Transplant shock is caused by various types of abiotic stress, limiting stand establishment and productivity of many vegetable crops. Although postplanting stress can be minimized under well-managed field conditions, mechanical stress is unavoidable during the transport and transplanting of seedlings. Mechanical stress stimulates ethylene production, which in turn, induces overall growth retardation as a stress adaptation strategy. We hypothesized that, under optimum field conditions, transplant shock is caused primarily by ethylene-induced stress responses, and that inhibiting ethylene action can reduce transplant shock by maintaining uninterrupted growth. In this study, a new spray formulation of 1-methylcyclopropene (1-MCP) was used to inhibit ethylene perception in tomato (Solanum lycopersicum L.) seedlings. A bioassay experiment demonstrated reduced ethylene sensitivity in 1-MCP–treated (1 mg·L−1) seedlings using leaf epinasty and chlorosis as measured responses. Field experiments evaluated growth, physiological, and yield responses to preplant spray treatment of 1-MCP (12.5–50 mg·L−1) under optimum field conditions. Postplanting growth modulation by 1-MCP at the flowering stage was characterized by enhanced height growth and suppressed stem diameter growth, indicating the inhibition of ethylene-induced stress responses. At the fruit harvest stage, preplant 1-MCP treatment increased shoot biomass by 23% and flower production by 22%, while improving photosynthetic capacity on a whole-plant basis. As a result, 1-MCP–treated plants produced 13% to 24% higher total marketable fruit yields than untreated plants in two consecutive growing seasons. Correlation analyses revealed that flower number increased proportionally to shoot biomass, and marketable fruit number increased proportionally to flower number. These results support our hypothesis and propose that preplant 1-MCP treatment is a new stress-management approach to reducing transplant shock. Importantly, this new technique is easily implementable by commercial transplant nurseries with no negative side effect on transplant quality and fruit development.
Shinsuke Agehara and Daniel I. Leskovar
Vegetable transplants grown in commercial high-density trays can quickly outgrow the optimal size for shipping and transplanting, limiting transplant performance, and marketing flexibility for commercial nurseries. Abscisic acid (ABA) and uniconazole can suppress shoot growth by inducing stress-adaptive responses and inhibiting gibberellin synthesis, respectively. We evaluated the effectiveness of the two growth regulators in prolonging marketability of ‘Florida 91’ and ‘Mariana’ tomato (Solanum lycopersicum L.) transplants at commercial nursery greenhouses in Texas and Florida. Spray treatments in the Texas experiment were 0 and 3.8 mm ABA at 7, 5, 3, or 1 days before maturity (DBM), and those in the Florida experiment were no spray control, 3.8 mm ABA at 7, 5, 3, or 1 DBM, and 34 μm uniconazole at 4 DBM. Both ABA and uniconazole showed minimal cultivar-specific effects. Different growth modifications were induced by ABA and uniconazole. First, suppression of stem elongation by ABA was reversible by 7 days after maturity (DAM), whereas that by uniconazole lasted for 20 days or until 16 DAM with up to 15% suppression in stem elongation. Second, only ABA inhibited leaf expansion and shoot dry matter accumulation. The primary growth-modulating effect of uniconazole was limited to height control, which is beneficial for producing compact transplants, rather than as a growth holding strategy. By contrast, the overall growth suppression by ABA is desirable for prolonging transplant marketability. Importantly, the magnitude of this growth suppression was moderate (up to 22% shoot biomass reduction at 8 DAM) and transient, followed by a rapid recovery. Furthermore, ABA caused relatively smaller inhibition in root growth, allowing sufficient root development and increasing the root-to-shoot ratio at 0 to 8 DAM. The growth suppression by ABA was maximal when it was applied at 7 to 5 DBM, indicating the age-dependent sensitivity of tomato seedlings to exogenous ABA. Although leaf chlorosis was induced by ABA in a similar age-dependent manner, it was transient and reversible by 7 DAM. These results suggest that ABA application 7 to 5 DBM is an effective growth holding strategy for tomato transplants.
Shinsuke Agehara and Daniel I. Leskovar
Height control is important to produce compact vegetable transplants that are suitable for shipping and transplanting. Although abscisic acid (ABA) inhibits stem elongation, it can also induce other growth modifications. To optimize its application timing for effective height control, we examined age-dependent sensitivity of various growth variables to ABA in diploid ‘Summer Flavor 800’ and triploid ‘Summer Sweet 5244’ watermelon [Citrullus lanatus (Thunb.) Matsum & Nakai]. Seedlings were sprayed once with 1.9 mm ABA at 25, 18, or 11 days before transplanting (DBT) or twice with 0.95 mm ABA at 25 and 18 DBT. The application rate was 0.55 mg ABA per plant with a spray volume of 0.61 L·m−2 (1.1 mL/plant). Only the single-spray treatment at 25 DBT (cotyledon stage) suppressed plant height by inhibiting petiole elongation. This effect was similar in both cultivars with 13% to 14% reductions at the transplanting stage compared with the untreated control. Undesirable growth modifications were also induced by ABA. In both cultivars, all ABA treatments caused 16% to 23% shoot biomass reductions mainly by inhibiting leaf expansion. Additionally, ABA treatments reduced stem diameter and root biomass in ‘Summer Flavor 800’. The double-spray treatment had similar growth-modulating effects as the single-spray treatments, except that it induced cotyledon abscission in ‘Summer Flavor 800’. These results suggest that although ABA applied at the cotyledon stage can reduce watermelon transplant height, the benefit is limited because of overall growth reductions, which can occur regardless of application timing. On the other hand, in triploid ‘Summer Sweet 5244’, moderate shoot growth delay by ABA may be of value as a growth-holding strategy when transplanting is delayed because of inclement weather at the time of field establishment. Importantly, field evaluations demonstrated that the growth modulation by ABA is only transient with no negative impact on marketable yield and fruit quality.
Stephen S. Deschamps and Shinsuke Agehara
Black plastic mulch is used predominantly for winter strawberry (Fragaria ×ananassa Duch.) production in Florida because of its warming effects. However, black plastic mulch can increase heat stress during establishment, especially when growers advance planting dates (e.g., late September) to improve earliness. Consequently, we designed a new plastic mulch film that has a metalized center stripe with black shoulders. We hypothesized that metalized-striped mulch can minimize heat stress during establishment, while maintaining the warming effects of black mulch during winter. We conducted field trials over two seasons to evaluate black mulch, fully metalized mulch, and metalized-striped mulch using two cultivars differing in heat stress tolerance and fruit production patterns: ‘Florida Radiance’ and ‘Florida Beauty’. The effect of plastic mulch type on plant growth and yield was generally consistent across both seasons. Compared with black mulch, metalized-striped mulch reduced afternoon root-zone temperature (RZT) by up to 3.1 °C and reduced the duration of heat stress conditions (RZT > 30 °C) by 119 hours across October and November, but exhibited equivalent soil warming during winter. Yield increases by metalized-striped mulch compared with black mulch ranged from 19% to 34% in the early season (November–January), 6% to 20% in the late season (February–March), and 12% to 26% over the entire season. Statistical significance was detected for the 2016–17 early-season yield and when yield data were expressed on a weekly basis. Compared with black mulch, metalized-striped mulch improved fruit number significantly without affecting fruit weight or canopy area, suggesting that heat stress on black mulch negatively affects flower and fruit development more than plant growth. Weekly fruit yield data indicate that metalized-striped mulch can produce greater yields than fully metalized mulch. Metalized-striped mulch is an easily implementable strategy for reducing establishment heat stress and improving fruit earliness in subtropical winter strawberry production regions.
Shinsuke Agehara and Daniel I. Leskovar
Excess transpiration relative to water uptake often causes water stress in transplanted vegetable seedlings. Abscisic acid (ABA) can limit transpirational water loss by inducing stomatal closure and inhibiting leaf expansion. We examined the concentration effect of exogenous ABA on growth and physiology of muskmelon (Cucumis melo L.) seedlings during water stress and rehydration. Plants were treated with seven concentrations of ABA (0, 0.24, 0.47, 0.95, 1.89, 3.78, and 7.57 mm) and subjected to 4-day water withholding. Application of ABA improved the maintenance of leaf water potential and relative water content, while reducing electrolyte leakage. These effects were linear or exponential to ABA concentration and maximized at 7.57 mm. Gas-exchange measurements provided evidence that such stress control is attributed to ABA-induced stomatal closure. First, net CO2 assimilation rate and stomatal conductance initially decreased with increasing ABA concentration by up to 95% and 70%, respectively. A follow-up study (≤1.89 mm ABA) confirmed this result with or without water stress and further revealed a close positive correlation between intercellular CO2 concentration and net CO2 assimilation rate 1 day after treatment (r 2 > 0.83). In contrast, ABA did not affect leaf elongation, indicating that stress alleviation was not mediated by leaf area adjustment. After 18 days of post-stress daily irrigation, dry matter accumulation showed a quadratic concentration-response, increasing up to 1.89 mm by 38% and 44% in shoot and roots, respectively, followed by 16% to 18% decreases at >1.89 mm ABA. These results suggest that excess levels of ABA delay post-stress growth, despite the positive effect on the maintenance of water status and membrane integrity. Another negative side effect was chlorosis, which accelerated linearly with increasing ABA concentration, although it was reversible upon re-watering. The optimal application rate of ABA should minimize these negative effects, while keeping plant water stress to an acceptable level.
Syuan-You Lin and Shinsuke Agehara
In subtropical climates, inadequate winter chill limits blackberry (Rubus L. subgenus Rubus Watson) production by causing poor and erratic floral budbreak. To compensate for a lack of chilling, bud dormancy-breaking agents must be developed for subtropical blackberry production. Our previous study showed that gibberellic acid (GA3) promotes budbreak in three blackberry cultivars but has potential negative side effects on floral development in ‘Natchez’. 6-benzyladenine (6-BA) is a synthetic cytokinin that can act as an antagonist of gibberellins during floral transition. The objectives of this study were to evaluate cultivar × exogenous GA3 interactions, characterize dose effects of exogenous GA3, and examine synergistic effects of GA3 and 6-BA. Three field experiments were conducted in west central Florida. All spray treatments were applied at the end of the chilling period. In the first experiment, ‘Natchez’, ‘Navaho’, and ‘Ouachita’ were treated with GA3 at 0 or 99 g·ha−1. Budbreak was promoted by exogenous GA3 in all three cultivars (0.9% to 4.5% vs. 42.9% to 69.4%), but yield responses varied considerably. Exogenous GA3 increased the yield of ‘Navaho’ and ‘Ouachita’ by 560% to 931%, whereas it induced flower abortion and caused a 15% yield reduction in ‘Natchez’. In the second experiment, ‘Natchez’ was treated with GA3 at 0, 25, 99, or 198 g·ha−1. Budbreak increased linearly with GA3, but yield decreased exponentially with GA3 because of dose-dependent flower abortion. In the third experiment, ‘Natchez’ was subjected to five treatments: 1) water control; 2) GA3 spray application; 3) 6-BA spray application; 4) combined spray application of GA3 and 6-BA; and 5) sequential spray application of 6-BA at 9 days after GA3 application. Application rates were 99 and 47 g·ha−1 for GA3 and 6-BA, respectively. Exogenous 6-BA suppressed GA3-induced flower abortion only to a limited extent. As a result, GA3-containing treatments caused 65% to 83% yield reductions compared with the control (2382 vs. 410–823 g/plant). These results demonstrate that GA3 is a highly effective bud dormancy-breaking agent for blackberry. However, the drawback of GA3 is cultivar-dependent flower abortion, which cannot be fully mitigated by 6-BA. The use of GA3 can be an important management practice for subtropical blackberry production, but its practical implementation must consider cultivar-dependent responses.
Syuan-You Lin and Shinsuke Agehara
In subtropical blackberry (Rubus L. subgenus Rubus Watson) production, inadequate winter chill causes poor and erratic budbreak, whereas high temperatures and heavy rainfall deteriorate late-season fruit quality. We examined the effects of four defoliants [zinc sulfate (ZS), potassium thiosulfate (KTS), urea, and lime sulfur (LS)] on defoliation, budbreak, yield, and fruit quality of ‘Natchez’ blackberry grown under inadequate chilling conditions in two consecutive growing seasons. Plants were treated with defoliants at 187 kg·ha−1 via spray application (1870 L·ha−1) at the beginning of chill accumulation (late December). A nonionic surfactant (Agri-Dex) was added at 0.5% (v/v) to all treatments including the water control. Cumulative chilling hours (<7.2 °C) at the experiment site were 209 and 134 in the first and second growing seasons, respectively. Defoliation was only 40.2% to 55.5% in the control, but it was induced moderately by LS (69.7% to 84.7%) and severely by the other defoliants (81.7% to 94.7%). Budbreak was induced most rapidly by urea application, followed by LS, KTS, and ZS, advancing by 17 to 66 days compared with the control. Consequently, urea, KTS, LS, and ZS increased early season yield by 2.79, 2.55, 0.87, and 0.31 t·ha−1, respectively, compared with the control (0.12 t·ha−1). By contrast, the final percentage of budbreak and total-season yield did not show significant treatment effects. KTS caused cane dieback and increased bud mortality, resulting in the lowest total-season yield among the treatments. Importantly, defoliants had no negative impact on berry size and soluble solids concentration. These results suggest that urea, LS, and ZS are effective bud dormancy-breaking agents for blackberry and that they could be an important adaptation tool for subtropical blackberry production. Among the three defoliants, urea appears to be the ideal chemical option because of its consistent efficacy, favorable safety profile, and low application cost.
Daniel I. Leskovar, Chenping Xu, and Shinsuke Agehara
Globe artichoke is typically grown in Mediterranean and coastal areas. Because of the high profitability as a specialty crop, demand to develop production systems optimized for other semiarid and water-limited regions is rising. Field experiments were conducted over three seasons (2008–09, 2010–11, and 2011–12) in southwest Texas to investigate plant growth, physiology, and yield of artichoke grown as an annual system. Three strategies were evaluated: planting configuration (single and double lines per bed), plasticulture (bare soil and black plastic mulch), and cultivars differing in maturity (‘Imperial Star’, early; ‘Green Globe Improved’, late). Each fall, transplants were established in the field at 2.03 m between rows and 0.90 m between plants (single line) or 4.06 m between rows and 0.90 m between plants (double line). In both cultivars, black plastic mulch enhanced plant growth (leaf number, plant height and width) and increased early yield; however, its effect on total yield and yield components was not consistent. Single line per bed significantly increased head number of jumbo and large size per plant as compared with double line in the 2009 season. Chlorophyll index was unaffected by either planting configuration or plastic mulch. Comparing cultivars, ‘Green Globe Improved’ had lower marketable yield but bigger head size than ‘Imperial Star’ in one and two seasons, respectively. Our results indicate that single line with black plastic mulch can be recommended to improve earliness and water savings as compared with the bare soil system for annual artichoke production.
Daniel Leskovar, Shinsuke Agehara, and Smiljana Goreta
The objective of this study was to determine the effects of 1-MCP preharvest spray application on harvest synchrony, maturity, fruit quality, and marketable yield of cantaloupe. Seeds were planted in a commercial field on 16 Mar. (early planting, cv. Caravelle) and 4 Apr. (late planting, cv. Mission) 2005. Standard plant population, fertilization, irrigation, and pest control practices were followed. We evaluated three 1-MCP rates (5, 10, or 25 g·ha-1 a.i.) at three preharvest spraying times for the early (22, 15, and 7 days before harvest, DBH) or once for the late planting experiment (4 DBH). An additional test (late planting) compared fruit quality after storage for melons dipped with 1-MCP (0 or 10 mg·L-1). Fruits were harvested six times during June 2005 (early planting) and once on 19 July 2005 (late planting) and fruit quality parameters were measured at harvest and after storage. The preharvest 1-MCP application slightly delayed maturity and improved early harvest synchrony, but did not affect total marketable or yield by fruit size regardless of timing or rate of application. There was no effect of 1-MCP rate or application timing on fruit quality at harvest or after cold storage, except for an increased in fruit firmness (10%) in one of the six harvests. However, fruits treated with 1-MCP spray at 25 g·ha-1 a.i. (late planting) had higher firmness than those treated with lower rates after 9 days of storage. In addition, 1-MCP postharvest dipping significantly improved fruit firmness; however, a `greening' was evident in the fruit surface. Our results suggest that cantaloupe fruit quality was less affected by early preharvest spray application of 1-MCP applied at less than 25 g·ha-1 a.i. as compared to postharvest applications.
Daniel I. Leskovar, Shinsuke Agehara, Kilsun Yoo, and Nuria Pascual-Seva
Agricultural communities in the semiarid regions of the world are constantly being affected by water scarcity, increased regulations restricting water use, strong competition for irrigation water with the urban sector, and severe drought periods. Conversely, the consumer demand for high-quality and nutritious foods is increasing rapidly. A 2-year field study evaluated growth, yield, and bulb quality in response to precision planting density and deficit irrigation of onion (Allium cepa L.) in southwest Texas. Seeds of short-day sweet onion cv. Texas Grano 1015Y were planted in the field on 11 Nov. 2007 and 30 Oct. 2008 at two planting densities (PDs), 397,000 (standard) and 484,000 (high) seeds/ha. Three irrigation rates using growth stage-specific crop coefficients and subsurface drip were imposed after plants were fully established, 100%, 75%, and 50% crop evapotranspiration rates (ETc). Total rainfall plus irrigation received for each irrigation rate were 594, 501, and 413 mm in 2008 and 662, 574, and 486 mm in 2009. In both seasons, there were consistent trends in growth, yield, and quality parameters. Leaf fresh weight was unaffected by PD but was reduced by deficit irrigation at 50% ETc. Although increasing planting density reduced the average bulb size by 12%, it increased the number of marketable bulbs by 21% to 33% and marketable yield by 7% to 14%. In contrast, deficit irrigation showed a trend to reduce both the number of bulbs and bulb size with yield reductions of 8% to 13% at 75% ETc and 19% to 27% at 50% ETc. Neither planting density nor deficit irrigation rate had a significant effect on soluble solids content, pungency, or quercetin contents. These results suggest that growers of short-day onions in semiarid regions could adjust PDs to target high-value bulb sizes. Implementing water-conserving practices (deficit irrigation at 75% ETc rate) would result in a decrease of high-value bulb grades and modest losses in yield but not flavor or nutritional components.