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Abstract
Choice of the most appropriate cultivars is a key decision that vegetable growers face every growing season, and one on which the profitability of the crop depends. Evaluation of cultivars for adaptation to local growing conditions is therefore of crucial importance to extension and research personnel that serve the vegetable industry and the companies that are developing and releasing vegetable cultivars. The present-day climate of restricted budgets and pressure to move from applied to more basic research has forced experiment station and university personnel in North America to abandon or greatly reduce vegetable cultivar testing. The seed industry has traditionally relied on both public institutions and its own grower–cooperators to evaluate the merits of new lines. Increasingly, seed companies face requests for funding of these public trials, or are asked to pay entry fees to submit new cultivars for testing.
When pumpkins are grown in elevated temperatures (32/27 °C day/night) for 1 week during flower development, fewer female flower buds are formed than at normal temperatures (20/15 °C) and only a small percentage of these reach anthesis. To determine if application of the ethylene-releasing compound ethephon can overcome the suppression of female flowers at high temperatures, `Baby Bear' pumpkin plants were sprayed at the two-leaf stage with 100 or 300 μL L–1 ethephon and then grown in hot and cool greenhouse compartments. At 20/15 °C, 17% of the first 15 main stem nodes produced female flower buds on control plants and virtually all of these developed into open flowers. The higher rate of ethephon increased female bud percentage to 37%. At 32/27 °C, only 3% of the nodes formed female flower buds and 2% flowered. Application of ethephon did not significantly increase female expression at high temperature, and none of the buds reached anthesis. Treatment with the inhibitor of ethylene action silver thiosulfate reduced female flower bud formation at the low temperature and entirely suppressed female flower buds at high temperature. In two additional experiments, these treatments were applied to two cultivars grown at a less extreme 32/20 and at 20/15 °C. Female buds and open flowers were moderately increased by ethephon in the high temperatures, suggesting that ethephon might foster female flowering in less extreme temperatures. Further work is needed to determine if ethephon treatment can overcome the heat-induced inhibition of female flowers in pumpkin under field conditions.
In one greenhouse and two field experiments, eight or ten pepper (Capsicum annuum L.) cultivars were subjected to low-light stress by use of shade cloth (reducing light by 80%) or to foliar sprays of ethephon at 75 or 150 pi-liter-]. Both low-light stress and ethephon identified `Ace', 'Canape', and. `Belrubi' as less susceptible to flower and flower bud abscission than other cultivars in the first field experiment. In the 2nd year, air mean maxima of 32C caused severe abscission in controls and shaded plants, and complete loss of flowers in those sprayed with ethephon. Abscission of disbudded pedicels was not related to abscission susceptibility of eight cultivars when subjected to shade. While ethephon spray can serve as a satisfactory abscission screening tool under unstressed growing conditions, low-light stress imposed by shading may be used under a wider range of conditions. Chemical name used: 2-chloroethyl phosphoric acid (ethephon).
Abstract
Vegetable cultivar testing in the tropics is faced with fundamental problems that occur less frequently in the temperate areas. First, it must be decided what species of vegetables should be tested. Second, availability of planting materials, not only for the trial but also for the producer, can be a major limiting factor. Third, the methodology of testing is frequently different than in the temperate world, both in the cultural practices used and in the characteristics evaluated. Each of these points will be elaborated in the following paper, placing particular emphasis on the situation in Southeast Asia.
Catfacing of tomato (Lycopersicon esculentum Mill.) fruit describes the enlarged blossom-end scar and ridged, flattened or irregular fruit shape often found on plants subjected to low temperature during ovary development. Experiments were conducted to determine if GA3 foliar sprays could be used as a screening tool for catfacing. Concentrations of 5 to 50 μM of GA3, applied once at transplanting, significantly increased catfacing incidence on the susceptible `Revolution', whereas the resistant `Valerie' was less affected. Two applications 8 days apart extended symptoms to later clusters formed on branches and may be useful for screening cultivars of a wide range of earliness. Plant apex removal may also be possible as a fruit catfacing screening tool. Chemical name used: gibberellic acid (GA3).
The blossom-end scarring of tomato fruit caused by exposure of the plant to cool weather during ovary formation, commonly termed catfacing, can also be induced by GA3 foliar sprays. To determine if GA3 treatment could serve as a cultivar screening tool to identify lines susceptible to the disorder, we compared the catfacing incidence in 14 fresh-market tomato cultivars after GAS sprays and in nontreated controls in two field experiments. In 1 year, removal of the plant's apex was also imposed. GA3 sprays (22 μm twice, applied 1 week apart to tomato seedlings ≈5 weeks old) increased catfacing incidence in both years and accentuated cultivar differences in the disorder. Topping did not increase catfacing significantly. The cultivars Valerie, Sunrise, and Basketvee were least affected by catfacing in the experiments, while `Starfire', `New Yorker', and `Olympic' had the highest percentage of catfaced fruit. The GA3 screening method shows promise for identifying cultivar differences in susceptibility to blossom-end scarring. Chemical name used: gibberellic acid (GA3).
When bell pepper plants are subjected to high temperatures or low irradiance during flowering, abscission of flowers and flower buds can be severe enough to limit yields. Both types of stresses may lead to reduction of carbohydrate levels in the reproductive structures, and evolution of the abscission-causing hormone ethylene. In two greenhouse experiments, plants were subjected to combinations of 30 or 20 C air temperature and unshaded or heavily shaded conditions for one week at anthesis of the first flower. In a third experiment, plants were subjected to total darkness and temperatures of 30, 20 or 15 C for one week at the same stage of growth. In all experiments, levels of soluble carbohydrates and starch declined under low or zero light conditions, with rate of decrease proportional to the air temperature. Abscission of reproductive structures was not well correlated with carbohydrate levels: in spite of low sugars and starch, plants darkened for 7 days at 15 C showed no abscission. Levels of the ethylene precursor ACC in the buds, though variable, rose just before abscission began, but remained low in non-abscising treatments.