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  • Author or Editor: James P. Syvertsen x
  • Journal of the American Society for Horticultural Science x
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Mechanical harvesting of citrus trees can cause physical injuries, such as shedding of leaves, exposing roots, and scuffing bark. Although mechanical harvesting usually has not reduced yield, physiological consequences to the tree from these visible injuries have not been investigated. We hypothesized that physical injuries to tree canopies and root systems from a properly operated trunk shaker would not cause short-term physiological effects. Tree water status and leaf gas exchange of mature `Hamlin' and `Valencia' sweet orange [Citrus sinensis (L.) Osb.] trees that were harvested by a trunk shaker were compared to hand-harvested trees. A trunk shaker was operated with adequate duration to remove >90% of mature fruit or with excessive shaking time under various environmental conditions and drought stress treatments throughout the harvest season. Mid-day stem (Ψstem) and leaf (Ψleaf) water potentials along with leaf gas exchange were measured before and after harvest. Trees harvested by the trunk shaker did not develop altered water status under most conditions. Trees harvested with excessive shaking time and/or with limited soil water supply developed low Ψstem resembling Ψstem of drought-stressed trees. However, water potential of all treatments recovered to values of the well-irrigated, hand-harvested trees after rainfall. In addition, mechanical harvesting did not reduce CO2 assimilation, transpiration, stomatal conductance, water use efficiency, or photosystem II efficiency as measured by chlorophyll fluorescence. Thus, despite visible injuries, a properly operated trunk shaker did not result in any measurable physiological stress.

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Effects of foliar sprays of a kaolin clay particle film (Surround WP) on leaf temperature (Tlf), net gas exchange, chlorophyll fluorescence and water relations of sun-exposed leaves on field-grown grapefruit trees (Citrus paradisi L.) were studied during Summer and Fall 2001. Trees were sprayed twice a week for 3 weeks with aqueous suspensions of kaolin (Surround) at 60 g·L-1. Physiological effects of kaolin application were most prominent around midday on warm sunny days than in mornings, evenings or cloudy days. Kaolin sprays increased leaf whiteness (62%), reduced midday leaf temperature (Tlf; ≈3 °C) and leaf to air vapor pressure differences (VPD; ≈20%) compared to water-sprayed control leaves. Midday reductions in Tlf and VPD were accompanied by increased stomatal conductance (gs) and net CO2 assimilation rates (ACO2) of kaolin sprayed leaves, suggesting that gs might have limited ACO2 in water-sprayed control leaves. Midday photoinhibition of photosynthesis was 30% lower in kaolin-sprayed leaves than in control leaves. Midday water use efficiency (WUE) of kaolin-sprayed leaves was 25% higher than that of control leaves. However, leaf transpiration and whole-tree water use were not affected by kaolin film sprays. Increased WUE was therefore, due to higher ACO2. Leaf intercellular CO2 partial pressures (Ci) were similar in control and kaolin-sprayed leaves indicating that stomatal conductance was not the major cause of reduced ACO2. These results demonstrate that kaolin sprays could potentially increase grapefruit leaf carbon uptake efficiency under high radiation and temperature stress.

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We studied whether foliar-applied N uptake from a single application of low-biuret N-urea or K NO to citrus leaves was affected by N source, leaf age, or whole-shoot N content. In a glasshouse experiment using potted 18-month-old Citrus paradisi (L.) `Redblush' grapefruit trees grown in full sun, 2- and 6-month-old leaves on single shoots were dipped into a 11.2 g N/liter (1.776% atom excess N-urea) solution with 0.1% (v/v) Triton X-77. Two entire trees were harvested 1.5,6,24, and 48 hours after N application. Uptake of N per unit leaf area was 1.6- to 6-fold greater for 2-month-old leaves than for older leaves. The largest proportion of N remained in the treated leaf, although there was some acropetal movement to shoot tips. In a second experiment, 11.2 g N/liter (3.78% atom excess) urea-15N and 3.4 g N/titer (4.92% atom excess) KNO solutions of comparable osmotic potential were applied to 8-week-old leaves on 5-year-old `Redblush' grapefruit field-grown trees of differing N status. Twenty-four percent of the applied N-urea was taken up after 1 hour and 54% after 48 hours. On average, only 3% and 8% of the K NO was taken up after 1 and 48 hours, respectively. Urea increased leaf N concentration by 2.2 mg N/g or 7.5% of total leaf N after 48 hours compared to a 0.5 mg N/g increase (1.8% of total leaf N) for KNO. Foliar uptake of N from urea, however, decreased (P < 0.05) with increasing total shoot N content after 48 hours (r = 0.57).

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We examined how N supply affected plant growth and N uptake, allocation and leaching losses from a fine sandy soil with four Citrus rootstock species. Seedlings of `Cleopatra' mandarin (Citrus reticulata Blanco) and `Swingle' citrumelo (C. paradisi × P. trifoliata) were grown in a glasshouse in 2.3-liter pots of Candler fine sand and fertilized weekly with a complete nutrient solution containing 200 mg N/liter (20 mg N/week). A single application of 15NH4 15NO3(17.8% atom excess 15N) was substituted for a normal weekly N application when the seedlings were 22 weeks old (day O). Six replicate plants of each species were harvested at 0.5, 1.5, 3.5, 7, 11, and 30 days after 15N application. In a second experiment, NH4 NO3 was supplied at 18,53, and 105 mg N/week to 14-week-old `Volkamer' lemon (C. volkameriana Ten. & Pasq.) and sour orange (C. aurantium L.) seedlings in a complete nutrient solution for 8 weeks. A single application of 15NH4 15NO3 (23.0% 15N) was substituted at 22 weeks (day 0), as in the first experiment, and seedlings harvested 3,7, and 31 days after 15N application. Nitrogen uptake and partitioning were similar among species within each rate, but were strongly influenced by total N supply and the N demand by new growth. There was no 15N retranslocation to new tissue at the highest (105 mg N/week) rate, but N supplies below this rate limited plant growth without short-term 15N reallocation from other tissues. Leaf N concentration increased linearly with N supply up to the highest rate, while leaf chlorophyll concentration did not increase above that at 53 mg N/week. Photosynthetic CO2 assimilation was not limited by N in this study; leaf N concentration exceeded 100 mmol·m-2 in all treatments. Thus, differences in net productivity at the higher N rates appeared to be a function of increased leaf area, but not of leaf N concentration. Hence, N use efficiency decreased significantly over the range of N supply, whether expressed either on a gas-exchange or dry weight basis. Mean plant 15N uptake efficiencies after 31 days decreased from 60% to 47% of the 15N applied at the 18,20, and 53 mg N/week rates to less than 33% at the 105 mg N/week rate. Leaching losses increased with N rate, with plant growth rates and the subsequent N requirements of these Citrus species interacting with residual soil N and potential leaching loss.

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Correlations between extractable leaf chlorophyll (Chl) concentration and portable, nondestructive leaf “greenness” meter readings imply that such estimates can be used as surrogate measurements of leaf nitrogen (N) status. However, few studies have actually found a direct relationship between Chl meter readings and leaf N. We evaluated the utility of two handheld transmittance-based Chl content meters (SPAD-502, Minolta Corp. and CCM-200, Opti-Sciences) and one reflectance-based meter (Observer, Spectrum Technologies), in estimating Chl and N concentrations in intact leaves of several citrus cultivars. Total Chl determined analytically, correlated well with nondestructive Chl meter readings (r 2: 0.72 to 0.97; P < 0.0001), but regression models differed among cultivars using the same meter and also among meters for a given cultivar. The relationships were generally more linear and stronger at low Chl concentrations (<0.5 mmol·m-2) than at higher Chl concentrations, reflecting increased variability in Chl meter readings with increasing leaf Chl. Significant relationships between Chl meter readings and measured leaf N concentrations were also found in all the cultivars tested (r 2: 0.23 to 0.69; P < 0.01), but the data were more variable than those for Chl. Field-grown leaves were significantly thicker and had higher Chl meter readings than greenhouse-grown leaves of similar Chl or N concentrations. The results suggest that nondestructive Chl content meters can overestimate Chl and N in thicker leaves and/or leaves with high Chl concentrations. A single prediction equation derived from a wide range of Chl or N concentrations could be applicable across the range of citrus cultivars when grown in the same environment. Potential limitations associated with leaf thickness as influenced by environmental factors may necessitate the development of more specific calibration equations.

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Although urea can be an effective adjuvant to foliar sprays, we examined effects of additional surfactants on urea penetration through leaf cuticles along with the effect of urea with and without surfactants on net gas exchange of leaves of `Marsh' grapefruit (Citrus paradisi Macf.) trees budded to Carrizo citrange (C. sinensis L. Osbeck × Poncirus trifoliata L. Raf.) rootstock. Various combinations of urea, a nonionic surfactant (X-77), and an organosilicone surfactant (L-77), were applied to grapefruit leaves and also to isolated adaxial cuticles. When compared to X-77, L-77 exhibited superior surfactant features with smaller contact angles of droplets deposited on a teflon slide. Both L-77 and X-77 initially increased penetration rate of urea through cuticles, but the effect of X-77 was sustained for a longer period of time. The total amount of urea which penetrated within a 4-day period, however, was similar after addition of either surfactant. Solutions of either urea, urea + L-77, urea + X-77, or L-77 alone decreased net assimilation of CO2 (ACO2) for 4 to 24 hours after spraying onto grapefruit leaves. A solution of X-77 alone had no effect on ACO2 over the 4-day period. Although reductions in ACO2 were similar following sprays of urea formulated with two different surfactants, the underlying mechanisms may not have been the same. For the urea + X-77 treatment, X-77 increased the inhibitory effects of urea on ACO2 indirectly by increasing penetration of urea into leaves. For the urea + L-77 formulation, effects of L-77 on ACO2 were 2-fold, direct by inhibiting ACO2 and indirect by increasing urea penetration. One hour after application, scanning electron microscopy (SEM) of leaf surfaces treated with X-77 revealed that they were heavily coated with the residue of the surfactant, whereas leaves treated with L-77 looked similar to nontreated leaves with no apparent residues on their surfaces. The amount of X-77 residue on the leaves was lower 24 hours after application than after 1 hour as observed by SEM.

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Effects of air temperature, relative humidity (RH), and leaf age on penetration of urea through isolated leaf cuticles of `Marsh' grapefruit (Citrus×paradisi Macfad.) trees on `Carrizo' citrange (C. sinensis L. Osbeck × Poncirus trifoliata (L.) Raf. rootstock were examined. Intact cuticles were obtained from adaxial surfaces of `Marsh' grapefruit leaves of various ages. A finite dose diffusion system was used to follow movement of 14C-labeled urea from urea solution droplets across cuticles throughout a 4-day period. Within the first 4 to 6 hours after urea application, the rate of urea penetration increased as temperature increased from 19 to 28 °C, but there was no further increase at 38 °C. Increasing relative humidity increased urea penetration at 28 °C and 38 °C. Cuticle thickness, cuticle weight per area, and the contact angle of urea solution droplets increased as leaves aged. Cuticular permeability to urea decreased as leaf age increased from 3 to 7 weeks, but permeability increased in cuticles from leaves older than 9 weeks. Contact angles decreased with increased urea solution concentration on leaf surfaces that were 6 to 7 weeks old, but solution concentration had no effect on contact angle on cuticles from younger and older leaves. Changing urea solution pH from 8.0 to 4.0 could have an effect on the amount of urea penetrating the cuticle through the loss of urea from breakdown possibly due to hydrolysis. Results from this study define leaf age, environmental conditions, and formulation for maximum uptake of foliar-applied urea.

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15Nitrogen uptake, allocation, and leaching losses from soil were quantified during spring, for 4-year-old bearing `Redblush' grapefruit (Citrus × paradisi Macf.) trees on rootstocks that impart contrasting growth rates. Nine trees on either the fast-growing `Volkamer' lemon (VL) (C. volkameriana Ten & Pasq.) or nine on the slower-growing sour orange (SO) (C. aurantium L.) rootstocks were established in drainage lysimeters filled with Candler fine sand and fertilized with 30 split applications of N, totaling 76, 140, or 336 g·year-1 per tree. A single application of double-labeled ammonium nitrate (15NH 15 4NO3, 20% enriched) was applied at each rate to replicate trees, in late April. Leaves, fibrous roots, soil, and leachates were intensively sampled from each treatment over the next 29 days, to determine the fate of the 15NH 15 4NO3 application. Newly developing spring leaves and fruit formed dominant competitive sinks for 15N, accounting for between 40% and 70% of the total 15N taken up by the various treatments. Large fruit loads intercepted up to 20% of this 15N, at the expense of spring flush development, to the detriment of overall tree N status in low-N trees. Nitrogen supply at less than the currently recommended yearly rate of 380 g/tree exceeded the requirements of 4-year-old grapefruit trees on SO rootstock; however, larger trees on VL rootstock took up the majority of 15N from this rate over the 29-day period. Nitrogen-use efficiency declined with increasing N rate, irrespective of rootstock. The residual amounts of 15N remaining in the soil profile under SO trees after this time represented a significant N leaching potential from these sandy soils. Therefore, under these conditions, present N recommendations appear adequate for rootstocks that impart relatively fast growth rates to Citrus trees, but seem excessive for trees on slower-growing rootstock species.

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The combined effects of O3 and acid rain on freeze resistance, growth, and mineral nutrition were studied using broadleaf-evergreen citrus and avocado trees. Using a factorial design, `Ruby red' grapefruit (Citrus paradisi L.) trees on either Volkamer lemon (Citrus volkameriana Ten. & Pasq.) or sour orange (Citrus aurantium L.) rootstock and `Pancho' avocado trees (Persea americana Mill.) on `Waldin' rootstock were exposed to O3 and acid rain for 8 months in open-top chambers under field conditions. The O3 treatments were one-third ambient (0.3X), ambient (1X), twice ambient (2X), or thrice ambient (3X). Ambient O3 concentrations averaged 39.1 nl·liter-3 over a 12-hour day. The acid rain treatments had a pH of 3.3, 4.3, or 5.3 and were applied to simulate long-term rainfall averages. In general, the effects of acid rain on growth and freeze resistance were small. Rain of high acidity (pH = 3.3) offset the negative effects of O3 on growth (total leaf mass) in avocado and grapefruit/Volkamer lemon trees. In contrast, rain of high acidity magnified the detrimental effects of O3 on electrolyte leakage of leaf disks at subzero temperatures, especially for citrus. Freeze resistance, determined by stem and whole-plant survival following freezing temperatures, was lower in the most rapidly growing trees. Consequently, for trees exposed to a combination of O3 and acidic rain, leaf electrolyte leakage did not correlate significantly with stem survival of freezing temperatures. We conclude that the danger of acid rain to citrus and avocado in Florida is rather slight and would only present a potential problem in the presence of extremely high O3.

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