Photosynthetic Rate and Stomatal Conductance are Related to Heat Tolerance in Pima Cotton

Breeding for high yield in hot environments (heat tolerance) has tripled the yield of Pirna cotton since 1949. We compared six strains (one primitive non-cultivated line, four cultivars representing advancing stages in the breeding process, and one unreleased advanced line) for their gas exchange properties in the greenhouse. Both photosynthetic rate and stomatal conductance increased with improving genetic yield potential. Photosynthetic rate was enhanced more in the morning than in the afternoon. Stomatal conductance did not limit photosynthesis; rather, the changes resulted from alterations of characteristics of the green mesophyll cells. There is no evidence that increased yield results from the enhanced photosynthetic rates of single leaves. However, the increased stomatal conductance in modem lines was also expressed in the field in 1990, allowing increased transpiration rate and evaporative cooling of leaves. Heat tolerance in Pima cotton may be related to the ability of plants to cool themselves by transpiration