Plant materials and growth conditions.

‘Roma VF’ tomato (Westar Seeds International, El Centro, CA) was used in this study. ‘Roma VF’ is not a commercial rootstock but we employed this cultivar for our study as a model system because of the abundant availability of seeds. Seeds were sown into 98-cell seedling trays (28 × 55 cm, one seed per cell) filled with moist commercial substrate (Sunshine Professional Mix 3; Sun Gro Horticulture, Agawam, MA) covered with a thin layer of vermiculite. The seeded trays were then covered with a thin plastic film and placed for 2 d under darkness in a growth chamber (model 2015; VWR International, Radnor, PA) controlled at 29 ± 1 °C air temperature. Two days after seeding, the trays were placed in the greenhouse and irrigated every 2 d with tap water. After day 11, the plants were irrigated daily with tap water and twice weekly with nutrient solution (EC 2.0 to 2.4 dS·m⁻¹, pH 5.9 to 6.3) containing 90 mg·L⁻¹ nitrogen (all in nitrate form), 47 mg·L⁻¹ phosphorus, 144 mg·L⁻¹ potassium, 160 mg·L⁻¹ calcium, 60 mg·L⁻¹ magnesium, and 113 mg·L⁻¹ sulfur, as well as micronutrients. The greenhouse used in the present experiments was located in Tucson, AZ, covered by double-layered acrylic roof and walls, and equipped with pad-and-fan cooling and overhead gas heating systems. The daytime and nighttime set points of air temperature were 28 and 21 °C, respectively, using a commercial greenhouse controller (Wadsworth Control Systems, Arvada, CO). After 17 or 18 d from seeding, when seedlings were at the three-true-leaf stage, uniform seedlings were selected for the experiments.

Fatty alcohol.

A commercially available fatty alcohol compound (N-TAC^®; Fair Products, Cary, NC), consisting of 36.2% C₈, 48.2% C₁₀, 0.3% C₁₂ fatty alcohol, and 15.3% other ingredients, was used in this study. Fatty alcohol was diluted with ion-exchanged water to prepare emulsions having product concentrations from 0.5% to 15% (v/v) for the experiments described below.

Fatty alcohol treatment to axillary buds (Expt. 1).

Seventeen days after seeding (5 May 2014), plants were treated with 0%, 2%, 5%, 10%, or 15% (v/v) concentration of fatty alcohol emulsions. Using a pipette, 5 µL of the emulsion was applied carefully to each cotyledonary axil area of all the seedlings. The application volume was selected so that it thoroughly covers the axillary bud without overflowing to contact the stem. To assess the effectiveness of the fatty alcohol treatment, plants were pinched about 1 cm above cotyledons 1 d after the treatment to force undamaged axillary buds to develop shoots. Plants were visually inspected for any sign of chemical damage or axillary shoot extension 1, 7, 14, and 22 d after treatment. Because the incidence of axillary shoot extension became unchanged after day 14, we conducted the inspection up to day 22. To examine the degree of damage to the axillary buds caused by fatty alcohol treatment, the axillary buds of three to five representative plants per treatment were observed under stereomicroscope (Vista Vision, VWR International).

Fatty alcohol treatment to extended axillary shoots (Expt. 2).

Eighteen days after seeding (6 May 2014), plants were pinched about 1 cm above cotyledons to force cotyledonary axillary shoots to develop. Seven days after pinching, axillary shoots in length from 1.8 to 3.2 cm were treated with 0%, 0.5%, 1%, 2%, 4%, or 5% (v/v) concentration of fatty alcohol emulsions. These concentrations examined in this experiment were selected based on the phytotoxicity observed in Expt. 1. Using a pipette, 250 µL of the emulsion was applied to each axillary shoot. The application volume was selected so that the axillary shoot was covered thoroughly with emulsion. Plants were visually inspected for any sign of chemical damage and axillary shoot growth 1, 7, 15, and 21 d after treatment.

Experimental design.

The study was conducted from 18 Apr. to 3 June 2014. For both experiments (Expt. 1 and 2), each treatment consisted of 10 plants and the treatments were repeated three times using a seedling tray (98 cells) as a replication. Five (Expt. 1) and 6 treatments (Expt. 2) of different fatty alcohol concentrations were randomly assigned within each seedling tray. Positions of three trays over the bench inside the greenhouse were rotated systematically every day to improve the uniformity of environments. JMP Pro (version 9.0; SAS Institute, Cary, NC) was used to analyze data. Percent data were arcsine-transformed and their means were separated by Tukey’s honest significant difference test at P ≤ 0.05.

Greenhouse environments.

Average day/night air temperatures in the greenhouse were 24.6 ± 2.4/18.5 ± 2.1 °C in Expt. 1 and 25.2 ± 2.7/19.0 ± 2.4 °C in Expt. 2. Average DLIs in the greenhouse were estimated from the solar radiation recorded in the weather station (The University of Arizona, 2014) and the predetermined greenhouse light transmission of 42%: 25.2 ± 2.6 mol·m⁻²·d⁻¹ in Expt. 1 and 25.0 ± 3.2 mol·m⁻²·d⁻¹ in Expt. 2.

Expt. 1.

Seven days after fatty alcohol treatment, the control plants (treated with 0% fatty alcohol) had 100% of plants with axillary shoot extension, induced by pinching (Table 1; Fig. 1A). At 2%, 5%, and 10% concentrations, the percentage of plants with axillary shoot extension increased over the first 14 d but remained the same after day 14 (Table 1). Twenty-two days after fatty alcohol treatment, the percentage of plants with axillary shoot extension at 2% fatty alcohol concentration was nonsignificantly different from that in the control. The percentage of plants with axillary shoot extension at 5% was significantly lower than that in the control but at an unacceptable level (23.3%). The high concentrations (10% and 15%) of fatty alcohol treatment significantly suppressed the percentage of plants with axillary shoot extension to below 7% throughout the experiment (Table 1). Under the stereomicroscope, the plants with no axillary shoot extension had the necrotic and desiccated buds in the cotyledonary axils (Fig. 1B).

Table 1.

Effects of fatty alcohol at various concentrations applied to cotyledonary axillary buds in pinched tomato seedling on the resulting percentage of plants with cotyledonary axillary shoot extension (7, 14, or 22 d after treatment) and the percentage of plants with chemical damage (22 d after treatment) (Expt. 1).

View Table

View Full Size

Tomato seedlings with fatty alcohol at various concentrations applied to cotyledonary axillary buds [A–D (Expt. 1)] or extended cotyledonary axillary shoots [E–H (Expt. 2)]. (A) A seedling with axillary shoot extension from cotyledons 7 d after 0% fatty alcohol treatment. (B) A seedling with necrotic axillary buds 7 d after 5% fatty alcohol treatment. (C) A seedling with adventitious shoots at the stem between the cut stem end and cotyledons 22 d after fatty alcohol treatment. (D) A seedling with cotyledon damage caused by 15% fatty alcohol treatment. (E) A seedling with surviving cotyledonary axillary shoots, showing growth 7 d after 0% fatty alcohol treatment. (F) A seedling with damaged cotyledonary axillary shoots, showing no growth 7 d after 4% fatty alcohol treatment. (G) Cotyledonary axillary shoots with necrotic shoot tip, leaves and petioles caused by 4% fatty alcohol treatment. (H) A collapsed seedling caused by 5% fatty alcohol treatment.

Citation: HortTechnology hortte 25, 4; 10.21273/HORTTECH.25.4.569

• Download Figure
• Download figure as PowerPoint slide

Fourteen days after treatment, adventitious secondary shoots were observed in some plants with no axillary shoot extension (data not shown). At day 22, the number of plants with adventitious shoots increased, and almost all plants with no axillary shoot extension had these adventitious shoots (data not shown). These adventitious shoots appeared to develop at either the pinched surface or the stem between the cut stem end and cotyledons, and usually they grew in clumps (Fig. 1C), unlike the axillary shoots developed from cotyledons (Fig. 1A).

The adventitious shoot development was presumably because of the lack of active apical and axillary meristems that produce auxin. Tomato seems to produce many adventitious shoots from cut surfaces of stems when apical and axillary meristems were removed. For example, tomato in vitro explants grown on solid tissue culture medium without growth regulators regenerated 2.9–5.3 shoots on the cut surface after removing the apical and axillary meristems (Pozueta-Romero et al., 2001). Also, tomato plants grown in a greenhouse reportedly regenerated many adventitious shoots from the cut surface of the stem and axillary shoots after the plants were decapitated and the axillary shoots that emerged after decapitation were excised (Harada et al., 2005; Johkan et al., 2008). If rootstock axillary buds of grafted plants were treated, the problematic adventitious shoot development would be less, as the growing scion could provide auxin to prevent adventitious shoot development. Tezuka et al. (2011) showed that adventitious shoot regeneration after decapitation was inhibited by the presence of axillary shoots or by application of 1-naphthaleneacetic acid in decapitated tomato plants grown in a greenhouse. However, Bausher (2011) found that rootstock shoot regrowth from cotyledons recurred even after original axillary shoots were pruned in grafted tomato plants especially when grafted above the interspecific tomato rootstock cotyledons. Therefore, adventitious shoots might occur even in grafted plants treated with high concentration of fatty alcohol. In that case, multiple applications of fatty alcohol after grafting and healing or alternative methods might be required to control axillary shoot and adventitious shoot growth from rootstock.

The degree of adventitious shoot development seems to be species specific. Commercial tomato rootstocks are generally classified in two types—intraspecific hybrid rootstocks (S. lycopersicum) and interspecific rootstocks (S. lycopersicum × S. habrochaites). Steinitz et al. (2006) evaluated the difference in ability for seedlings to regenerate adventitious shoots among nine species of wild tomato relatives and showed that S. habrochaites generated adventitious shoots at a significantly greater extent than S. lycopersicum when apical and axillary meristems were removed. Therefore, interspecific hybrid rootstock may have more chance to develop adventitious shoots after killing axillary shoots by fatty alcohol treatment than what we observed with ‘Roma VF’ (S. lycopersicum). Further study is needed to investigate possible development of rootstock adventitious shoots occurring after fatty alcohol treatment using grafted tomato plants.

Other chemical damage on the plants such as the stem scarring and the cotyledon damage (Fig. 1D) caused by fatty alcohol treatment were observed 1 d after treatment. Twenty-two days after treatment, the plants treated with 2%, 5%, 10%, and 15% fatty alcohol had 30.0%, 76.7%, 100%, and 100% incidence of the plants exhibiting stem scarring, respectively (Table 1). For the 5%, 10%, and 15% fatty alcohol treatments, 10.0%, 33.3%, and 50.0% of the plants had cotyledon damage, respectively. However, retaining cotyledons is not needed for tomato grafting. Also stem scarring and cotyledon damage did not seem to affect growing ability of plants after the fatty alcohol treatment in our experiment.

Expt. 2.

Seven days after fatty alcohol treatment, the control plants (treated with 0% fatty alcohol) had 100% of plants with surviving cotyledonary axillary shoots (Table 2, Fig. 1E). Nearly 100% of plants treated with 2% or lower concentration fatty alcohol had surviving axillary shoots, suggesting that the concentrations or dose of fatty alcohol was not enough to kill extended axillary shoot (Table 2). Fatty alcohol treatments at 4% and 5% concentrations significantly reduced axillary shoot survival and suppressed axillary shoot growth during 21 d after treatment, and the percentage of plants with surviving axillary shoots were 40% and 20%, respectively, 21 d after treatment (Table 2; Fig. 1F). However, these percentages are unacceptably high for controlling cotyledonary axillary shoots from rootstock. This result indicates that fatty alcohol concentration needs to be higher than 5% to sufficiently eliminate extended cotyledonary axillary shoots. It was observed that the axillary shoots with no growth had necrotic shoot tip, leaves, and petioles (Fig. 1G).

Table 2.

Effects of fatty alcohol at various concentrations applied to extended cotyledonary axillary shoots of pinched tomato seedling on the resulting percentage of plants with surviving axillary shoot (7, 15, or 21 d after treatment) and the percentage of plants collapsed (21 d after treatment) (Expt. 2).

View Table

However, at 2% or greater concentrations, some plants collapsed after the treatment (Fig. 1H). The plants treated with 0%, 0.5%, and 1% fatty alcohol had no incidence of collapse during 21 d of observation (Table 2). For the 2%, 4%, and 5% fatty alcohol treatments, 16.7%, 30.0%, and 43.3% of the plants collapsed after 21 d (Table 2). These levels of damage are unacceptably high even though they were nonsignificantly different from that of the control except for 5% treatment. These plants collapsed at the base of the stem, likely due to that excessive amount of fatty alcohol flowing down the stem to the substrate, causing catastrophic damage of the base of stem or the roots.

This collapse was observed in Expt. 2 but not in Expt. 1, despite that higher concentrations were applied in Expt. 1. This is probably because the volume of fatty alcohol applied to the plants in Expt. 2 was greater than that in Expt. 1. In Expt. 2, 250 µL of fatty alcohol emulsion was applied to cover the axillary shoot thoroughly and it was observed that fatty alcohol emulsion flowed down the stem, which was not observed in the Expt. 1 where only 5 µL was applied to the target axil. From these experiments, it is suggested that the degree of chemical damage may differ depending on the dose and application method of fatty alcohol. Two percent and higher concentration fatty alcohol could cause collapse at a higher than 10% chance when fatty alcohol spills from the target shoot and flows down stem.

This experiment demonstrates the challenge of controlling already extended axillary shoots with fatty alcohol, because the effective concentration in this type of application seems to be higher than 5%, what was examined in this experiment, yet is enough to collapse whole plants because of the volume of fatty alcohol required to cover the axillary shoots. For this reason, the effective concentration of fatty alcohol has to be applied exclusively to target axillary buds without excessive fatty alcohol overflow to prevent undesirable damage. Sensitivity of true leaves and stems to fatty alcohol was also demonstrated in our separately conducted experiment examining fatty alcohol application on wide area of whole plants using an ordinary spray bottle. The result showed that spraying fatty alcohol on stems at 2% or higher or on leaves at 4% or higher concentration caused notable damage, resulting in the collapse of more than half of plants (data not shown).

Steffens et al. (1967) showed that fatty alcohol effectively damaged meristematic tissues, but caused little or no visible injury to more mature tissues in tobacco due to the protective action of surfactants. In tobacco production, commercially available fatty alcohol compounds at about 3% to 6% (v/v) product concentration have been used to control sucker growth after topping. The tobacco topping and sucker control are applied conventionally after the production of 18 to 22 leaves. For young tomato seedlings at three-true-leaf stage, 5% and lower concentration of fatty alcohol did not suppress cotyledonary axillary shoot growth sufficiently in pinched plants but caused true leaf damage and significant percent plant collapse despite of the inclusion of surfactant in the fatty alcohol product. This indicates that controlling cotyledonary axillary shoot growth by fatty alcohol treatment without damaging stems and true leaves is difficult possibly due to the relatively young stage of plants upon application. However, the previous reports (Maw, 1977; Tucker and Maw, 1975) showed that fatty alcohol must be applied exclusively to leaf axils, to control axillary shoot growth without injury to mature leaves in tomato at age of 5, 6, or 9 weeks, suggesting that tomato plants are possibly susceptible to this type of chemical regardless of physiological age of tissue, or that surfactants used in fatty alcohol products for tobacco might not work for protecting mature tissue of tomato. Steffens and Cathey (1969) reported that the selective action of fatty acid methyl ester and fatty alcohol as chemical pruning agents and tobacco sucker control agents depends on the class and amount of surfactant used to emulsify these chemicals. The appropriate ratio of fatty ester or alcohol to surfactants, emulsion concentration, and surfactant type may be different among plant species.

To protect mature tissues, Maw (1977) used a small brush and a felt tip marker to apply fatty alcohol only to the axillary shoots in 5 or 6 weeks of tomato plants without injury to other parts of plant. Logendra et al. (2004) used a syringe attached to a pipette to apply fatty acid methyl ester or fatty acid only to leaf axils of tomato plants after 45 or 55 d of seeding. In commercial propagation setting, an innovative, rapid and effective application method targeting the cotyledonary axils without excessive fatty alcohol overflow needs to be developed to avoid undesirable damage to the stems and true leaves. Application timing of fatty alcohol to the rootstock axillary buds may be critical as the effective concentration to kill the buds is highly toxic to leaves and stems. Based on our preliminary study, application after the healing and acclimatization of grafted plants seems to be less problematic than application before or during the grafting.