Non-irrigated crops in temperate climates and irrigated crops in arid climates are subjected to continuous cycles of water stress and re-watering. Thus, fast and efficient recovery from water stress may be among the key determinants of plant drought adaptation. The present study was designed to comparatively analyze the roles of drought resistance and drought recovery in drought adaptation and to investigate the physiological basis of genotypic variation in drought adaptation in maize (Zea mays) seedlings. As the seedlings behavior in growth associate with yield under drought, it could partly reflect the potential of drought adaptability. Growth and physiological responses to progressive drought stress and recovery were observed in seedlings of 10 maize lines. The results showed that drought adaptability is closely related to drought recovery (r = 0.714**), but not to drought resistance (r = 0.332). Drought induced decreases in leaf water content, water potential, osmotic potential, gas exchange parameters, chlorophyll content, Fv/Fm and nitrogen content, and increased H2O2 accumulation and lipid peroxidation. After recovery, most of these physiological parameters rapidly returned to normal levels. The physiological responses varied between lines. Further correlation analysis indicated that the physiological bases of drought resistance and drought recovery are definitely different, and that maintaining higher chlorophyll content (r = 0.874***) and Fv/Fm (r = 0.626*) under drought stress contributes to drought recovery. Our results suggest that both drought resistance and recovery are key determinants of plant drought adaptation, and that drought recovery may play a more important role than previously thought. In addition, leaf water potential, chlorophyll content and Fv/Fm could be used as efficient reference indicators in the selection of drought-adaptive genotypes.

In plants, water deficiency can result from a deficit of water from the soil, an obstacle to the uptake of water or the excess water loss; in these cases, the similar consequence is the limitation of plant growth and crop yield. Silicon (Si) has been widely reported to alleviate the plant water status and water balance under variant stress conditions in both monocot and dicot plants, especially under drought and salt stresses. However, the underlying mechanism is unclear. In addition to the regulation of leaf transpiration, recently, Si application was found to be involved in the adjustment of root hydraulic conductance by up-regulating aquaporin gene expression and concentrating K in the xylem sap. Therefore, this review discusses the potential effects of Si on both leaf transpiration and root water absorption, especially focusing on how Si modulates the root hydraulic conductance. A growing number of studies support the conclusion that Si application improves plant water status by increasing root water uptake, rather than by decreasing their water loss under conditions of water deficiency. The enhancement of plant water uptake by Si is achievable through the activation of osmotic adjustment, improving aquaporin activity and increasing the root/shoot ratio. The underlying mechanisms of the Si on improving plant water uptake under water deficiency conditions are discussed.

Although silicon (Si) has been widely reported to alleviate plant nutrient deficiency, the underlying mechanism in potassium (K) deficiency is poorly understood. In this study, sorghum seedlings were treated with Si under a K deficiency condition for 15 days. Under control conditions, plant growth was not affected by Si application. The growth and water status were reduced by K-deficient stress, but Si application significantly alleviated these decreases. The leaf gas exchanges, whole-plant hydraulic conductance (K plant ), and root hydraulic conductance (Lp r ) were reduced by K deficiency, but Si application moderated the K-deficiency-induced reductions, suggesting that Si alleviated the plant hydraulic conductance. In addition, 29% of Si-alleviated transpiration was eliminated by HgCl 2 treatment, suggesting that aquaporin was not the primary cause for the reversal of plant hydraulic conductance. Moreover, the K + concentration in xylem sap was significantly increased and the xylem sap osmotic potential was decreased by Si application, suggesting that the major cause of Si-induced improvement in hydraulic conductance could be ascribed to the enhanced xylem sap K + concentration, which increases the osmotic gradient and xylem hydraulic conductance. The results of this study show that Si mediates K + accumulation in xylem, which ultimately alleviates the plant-water status under the K-deficient condition.