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Qingzhang Xu, Bingru Huang, and Zhaolong Wang

Heat injury in creeping bentgrass (Agrostis stolonifera var. palustris Huds) has been associated with decreases in carbohydrate availability. Extending light duration may increase carbohydrate availability and thus improve growth of creeping bentgrass under heat stress. The objective of this study was to investigate whether turf performance and carbohydrate status could be improved by extending daily light duration for creeping bentgrass exposed to supraoptimal temperature conditions. `Penncross' plants were initially grown in growth chambers set at a day/night temperature of 20/15 °C and 14-hour photoperiod and then exposed to a day/night temperature of 33/28 °C (heat stress) and three different light durations: 14 (control), 18, and 22 hours (extended light duration) for 30 days. Turf quality and tiller density decreased with the duration of heat stress, as compared to the initial level at 20 °C, regardless of the light duration. However, both parameters increased with extended light duration from 14 to 18 or 22 hours. Extended light duration, particularly to 22 hours, also improved canopy net photosynthetic rate from -1.26 to 0.39 μmol·m-2·s-1 and daily total amount of carbon assimilation from -6.4 to 31.0 mmol·m-2·d-1, but reduced daily total amount of carbon loss or consumption to 50% through dark respiration compared to 14 hours treatment by the end of experiment. In addition, extending light duration from 14 to 22 hours increased water-soluble carbohydrate content in leaves both at the end of light duration and the dark period. These results demonstrated that extending light duration improved turf performance of creeping bentgrass under heat stress, as manifested by the increased tiller density and turf quality. This could be related to the increased carbohydrate production and accumulation. Supplemental lighting could be used to improve performance if creeping bentgrass is suffering from heat stress.

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Jeffrey A. Anderson and Sonali R. Padhye

Although heat stress injury is known to be associated with membrane dysfunctions, protein structural changes, and reactions of activated forms of oxygen, the underlying mechanisms involved are poorly understood. In this study, the relationships between thermotolerance and hydrogen peroxide (H2O2) defense systems, radical scavenging capacity [based on 1,1-diphenyl-2-picrylhydrazyl (DPPH) reduction], and protein aggregation were examined in vinca [Catharanthus roseus (L.) G. Don `Little Bright Eye'], a heat tolerant plant, and sweet pea (Lathyrus odoratus L. `Explorer Mix'), a heat susceptible plant. Vinca leaves were 5.5 °C more thermotolerant than sweet pea leaves based on electrolyte leakage analysis. Vinca leaf extracts were more resistant to protein aggregation at high temperatures than sweet pea leaf extracts, with precipitates forming at ≥40 °C in sweet pea and at ≥46 °C in vinca. Vinca leaves also had nearly three times greater DPPH radical scavenging capacity than sweet pea leaf extracts. Two enzymatic detoxifiers of H2O2, catalase (CAT) and ascorbate peroxidase (APOX), demonstrated greater activities in vinca leaves than in sweet pea leaves. In addition, CAT and APOX were more thermostable in vinca, compared with sweet pea leaves. However, tissue H2O2 levels did not differ between controls and tissues injured or killed by heat stress in either species, suggesting that H2O2 did not play a direct role in acute heat stress injury in vinca or sweet pea leaves. Greater thermotolerance in vinca, compared with sweet pea, was associated with greater DPPH radical scavenging capacity, indicating that AOS other than H2O2 may be involved in acute heat stress injury.

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Emily B. Merewitz and Sha Liu

, which causes high input requirements when managing creeping bentgrass. Heat stress in summer months is a major problem for creeping bentgrass growing in temperate areas. There are many known physiological effects of heat stress on creeping bentgrass

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Aref A. Abdul-Baki, S. A. Haroon, and R. N. Huettel

Susceptibility of tomato (Lycopersicon esculentum Mill) genotpyes to the root-knot nematode Meloydogyne incognita and to heat stress can be evaluated in a single labor- and time-saving operation using a nondestructive in vitro excised root technique. Seeds are sterilized and germinated for 2 days on 1% water agar. Five-mm root sections are grown at 28 and 35 C for 30 days on Gamborg-B medium with and without nematode inoculum. Evaluation criteria include fresh and dry weight and the appearance of juveniles, adults, gulls, and egg masses. Evidence will be presented on the breakdown of resistance to M. incognita under high temperature stress.

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Michael Wisniewski, Jörg Sauter, Valerie Stepien, and Les Fuchigami

Sublethal heat stress has been shown to decrease or eliminate deep supercooling of flower buds in woody plants and to release plants from endodormancy. Experiments were conducted to characterize the effect of heat stress on endodormancy and ecodormancy in peach (cv Loring) and two hybrid poplars. Protein synthesis (de novo) and patterns of protein expression were also monitored. In order to determine optimum treatment temperatures, shoots, collected September-March, were exposed to a range of temperatures (35-60 C) under wet or dry conditions for 1-6 h. Shoots were then placed in the greenhouse and cumulative budbreak was monitored over 4 weeks. Samples of bud and bark tissues were collected during and up to 72 h after heat treatment for SDS-PAGE analysis. Data indicate: 1) twigs must be immersed in water for the heat treatments to be effective; 2) heat treatments resulted in a release from endodormancy and a decrease in thermal units needed for budbreak during ecodormancy; 3) 40 C for 2-4 h was optimum in fall and late winter whereas 45 C was the optimum temperature to induce budbreak in midwinter; 4) optimum temperature for peach floral buds (37.5 C/2h) was lower than for vegetative buds (40 C/4h), and 5) heat treatments also decreased cold hardiness. Protein synthesis decreased significantly following heat treatment but was significantly greater than controls (room temp) 24-48 h after heat treatment.

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A.M. Shirazi, Y.M. Moreno, L.H Fuchigami, and L.S. Daley

Previously, we reported recovery of plants from “Near-Lethal” (NL) (Sub-Lethal) stresses was dependent on stage of development and post-stress environment Dormant plants exposed to NL-heat, freezing, and hydrogen cyanamide either died or were severely injured when stored at 0°C or recovered at 23°C and natural condition. This study reports on the changes in the evolution of metabolic heat in dormant red-osier dogwood (Cornus sericea L.) stem tissues after beat stress. Heat stress (51°C for half an hour) was followed by one of two post-stress environment (PSE) (0° or 23°C dark condition). Isothermal measurements of the heat of metabolism of the tissues were taken after 0, 1, 2, 5, 7 and 11 days of PSE. A significant reduction of metabolic heat generation occured in heat stressed plants at 0°C PSE from one to 11 days of incubation as compared to the non-stressed tissues. At 23°C PSE, no significant differences of heat generation between stressed and non stressed tissues were found within 7 days of incubation. The rate of metabolic. heat measured by decreasing temperature scanning microcalorimetry (21° to 1°C) were lower in beat stressed tissues. Arrhenius plots of metabolic heat rate gave a linear slope for non-stressed tissues and a complex slop for NL-stressed tissues at lower temperatures. Energy of activation (Ea) between 1°-8°C were 15.45 and 83.882 KJ mol-1 for NL-heat and non-stressed tissues, respectively.

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Yiwei Jiang and Bingru Huang

Heat and drought are two major factors limiting growth of cool-season grasses during summer. The objective of this study was to compare the effects of heat stress alone (H) or in combination with drought (H+D) on photosynthesis, water relations, and root growth of tall fescue (Festuca arundinacea L.) vs. perennial ryegrass (Lolium perenne L.). Grasses were exposed to H (35 °C day/30 °C night) or H+D (induced by withholding irrigation) in growth chambers for 35 days. Soil water content declined under H+D for both grasses but to a greater extent for fescue than for ryegrass. Declines in canopy net photosynthetic rate (Pn), leaf photochemical efficiency (Fv/Fm), and leaf relative water content (RWC) and the increase in electrolyte leakage (EL) were much more severe and occurred earlier for ryegrass than fescue subjected to both H and H+D and for both species than under H+D then H. Evapotranspiration (ET) rate increased to above the control level within 3 or 6 days of H and H+D for both species, but fescue had a higher ET rate than ryegrass at 3 and 6 days of H and 6 days of H+D. Root dry weight and viability in all soil layers decreased under H and H+D for both species. However, fescue had higher root dry weight and viability than ryegrass in the 20-40 cm layer under H and in both the 0-20 and 20-40 cm layers under H+D. The results indicated that maintenance of higher Pn, Fv/Fm, ET, RWC, and root growth and lower EL would help cool-season turfgrass survive summer stress, and that their characteristics could be used for selecting stress tolerant species or cultivars.

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Magaji G. Usman, Mohd Y. Rafii, Mohd Razi Ismail, Mohammad Abdul Malek, and Mohammad Abdul Latif

agricultural regions ( Sood et al., 2009 ). Temperature increase resulting from changing climatic condition is a serious threat ( Jones et al., 1999 ) that affects crop production. Heat stress occurs when temperatures are high enough to cause irreversible

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M.M. Peet, C. Clement, and S. Sato

Starting 2 weeks before anthesis of the first flower, tomato cultivars (Lycopersicon esculentum Mill.) differing in heat tolerance were exposed to mild heat stress (31/24 vs. 28/22 °C) at three levels of relative humidity (30%, 60%, and 90%) in controlled environment chambers at the Duke Univ. Phytotron. Pollen development in the anthers was followed cytologically, pollen release was measured at anthesis, and seed production and fruit weight were measured as fruit matured. Fruit and seed development were best at 60%RH and 28/22 °C and worst at 90% RH and 31/24. Seed development was poor at 31/24 °C at all humidity levels. It was also poor at 28/22 in the 90% RH treatment. Low relative humidity had a greater negtive effect on fruit and seed production and on cytological development in plants grown at high temperature. Pollen release was also reduced at 90% RH, with virtually no pollen released at 31/24 °C. Cytological examinations revealed developmental anomolies in pollen in some, but not all cultivars at 90% and 30% RH. Plant height was also affected by the treatments, with much taller plants in the high-temperature, high-humidity treatments.

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Sanalkumar Krishnan, Yingmei Ma, and Emily Merewitz

wounding stress other than mowing. In addition, how the major hormones described in this paper respond to heat stress in turfgrass species is not well documented. Therefore, the objectives of the study were to evaluate hormone profiles in response to