Purple versus green‐leafed Ocimum basilicum : Which differences occur with regard to photosynthesis under boron toxicity?

This study was undertaken to investigate how different cultivars of sweet basil ( Ocimum basilicum ) responded to boron (B) excess. Two purple‐leafed and eight green‐leafed cultivars were hydroponically grown for 20 d with 0.2 or 20 mg L –1 B in the nutrient solution. Leaf B concentration, gas exchanges, chlorophyll a fluorescence, and oxidative stress were determined at the end of the treatment along with the severity of leaf necrosis. A range of tolerance to B toxicity was found: the green cultivars were more susceptible than the purple‐leafed ones characterized by a higher constitutive anthocyanin concentration. In all the genotypes B excess resulted in oxidative stress as determined by accumulation of malondialdehyde by‐products (MDA), reduced photosynthesis, and the occurrence of leaf burn. A close correlation was found between leaf B accumulation and oxidative stress, as well as between oxidative stress and the severity of leaf burn. Net photosynthesis ( P n ) was reduced due to both stomatal and nonstomatal limitations in the green cultivars whereas the reduction of P n in the purple leaves was only attributable to stomatal factors. Chlorophyll a fluorescence revealed a decrease in the maximum quantum yield of PSII ( F v / F m ) and in the electron transport rate (ETR) in plants grown with B excess although less reduction was observed in the purple genotypes. The quantum yield of PSII (Φ PSII ) decreased as a result of B toxicity only in the green cultivars. It is concluded that anthocyanins are involved in attenuation of the negative effects of B toxicity.