Analysis of dynamic variations in crown characteristics ofdifferent root-type alfalfa plants

Alfalfas are divided into tap-rooted, branch-rooted, creep-rooted and rhizome-rooted plant types. Rhizome-rooted alfalfa is a relative independence germplasm resource with great genetic distance between rhizome-rooted alfalfa and international registry of other alfalfa varieties. It is an important gene-pool for alfalfa breeding and varietal improvement. Retrospective review on past papers shows that a huge amount of work have focused on introduction, breeding, productive performance, resistance and genetic diversity of tap-rooted and creeping-rooted alfalfas. Also the mechanisms, genetic characters and environment impacts of branch-rooted alfalfa have been extensively documented. However, only a few studies have focused on rhizome-rooted alfalfa and fewer still on the contrasts among different root-type alfalfas. Therefore, four alfalfa materials with three root-types (rhizome-rooted Medicago sativa L. cv. "Qingshui", creep-rooted native M. falcata material, creep-rooted M. varia Martin.cv. "Gannong No.2" and tap-rooted M. sativa L.cv. "Longdong") were used to determine the differences in crown characteristics in the semi-humid region of Tianshui, semi-arid region of Lanzhou and arid region of Wuwei in Gansu Province. The results showed significant differences among different root-type alfalfas regarding crown characteristics, crown diameter and crown branch diameter. Rhizome-rooted alfalfa was relatively thinner with deeper crown depth and more crown branches. Crown diameter and crown branch diameter of creep-rooted alfalfa were relatively thicker with shallower crown depth. There was no significant difference in crown branches between rhizome-rooted and creep-rooted alfalfa. The indexes of tap-rooted alfalfa were between those of rhizome-rooted and creep-rooted alfalfas, but were much closer to those of creep-rooted alfalfa. Experiments showed that temperature, fixed growth number of years and ecological zones obviously influenced alfalfa crown characteristics. Within 3 years of growth, crown diameter of all the root-type alfalfas was remarkably influenced by growth temperature. Crown diameter growth was generally faster in summer/autumn than in winter/spring. Horizontal extensions of creep-rooted and rhizome-rooted alfalfas were not detected at the different experimental sites and soil textures in the first and second years of growth. In the third year, plants appeared rhizome and creeping characters at three experimental sites but with different rhizome or creeping rates. Rhizome rate of rhizome-rooted alfalfa was highest in Tianshui and lowest in Wuwei. Creeping rate of creep-rooted alfalfa was higher in Wuwei and Lanzhou, and lower in Tianshui. Both rhizome and creeping rates increased with growth stage. Rhizome rate of rhizome-rooted M. sativa L. cv. "Qingshui" at podding stage reached 76.7%, 43.5% and 39.0% in Tianshui, Lanzhou and Wuwei, respectively. Creeping rates of creep-rooted native M. falcata material and creep-rooted M. varia Martin. cv. "Gannong No.2" reached 61.2 % and 50.6% in Lanzhou, 58.7% and 54.7 % in Wuwei and 44.5% and 36.3% in Tianshui, respectively.