Leaf anatomy, photosynthesis, and chloroplast ultrastructure of Heptacodium miconioides seedlings reveal adaptation to light environment

Abstract

Light intensity is a critical factor for the plant survival and early growth. Heptacodium miconioides , a well-accepted ornamental plant, has been endangered due to fragmented habitats in the wild. To better understand the light adaptation mechanisms of H. miconioides , we investigated the morphology, photosynthesis, chloroplast ultrastructure, and chloroplast gene expression of the leaves under high light (HL), moderate light (ML), and low light (LL) conditions. With decreasing light intensity, the specific leaf area and chlorophyll contents of H. miconioides increased, whereas the leaf size first increased followed by a decrease. The maximum net photosynthetic rate under ML and HL treatments were significantly higher than that under LL treatment. However, leaves under ML treatment exhibited the lower dark respiration rate and light compensation point than those under HL treatment. The leaf thickness, palisade tissue, spongy tissue, and stomatal density were obviously reduced with decreasing light intensity. Chloroplast ultrastructure showed the better development in leaves under ML condition, and the size of chloroplast, the number of starch grains and plastoglobules increased significantly. In addition, the expression levels of genes involved in chlorophyll biosynthesis ( HEMA , CLH , CHLI , CHLH , CHLG ) were higher under ML than LL treatment, but lower under HL treatment. The up-regulation of the photosystem II ( PsbB , PsbD , PsbO ) genes expression play an important role in under ML and LL treatments. Taken together, we proposed that the optimal growth of H. miconioides seedlings varies between HL and ML but had poor growth under LL condition. The improvement in H. miconioides growth under moderate shade was mainly through its effects on leaf morphology, chloroplast development, and light capture efficiency of PSII phase. • Adaptation mechanisms to different light intensity were investigated in Heptacodium miconioides . • Seedlings grown under moderate and high light intensities exhibited higher photosynthetic capacity. • Moderate light intensity increased the size of chloroplast and the accumulation of starch grains and plastoglobules. • Moderate light intensity up-regulated chlorophyll biosynthesis and photosystem II (PSII) related gene expression.