Effects of Heat Acclimation on Photosynthesis, Antioxidant Enzyme Activities, and Gene Expression in Orchardgrass under Heat Stress

The present study was designed to examine the effects of heat acclimation on enzymatic activity, transcription levels, the photosynthesis processes associated with thermostability in orchardgrass (Dactylis glomerata L.). The stomatal conductance (Gs), net photosynthetic rate (Pn), and transpiration rates (Tr) of both heat-acclimated (HA) and non-acclimated (NA) plants were drastically reduced during heat treatment [using a 5-day heat stress treatment (38/30 degrees C. day/night) followed by a 3-day recovery under control conditions (25/20 degrees C. day/night), in order to consolidate the second cycle was permitted]. Water use efficiency increased more steeply in the HA (4.9 times) versus the NA (1.8 times) plants, and the intercellular CO2 concentration decreased gently in NA (10.9%) and HA (25.3%) plants after 20 d of treatments compared to 0 days'. Furthermore, heat-acclimated plants were able to maintain significant activity levels of superoxide disumutase (SOD), catalase (CAT), guaiacol peroxidase (POD), and transcription levels of genes encoding these enzymes; in addition, HA plants displayed lower malondialdehyde content and lower electrolyte leakage than NA plants. These results suggest that maintenance of activity and transcription levels of antioxidant enzymes as well as photosynthesis are associated with variable thermostability in HA and NA plants. This likely occurs through cellular membrane stabilization and improvements in water use efficiency in the photosynthetic process during heat stress. The association between antioxidant enzyme activity and gene expression, both of which may vary with genetic variation in heat tolerance, is important to further understand the molecular mechanisms that contribute to heat tolerance.