| Alfalfa (Medicago sativa L.) is a major perennial legume forage, with the features of rapid growth, high forage yield, high protein content, strong stress tolerance, adapting to wide range of regions, good palatability and so on. It is cultivated and utilized widely around the world, and known as the "Queen of the Forage". However, the cultivation of alfalfa in South China is limited due to the sticky soil texture, the large amount of annual precipitation and the high underground water level.In order to investigate the effect of excessive moisture on growth of alfalfa, this research studied the effect of waterlogging on the structure of root in eight cultivars of alfalfa, which were Sadie7, Icon, Hunter River, WL-525HQ, Eureka, Supersonic, WL-903and Sequel, respectively.The results indicated that the waterlogging treatment inhibited the growth of alfalfa taproot, made the taproot length and diameter obviously less than the normal controls’, moreover, it was the important reason which leaded to the taproot hurted and shortened. However, waterlogging promoted the lateral roots, and the number of root tips was significantly higher than the normal control’s after7th leaf stage. In the normal watering and waterlogging conditions, alfalfa’s lateral roots had already been grown on the taproot in3rd leaf stage, and in8th leaf stage there have been many secondary lateral roots. The lateral roots were concentrated at the taproot segment near the collar, and in waterlogging condition this trend was more obvious. Although the number of lateral roots increased significantly in the later period in waterlogging condition, the root volume and surface area were still smaller than the normal controls’due to the obviously shortened taproot in waterlogging condition.From the observation of paraffin sections, it was showed that the air cavity in taproot of each cultivar had already begun to be obvious in4leaf stage in normal growth condition. And waterlogging treatment could promote the formation of cortex aerenchyma in taproot, and prolong the time of aerenchyma formation. In the waterlogging treatment, the proportion of aerenchyma accounting for transverse section in all cultivars of alfalfa taproot reached to maximum generally in7th leaf stage. The proportion of cortex accounting for transverse sectional radius in taproot was obviously thickened, the number and area of vessel were reduced significantly. This phenomenon might be an adaptation of alfalfa’s water conveyance organizations to the waterlogging environment.Comparison between all cultivars, the extent of taproot injury in waterlogging condition was different. The taproot lengths of cultivars WL-525HQ and Icon were most sensitive to waterlogging, followed by Sadie7, Hunter River, Eureka and WL-903, Supersonic and Sequel were least sensitive. There were two important ways that could help alfalfa to alleviate the damage of waterlogging to the taproot, increasing the number of lateral roots and developing the aerenchyma, and the the former effect might be greater than the latter after6th leaf stage. Therefore, the cultivars which had many lateral roots and developed aerenchyma should be chosen when alfalfa was cultivated in Southern China.In summary, the growth of alfalfa root was affected and the structures were changed in waterlogging condition. The taproot growth was inhibited and the length was shortened. However, the lateral roots were promoted, and the positions of lateral roots were mainly concentrated at the taproot segment near the collar in order to absorb oxygen better. Meanwhile, the waterlogging promoted the formation and development of cortex aerenchyma in taproot. The root of alfalfa made a good corresponding to waterlogging from apparent structure and anatomical structure, and then it could adapt to waterlogging condition better. |
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