Abstract:

This research studied the variety specific morphological and physiological traits of leaves and roots of two different alfalfa cultivars， WL363HQ and Magnum Ⅶ， under drought in the greenhouse. Measurements made included plant height，branch number and biomass； leaf and root malondialdehyde （MDA） and proline concentration， antioxidant enzyme activities， C and N contents， C∶N ratio， δ¹³C， and δ¹⁵N. Drought stress significantly reduced the dry weight of the above-ground and root， branch number of the tested varieties （P<0.05）. Drought stress significantly reduced the plant height of Magnum Ⅶ （P<0.05）， but increased the root-to-shoot ratio； while the opposite result was observed for WL363HQ， indicating that plant height and root-to-shoot ratio of the tested varieties were variety specific traits under drought stress. Drought stress increased the concentrations of MDA and proline， and the activity of antioxidant enzymes in leaves and roots of both WL363HQ and Magnum Ⅶ， and again there were variety specific responses at the organ level. The MDA content of the leaf of Magnum Ⅶ increased significantly under drought stress （P<0.05）， while in WL363HQ MDA levels of roots were significantly increased （P<0.05）. Leaf proline content， peroxidase （POD） and superoxide dismutase （SOD） activities， and root SOD activity of WL363HQ were significantly increased under drought stress （P<0.05）， while leaf SOD activity， and root proline content， and root POD activity of Magnum Ⅶ were increased significantly （P<0.05）. Although drought stress had no significant effect on the C and N concentrations of the leaf and root of the tested varieties （P>0.05）， drought stress significantly increased the δ¹³C of roots of WL363HQ and Magnum Ⅶ （P<0.05）， while the δ¹⁵N values of leaves of WL363HQ were significantly higher than those of Magnum Ⅶ （P<0.05）. Also， drought stress significantly increased the C∶N ratio of the leaves and roots of Magnum Ⅶ （P<0.05）. The metabolic parameters for C and N of the tested varieties under drought stress did not show obvious variety specific differences at the leaf and root level， and the underlying mechanism explaining the observed results needs to be further explored. The results will provide a theoretical basis for further understanding of the integrated drought resistance mechanisms of alfalfa leaves and roots， relevant to the breeding of new drought-resistant and high-yielding alfalfa varieties.

Key words: alfalfa, drought stress, leaf and root, variety specificity, malondialdehyde, proline, δ¹³C and δ¹⁵N