Effects of blue light deficiency on acclimation of light energy partitioning in PSII and CO2 assimilation capacity to high irradiance in spinach leaves

Blue light effects on the acclimation of energy partitioning characteristics in PSII and CO(2) assimilation capacity in spinach to high growth irradiance were investigated. Plants were grown hydroponically in different light treatments that were a combination of two light qualities and two irradiances, i.e. white light and blue-deficient light at photosynthetic photon flux densities (PPFDs) of 100 and 500 mol m(2) s(1). The CO(2) assimilation rate, the quantum efficiency of PSII (Phi PSII) and thermal dissipation activity (F(v)/F(m) - F(v)'/F(m)') in young, fully expanded leaves were measured under 1,600 mol m(2) s(1) white light. The CO(2) assimilation rate and PSII were higher, while F(v)/F(m) - F(v)'/F(m)'was lower in plants grown under high irradiance than in plants grown under low irradiance. These responses were observed irrespective of the presence or absence of blue light during growth. The extent of the increase in the CO(2) assimilation rate and PSII and the decrease in F(v)/F(m) - F(v)'/F(m)'by high growth irradiance was smaller under blue light-deficient conditions. These results indicate that blue light helps to boost the acclimation responses of energy partitioning in PSII and CO(2) assimilation to high irradiance. Similarly, leaf N, Cyt f and Chl contents per unit leaf area increased by high growth irradiance, and the extent of the increment in leaf N, Cyt f and Chl was smaller under blue light-deficient conditions. Regression analysis showed that the differences in energy partitioning in PSII and CO(2) assimilation between plants grown under high white light and high blue-deficient light were closely related to the difference in leaf N.