Photoinactivation of PSII is thought to be caused by the excessive light energy that is neither used for photosynthetic electron transport nor dissipated as heat. However, the relationship between ...the photoinactivation rate and excess energy has not been quantitatively evaluated. Chenopodium album L. plants grown under high-light and high-nitrogen (HL-HN) conditions show higher tolerance to photoinactivation and have higher photosynthetic capacity than the high-light and low-nitrogen (HL-LN)- and low-light and high-nitrogen (LL-HN)-grown plants. The rate of photoinactivation in the LL-HN plants was faster than that in the HL-LN, which was similar to that in the HL-HN plants, while the LL-HN and HL-LN plants had similar photosynthetic capacities Kato et al. (2002b) Funct. Plant Biol. 29: 787. We quantified partitioning of light energy between the electron transport and heat dissipation at the light intensities ranging from 300 to 1,800 µmol m–2 s–1. The maximum electron transport rate was highest in the HL-HN plants, heat dissipation was greatest in the HL-LN plants, and the excess energy, which was neither consumed for electron transport nor dissipated as heat, was greatest in the LL-HN plants. The first-order rate constant of the PSII photoinactivation was proportional to the magnitude of excess energy, with a single proportional constant for all the plants, irrespective of their growth conditions. Thus the excess energy primarily determines the rate of PSII photoinactivation. A large photosynthetic capacity in the HL-HN plants and a large heat dissipation capacity in the HL-LN plants both contribute to the protection of PSII against photoinactivation.
Photosynthetic characteristics in rice (Oryza sativa L.) leaves were examined after treatment with low temperature (15 deg C) and high irradiance (1,500 mu mol quanta m(-2) s(-1)). Decreases in ...quantum efficiencies in PSII (phi PSII) and PSI (phi PSI) and in the rate of CO2 assimilation were observed with a decrease in the maximal quantum efficiency of PSII (Fv/Fm) by simultaneous measurements of Chl fluorescence, P700(+) absorbance and gas exchange. The decreases in phi PSII were most highly correlated with those in CO2 assimilation. Although the initial (the activity immediately measured upon extraction) and total (the activity following pre-incubation with CO2 and Mg(2+)) activities of ribulose-1,5-bisphosphate (RuBP) carboxylase/oxygenase (Rubisco) decreased slightly, the maximal activity (the activity following treatment with SO4(2-)) of Rubisco remained almost constant. These results indicate that the decrease in CO2 assimilation rate with the decreasing Fv/Fm was not caused by a decrease in Rubisco activity but rather by a decrease in RuBP regeneration capacity which resulted from the decrease in the rate of the linear electron transport. On the other hand, the decrease in (phi PSI was very small and the ratio of phi PSI to phi PSII increased. The de-epoxidation state of xanthophyll cycle pigments also increased. Thus, the cyclic electron transport around PSI occurred in photoinhibited leaves.
Effects of low temperature on chlorophyll (Chl) fluorescence, gas exchange rate, the amounts of xanthophyll cycle pigments (Xp) and the activities of several antioxidant enzymes were examined in the ...8th leaf of two rice (Oryza sativa L.) cultivars (japonica and indica types) and rbcS antisense rice. All plants were grown hydroponically at 25/20°C (day/night), and then exposed to 20/17°C (day/night) after full expansion of the 8th leaf, or exposed to either 20/17°C or 15/13°C (day/night) during the expansion of the 8th leaf. All plants exposed to low temperatures showed a decrease in CO2 assimilation rate without photoinhibition, and increases in the fraction of thermal dissipation in PSII, and in the electron flux through the water–water cycle (WWC) were observed. Although the increase of thermal dissipation was associated with increases in the ratio of carotenoids to Chl, the ratio of Xp to carotenoids and the de-epoxidation state of Xp, the increase of the electron flux of WWC was not accompanied by an increase in the activities of antioxidant enzymes. Such photoprotective responses did not differ between during and after full expansion of the leaf, and did not differ among the three genotypes. Quantitative analyses on the dissipation of excess light energy showed that thermal dissipation makes a larger contribution than WWC. Thus, although low temperature led to a decrease in CO2 assimilation, rice potentially coped with the excess light energy by increasing the thermal dissipation and the electron flux of WWC under low temperature irrespective of leaf development and genotypes.
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