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Rajeev Arora, S.P. Dharmalingam, and B.C. Bearce

136 ORAL SESSION 29 (Abstr. 205–210) Stress–Heat and Water

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Mary E. Mangrich and Mikal E. Saltveit

Chilling 10-mm cucumber (Cucumis sativus L. `Poinsett 76') radicles at 2.5 °C reduced their subsequent growth during 3 days at 25 °C. The reduction in radicle growth was linear for 1 to 3 days of chilling but then increased substantially until subsequent radicle growth was all but eliminated by 6 days of chilling. Heat shocks of 40 °C applied for 4 to 12 min increased chilling tolerance such that 4 days of chilling caused only a 36% decrease in radicle growth compared to 66% for seedlings not heat shocked, which brought the response in line with the responses of the non-heat-shocked seeds chilled for 1 to 3 days. Eight-minute heat shocks applied before 5 days of chilling resulted in a 45% inhibition of subsequent growth, compared to 82% for chilled non-heat-shocked controls. Heat shocks applied before 3 days of chilling did not result in a significant increase in subsequent growth compared to the non-heat-shocked controls chilled for 3 days. Heat shocks were only able to protect that part of radicle growth that was in excess of the linear decrease in radicle growth. There appears to be two effects of chilling on radicle growth. The first is linear and cannot be affected by heat shocks. The second is much more severe and can be prevented by heat shocks. Seeds were selected for three categories of vigor according to the rate at which their radicles grew to 10 mm. Seeds classified with different vigors neither responded significantly differently to 3 days exposure to 2.5 °C nor did they respond differently to chilling stress following application of heat shocks.

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D.M. Yeh and H.F. Lin

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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Thomas G. Ranney and John M. Ruter

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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Seenivasan Natarajan* and Jeff S. Kuehny

The demand for new and/or improved herbaceous annuals and perennials continues to increase, making information on production and viability of these plants a necessity. In Louisiana and the Southern U.S., one of the greatest impediments to production of marketable herbaceous plants and their longevity is high temperature. Herbaceous plants have various stages of vegetative growth and flowering; high temperatures during these developmental stages can have a tremendous impact on plant metabolism, and thus plant growth and development. The goal of this research was to better understand the differences between heat tolerant (HT) and heat sensitive (HS) species and cultivars at various high temperatures in terms of whole plant growth, flowering, photosynthesis, carbohydrate content, electrolyte leakage, chlorophyll content and plant small heat shock proteins (HSP) expression levels. Salvia splendens Vista Series (HT), Sizzler series (HS); Viola witrokiana `Crystal Bowl Purple' (HT), `Majestic Giant Red Blotch' (HS), F1 Nature Series (HT) and F1 Iona Series (HS); Gaillardia × grandiflora `Goblin' (HT) and Coreopsis grandiflora `Early Sunrise' (HS) were grown from seed in growth chambers under 25/18 °C (day/night) cycles. Plants at 4, 6, and 8 weeks after germination were subjected to different high temperature treatments of 25 (control), 30, 35, 40, and 45 °C for 3 h. Results show that there was a significant difference in net photosynthesis, electrolyte leakage, soluble carbohydrate content and HSP levels between HT and HS cultivars. Effects of high temperature on plant growth, chlorophyll content, and number of days to flower, flower size, and marketable quality were also significantly different.

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Abha Upadhyaya, Tim D. Davis, and Narendra Sankhla

Moth bean (Vigna aconitifolia Jacqu. Marecbal cv. Jaadia) seeds were germinated in 0, 0.1, 1, or 2 μm EBL. After 72 hours, seedlings were exposed to 22 or 48C for 90 minutes. At 48C, EBL increased total electrolyte, K+, and sugar leakage from the seedlings relative to the control. Following exposure to 48C, EBGtreated seedlings bad higher malondialdebyde concentrations than controls, indicating that EBL enhanced high-temperature-induced lipid peroxidation. At 48C, EBL increased ascorbic acid oxidase activity but decreased superoxide dismutase activity relative to the control. Taken collectively, these data do not support a hypothesis that brassinosteroids confer beat shock tolerance to moth bean. Chemical name used: 24-epibrassinolide (EBL).

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Kate A. Nishijima, Harvey T. Chan Jr., Suzanne S. Sanxter, and Edward S. Linse

A reduced heat shock period for `Sharwil' avocado (Persea americana Mill.) before quarantine cold treatment is described. The shortened heat pretreatment period of 8 to 12 hours, rather than the originally recommended 18 hours at 38C, is effective in reducing chilling injury symptoms when the pulp is at ≤2.2C during 16 days of storage. The reduced durations and the range of pretreatment hours affords packinghouses greater efficiency and more flexibility and will reduce handling costs relative to the longer exposure.

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Suzanne S. Sanxter, Kate A. Nishijima, and Harvey T. Chan Jr.

Chilling injury symptoms were reduced when `Sharwil' avocados (Persea americana Mill.) were held at 37 to 38C for 17 to 18 hours and then air-cooled at 20C for 4 hours before storage at 1.1C for ≥14 days. In contrast, nonheated fruit developed severe surface discoloration and pitting. Chilling injury symptoms were reduced further when the heated fruit were stored in perforated polyethylene bags during 1.1C storage. No treatment equaled or surpassed the quality of fruit in nontreated controls.

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Douglas D. Archbold and Ann M. Clements

Several components of whole-plant growth were compared among accessions of Fragaria chiloensis (FC) and F. virginiana (FV) grown at 23 and 31 °C daytime temperatures. The accessions loosely represented North American (NA) and South American (SA) provenances of FC and Kentucky (KY) and eastern Canadian (CN) provenances of FV. Differences in component values between species and by provenance and accession within species were observed at each temperature. Using the ratio of the component value at 31 °C to that at 23 °C as a basis for comparisons, whole-plant relative growth rate (RGR), leaf net assimilation rate (NAR), root RGR, and root: shoot ratio were reduced relatively more by high temperature in FC than FV, while crown RGR, leaf RGR, and leaves produced per day were not consistently affected by temperature or and did not differed significantly between species. While the SA FC exhibited higher values for nearly all components than the NA FC at both temperatures, both were affected similarly by high temperature. The CN FV exhibited somewhat greater sensitivity to high temperature than the KY FV, with significantly lower leaf NAR, crown RGR, and leaves produced per day in the former group.