Effects of B excess on some physiological and anatomical parameters of 'Navelina' orange plants grafted on two rootstocks

A greenhouse experiment was carried out in order to study the effects of B excess (2.50 mg B l(-1)) on physiological (B concentration and distribution, gas exchange parameters, chlorophyll concentration and fluorescence) and anatomical (leaf structure and chloroplast ultrastructure) characteristics of 'Navelina' orange plants grafted on sour orange and Swingle citrumelo rootstocks. Boron concentrations of all plant parts increased with increasing B concentration in the nutrient solution, following the order: old leaves > young leaves > bark of rootstock > root > stems of scion > stem of rootstock > wood of rootstock. 'Navelina' plants were less sensitive to excess B when grafted on Swingle citrumelo than on sour orange. This was due to the lesser absorption of B from the nutrient solution and its greater total retention in the rootstock stem of plants grafted on Swingle citrumelo than those grafted on sour orange rootstock, thus limiting the amount of B that reached the leaves of plants grafted on Swingle citrumelo rootstock. Increased B concentration in the nutrient solution brought about a decrease of leaf thickness, mainly due to reduced spongy parenchyma tissue. Although the size of chloroplasts decreased in response to high B concentration in the nutrient solution, their shape was not altered. Excess B further decreased the relative volume of chloroplast thylakoids and increased the relative volume of plastoglobuli. Finally, the leaves of 'Navelina' plants grown under 0.25 mg B l(-1) exhibited higher concentrations of chlorophyll as well as higher values of chlorophyll fluorescence parameters (F-v/F-m, F-v/F-0) and of photosynthetic rates in contrast to the plants grown under 2.50 mg B l(-1). (C) 2003 Elsevier B.V. All rights reserved.