CO2 assimilation, photosystem II photochemistry, carbohydrate metabolism and antioxidant system of citrus leaves in response to boron stress

Abstract: Seedlings of Citrus grandis were fertilized every other days for 15 weeks with nutrient solution containing 0 (deficiency), 10μM (control) or 500μM (excess) H3BO3. CO2 assimilation and chlorophyll (Chl) content decreased to a greater degree in B-deficient than in B-excess leaves, but photosynthetic enzyme activities were similarly decreased. Starch accumulated in B-deficient leaves, but not in B-excess ones. Chlorophyll a fluorescence transient showed that the positive L- and K-steps were more pronounced in B-excess than in B-deficient leaves. Maximum quantum yield of primary photochemistry (F v/F m), maximum variable fluorescence (F v), oxygen-evolving complex (OEC) were less decreased in B-deficient than in B-excess leaves, whereas minimum fluorescence (F 0) was less increased in B-deficient leaves. Boron-deficient leaves displayed higher or similar antioxidant enzyme activities and higher ascorbate (AsA) and reduced glutathione (GSH) contents compared to B-excess leaves. Content of thiobarbituric acid (TBA) reactive compounds was less increased by B-deficiency than by B-excess. We conclude that B-deficient leaves are less damaged by oxidative stress than B-excess leaves due to their higher ability to scavenge reactive oxygen species. Both the donor (i.e. the OEC) and the acceptor sides of photosystem II were less photoinhibited by B-deficiency than by B-excess. The greater decrease in CO2 assimilation and Chl content in B-deficient leaves may be caused by the excessive accumulation of starch. The reduction of CO2 assimilation by B-excess is probably caused by a combination of factors such as oxidative damage, reduced photosynthetic enzyme activities and impaired electron transport capacity. [Copyright &y& Elsevier]