At the time of the freeze event on 21 Apr 2021, KG, KM, KC, and MC trees were at the leaf expansion stage with tiny catkins (Fig. 1B–E), and the PP trees were more advanced, at the early flowering stage with larger catkins and few leaves (Fig. 1A). Twenty branch samples of each scion–rootstock combination were collected randomly from the middle canopy of different trees on 19 Apr 2021, 2 d (calendar days) before the expected freezing event in Perkins, OK, USA. The collected branch samples were ∼30 cm long with a terminal bud; they were immediately placed into water to prevent wilting. To check temperature conditions in the orchard, we used the Mesonet website (Mesonet 2023) and HOBO data loggers (Onset RS3-B Solar Radiation Shield; Onset, Bourne, MA, USA) which were installed in the same block. The freeze occurred on 21 Apr 2021 (early morning near 1:00 AM); during that time, the temperature was below 0 °C for 6 h (range, −1.6  to −0.5 °C) (Fig. 2). During daylight hours on 21 Apr 2021, trees of each scion–rootstock combination were observed for visible signs of damage, and 20 more branch samples per scion–rootstock combination were collected from the middle canopy of different trees. Branches were transported approximately 20 miles to Stillwater, where three of the 20 branches collected each time were immediately processed for carbohydrate analysis. The rest of the branches were placed in growth chambers under conditions identified in Table 1. Catkins were collected from these branches after growth chamber incubation in FAA solution (70% ethanol: acetic acid: formalin in 16:1:1 volume/volume/volume) on 24 Apr 2021, for microscopy analysis.

Fig. 1.
Fig. 1.

(A–E) Green terminal tips, catkins, and leaves before the freeze event (on 19 Apr 2021, 2 d before the expected freezing event). (F, H) Damage to the terminal leaves after the freeze event (on 21 Apr 2021). (G) Almost no damage to leaves and catkins after the freeze event. (I, J) Terminal bud, catkins, and leaves damage after the freeze event. Black and red arrow in (F–J) indicate leaves and catkins, respectively. For all the scion–rootstock combinations, the visual observations and images were obtained on the same days previously mentioned before and after the freeze events.

Citation: HortScience 58, 10; 10.21273/HORTSCI17258-23

Fig. 2.
Fig. 2.

The minimum temperature (in °C) from 19 Apr 2021 (2 d before the expected freezing event) until 21 Apr 2021 (the day of the freeze event) in Perkins, OK, USA (https://www.mesonet.org/). The temperature was recorded hourly.

Citation: HortScience 58, 10; 10.21273/HORTSCI17258-23

Table 1.

Controlled temperature, humidity, and light conditions in the growth chambers.

Table 1.

Freeze damage observations in the field on 21 Apr 2021.

Visible freeze damage was classified into the following three groups based on the severity of the damage: branches with terminal buds exhibiting no damage; branches with terminal buds that exhibited only damaged leaves (wilted or burnt); and branches with terminal buds exhibiting damage to both leaves and catkins. Observations included 50 PP, 50 KG, 50 MC, 30 KM, and 30 KC (for KC and KM, there were a total 30 trees in our orchard) trees.

Soluble sugar and starch tests of bark and wood.

Three branch samples per scion–rootstock combination were used to assess the carbohydrate contents of wood (mostly xylem transport tissues) and bark (mostly phloem transport tissues) of branch samples taken randomly from trees before and after the freeze event and after incubation in the growth chamber. The top section (4–5 cm in length from the terminal bud) from each of the branches was used. The wood and bark of each section were separated, chopped using pruning shears, and dried in the oven (Isotemp oven model 655F; Fisher Scientific, Pittsburgh, PA, USA) for 2 to 3 d at 75 °C. After drying, the samples were kept at room temperature until further processing. The bark and wood tissues were ground into small pieces using a Willey mill (Arthur K. Thomas Co., Philadelphia, PA, USA), and then into a fine powder in 2-mL Eppendorf tubes with 4-mm bearing balls (Precision Chrome Steel G25; UXCELL, Hong Kong, China) using a Mini-Beadbeater 96 (Biospec Products, Bartlesville, OK, USA).

Fine powder samples (25–27 mg) were incubated in 1 mL of ultra-pure water (resistivity of ≤18.2 MΩ cm−1 and total organic content less than five parts per billion) at 70 °C for 15 min followed by centrifugation for 10 min at 15,000 rpm. The supernatant was diluted into ultra-pure water (1:20 volume/volume), and soluble sugars were quantified using anthrone as the reagent. For the water-insoluble starch analysis, the remaining pallet was washed with 95% ethanol, washed with water, incubated at 100 °C for 10 min to allow starch gelatinization, and digested with amylo-glucosidase (700 units/mL), alpha-amylase (70 units/mL), and sodium acetate (0.2 M, pH 5.5) for 4 h at 37 °C in a Roto-ThermTM Plus Incubated Rotator (H2024; Benchmark Scientific, Sayreville, NJ, USA). After incubation, samples were centrifuged for 5 min at 15,000 rpm. The supernatant was diluted with ultra-pure water (1:4 volume/volume) and used for quantification.

Colorimetric anthrone reactions were conducted in 96-well microplates. Absorbance readings were obtained using a microplate reader (Epoch; Bio-TEK, Instruments Inc., Winooski, VT, USA) at 620 nm and compared to a glucose standard. Gen5 3.10.lnk software was used to convert absorbance readings into glucose equivalents using a standard curve generated with each 96-well microplate.

Light microscopy.

Male flower samples were fixed in FAA solution (70% ethanol: acetic acid: formalin in a 16:1:1 volume/volume/volume) at room temperature until further processed for the microscopy analysis. Samples were dehydrated by an ethanol series (i.e., 50%, 70%, 90%, 95%, and 100%). Histochoice was used as the paraffin intermediate solvent before infiltration and embedding in liquid paraffin at 58 °C (Ruzin 1999). The embedded samples were sectioned at a thickness of 10 μm using a microtome (Leica RM2135). The sections were stained with 1% aqueous safranin (Supplemental Table 1) and observed using a Nikon 80i compound microscope (Nikon Corporation, Minato City, Tokyo, Japan). Micrographs were obtained using a DS-Ri1 camera and software (NIS-Elements BR 4.40.00, Nikon Corporation).

Statistical analysis.

For soluble sugars and starch, the total carbohydrate content was determined based on glucose equivalents and expressed as mg/g dry weight (DW). The data were analyzed using PROC GLM (SAS version 9.4; SAS Institute Inc., Cary, NC, USA). An analysis of variance was performed to determine the effects of a freeze on soluble sugar and starch concentrations in different scion–rootstock combinations, and the treatment differences were analyzed using the LSMEANS with LINES statement (α = 0.05). The percentage of normal anthers was calculated and used for the t test using SAS (version 9.4; SAS Institute Inc., Cary, NC, USA) to compare the catkin samples before and after the freeze.

Results

Freeze damage observations in the field on 21 Apr 2021.

Figure 1A–E shows the representative shoots of the five scion–rootstock combinations 1 d before and after the freeze event. The Pawnee scion is protandrous (type I) and produces mature pollen before the female flowers are receptive. Kanza and Maramec are protogynous (type II) scions that shed pollen after the female flowers bloom. On average, the Pawnee scion (PP) had the largest catkins (but smaller terminal leaves), followed by the Maramec (MC) and Kanza scions (KG, KM, and KC), with smaller catkins but larger leaves.

After the freeze occurred, apparent differences in damage of the terminal bud, leaf, and catkin were observed (Table 2). The Kanza scion appeared to be most damaged by the spring freeze event, with between 50% to 60% of branches exhibiting some visible signs of damage. Conversely, the Pawnee and Maramec scions exhibited less than 10% visible signs of damage after the freeze event. Within the Kanza scion, there appeared to be some differences in the most prevalent type of damage, dependent on the rootstock. Although KM and KC exhibited more damage encompassing all three observed components of the terminal bud, leaf, and catkin, KG exhibited damage mostly confined to leaves.

Table 2.

Damage observed in pecan scion–rootstock combinations after the freeze (based on 20 randomly collected branches from each).

Table 2.

For all combinations, we observed that lower canopy branches had a higher level of damage than the upper canopy branches. This likely resulted from colder air settling to the lower canopy.

Bark and wood soluble sugar and starch.

A two-way scion-rootstock × freeze (samples collected before and after the freeze) interaction was significant for the bark soluble sugar, wood soluble sugar, and wood starch concentrations, but not for the bark starch concentration (Table 3). There were no differences in the bark soluble sugar concentration before the freeze event, but the soluble sugar concentrations of all five scion–rootstock combinations increased significantly after the freeze event (Fig. 3). After the freeze, the soluble sugar concentration was lower in the Pawnee and Kanza scion–rootstock combination. The wood soluble sugar concentration was the same in Maramec and Pawnee, but it differed in all Kanza scion–rootstock combinations before and after the spring freeze (Fig. 4). Within the Kanza scion, the wood soluble sugar concentrations were higher for KC and KM but lower for KG before the spring freeze. The wood starch concentration followed a pattern similar to that of soluble sugars, with Maramec and Pawnee scions exhibiting no difference and the Kanza scion exhibiting higher differential responses with KC and KM and the same differential response with KG for the starch concentration before the freeze (Fig. 5). The bark starch concentration was only different after the spring freeze for the Pawnee scion; in this scion, the starch concentration decreased after the freeze (Fig. 6).

Table 3.

Summary analysis of variance table for bark and wood soluble sugar and starch contents.

Table 3.
Fig. 3.
Fig. 3.

Soluble sugar content in bark for five pecan scion–rootstock combinations before (on 19 Apr 2021) and after the spring freeze (on 21 Apr 2021). Means with the same letter are not significantly different at P < 0.05. Bars above each scion–rootstock combination represent the SEM of three replications.

Citation: HortScience 58, 10; 10.21273/HORTSCI17258-23

Fig. 4.
Fig. 4.

Soluble sugar concentration in wood for five pecan scion–rootstock combinations before (on 19 Apr 2021) and after the spring freeze (on 21 Apr 2021). Means with the same letter are not significantly different at P < 0.05. Bars above each scion–rootstock combination represent the SEM of three replications.

Citation: HortScience 58, 10; 10.21273/HORTSCI17258-23

Fig. 5.
Fig. 5.

Starch concentration in wood for five pecan scion–rootstock combinations before (on 19 Apr 2021) and after the spring freeze (on 21 Apr 2021). Means with the same letter are not significantly different at P < 0.05. Bars above each scion–rootstock combination represent the SEM of three replications.

Citation: HortScience 58, 10; 10.21273/HORTSCI17258-23

Fig. 6.
Fig. 6.

Starch concentration in bark for five pecan scion–rootstock combinations before (on 19 Apr 2021) and after a spring freeze (on 21 Apr 2021). Means with the same letter are not significantly different at P < 0.05. Bars above each scion–rootstock combination represent the standard error for the mean of three replications.

Citation: HortScience 58, 10; 10.21273/HORTSCI17258-23

Microscopic catkin pollen observations.

The Kanza and Maramec scion catkins did not have sufficiently advanced development to have produced mature pollen grains at the time of the spring freeze; therefore, no comparisons of pollen damage could be performed. For PP, damage to catkins and anthers was observed based on the tapetum, anther wall, and pollen grains. We observed severe tapetum degeneration in anthers of the catkin section after the freeze, whereas anthers of the catkin before the freeze had intact tapetum (Fig. 7A, C, and D). Based on the anther wall, the less stained anther wall of the sample obtained after the freeze indicated freeze damage (Fig. 7B, D, and F). Pollen grains appeared nonstained or lightly stained after the freeze, whereas they were darkly stained before the freeze. The numbers of normal and abnormal anthers were calculated based on the tapetum, anther wall, and pollen observations. The PP catkin samples obtained before the freeze had higher percentages of normal anthers (88.89%, 83.67%, and 82.19%) than those of samples obtained after the freeze (27.27%, 35.89%, and 37.89%) (Table 4).

Fig. 7.
Fig. 7.

(A, C, E) Pawnee–Peruque catkin and staminate flower section collected before the freeze event. (B, D, F) Pawnee–Peruque catkin and staminate flower section collected after the freeze event under microscopy (10×). Red, yellow, and black arrows indicate anther wall (epidermis and endothecium), tapetum, and pollen grains, respectively.

Citation: HortScience 58, 10; 10.21273/HORTSCI17258-23

Table 4.

Comparison of normal and abnormal anthers collected before and after the freeze from Pawnee–Peruque (PP) catkins.

Table 4.

Discussion

Freeze damage observations in the field on 21 Apr 2021.

Flower morphogenesis is an essential developmental process influenced by abiotic factors such as temperature, photoperiod, and precipitation. The damage to pecans by spring freezes has been observed previously. Wells (2007) reported severe damage to pecan leaves and catkins by the late spring freeze with a temperature of −2.2 °C. Some cultivars (Pawnee, OK642, and Mohawk) showed more injuries by extreme freeze events (−6 °C for 8 h on 7 Apr 2009), whereas other cultivars (Giles, Kanza, and Mount) showed less bud damage (Smith and Cheary 2010). Rootstocks affect the resistance or susceptibility of cultivars to low-temperature damage during freeze events (Sanderlin 2000). The scion–rootstock combination significantly alters the frozen impact, indicating a close relationship between the effect of the rootstock on scion phenology and freeze tolerance (Grauke and Pratt 1992). Differences in freeze damage of scions at uniform stages of bud growth were apparent at bud swell and leaf burst. At the bud swell stage, buds of the Candy (used as the scion on different rootstock) cultivar were more damaged than those of ungrafted control seedlings (at the same bud growth stage). In the case of rootstocks, at the inner scale split stage, Sioux was more severely damaged than Burkett. However, the rootstock did not show any differences in freeze damage at the bud dormant and swell stages (Grauke and Pratt 1992). During the present experiment, similar results were observed. The degree of damage was different across scion–rootstock combinations. The Kanza scion grafted on three different rootstocks (Mount, Colby, and Giles) had different extents of damage in terminal leaves, buds, and catkins. Scions Maramec and Kanza grafted on the same rootstock, Colby, had differing damage levels. MC had rare damage in leaves and catkins, whereas KC had symptoms of severe destruction in leaves, terminal buds, and catkins. The scion and rootstock affect the budbreak date, which is a heritable trait associated with their provenance. The rootstock that breaks their bud dormancy later than the other rootstocks can result in a late budbreak of the scion grafted on them (Grauke and Pratt 1992). In this study, at the time of the freeze event, KG, KM, KC, and MC trees were at the leaf expansion stage with tiny catkins (they are protogynous; female flowers mature first before the maturation of catkins) (Fig. 1B–E). However, the PP trees were at the early flowering stage, with larger catkins and few leaves (because it is a protandrous cultivar in which catkins mature first, before female flowers) (Fig. 1A). Furthermore, Kanza, Mount, and Colby scions break their bud dormancy later than other scions (personal observation from the field). At the time of the freeze event (21 Apr 2021), all Kanza scions with rootstocks Mount, Colby, and Giles had late bud and leaf growth compared with PP and MC. The Kanza scions had younger and more tender leaves than Pawnee and Maramec, which caused more severe damage to KG, KC, and KM compared with MC and PP. However, the opposite trend was observed by Smith and Cheary (2010) after the freeze on 9 Apr 2009; Kanza, Mount, and Colby had less bud damage than Pawnee (all grafted on Giles). The difference in damage by these two freeze events (2009 and 2021) could be attributable to the differences in the times of the freeze events and developmental stages of buds and flowers. During the 2009 freeze, they recorded when Kanza was at the least advanced bud growth stage (outer bud scale intact) and Pawnee was at a more advanced bud stage (outer bud scale shed). The outer bud scale intact stage is considered more resistant to low temperatures than the outer bud scale shed stage.

Another possible crucial factor for differences in the damage of scions is their location in the orchard. KC and KM trees are planted far (∼356.5 m and 528.8 m, respectively) from the MC, PP, and KG trees in the experimental orchard so that there is a visual 2- to 3-m difference in altitude. The spring frost injury has been shown to vary within an orchard (Miranda et al. 2005). OK642, an advanced selection from Oklahoma, showed minor bud damage in the east field, which had higher elevation, but tremendous bud damage in the west field after the freeze in Apr 2009 (Smith and Cheary 2010).

Pecan trees usually have a wide canopy (nearly 12 m) and height (21 m). Because of this vast structure, temperature and growth vary within the tree canopy. A radiation freeze, whereby cold air is close to the ground and branches in a lower position are subjected to colder temperatures than branches in an upper area, increases the temperature gradient from the bottom to the top of the canopy. This causes more damage in the lower branches than in the upper branches (Charrier et al. 2015; Graham 2020; Reid 2020a, 2018a, 2018b; Sparks 2005). During the present experiment, we also observed that levels of damage differed between the upper and lower canopy after the spring freeze. The lower canopy branches showed more damage than the upper branches. A similar damage trend has been observed previously (Reid 2020a; Wells 2007). Wells (2007) observed that damage often occurs in low areas in the orchard, on small trees, or on the lower limbs of larger trees after a late spring freeze event. A branch only 2.4 m above the ground had catkins that were brown and dead and shoots from a limb 6.1 m from the ground that had healthy catkins (after the 18 Apr 2020 freeze) (Reid 2020a). Another reason for the greater damage to the buds of the lower canopy could be that the budbreak of pecan occurs from the bottom of the canopy to the top (i.e., lower canopy budbreak earlier than that in the upper canopy). This aids in the freezing protection of the upper canopy compared with that of the lower canopy, which is at an advanced bud growth stage. Therefore, when collecting samples, we evaluated and collected the shoots from the middle tree canopy, approximately from the same height, of all scion–rootstock combination trees.

Bark soluble sugar increased and wood soluble starch decreased as a response to the freeze.

Soluble sugar and starch contents of pecan tissues have been analyzed and reported previously (Kim and Wetzstein 2005; Meléndez et al. 2021; Su et al. 2021; Taylor 1970; Wood 2014). Taylor (1970) reported that the soluble sugar contents ranged from ∼46 to 97 mg⋅g−1 DW and starch contents ranged from 4 to 23 mg⋅g−1 DW from pecan stem sections. Meléndez et al. (2021) recently reported soluble sugars of ∼120 mg⋅g−1 DW and starch contents of leaves ranging from 30 to 40 mg⋅g−1 fresh weight (FW) from 9-year-old pecan trees. During our experiment, we observed similar higher soluble sugar levels and lower starch levels in the branch samples from the current-year branch. The soluble sugar content of pecan bark ranged from 60 to 83 mg⋅g−1 DW and that of wood ranged from 40 to 60 mg⋅g−1 DW, whereas the starch content of bark tissue ranged from 9 to 13 mg⋅g−1 DW, and that of wood tissue ranged from 9 to 45 mg⋅g−1 DW. At the flowering stage, a lower soluble sugar content (∼26 mg⋅g−1 DW) and starch content (25 mg⋅g−1 DW) were observed in the stem (current-year growth and 1-year-old shoot) samples from 15-year-old Stuart pecans (Kim and Wetzstein 2005). The starch contents of leaf samples were also studied, and it was found that the content reached as high as 60 mg⋅g−1 DW. In the case of grafted pecan stem samples from 1-year-old seedlings, a lower sugar content (20–39 mg⋅g−1 FW) and higher starch content (30–80 mg⋅g−1 DW) were observed by Su et al. (2021). These studies proved that when soluble sugars were higher in tissues, the starch contents were lower, indicating a relationship between soluble sugars and starch in tissues.

The cold tolerance research of pistachio rootstocks showed rare soluble sugar changes in the leaves of scion when samples of different rootstocks before the freeze were compared (Sorkhan et al. 2011). These findings are similar to our results. There was no significant difference in bark soluble sugar levels among different scion–rootstock combinations 1 d before the freeze event (Fig. 3). Interestingly, the bark soluble sugar contents in all the scion–rootstock combinations substantially increased after the freeze. There was a significant reduction in the bark starch content immediately after the freeze day, especially for the PP combination. A decrease in starch and a simultaneous increase in sugars have also been reported for grapefruit trees under low-temperature conditions (Rodrigues and Ryan 1960). Starch depletion of all tree organs was associated with increased soluble carbohydrate levels. The interconversion of starch and soluble carbohydrates is classic and has been reported by many investigations (Charrier et al. 2013). The relationship between frost hardiness and glucose plus fructose plus sucrose in the walnut branches has been reported (Charrier et al. 2013). Higher carbohydrate contents (as cryoprotectants) and less intracellular water are crucial for plant survival at low temperatures (Charrier et al. 2015).

The inverse levels of sugars in stems and leaves have been observed during the pecan bloom stage. The transport of sugars from leaves to support stem growth can cause a reduction in sugars in leaves and increases in stem sugars (Kim and Wetzstein 2005). The phloem transports the sugars from leaves to other parts and sugar decreases in stem. During this research, we observed an increase in phloem sugars after the freeze event. This indicates phloem is transporting higher sugars to apical tissues to meet the extra energy demand to bear the freeze injury. Another reason could be that they are using wood soluble sugar and starch (in KM and KC) contents because wood soluble sugar and starch decreased significantly in KM and KC after the freeze (these two were the most damaged cultivars).

The total carbohydrate content represents the pool of carbon that can be used either to improve cold hardiness via hydrolysis to soluble carbohydrate forms or to support the tree’s metabolic processes (Morin et al. 2007). After the freeze, the contents of bark soluble sugar differed among the scion–rootstock combinations. All the Kanza scions on the three different rootstocks (i.e., Mount, Colby, and Giles) had significantly higher soluble sugar than PP. This might be attributable to the Kanza scion breaking dormancy later and growing and shooting slightly after PP and MC. This caused less utilization of sugar in Kanza scions to support the leaves and flower growth. However, PP consumed sugar more quickly because it had catkins growth. The difference in the levels of starch and soluble sugar among the scions has been reported for oak trees (Morin et al. 2007). During the bud dormancy release time (1 and 20 Mar 2014), the late mature genotype had the highest content of soluble sugar compared with the early and midseason mature walnut genotypes, although there was an opposite trend for the starch content (Farokhzad et al. 2018).

Regarding wood soluble sugar, there was a significant change in the sugar content of the Kanza scion grafted on the three different rootstocks after the low temperature. However, the difference in wood soluble sugar before and after the freeze was not significant for PP and MC. The lower wood sugar contents of KC and KM after the freeze may indicate the utilization of sugars to support their branch growth as well as protect them from the low temperatures.

There was significantly higher wood starch in KM and KC compared with that in PP and MC in samples collected 1 d before the freeze. One reason for this might be the presence of higher stored carbohydrates from the previous season and comparatively less demand during the current season to support buds and leaves. As in pecans, branch and root tissues are the sites for carbohydrate reserves produced during the current season in summer and early autumn (Lockwood and Sparks 1978). These stored carbohydrates serve as a substrate for flower growth during the spring season (Lockwood and Sparks 1978). There was a significant decrease in wood starch in KM and KC after the freeze event. The rapid decline in the starch level at the bloom time of pecans indicates its significance to flowering and fruiting (Smith and Waugh 1938). The stored starch in xylem parenchyma is converted to soluble carbohydrates and released into the xylem sap during bud growth in the spring (Tixier et al. 2017). During our experiment, after the freeze event, wood starch decreased, and bark soluble sugar increased. In avocado, a decrease in the starch content coincided with an increase in glucose and sucrose under low temperatures (Rodrigues and Ryan 1960).

The soluble carbohydrate concentrations are associated with higher resistance to cold stress (Charrier et al. 2015; Levitt 1980). Accumulated carbohydrates (in the apoplastic/extracellular region) reduced the freezing point of cells and delayed ice crystal formation (Mutlu et al. 2013). For Arabidopsis, it was observed that accessions with very low sugar levels had low freezing tolerance (Zuther et al. 2012). However, we observed the opposite pattern; the most damaged scion–rootstock combinations had the maximum soluble sugar content, whereas the least damaged had a lower soluble sugar content. This might be attributable to differences in the physiology of the model plant and nut tree. Furthermore, differences in the soluble sugar content may be caused by differences in the growth stage (budbreak time) of these scion–rootstock combinations.

During the present experiment, the soluble sugar concentration was higher in the pecan bark than in wood tissue. A similar trend has been reported for other trees such as J. regia and oak trees for bark and wood sugars during the spring season (Bazot et al. 2013; Bonhomme et al. 2010; Tixier et al. 2017). However, xylem exhibited a higher starch content than the phloem. The xylem parenchyma plays an important part in starch storage and accumulates soluble sugars during the spring (Tixier et al. 2017).

The complex scion–rootstock interaction affects various traits of grafts, such as flowering time (Meimand and Shamshiri 2019), allocation of carbon assimilates between these tissues (of almond) (Khadivi-Khub and Anjam 2016), trunk diameter of the scion (Barbera et al. 1993), and water and nutrient uptake (Vahdati et al. 2021). The regulatory feedback between the rootstock and scion ultimately modulates the final tree architecture. During this experiment, we observed differences in the carbohydrate contents of different scions grafted on the same rootstock as well as the same scion grafted on different rootstocks. Before the freeze event, there was a significant difference in wood soluble sugar and starch among the scions (Kanza and Maramec) grafted on same rootstock (Colby). The Kanza scion grafted on three different rootstocks also had significant differences in wood soluble sugar contents; Kanza grafted on Colby and Mount had significantly higher wood soluble sugar and starch contents than Kanza grafted on Giles. Similarly, after the freeze event, the Kanza grafted on Colby had significantly decreased wood soluble sugar and starch contents, whereas Maramec (grafted on Colby) had no change in wood soluble sugar and starch contents. Furthermore, the Kanza scion grafted on Colby and Mount had significantly decreased wood soluble sugar and starch, whereas Kanza on Giles rootstock had significantly increased wood soluble sugar and no change in wood starch. In the case of Pawnee grafted on Peruque (PP), before the freeze event, it had significantly less wood soluble sugar contents than KM and significantly different wood starch contents than KC, KM, and MC. After the freeze, the bark soluble sugar contents were significantly increased in all scion–rootstock combinations; however, the PP soluble sugar content was still significantly less than that of all Kanza scions (KC, KM, KG). Before the freeze event, even though the Kanza and Maramec grafted on Colby had similar bark starch contents, Maramec (grafted on Colby) had a significantly higher bark starch content than KG and PP. These observations suggest that rootstock–scion interactions affect the wood and bark soluble sugar and starch contents of pecans. The mRNA studies also showed that mRNA transferred from rootstock to scion (for example, GAI, can cause delayed flowering of Pyrus) and from scion to rootstock (BEL5 can increases tuber yield) can affect each other’s physiology and function (Banerjee et al. 2009; Zhang et al. 2011).

Abnormal catkins observed through paraffin sectioning.

The different catkin development stages of pecan have been observed by Grauke and Thompson (1996), Haulik and Holtzhausen (1988), and Zhang et al. (2016). Each catkin is composed of many individual staminate flowers (varied among protogynous and protandrous). A staminate flower consists of a central bract, two lateral bracteoles, and three to seven stamens. The stamen is composed of an anther and a filament. In a mature anther, four pollen sacs enclose a large number of pollen grains. The mature anther wall is two cell layers thick, with an outer epidermis and inner endothecium (Grauke and Thompson 1996). The mature pecan pollen grains are triporate in shape.

With PP, we observed visual differences in anther and catkin structures before and after the freeze event. The differences were observed based on the tapetum, anther wall, and pollen. The samples before the freeze had an intact or less degenerated tapetum and darkly stained anther wall and pollen grains. In contrast, the samples after the freeze had a more degenerated tapetum and less stained anther wall and pollen grains, which indicate damage. The percentage of normal anthers of catkins before the freeze was significantly higher than that of those after the freeze. This indicates that catkin samples before the freeze were developing well, whereas catkins and anthers after the freeze were impacted.

Regarding KG, KC, KM, and MC, a mass of undivided cells in anthers was observed because they were at an early stage of development (Fig. 8). Regarding PP, catkins were longer and wider (Fig. 9A), i.e., had more advanced development, because PP is protandrous. In comparison, protogynous KG, KC, KM, and MC had smaller catkins (Fig. 9B and C) at an early stage of development. The protogynous cultivars do not seem to be affected by the freeze. Similar observations have been made by Garcia-Mozo et al. (2001) for oak trees; pollen grains at early stages of development are more tolerant to low temperatures than pollen grains at later developmental stages.

Fig. 8.
Fig. 8.

Kanza–Giles (A) and Kanza–Colby (B) catkin and staminate flower section collected before the freeze event under microscopy (4×). Maramec–Colby (C) catkin and staminate flower section collected before the freeze event under microscopy (10×).

Citation: HortScience 58, 10; 10.21273/HORTSCI17258-23

Fig. 9.
Fig. 9.

Branches bearing catkins of Kanza–Mount (A) and Pawnee–Peruque (B). (C) Comparison of Kanza–Giles and Pawnee–Peruque catkins 2 d before the expected freezing event. (Image by Amandeep Kaur, obtained on 19 Apr 2021.)

Citation: HortScience 58, 10; 10.21273/HORTSCI17258-23

Conclusion

The differences in the extent of damage of terminal buds, leaves, and catkins were observed among five scion–rootstock combinations by the spring freeze that occurred on 21 Apr 2021. The Kanza scion grafted on three different rootstocks (Mount, Colby, and Giles) showed variations in the extent of damage. For example, Kanza–Mount showed the most damage to terminal buds, leaves, and catkins, whereas Kanza–Giles had comparatively less damage to the leaves. Maramec and Kanza on the same rootstock, Colby, had different levels of damage; Maramec–Colby had almost no damage to leaves, buds, and catkins, whereas Kanza–Colby had damaged leaves and catkins. These observations indicate that the types of scion and rootstock and growth stage affect the extent of damage by the freeze. Furthermore, the scion–rootstock interaction affects the wood and bark starch and soluble sugar contents. A significant increase in the bark soluble sugar was observed in all scion–rootstock combinations 1 d after the freeze. The increased soluble sugar might reflect a physiological response to the freeze, which could provide protection to the plants under low temperature conditions. The evaluation of the damage to buds and bloom in different pecan scion–rootstock combinations by the spring freeze is important for understanding the effect of the freeze on pecans and may help develop strategies for minimizing such effects.

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  • Fig. 1.

    (A–E) Green terminal tips, catkins, and leaves before the freeze event (on 19 Apr 2021, 2 d before the expected freezing event). (F, H) Damage to the terminal leaves after the freeze event (on 21 Apr 2021). (G) Almost no damage to leaves and catkins after the freeze event. (I, J) Terminal bud, catkins, and leaves damage after the freeze event. Black and red arrow in (F–J) indicate leaves and catkins, respectively. For all the scion–rootstock combinations, the visual observations and images were obtained on the same days previously mentioned before and after the freeze events.

  • Fig. 2.

    The minimum temperature (in °C) from 19 Apr 2021 (2 d before the expected freezing event) until 21 Apr 2021 (the day of the freeze event) in Perkins, OK, USA (https://www.mesonet.org/). The temperature was recorded hourly.

  • Fig. 3.

    Soluble sugar content in bark for five pecan scion–rootstock combinations before (on 19 Apr 2021) and after the spring freeze (on 21 Apr 2021). Means with the same letter are not significantly different at P < 0.05. Bars above each scion–rootstock combination represent the SEM of three replications.

  • Fig. 4.

    Soluble sugar concentration in wood for five pecan scion–rootstock combinations before (on 19 Apr 2021) and after the spring freeze (on 21 Apr 2021). Means with the same letter are not significantly different at P < 0.05. Bars above each scion–rootstock combination represent the SEM of three replications.

  • Fig. 5.

    Starch concentration in wood for five pecan scion–rootstock combinations before (on 19 Apr 2021) and after the spring freeze (on 21 Apr 2021). Means with the same letter are not significantly different at P < 0.05. Bars above each scion–rootstock combination represent the SEM of three replications.

  • Fig. 6.

    Starch concentration in bark for five pecan scion–rootstock combinations before (on 19 Apr 2021) and after a spring freeze (on 21 Apr 2021). Means with the same letter are not significantly different at P < 0.05. Bars above each scion–rootstock combination represent the standard error for the mean of three replications.

  • Fig. 7.

    (A, C, E) Pawnee–Peruque catkin and staminate flower section collected before the freeze event. (B, D, F) Pawnee–Peruque catkin and staminate flower section collected after the freeze event under microscopy (10×). Red, yellow, and black arrows indicate anther wall (epidermis and endothecium), tapetum, and pollen grains, respectively.

  • Fig. 8.

    Kanza–Giles (A) and Kanza–Colby (B) catkin and staminate flower section collected before the freeze event under microscopy (4×). Maramec–Colby (C) catkin and staminate flower section collected before the freeze event under microscopy (10×).

  • Fig. 9.

    Branches bearing catkins of Kanza–Mount (A) and Pawnee–Peruque (B). (C) Comparison of Kanza–Giles and Pawnee–Peruque catkins 2 d before the expected freezing event. (Image by Amandeep Kaur, obtained on 19 Apr 2021.)

  • Barbera G, Marco LD, Mantia TL, Schirra M. 1993. Effect of rootstock on productive and qualitative response of two almond varieties. Acta Hortic. 373:129134. https://doi.org/10.17660/ActaHortic.1994.373.17.

    • Search Google Scholar
    • Export Citation
  • Banerjee AK, Lin T, Hannape DJ. 2009. Untranslated regions of a mobile transcript mediate RNA metabolism. Plant Physiol. 151:18311843. https://doi.org/10.1104/pp.109.144428.

    • Search Google Scholar
    • Export Citation
  • Bazot S, Barthes L, Blanot D, Fresneau C. 2013. Distribution of non-structural nitrogen and carbohydrate compounds in mature oak trees in a temperate forest at four key phenological stages. Trees (Berl). 27:10231034. https://doi.org/10.1007/s00468-013-0853-5.

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Amandeep Kaur Department of Horticulture and Landscape Architecture, Oklahoma State University, Stillwater, OK 74078, USA

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Lu Zhang Department of Horticulture and Landscape Architecture, Oklahoma State University, Stillwater, OK 74078, USA

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Ming Yang Department of Plant Biology, Ecology, and Evolution, Oklahoma State University, Stillwater, OK 74078, USA

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Niels Maness Department of Horticulture and Landscape Architecture, Oklahoma State University, Stillwater, OK 74078, USA

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Charles J. Graham Noble Research Institute, Ardmore, OK, 73401, USA

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Yanwei Sun Department of Horticulture and Landscape Architecture, Oklahoma State University, Stillwater, OK 74078, USA

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Srijana Panta Department of Horticulture and Landscape Architecture, Oklahoma State University, Stillwater, OK 74078, USA

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Louise Ferguson Department of Plant Sciences, University of California Davis, Davis, CA 95616, USA

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Contributor Notes

The authors declare that they have no conflict of interest.

The work was funded by the Oklahoma Department of Agriculture, Food, and Forestry Specialty Crop Grant Program: Flowering Management: Minimizing the Harms Caused by Spring Freeze in Pecans, J.D. (Scotty) Scott Horticulture Research Endowed Professorship and Oklahoma Pecan Growers’ Association.

L.Z. is the corresponding author. E-mail: luzhang@okstate.edu.

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  • Fig. 1.

    (A–E) Green terminal tips, catkins, and leaves before the freeze event (on 19 Apr 2021, 2 d before the expected freezing event). (F, H) Damage to the terminal leaves after the freeze event (on 21 Apr 2021). (G) Almost no damage to leaves and catkins after the freeze event. (I, J) Terminal bud, catkins, and leaves damage after the freeze event. Black and red arrow in (F–J) indicate leaves and catkins, respectively. For all the scion–rootstock combinations, the visual observations and images were obtained on the same days previously mentioned before and after the freeze events.

  • Fig. 2.

    The minimum temperature (in °C) from 19 Apr 2021 (2 d before the expected freezing event) until 21 Apr 2021 (the day of the freeze event) in Perkins, OK, USA (https://www.mesonet.org/). The temperature was recorded hourly.

  • Fig. 3.

    Soluble sugar content in bark for five pecan scion–rootstock combinations before (on 19 Apr 2021) and after the spring freeze (on 21 Apr 2021). Means with the same letter are not significantly different at P < 0.05. Bars above each scion–rootstock combination represent the SEM of three replications.

  • Fig. 4.

    Soluble sugar concentration in wood for five pecan scion–rootstock combinations before (on 19 Apr 2021) and after the spring freeze (on 21 Apr 2021). Means with the same letter are not significantly different at P < 0.05. Bars above each scion–rootstock combination represent the SEM of three replications.

  • Fig. 5.

    Starch concentration in wood for five pecan scion–rootstock combinations before (on 19 Apr 2021) and after the spring freeze (on 21 Apr 2021). Means with the same letter are not significantly different at P < 0.05. Bars above each scion–rootstock combination represent the SEM of three replications.

  • Fig. 6.

    Starch concentration in bark for five pecan scion–rootstock combinations before (on 19 Apr 2021) and after a spring freeze (on 21 Apr 2021). Means with the same letter are not significantly different at P < 0.05. Bars above each scion–rootstock combination represent the standard error for the mean of three replications.

  • Fig. 7.

    (A, C, E) Pawnee–Peruque catkin and staminate flower section collected before the freeze event. (B, D, F) Pawnee–Peruque catkin and staminate flower section collected after the freeze event under microscopy (10×). Red, yellow, and black arrows indicate anther wall (epidermis and endothecium), tapetum, and pollen grains, respectively.

  • Fig. 8.

    Kanza–Giles (A) and Kanza–Colby (B) catkin and staminate flower section collected before the freeze event under microscopy (4×). Maramec–Colby (C) catkin and staminate flower section collected before the freeze event under microscopy (10×).

  • Fig. 9.

    Branches bearing catkins of Kanza–Mount (A) and Pawnee–Peruque (B). (C) Comparison of Kanza–Giles and Pawnee–Peruque catkins 2 d before the expected freezing event. (Image by Amandeep Kaur, obtained on 19 Apr 2021.)

 

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