Boron toxicity in 'Clementine' mandarin plants grafted on two rootstocks

The effects of high B concentration in the nutrient solution (2.50 mg B l(-1)) on various morphological and physiological parameters of `Clementine' mandarin plants grafted on sour orange and Swingle citrumelo were investigated. These parameters included plant growth, B estimation, chlorophyll, proline and carbohydrate determinations, gas exchange (photosynthetic rate, stomatal conductance, P-n/C-i ratio) and chlorophyll fluorescence (F-v/F-m and F-v/F-0 ratios). Furthermore, micrographs of leaf structure and chloroplast ultrastructure as well as relevant morphometric assessments were presented. Boron toxicity resulted in a diminution of the proline and carbohydrate concentrations in the leaves, whereas the respective values for the roots were not affected. Boron toxicity also resulted in a decrease of the leaf chlorophyll concentration, photosynthetic rate, stomatal conductance, F-v/F-m, F-v/F-0 and P-n/C-i ratios, entire lamina and spongy parenchyma thickness, size of mesophyll cell chloroplasts, and relative volumes of chloroplast thylakoids and starch grains. Both stomatal and non-stomatal factors were responsible for the reduction of the photosynthetic rate, under B excess. The rootstock was found to play an important role in B toxicity. `Clementine' mandarin plants grafted on sour orange proved to be more sensitive to B toxicity than those grafted on Swingle citrumelo. (C) 2003 Elsevier Ireland Ltd. All rights reserved.