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capacity of pollen to fertilize female germ cells. Instead, pollen germination data are required to estimate heat tolerance of pepper pollen ( Mercado and Quesadac, 1994 ). In vitro pollen germination may be affected by genotype-specific differences in

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. From these early trials and subsequent investigations, several major traits have been identified for improvement. PRIMOCANE BLACKBERRIES: THE CHALLENGING TRAITS Heat tolerance of flowers. The first trait identified for need of

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far, differences in heat tolerance have been detected among rose cultivars in their ability to maintain consistent flower size and numerous flowers under heat stress in the field and in flower abscission and leaf necrosis in response to a heat

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Direct heat injury to plant parts may occur in areas of high insolation and high humidity where transpiration is low. Using electrolyte leakage procedures, critical high temperatures of detached leaves of ‘Glen’ citrange [Citrus sinensis L. (Osbk.) × Poncirus trifoliata L. (Raf.)], ‘Swingle’ citrumelo [C. paradisi Macf. × P. trifoliata L. (Raf.)], and ‘Hamlin’ orange [C. sinensis L. (Osbk.)] were determined by exposure to temperatures between 25° and 65°C. Lethal temperatures for a 20 min exposure ranged from 54.3° ± 0.5° for ‘Glen’ citrange to 56.1° ± 0.4° for ‘Swingle’ citrumelo. Maximum canopy temperatures of 36.6° were recorded. Therefore, it appears that under field conditions in Florida, these cultivars are normally not subjected to temperatures that would cause direct heat injury.

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Temperature sensitivity of CO2 assimilation (ACO2), dark respiration, and chlorophyll fluorescence was evaluated among three taxa of hollies including I. aquifolium L., I. cornuta Lindl. & Paxt., and I. rugosa Friedr. Schmidt. Variations in foliar heat tolerance among these species were manifested in temperature responses for ACO2. Temperature optima of ACO2 for I. rugosa, I. cornuta, and I. aquifolium were 22.0, 26.3, and 27.9 °C, respectively (LSD0.05 = 2.9). Temperature responses of respiration were similar among taxa and did not appear to be contributing factors to variations in ACO2. At 40 °C, potential photosynthetic capacity, measured under saturating CO2, was 4.1, 9.4, and 14.8 μmol·m-2·s-1 for I. rugosa, I. aquifolium, and I. cornuta, respectively (LSD0.05 = 5.1). Variations in the relative dark-acclimated fluorescence temperature curves were used to assess thresholds for irreversible heat injury. The critical fluorescence temperature threshold (TC) was similar (48.0 °C) for all taxa. The fluorescence temperature peaks (TP) were 52.0, 52.8, and 53.5 °C for I. rugosa, I. cornuta, and I. aquifolium, respectively (LSD0.05 = 0.9). Based on these results, I. rugosa was the most heat-sensitive species, followed by I. aquifolium and I. cornuta. Ilex cornuta also had substantially greater potential photosynthetic capacity than the other species at 40 °C, indicating superior metabolic tolerance to high temperatures.

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cellular integrity, measures of membrane stability before and after exposure to heat stress serve to provide quantitative data that have been found to be well-correlated with heat tolerance ( Wahid et al., 2007 ). The occurrence of wide variation in the

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Heat tolerance and endogenous ABA levels in leaves and cultured grape cells (Vitis spp., cultivars Venus and Veeblanc) were evaluated during beat acclimation. Plants and cultured cells were acclimated at 38 and 36C, respectively. Heat tolerance increased rapidly after exposing plants or cells to acclimation temperatures, reaching a maximum after 10 to 16 hours and 10 to 12 hours for leaves and cultured cells, respectively. Free and bound ABA levels increased sharply during the first hour of heat acclimation, before leaves and cultured cells reached their maximum beat tolerance. The increase in ABA during heat acclimation was 2- to 3-fold that of the nonacclimated control, and the time of the ABA accumulation peak in tissue roughly corresponded to the maximum heat tolerance in leaves and cultured cells. Heat tolerance was induced in cultured cells by exogenous ABA application. Heat tolerance increased significantly after 24 hours of ABA application at 7.6 or 9.5 μm. The results suggest that ABA may be a factor in high-temperature acclimation and beat-tolerance induction in grapes. Chemical name used: abscisic acid (ABA).

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Abstract

Leaf heat tolerances of ‘Saladette’ (heat tolerant) and ‘UC-82B’ (less heat tolerant) tomato (Lycopersicon esculentum, Mill) were evaluated after heat acclimation. When plants of both genotypes were grown in a temperature regime below 30°C, there was no difference in heat tolerance. When plants of both genotypes were exposed to a temperature regime of 35° (day/night), ‘UC-82B’ could reach a higher level of heat tolerance, similar to ‘Saladette,’ but ‘UC-82B’ required 6 cycles of high temperature exposure, whereas ‘Saladette’ needed only a single cycle.

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Abstract

The thermostability of Ilex crenata Thunb. ‘Helleri’ and Ilex vomitoria Ait. ‘Schellings’ root cell membranes at supraoptimal temperatures were determined using electrolyte leakage procedures with excised roots. Mathematical models were developed to describe effects of treatment temperature and exposure time interactions on membrane thermostability. Critical temperature, Tc, decreased linearly as exposure duration increased exponentially, and predicted Tc for ‘Helleri’ and ‘Schellings’ were 51.0 ± 0.8 and 52.6 ± 0.7°C, respectively, for a 30-min exposure and 43.9 ± 0.8 and 46.7 ± 0.3° for a 300-min exposure. Three dimensional plots of the fitted mathematical functions are presented.

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High-temperature fruit set (heat tolerance) is a critical trait of tomato (Lycopersicon esculentum Mill.) cultivars targeted for lowland wet season production in the tropics and subtropics. Heat-tolerant Asian Vegetable Research and Development Center (AVRDC) tomato line CL5915-93D4-1-0-3 (CL5915) is a valuable source of heat-tolerance genes for tomato genetic improvement. The gene action of heat tolerance in CL5915 was determined by evaluating the F1, F2, BCP1, and BCP2 of a cross between CL5915 and heat-sensitive line UC204A for fruit set traits in two wet-season trials at AVRDC. Parent-offspring regression of F2-derived F3 (F2:3) family means on the F2 plants from CL5915 × UC204A was used to estimate the heritability of F2 single plant selection for heat tolerance. Mean percentage of fruit set and fruit number per cluster of the F1 and BCP1 exceeded midparent values and were not significantly different from those of CL5915, indicating complete dominance for heat tolerance. Generation means analyses indicated that a model including simple additive and dominance effects adequately explained the inheritance of mean fruit number per cluster both years. For mean percentage of fruit set, a model including simple additive-dominance effects produced an adequately fitting model in the 1996 season but the best-fitting model included an epistatic component in the 1997 season. Heritabilities estimated for fruit set traits in 1996 and 1997, respectively, were: 0.31 and 0.21 for percentage of fruit set; 0.28 and 0.14 for mean fruit number per cluster; and 0.53 and 0.15 for flower number per cluster. The low heritabilities for percentage of fruit set and mean fruit number per cluster under high temperatures imply that single plant selection in the F2 for heat tolerance from crosses involving CL5915 is not effective and that selection should be based on replicated family testing in the F3 and later generations.

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