Effect of zinc on maize leaf cell ultra-structure under different soil moistures

Zinc (Zn) deficiency is a major nutrient problem in calcareous soils of the North China, where drought is also a considerable stress factor in crop production. Pot trials under greenhouse conditions were conducted using Zn-deficient cumulic cinnamon soils sampled from Shaanxi Province of the Northwest China. With two levels of Zn 0 and 5.0 mg(Zn)·kg^-1(soil) and two soil moisture (40%~45% and 70%~75% of saturated soil water content) regimens, a completely randomized factorial design (2 Zn treatments × 2 water levels × 3 replicates) was set. Ultra-structure of cells of maize plant leaves under different soil water conditions and Zn doses were examined using transmission electron microscope. The objective of the study was to investigate the mechanisms of maize plants responding to Zn under different soil moisture conditions. The results showed that Zn increased plant biomass and Zn content more notably under well-watered conditions than under drought conditions. The structures of maize mesophyll and bundle sheath cells remained integrated and normal under adequate soil water and Zn supply. Under adequate soil water supply, chloroplast plasmolemma distortion, starch accumulation and thylakoid disturbance were noticed in vascular bundle sheath cells of Zn-deficient leaves. Moreover, vascular bundle sheath cells collapsed when plants were subjected to severe Zn deficiency. Thylakoid shrinking and much less lamella stacking occurred in leaf mesophyllic cells of maize plants grown under adequate soil water and severe Zn deficiency. Under drought conditions, thylakoid shrinking and much less grana stacking occurred in Zn deficient leaf mesophyllic cells. Chloroplast structure in vascular bundle sheath and mesophyllic cells of Zn-sufficient leaves remained normal. With Zn application under drought treatments, the number of mitochondria increased around the chloroplast in vascular bundle sheath cells. Also numerous plastoglobuli were noted in the chloroplast of mesophyllic cells of maize plants. It suggested that Zn application alleviated disruptions in leaf cells by drought stress conditions. However, the ultra-structures of Zn-deficient leaf cells were more easily harmed under adequate soil water supply than that under drought stress conditions.