METHOD: Gas Exchange CONDITIONS: Growth Chamber - 14h light at 25/20C or 40/30C METHOD NARR: Control rates of photosynthesis were measured at 25C. Oxygen evolution was measured on fresh leaf discs at saturating CO2 and humidity electrode. Rates of oxygen evolution were normalized to the total Chl a + b content of each leaf disc. Chl a & b were measured immediately after rates of oxygen had been recorded, on the same leaf discs. Carbon fixaion wa steady state gas exchange system. Leaf temperature was determined by a thermocouple contacting the leaf. RESULTS NARR: Heat tolerant (HT) accessions showed higher maximal rates of oxygen evolution than heat sensitive (HS) aleaves of the HS accession exhibited less pronounced effects of heat treatment than did mature, fully expanded leaves on the same plant. All accessions showed lower stomatal conductance at 40C than for 25C controls. Changes in leaf C to Chl b ratios contributed significantly to the decline of PS performance in chc. AUTHOR EXPLANATION: Results suggest that Chl loss, reduced stomatal conductivity and accelerated senescence were the major factors accoosynthetic performance. HS may be a manifestation of accelerated senescence. It appears that CO2 availability is the major factor determining the difference in photosynthesis between HT and HS accessions at 40C. Vafactor in reduced CO2 fixation at 40C.