The effect of high temperatures on the photosynthetic activity of intact barley leaves at low and high irradiance

摘要

Light curves of CO2 fixation by barley seedling leaves preliminarily heated at 30-43degreesC for 5 min were measured. The slope of the linear part of the light curve decreased after leaf heating at temperatures above 35degreesC; whereas, at a high light level, the photosynthesis rate decreased only at temperatures of 40degreesC and higher, The linear relationships between the photosynthetic CO2-fixation rate and a photon flux density up to 1400 mumol/(m(2) s) were found in leaves preheated at 42degreesC; this indicates the strong nonphotochemical dissipation of absorbed light quanta. The lowering of the oxygen concentration from 21 to 1% led to a CO2 fixation maximum quantum yield and a photosynthesis-rate increase at the highest light intensity in leaves preheated at temperatures above 40degreesC as compared to the control leaves. Nevertheless. the linear relationship between the photosynthetic CO2 fixation and the light intensity was found in leaves heated at 42degreesC at O-2 concentrations or both 21 and 1%. The latter fact suggests that the proton gradient of the thylakoid membrane, which causes an increase in the nonphotochemical dissipation of the quanta absorbed, could also be formed due to the cyclic electron transport over photosystem I. [References: 18]