Title : Changes on growth and abscisic acid contents of Medicago sativa to drought at different growth stages
The morphological and physiological responses in plants to water stress varies at different stages of growth as affected by hormones and their relative concentrations, leading to different outcomes for crop production. Cereal crops harvested for grain suffer severer yield reduction from water stress during the reproductive stage than at the vegetative stage. In contrast, forage crops, with vegetative tissues the harvested product, should examined differently to cereal crops in experiments on the effect of water stress on production. ABA is a hormone closely related to stresses and, therefore, can improve the tolerance of crops to various biotic and abiotic stresses. ABA accumulates in the leaves when plants are under water stress, and reduces stomata opening and water loss from stomata, thereby maintaining the balance of water uptake and loss in plants and increasing the adaptation of plants to dry environments. Previous studies on drought tolerance of alfalfa focused on responses of plant morphological traits, osmotic pressure regulatory substances and peroxide scavenging systems. The effect of water stress on ABA metabolism remains unclear. Alfalfa (Medicago sativa L.) is an important forage species with high nutritional quality and yield around the world. As a legume plant, alfalfa is capable of fixing nitrogen to nitrate in nodules, by establishing a symbiotic relationship with rhizobium in the root system, making it an ideal crop for sustainable agriculture. The response of eco-physiological index of alfalfa at different growth stages on water stress remains unclear. Therefore, a pot experiment was designed and conducted in a greenhouse with three alfalfa (Medicago sativa) cultivars, Aohan, Zhongmu No.1 and Sanditi, to examine the morphological and physiological changes and response of alfalfa to water stress. The response of alfalfa to water stress at different growth stages was generally similar, but varied between cultivars. At the branching, flowering and podding stages, the shoot biomasses of Aohan and Zhongmu No. 1 were greatly affected by water stress, and these cultivars responded quickly to water stress. The shoot biomass of Sanditi was not affected by mild water stress, but had a slight response to moderate and severe water stress. The root/shoot ratios in Aohan and Zhongmu No. 1 were more sensitive to water stress than in Sanditi, with the root/shoot ratio in Aohan increasing most significantly. At flowering, the root/shoot ratio the highest and the effect of water stress the greatest. The abscisic acid (ABA) concentration in the roots of Aohan and Zhongmu No.1 increased under water stress, while in Sanditi there was only a slight or delayed response in ABA concentration. The objective of this study is to investigate the effect of water stress at different growth stages of alfalfa on growth and ABA concentration in roots, and to help to provide a better understanding of stress ecophysiology of alfalfa to assistance in the selection of drought-tolerant cultivars.