H igh temperature stress is an important yield limiting factor in both spring and winter wheat (Triticum aestivum L.). At the present rates of greenhouse gas emissions and population growth, it is expected that mean surface air temperatures will increase in the range of 1.8 to 5.8°C by the end of this century (Intergovernmental Panel on Climate Change, 2007). It is predicted that future climates will not only be associated with an increase in mean temperatures (Easterling et al., 1997) but also with an increase in the frequency of episodes of high temperatures . In addition, climate models foresee that there will be a relatively greater increase in nighttime temperatures as compared to daytime temperatures. Over the past century global daily minimum temperatures increased more than twice compared to increases in daily maximum temperatures (Easterling et al., 1997). Recent studies have shown that historical yields of rice (Oryza sativa L.; Peng et al., 2004) and wheat (Lobell et al., 2005) were strongly correlated with minimum (nighttime) temperatures, rather than daytime maximum temperatures. Decreasing rice yields in the Philippines were related to increasing nighttime temperatures (Peng et al., 2004), and increasing wheat yields in Mexico were related to decreasing nighttime temperatures (Lobell et al., 2005). ABSTRACTClimate models predict greater increases in nighttime temperature in the future. The impacts of high nighttime temperature on wheat (Triticum aestivum L.) are not well understood. Objectives of this research were to quantify the impact of high nighttime temperatures during reproductive development on phenology, physiological, vegetative, and yield traits of wheat. Two spring wheat cultivars (Pavon-76 and Seri-82) were grown at optimum temperatures (day/night, 24/14°C; 16/8 h light/dark photoperiod) from sowing to booting. Thereafter, plants were exposed to four different nighttime temperatures (14, 17, 20, 23°C) until maturity. The daytime temperature was 24°C across all treatments. There were signifi cant infl uences of high nighttime temperatures on physiological, growth, and yield traits, but no cultivar or cultivar by temperature interactions were observed. High nighttime temperatures (>14°C) decreased photosynthesis after 14 d of stress. Grain yields linearly decreased with increasing nighttime temperatures, leading to lower harvest indices at 20 and 23°C. High nighttime temperature (≥20°C) decreased spikelet fertility, grains per spike, and grain size. Compared to the control (14°C), grain fi lling duration was decreased by 3 and 7 d at night temperatures of 20 and 23°C, respectively. High nighttime temperature increased the expression of chloroplast protein synthesis elongation factor in both cultivars suggesting possible involvement of this protein in plant response to stress.

Determining mechanisms associated with heat tolerance and identifying screening methods are vital for improvement of heat tolerance in plants. The objectives of this study were to investigate the relationship between the heat stability of thylakoids and loss of chlorophyll in winter wheat (Triticum aestivum L.) under heat stress, and to examine whether chlorophyll loss can be used as an indicator of heat tolerance in wheat. We assessed heat tolerance and measured chlorophyll content in 12 cultivars of winter wheat at flowering stage during exposure to 16‐d‐long heat stress. Heat tolerance was assessed using fluorescence to determine the heat stability of thylakoids, and chlorophyll content was measured with a chlorophyll meter. Experiments were conducted under controlled conditions. Heat stress caused damage to thylakoids in all cultivars as indicated by the increase in the ratio of constant fluorescence (O) and the peak of variable fluorescence (P). Heat stress also caused a decline in chlorophyll content in most cultivars. A strong negative correlation between heat‐induced increases in O/P and chlorophyll content was seen. The results suggest that heat‐induced damage to thylakoids and chlorophyll loss are closely associated in winter wheat. Measurements of chlorophyll content with a chlorophyll meter could be useful for high throughput screening for heat tolerance in wheat.

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Rubisco activase (RCA) constrains the photosynthetic potential of plants at high temperatures (heat stress). Endogenous levels of RCA could serve as an important determinant of plant productivity under heat-stress conditions. Thus, in this study, the possible relationship between expression levels of RCA and plant yield in 11 European cultivars of winter wheat following prolonged exposure to heat stress was investigated. In addition, the effect of a short-term heat stress on RCA expression in four genotypes of wheat, five genotypes of maize, and one genotype of Arabidopsis thaliana was examined. Immunoblots prepared from leaf protein extracts from control plants showed three RCA cross-reacting bands in wheat and two RCA cross-reacting bands in maize and Arabidopsis. The molecular mass of the observed bands was in the range between 40 kDa and 46 kDa. Heat stress affected RCA expression in a few genotypes of wheat and maize but not in Arabidopsis. In wheat, heat stress slightly modulated the relative amounts of RCA in some cultivars. In maize, heat stress did not seem to affect the existing RCA isoforms (40 kDa and 43 kDa) but induced the accumulation of a new putative RCA of 45-46 kDa. The new putative 45-46 kDa RCA was not seen in a genotype of maize (ZPL 389) that has been shown to display an exceptional sensitivity to heat stress. A significant, positive, linear correlation was found between the expression of wheat 45-46 kDa RCA and plant productivity under heat-stress conditions. Results support the hypothesis that endogenous levels of RCA could play an important role in plant productivity under supraoptimal temperature conditions.

The design, synthesis, modeling and in vitro testing of channel-forming peptides derived from the cys-loop superfamily of ligand-gated ion channels are part of an ongoing research focus. Over 300 different sequences have been prepared based on the M2 transmembrane segment of the spinal cord glycine receptor α-subunit. A number of these sequences are water-soluble monomers that readily insert into biological membranes where they undergo supramolecular assembly, yielding channels with a range of selectivities and conductances. Selection of a sequence for further modifications to yield an optimal lead compound came down to a few key biophysical properties: low solution concentrations that yield channel activity, greater ensemble conductance, and enhanced ion selectivity. The sequence NK4-M2GlyR T19R, S22W (KKKKPARVGLGITTVLTMRTQW) addressed these criteria. The structure of this peptide has been analyzed by solution NMR as a monomer in detergent micelles, simulated as five-helix bundles in a membrane environment, modified by cysteine-scanning and studied for insertion efficiency in liposomes of selected lipid compositions. Taken together, these results define the structural and key biophysical properties of this sequence in a membrane. This model provides an initial scaffold from which rational substitutions can be proposed and tested to modulate anion selectivity.