Abstract
Diurnal changes in photosynthesis and transpiration in melons were examined, using '9-8' and 'Thai-7' lines and 'Ogon-makuwa' and 'Perlita' along with some of their hybrids. Vari-ations were found in both photosynthetic and transpiration rates among the genotypes, and there was a tendency for the genotypes with higher photosynthetic rates to have higher trans-piration rates. When the melon plants were stressed by exposing them to temperatures of 50°C for 30 min and 48°C for 60 min on 2 different dates, the stress resulted in a decrease in photosynthetic rates but not in transpiration rates of the melon leaves. This indicates the significance of mesophyll, not stomatal, resistance to photosynthesis. The photosynthetic rates of '9-8' line and its hybrids were less affected by the heat stress, suggesting the line's heat-tolerant potential and its heritability. Photosynthetic rates after heat stress did not correspond well with electrolyte leakage data. Since correlations between photosynthesis and a) leaf chlo-rophyll and b) nitrogen content were significant, high leaf chlorophyll and nitrogen content along with less-reduced photosynthetic rates after heat stress can possibly be used as indi-cators in selecting melon for high photosynthetic capacity under high temperature.
