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 (Pn) was reduced due to both stomatal and nonstomatal limitations in the green cultivars whereas the reduction of Pn in the purple leaves was only attributable to stomatal factors. Chlorophyll a fluorescence revealed a decrease in the maximum quantum yield of PSII (Fv/Fm) 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 (ΦPSII) 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.