国家重点基础研究发展计划（973计划）项目（2009CB118604）; 国际自然科学基金项目（40671175）; 黄土高原土壤侵蚀与旱地农业国家重点实验室开放基金项目（10501234）;省部共建河南大学科研课题项目(SBGJ090405)
采用盆栽实验方法研究了紫花苜蓿（品种：陇东和阿尔冈金）根系形态、生物量、蒸腾耗水量等对持续干旱的反应及与水分利用效率（WUE）间的关系，以期揭示紫花苜蓿对干旱胁迫的适应机制。结果表明：干旱胁迫使得紫花苜蓿根系形态特征在年季间、茬次间和品种间发生了显著变化，主要表现为主根伸长生长受到抑制、主根直径变细、侧根和根系总长度伸长生长则被促进、根系表面积和直径≥1 mm的侧根数目显著增加、根系生物量下降，这是紫花苜蓿对干旱逆境的适应策略，但这种适应性存在限度。另一方面，干旱胁迫条件下紫花苜蓿草产量和蒸腾耗水量也因生长年限、茬次和品种的不同而呈现不同程度的降低。紫花苜蓿根系形态性状(总根长、根系生物量与根冠比) 与植株水分利用效率间具有显著的相关性，其中根重对水分效率的影响是第一位的。WUE在根系形态与冠层水分消耗的协同变化下得到有限提高。对干旱的耐性最终表现为第2年＞第1年、第1茬和第2茬＞第3茬、陇东＞阿尔冈金。
Great challenge in agricultural production is how to harvest more with less water consumption. Improvement of water use efficiency (WUE) for plant itself becomes increasingly more important. Root system is essential to uptake water from soil for plant growth. However, it is still a divergent topic about the regulating mechanism between root morphological variations and shoots water consumption. To clarify a relationship between WUE change and root morphological characteristics further under water deficit and understand the adaptive traits to drought stress of root morphological components, the response of biomass, water dissipation and root morphology, including root length, root surface-area, root volume, basal diameter of main root to the continual drought stress conditions were studied in potted one- and two-year old alfalfa (cultivars: Long-Dong and Algonquin). Three levels of soil moisture including (75±5)%, (55±5)% and (35±5)% of field capacity were controlled with weighing method. The plants grown in rain-proof shed were irrigated by using tap water. Tests were carried out at branching and anthesis stages of one- and two-year old alfalfa plants. The results show that root morphology of alfalfa changed remarkably among plant age, harvest times and cultivars under water stress. The inhibition of main root elongation and the enhancement of lateral roots and total root elongation were stimulated by drought stress, demonstrated by enlarged root surface area and increased numbers of lateral roots (≥1 mm), with thinner main root and declined root biomass. All these root morphological changes，which differ among different seasons, harvest times or cultivars, increasing in adaptation to drought conditions is limited. On the other hand, the biomass and water dissipation amounts declined and differed with the change of season, biomass harvesting or cultivar when subjected to drought stress. Moreover, there was a significant positive correlation between root morphological characteristics and WUE, which indicated that root weight had largely effected on WUE. Therefore, the increase of WUE under water stress depended on the root morphology, including total root length, root biomass and root/shoot ratio etc., and canopy water consumption. Alfalfa tolerance to drought stress was stronger compared with two-year old plants than one-year old ones, with the first harvest and the second than third harvest, with Long-Dong than Algonquin. The adaptive changes of root morphology are the main strategies to endure drought stress in alfalfa. In general, this study explored the adaptive mechanism to drought and improvement of WUE in alfalfa.