Photosynthesis at High Temperature in Tuber-Bearing Solanum Species : A Comparison between Accessions of Contrasting Heat Tolerance.

Differences in the photosynthetic performance between pairs of heat tolerant (HT) and heat sensitive (HS) accessions of tuber-bearing Solanum species were measured at 40 degrees C, after treating plants at 40/30 degrees C. After 1 to 9 days of heat treatment, both HT and HS accessions showed progressive inhibitory effects, primarily decreased rates of CO(2) fixation, and loss of leaf chlorophyll. These effects were most pronounced in the HS accessions. Stomatal conductivity and internal CO(2) concentrations were lower for both accessions at 40 degrees C especially for the HS accessions, suggesting that at ambient CO(2) concentrations, stomatal conductance was limiting CO(2) availability at the higher temperature. In the HT accessions, stomatal limitations were largely attributed to differences in vapor pressure deficit between 25 degrees and 40 degrees C, while the HS accessions exhibited significant nonstomatal limitations. The young expanding leaves of the HS accession showed some HT characteristics, while the oldest leaves showed severe senescence symptoms after 9 days at 40/30 degrees C. The data suggest that differences in heat sensitivity between HT and HS accessions are the result of accelerated senescence, chlorophyll loss, reduced stomatal conductance, and inhibition of dark reactions at high temperature.