PHOTOSYNTHETIC ACTIVITY AND CONCENTRATION OF CHLOROPHYLLS, CAROTENOIDS, HYDROGEN PEROXIDE AND MALONDIALDEHYDE IN LOQUAT SEEDLINGS GROWING UNDER EXCESS BORON CONDITIONS

The effects of boron (B) excess on various physiological and biochemical parameters of loquat plants were investigated. For this purpose, one and a half year old loquat seedlings were grown for 147 days under hydroponic conditions. They were fertigated thrice a week with a full strength Hoagland’s nutrient solution containing either 25 or 400 μM B. At the end of the experiment, gas exchange parameters (photosynthetic rate, transpiration rate, stomatal conductance, water use efficiency) were measured in the top and basal leaves of six plants per treatment. Afterwards, appropriate samples from the top leaves were taken for the determination of chlorophyll, carotenoid, hydrogen peroxide and malondialdehyde (MDA) concentrations. None of the photosynthetic parameters measured in basal leaves were affected by B concentration in the nutrient solution with the exception of transpiration rate that decreased under high B. In contrast, photosynthetic and transpiration rates as well as water use efficiency of top leaves were all significantly reduced under excess B conditions. Moreover, the concentrations of chlorophylls (a, b, a+b, a/b), carotenoids and hydrogen peroxide in the top leaves were unaffected by B treatments. However, excess B conditions caused a two-fold increase of MDA concentrations in the top leaves. In conclusion, B toxicity seems to affect negatively the transpiration rate of leaves prior to the photosynthetic rate. Moreover, the reduction of photosynthesis, at least at the initial stages of B toxicity stress, was mainly due to non-stomatal factors.