Foliar phenotypic plasticity of a warm-temperate shrub, Vitex negundo var. heterophylla, to different light environments in the field

Light is an important factor affecting plant growth and species distribution. In the field, open forest, forest edge, forest gaps and forest understory are typical light environment for species to survive and growth, in which light intensity are different for 10 times and even more. Plant phenotypic plasticity, which allows plants to adapt to different light environments, plays an important role in how plants adapt to changes in lighting, and leaves play an especially important role. Leaves are the main organs plants use for gas exchange with the environment. Leaves have both structural and functional characteristics. Different species show different sensitivities to irradiance. Vitex negundo Linn. var. heterophylla (Franch.) Rehd. is the dominant shrub of the warm temperate zone in China. It is an important native and pioneer species for vegetation restoration of Shandong Province, because of the high soil and water conservation ability. But up to now, studies related to its adaptation to irradiance are lacking. This study provides information related to this shrub and its use in vegetation restoration in Luzhong Mountains of Shandong Province. We measured leaf photosynthetic light response curves, leaf chlorophyll fluorescence, leaf chlorophyll content, leaf nirtogen (N) and phosphorus (P) content and leaf morphology. We studied the response of V. negundo var. heterophylla to five different light environments in the field: open forest, broad-leaved forest edge broad-leaved forest understory, gaps in needle-leaf forests and needle-leaf forest understory. The results the leaf traits of V. negundo var. heterophylla acclimated well to different habitats. Leaf functional traits were adjusted by leaf structural traits. The species exhibited some shade tolerance based on its ability to modify the characteristics of its leaves. The species improved its light use efficiency and kept growing in low light by increasing its specific leaf area (SLA), its chlorophyll (Chl) content, and the maximal quantum yield of PSII, and by lowering its respiration rate, light saturation point, light compensation point, and Chl a to b ratio. Also, leaf structure, especially SLA, played an important role in the regulation of photosynthesis under high levels of irradiance. Most of the traits were only impacted by the daily total irradiance; however, the variation of SLA and leaf maximal fluorescence in the dark (F sub(m)) was related to the highest irradiance levels. SLA was sensitiv