Temperature response of whole-plant CO2 exchange rates of four upland cotton cultivars differing in leaf shape and leaf pubescence

It seems likely that CO2 exchange rates of cotton (Gossypium hirsutum L.) cultivars differing in leaf shape and leaf pubescence would respond differently to heat stress due to differences in boundary layer thickness. The objective of this study was to determine if CO2 exchange rates in commercially available cotton cultivars. differing in leaf shape and leaf pubescence respond differently to heat stress under well-watered conditions. CO2 exchange rates of whole plants of four cultivars ('FiberMax 832', 'Stoneville 474', 'DeltaPine 5690' and 'Paymaster 1220') were measured at eight different temperatures (6 to 34 degreesC) in 4 degreesC steps. Net photosynthesis (P-net) of the plants decreased at temperatures over 20C, while dark respiration (R-dark) increased exponentially with increasing temperature. Cotton cultivar did not influence the response of P-net or R-dark to high temperature stress. Carbon use efficiency (CUE) and net assimilation rate (NAR) were highest at lower temperatures and became negative at temperatures higher than 32 degreesC. The initial temperature response of CUE and NAR differed significantly between 'FiberMax 832' and the other cultivars. Carbon use efficiency and NAR of 'FiberMax 832' decreased at a greater rate with increasing temperature. Due to the fact that the okra leaf of 'FiberMax, 832' is deeply lobed and smooth. one would expect this cultivar to possess a thinner boundary layer and enhanced heat tolerance under well-watered conditions. which was not the case.