Micronutrient deficiencies accelerate leaf senescence in Amomum villosum

Fruit yield decreases gradually after several years of cultivation in Amomum villosum, an important Chinese medicinal herb, which is generally attributed to plant senescence. However, the reasons for plant senescence are still not clear until now. In this study, we investigated the effects of micronutrient deficiencies on leaf senescence. We hypothesized that micronutrient deficiencies may cause oxidative stress by disturbing cell metabolisms, which in turn leads to leaf senescence and decrease growth in A. villosum plants. Deficiencies of copper, zinc, and manganese decreased light-saturated photosynthetic rate and the contents of chlorophylls and soluble proteins but increased malondialdehyde content in A. villosum plants. The results indicated that oxidative stress and senescence occurred in leaves of A. villosum plants grown under micronutrient deficient conditions. Amomum plants were more susceptible to zinc deficiencies than to deficiencies of copper and manganese. The negative effects of micronutrient deficiencies on CO2 assimilation in chloroplast stroma were more serious than on electron transfer in thylakoids, increasing excess excitation energy. In most cases, micronutrient deficiencies decreased thermal dissipation of excess excitation energy and carotenoid content, leading to oxidative stress although antioxidant enzyme activities increased. Our results indicated that the micronutrient deficiencies caused oxidative stress, exacerbating leaf senescence and inhibiting biomass accumulation in Amomum plants.