Changes in photosynthetic properties and antioxidative system of pear leaves to boron toxicity

Seedlings of the Cuiguan cultivar of the Asian pear ( Pyrus pyrifolia ) were used to study the effects of boron toxicity on leaf photosynthetic properties and lipid peroxidation. The plants were grown hydroponically and treated with four concentrations of boron: 10 (CK), 100, 300 and 500 μmol·L -1 . After 16 weeks of treatment, we measured the leaf contents of boron and chlorophyll (Chl), the net photosynthetic rate (Pn), stomatal conductance (Gs), intercellular carbon dioxide concentration (Ci), the concentrations of malondialdehyde (MDA), H 2 O 2 , ascorbic acid (ASA) and glutathione (GSH) and the enzymatic activities of superoxide dismutase (SOD), ascorbate peroxidase (APX), catalase (CAT) and glutathione reductase (GR). Treatment with high levels of boron significantly increased the boron content of the pear leaves and reduced Chla, Chlb and carotenoid (Car) contents. Boron stress also reduced the Pn and Gs but increased the Ci. Furthermore, the leaf concentrations of MDA and H 2 O 2 increased with increasing concentrations of boron, whereas the enzymatic activities of SOD, APX, CAT and GR and the ASA and GSH levels first increased and then decreased. These results indicate that boron toxicity reduced the photosynthetic capacity of the pear plant, resulting in the accumulation of reactive oxygen species (ROS) and increased membrane lipid peroxidation. Moderate boron stress can therefore improve the vitality of a plant’s ROS scavenging system, but high concentrations will eventually overcome the system. Key words: Pear, boron toxicity, photosynthetic rate, lipid peroxidation.