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Red-osier dogwood sterns, Cornus sericea L., at ten different growth stages were subjected to a series of temperatures ranging from 25C to 60C by immersing them in a water bath for one hour. After heat treatments, the viability of internode tissues were determined by electrical conductivity and ethylene production. Heat tolerance was expressed as LT50, the temperature at which 50% of the tissues were injured. The results suggest that the LT50 of dormant plants remained relatively constant, approximately 53C. During dormancy, heat stress did not stimulate ethylene production from internode tissues. In contrast, tissues from non-dormant plants exposed to heat stress produced increasing levels of ethylene reaching a peak at 40C followed by a steady decrease at higher temperatures. Application of 1-aminocyclopropane-1-carboxylic acid (ACC) to stem segments from dormant plants, following heat treatment, enhanced production of ethylene.

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Carnation cultivars `German Red' and `Chabaud' were planted in the field in Dallas, Texas, on 26 May 1994. During the subsequent 3 months, the average daily high temperature was 33C, and the average daily low temperature was 22C. `German Red' plants increased in height and diameter several-fold during this period. In contrast, `Chabaud' did not increase in height or diameter. `German Red' plants began flowering in early August, and by 2 Sept., all of the plants were blooming. None of the `Chabaud' plants produced flowers, and only 50% of the original plants were still alive on 2 Sept. Mean shoot dry weight per plant on 2 Sept. was 71.6 g for `German Red' and only 2.4 g for `Chabaud'. These results document the extraordinary heat tolerance of `German Red' carnation. This plant not only survived the summer, but also grew and began blooming during the hottest time of the year.

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Leaf gas-exchange and chlorophyll fluorescence measurements were used as indexes for evaluating heat tolerance among five taxa of birch: paper (Betula papyrifera Marsh.), European (B. pendula Roth.), Japanese (B. platyphylla var. japonica Hara. cv. Whitespire), Himalayan (B. jacquemontii Spach.), and river (B. nigra L. cv. Heritage). Gas-exchange measurements were conducted on individual leaves at temperatures ranging from 25 to 40C. River birch maintained the highest net photosynthetic rates (Pn) at high temperatures, while Pn of paper birch was reduced the most. Further study of river and paper birch indicated that the reduced Pn at high temperatures and the differential sensitivity between taxa resulted from several factors. Inhibition of Pn at higher temperatures was due largely to nonstomatal limitations for both taxa. Increases in respiration rates, decreases in maximal photochemical efficiency of photosystem (PS) II (F V/F M), and possible reductions in light energy directed to PS II (F 0 quenching) were apparent for both taxa. The capacity of river birch to maintain greater Pn at higher temperatures seemed to result from a lower Q10 for dark respiration and possibly greater thermotolerance of the Calvin cycle as indicated by a lack of nonphotochemical fluorescence quenching with increasing temperatures. Thermal injury, as indicated by a rapid increase in minimal, dark-acclimated (F 0) fluorescence, was not evident for either paper or river birch until temperatures reached ≈49C and was similar for both taxa.

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High temperatures (>30°C day and/or >20°C night) in tropical lowlands and production areas in temperate zones reduce yield and quality in common bean (Phaseolus vulgaris L.). Tepary bean (P. acutifolius A. Gray) is a crop adapted to hot arid climates and is grown in the American Southwest and parts of Mexico under temperatures that are too high for pod formation in common bean. Interspecific hybridization may enable transfer of heat tolerance traits from tepary bean to common bean. Twenty-five tepary bean plant introductions (PI) with the ability to set seed under controlled-environment conditions were evaluated under high (35 °C day/32 °C night) and control (27 °C day/24 °C night) temperature treatments during reproductive development. Four accessions (PI 200902, PI 312637, PI 440788, and PI 440789) exhibited normal pod formation and comparatively high yield when exposed to high temperature, while common bean controls displayed zero pod and seed set. These four PIs showed a mean decrease in seed yield of 72.9% from control to high temperature treatment, as compared to 90.3% among all tepary beans. These accessions were hybridized with the dry bean cultivar `ICA Pijao', and the heat-tolerant bean cultivars `Carson' and `CELRK' and breeding line `Cornell 503'. Immature embryos were cultured to obtain interspecific hybrids. Fertility of F1 hybrids and generation of backcrosses are discussed.

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Small heat shock proteins (sHSP) are a specific group of highly conserved proteins produced in almost all living organisms under heat stress. These sHSP have been shown to help prevent damage at the biomolecular level in plants. One of the greatest impediments to production of marketable herbaceous plants and their longevity is high temperature stress. The objectives of this experiment were to study the plant responses in terms of sHSP synthesis, single leaf net photosynthesis, total water-soluble carbohydrates (WSC), and overall growth for two S. splendens cultivars differing in performance under heat stress. `Vista Red' (heat tolerant) and `Sizzler Red' (heat sensitive) were exposed to short duration (3 hours) high temperature stresses of 30, 35, and 40 °C in growth chambers. Increasing the temperature to about 10 to 15 °C above the optimal growth temperature (25 °C, control) induced the synthesis of sHSP 27 in S. splendens. Expression of these proteins was significantly greater in the heat-tolerant vs. the heat-sensitive cultivar. Soluble carbohydrate content was greater in `Vista Red', and in both the cultivars raffinose was the primary soluble carbohydrate in heat-stressed plants. Overall growth of plants was significantly different in the two cultivars studied in terms of plant height, stem thickness, number of days to flower, and marketable quality. The better performance of `Vista Red' under heat stress was attributed to its morphological characteristics, including short stature, thicker stems and leaves. sHSPs and WSC are also found to be associated with heat tolerance and heat adaptation in S. splendens.

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One of the greatest impediments to production of marketable ornamental herbaceous plants in southern U.S. is high temperature stress. Exposure of plants to sub-lethal temperature (heat preconditioning) before sustained heat stress helps some plants to tolerate subsequent heat stress a phenomenon often referred as acquired thermotolerance. The objective of this research was to examine various morphological, physiological and anatomical responses of `Vista red' (heat tolerant) and `Sizzler red'(heat sensitive) cultivars of Salvia splendens to heat preconditioning (HC) and subsequent heat stress treatments (challenging temperatures, CT). Cultivars of Salvia were subjected to short duration HC of 35 °C for 3 hours every third day until 5 weeks after germination and subsequent exposure to two CT treatments 30/23 °C and 35/28 °C (D/N) cycles in growth chambers for the next five weeks. Plant growth, marketable quality, stomatal conductance and net photosynthesis declined for Sizzler Red without HC treatment. Compared with nonpreconditioned plants, heat preconditioned Sizzler Red had 38.28% higher root dry weight, 95% greater leaf thickness, 50% higher marketable quality at 35/28 °C heat stress condition. Heat preconditioning helped both Vista Red and Sizzler to survive in both the heat stress treatments. Vista Red had greater heat tolerant traits than Sizzler Red, these traits exacerbated with heat preconditioning treatment. The results demonstrated that heat preconditioning enhanced heat tolerance in cultivars of Salvia, which could be related to maintenance of dense plant growth with shorter internodes, thicker stems, greater stomatal conductance, extensive root growth that compensated the transpirational water loss and overall cooling of plants.

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may carry heat tolerance genes. Heat-delay-insensitive chrysanthemum genotypes exist, but screening for the desired seedlings by greenhouse or field evaluation techniques is slow ( Anderson and Ascher, 2001 ; De Jong, 1989 ). Cell membrane

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to survive subsequent exposures to potentially lethal temperatures ( Krebs and Loeschficke, 1994 ; O'Connell, 1994 ; Vierling, 1991 ). The capacity of plants to acclimate and survive under high temperature is a critical factor in heat tolerance

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Identification of heat-tolerant chrysanthemum [Dendranthema ×grandifolia (Ramat.) Kitamura] genotypes for commercial production in hot areas of the world is desirable. The extent to which electrolyte leakage from chrysanthemum leaf discs, measured using a test for cell membrane thermostability (CMT), could be related to the delay in flowering induced by heat in the field-grown plants was determined. The relationship between the relative injury (RI) occurring in leaf tissue discs of chrysanthemum cultivars and treatment temperature was sigmoidal. A single temperature treatment at 50 °C resulted in injury values near the midpoint of the sigmoidal response curve and showed the greatest sensitivity in detecting genotypic differences in heat tolerance. The cultivars with a low RI value are those with the greater CMT and shorter heat-induced delay to flowering.

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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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