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Aref A. Abdul-Baki and John R. Stommel

Heat-tolerant and -sensitive Lycopersicon esculentum Mill. and L. pimpinellifolium (Jusl.) Mill. genotypes were grown in the greenhouse under optimum- (27/23C, day/night) and high-temperature (35/23C) stress regimes. Heat tolerance levels in the genotypes were established by determining percent fruit set at high and optimum temperatures. Under optimum temperature, fruit set ranged from 41% to 84% and from 45% to 91% in the heat-sensitive and heat-tolerant genotypes, respectively. Under high temperature, no fruit set in the most heat-sensitive genotypes. Fruit set in the heat-tolerant genotypes ranged from 45% to 65%. In vitro germination and tube growth of pollen taken from genotypes grown under optimum temperature conditions were determined before and after subjecting the pollen to 45C for 1, 2, and 4 hours. The response of pollen to heat treatments was genotype dependent and not a general predictor of fruit set under high-temperature stress.

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Ami N. Erickson and Albert H. Markhart III

Reduction of floral number in Capsicum annuum has been observed during growth at high temperature. To determine whether decreased flower production or increased flower abscission is a direct response to high temperatures or a response to water stress induced by high temperatures, we compared flowers and fruit produced and flowers aborted to leaf growth rate, osmotic potential, stomatal conductance, and chlorophyll fluorescence of two cultivars. To determine the stage(s) of floral development that are most sensitive to high temperatures, flower buds were wax-embedded and examined at each stage of development during heat treatment. Rate of floral development also was examined. At first visible floral bud initiation, plants were transferred to each of three controlled environment growth chambers with set temperatures and vapor pressure deficits (VPD) of 25°C, 1.1 kPa; 33°C, 1.1 kPa; and 33°C, 2.1 kPa. Flower bud production and leaf growth rate were not significantly affected by high temperatures. Pepper fruit set, however, was inhibited at 33°C at either VPD. Preliminary water relations data suggested that water potentials were more negative under high temperature conditions. Differences in leaf fluorescence were statistically significant for temperature treatments, but not for VPD. Temperature is the primary factor in the decrease of fruit production in pepper. Decreased production is due to flower abortion and not to decreased flower initiation or plant growth.

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Katsumi Suzuki, Tadashi Tsukaguchi, Hiroyuki Takeda, and Yoshinobu Egawa

Pod yield of `Kentucky Wonder' green bean (Phaseolus vulgaris L.) decreased at high temperatures due to a reduction of pod set. A highly positive correlation was observed between pod set and pollen stainability in flowers that were affected by heat stress about 10 days before anthesis. Pollen stainability was decreased by heat stress applied 8 to 11 days before flowering under controlled environment conditions. When mean air temperature during this period exceeded 28 °C, pollen stainability decreased under field conditions. Low pollen stainability indicated sensitivity to high temperatures about 10 days before flowering. A heat-tolerant cultivar showed higher pollen stainability than did heat-sensitive cultivars under high temperatures. These results demonstrated that heat tolerance at an early reproductive stage could be evaluated by analyzing pollen stainability using flowers developed under high temperatures.

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Marshall K. Elson, Ronald D. Morse, Dale D. Wolf, and David H. Vaughan

High summer temperatures may reduce plant stands of direct-seeded fall broccoli (Brassica oleracea var. italica Plenck). The influence of constant and diurnally alternating temperatures in the range of 5 to 42C on germination and emergence of `Packman' broccoli was evaluated. Germination was defined as protrusion of the radicle from the seedcoat, and emergence as 10 mm elongation of the radicle. The range of constant temperatures from 10 to 30C for 14 days was satisfactory for 90% germination and 75% emergence. However, alternating temperatures extended the acceptable emergence range to 5/17 through 20/32C. Since soil temperatures in warm climates often exceed 20/32C during the summer, high-temperature inhibition of seed germination and seedling emergence is a potentially important factor limiting direct-seeded broccoli stands.

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Abbas M. Lafta and James H. Lorenzen

Growth chamber and greenhouse experiments were conducted to investigate the effect of temperature and irradiance on foliar glycoalkaloids of three potato genotypes (Solanum tuberosum L.) that differ in glycoalkaloid content. Two genotypes (ND4382-17 and ND4382-19) produced the acetylated glycoalkaloids, leptine I and II, that contribute resistance to the Colorado potato beetle (CPB, Leptinotarsa decemlineata Say). The glycoalkaloids were separated and quantified by high performance liquid chromatography. Exposure of plants to high temperature (32/27 °C, 14-hour day/10-hour night) for 3 weeks under a 14-hour photoperiod with an irradiance of 475 μmol·m-2·s-1 significantly increased the levels of leptines I and II, solanine, and chaconine compared to that at low temperature (22/17 °C). Increases in foliar leptines and total glycoalkaloids at high temperature were 90% and 169%, respectively. Growing potato plants at low irradiance (75% reduction) for 2 or 4 weeks resulted in a significant reduction in the levels of leptine I and II (46%), solanine (43%), and chaconine (38%) compared to nonshaded plants. Transferring plants from high to low irradiance or from low to high irradiance for 2 weeks caused a decrease and an increase in glycoalkaloid concentration, respectively. Therefore, both temperature and irradiance influenced foliar levels of glycoalkaloids in potato plants without changing the leptines and solanine to chaconine ratios. Thus, irradiance and temperature influenced glycoalkaloid compounds that can effect resistance to CPB, especially leptine I and II.

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M. Oren-Shamir and Dela Gal

Changes in temperature during rose flower development, often cause a significant fading of flower color, decreasing its market value. We are studying the effect of transient high temperature stress on red roses (Rosa ×hybrida, `Jaguar'). We have found that a transient temperature stress of 39/18 °C day/night respectively for 3 days, in comparison to the growth temperature of 26/18 °C, caused a significant fading to flower color at a mature bud stage. The plant organ responsible for color fading is the flower bud only. When the stress was applied to the whole plant, not including the flower buds, there was no change on the mature bud color. We have also shown that there are specific flower developmental stages sensitive to the transient increase in temperature. Flower buds at the critical stage of development, that have been exposed to temperature increase have a faded pink-red color when matured. Total anthocyanin levels of faded flowers, due to temperature stress, decreased to ≈50%. In addition, the ratio between the two anthocyanidins composing the red color, cyanidin and pelargonidin, changed dramatically due to the temperature stress: flowers on plants that have not overcome a temperature stress had a ration of 1:1, while those that have faded due to the temperature stress have a ration of 2:1 of pelargonidin to cyanidin, respectively. These findings hint to specific stages of anthocyanin synthesis, that are hypersensitive to increased temperature. We are now in the process of identifying and characterizing these stages.

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Janice M. Coons, Robert O. Kuehl, and Nancy R. Simons

Water that may contain salt often is used to cool soil to help overcome high-temperature inhibition of lettuce germination. This study was done to determine how lettuce cultivars differ in their germination response to high temperature and NaCl. Ten lettuce (Lactuca sativa L.) cultivars (Grand Rapids, Climax, Coolguard, Empire, Great Lakes 659-700, Mesa 659, Salinas, Vanguard 75, Red Coach 74, and Wintersupreme) were germinated at 20, 25, 30, or 35C with 0.0, - 0.3, - 0.6, - 0.9, - 1.2, or - 1.5 MPa NaCl. With no NaCl, germination percentages and rates decreased significantly at 35C for all cultivars except `Salinas', which decreased at 30C. With higher concentrations of NaCl, decreases in germination percentages and rates were observed at lower temperatures. Cultivar differences in response to temperature were present with no NaCl but became larger in the presence of NaCl. `Great Lakes 659-700' and `Mesa 659' were most sensitive to high temperature and salt. `Coolguard' and `Empire' were most tolerant to high temperature and salt, with some tolerance also present in `Grand Rapids' and `Vanguard 75'. Relative tolerance of cultivars to NaCl as shown by germination percentages and rates was consistent with growth of roots.

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John Frett, W. Edward Kee, and Stephen Redcay

Lima bean yields are lower in Delaware than in other lima-bean-producing states. One of the factors that contributes to the low production is the high temperatures that occur during production. Six commercial varieties of lima beans, both fordhook and baby lima bean types, were grown in a glass greenhouse at either 25C or 35C daytime temperatures to screen for heat tolerance. Plants grown at high temperature were typically shorter and more bushy than plants grown at 25C. Few, if any, buds, flowers, or early pods remained on plants at harvest if the plants were grown at 25C, while plants grown at 35C were still producing buds and flowers. Lima bean yields were generally reduced at 35C. The magnitude of the effect on yield ranged from `F1072', which had a 100-fold decrease in yield, to `Early Thorogreen', which demonstrated a slight increase in yield in response to increased temperatures.

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John M. Ruter

Membrane thermostability of Heritage river birch (Betula nigra L. Heritage) was measured by electrolyte leakage from excised roots of plants grown in pot-in-pot (PIP) and conventional aboveground production systems (CPS). The predicted critical midpoint temperature (Tm) for a 30-min exposure was 54.6 ± 0.2 °C for PIP and 56.2 ± 0.6 °C for CPS plants. Plants grown PIP had a steeper slope through the predicted Tm, suggesting a decreased tolerance to high root-zone temperatures in relation to plants grown aboveground. Since the root systems of Heritage river birch grown PIP are damaged at lower temperatures than plants grown aboveground, growers should prevent exposure of root systems to high temperatures during postproduction handling of plants grown PIP.

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X. Liu, J.A. Anderson, N.O. Maness, and B. Martin

Pepper (Capsicum annuum L. `Early Calwonder') leaf disks were vacuum-infiltrated in distilled water (control), anisomycin, aurintricarboxylic acid, cycloheximide, ethionine, norvanine, or puromycin to determine whether protein synthesis inhibitors blocked high-temperature acclimation. After infiltration, one-half of the leaf disks were placed in an incubator at 24C as a control, and the other half were kept in a water bath at 38C for 2 h to induce acclimation. Test tubes containing the disks then were placed in a water bath at 50.5C for 0, 1, 5, 10, 15, 25, 35, or 50 minutes. Thermotolerance was evaluated using electrolyte leakage. High-temperature acclimation was blocked in all six protein synthesis-inhibitor treatments. Only control disks infiltrated with distilled water acclimated. It seems that protein synthesis is required for high-temperature acclimation in bell pepper leaves.