How to absorb more water and consume less water for plant has become increasingly more important because of water shortage. Root system is essential to uptake water from soil and stomata are important tissues to dismiss water for plant growth. However, one of great challenges in agricultural production is how to balance water absorption by roots and water use by shoots, especially under water deficit.
In this paper we are focus on the regulating mechanism between roots water absorption and shoots water consumption, to clarify eco-physiological basis of drought resistance and understand the adaptive traits to drought stress. In drought-stressed plants, roots system hydraulic conductivity (Lp_r), roots vigor, relative water content of leave and roots, water use efficiency and some parameters dedicated water use characteristics, such as stomatal conductivity, net photosynthetic rate, intercellular CO₂ concentration and transpiration rate, etc. of alfalfa (Medicago sativa L cultivars: Algonquin and Long-Dong) and sorghum (Sorghum bicolor L. cultivars: Kang-Si) were studied. Under variable water conditions of mimicked by PEG-6000, when tested plants undergo for 48 h stress (-0.2MPa water potential) and subsequent water recovery of 48 h, the experimental results indicated that roots water absorption (Lp_r) affected vitally water use and photosynthesis of shoots, especially at primary of water deficit and water recovery. Water absorption of alfalfa and sorghum was inhibited firstly by water deficit companying with decrease of Lp_r and root vigor and then, decline of stomatal conductivity. Net photosynthetic rate, relative water content of leave and transpiration rate etc. were induced. On the other hand, a limited increased in water use efficiency not in sorghum but in alfalfa are observed. It also shows that water deficit affect alfalfa stronger than that of sorghum. After rewatering from stress, Lp_r, root vigor, other parameters related to water characteristics (stomatal conductivity, net photosynthetic rate, relative water content of leave and transpiration rate and so on) and water use efficiency recovered gradually to the level of control treatment partly in alfalfa and completely in sorghum. These changes show that variable water conditions induced some significant differences, such as roots water absorption and shoots water consumption between two specie as well as two cultures. In this experiment, we also checked the role of aquaporins (AQPs) in alfalfa roots, results indicate that water deficit decrease Lp_r of alfalfa through inhibiting activity of AQPs. Comparing to the change extents of alfalfa and sorghum in Lp_r and other water parameters under variable water conditions, alfalfa is more sensitive to water stress than sorghum, and Algonquin is more sensitive than Long-Dong. All these data could increase our better understanding on the adaptive mechanism to drought and control role of root water absorption in water balance of alfalfa and sorghum.