Carotenoid composition and photon-use efficiency of photosynthesis inGossypium hirsutum L. grown under conditions of slightly suboptimum leaf temperatures and high levels of irradiance.

Cotton (Gossypium hirsutum L. var. DP 61) was grown at different temperatures during 12-h light periods, with either 1800-2000 μmol photons m-2 s-1 (high photon flux density, PFD) or 1000-1100 μmol m-2 s-1 (medium PFD) incident on the plants. Night temperature was 25°C in all experiments. Growth was less when leaf temperatures were below 30°C during illumination, the effect being greater in plants grown with high PFD (Winter and Königer 1991). Leaf pigment composition and the photon-use efficiency of photosynthesis were analysed to assess whether plants grown with high PFD and suboptimal temperatures experienced a higher degree of high irradiance stress during development than those grown with medium PFD. The chlorophyll content per unit area was 3-4 times less, and the content of total carotenoids about 2 times less, with the proportion of the three xanthophylls zeaxanthin + antheraxanthin + violaxanthin being greater in leaves grown at 20-21°C than in leaves grown at 33-34°C. In leaves from plants grown at 21°C and 1800-2000 μmol photons m-2 s-1, zeaxanthin accounted for as much as 34% of total carotenoids in the middle of the photoperiod, the highest level recorded in this study. This finding is consistent with a protective role of zeaxanthin under conditions of excess light. At the lower temperatures, the photochemical efficiency of photosystem II, measured as the ratio of variable to maximum fluorescence yield (F V/F M) after 12-h dark adaptation, was 0.76 in medium PFD plants and 0.75 in high PFD plants compared with 0.83 and 0.79, respectively, at the higher temperatures. The photon-use efficiency of O2 evolution (ϕ) based on absorbed light between 630 and 700nm, decreased with decrease in temperature from 0.102 to 0.07 under conditions of high PFD, but remained above 0.1 at medium PFD. Owing to compensatory reactions in these long-term growth experiments, sustained differences inF V/F M and ϕ were much less pronounced than the differences in chlorophyll content and dry matter, particularly in plants which had developed at high PFD and low temperature. In fact, in these plants, which exhibited pronounced photobleaching, a largely functional photosynthetic apparatus was still maintained in cells adjacent to the lower leaf surfaces. This was indicated by measurements of photon use efficiencies of photosynthetic O2 evolution with leaves illuminated first at the upper, and then at the lower surface.