Cultivating new alfalfa (Medicago sativa L.) varieties with high yield and quality is of great significance for improving alfalfa yield and promoting the development of the grass and livestock industry. Plant height is an important indicator of alfalfa yield and is closely related to photosynthetic capacity, harvest index and yield. However, the underlying cause of the variation in height among alfalfa plants is not clear. In this paper, we measured the phenotypic traits, photosynthetic physiology and endogenous hormone content of tall- and short-stalked alfalfa materials and analyzed the important external and internal factors that caused the difference in plant height of alfalfa. We found that the phenotypic traits of tall- and short-stalked alfalfa materials showed significant differences, and dwarf alfalfa showed significant shortening of the main stem internode length. There were also some differences in light and physiological indicators and endogenous hormone contents between tall- and short-stalked alfalfa materials. Through correlation analysis, we found that the phenotypic traits and physiological indicators significantly correlated with alfalfa plant height were the number of internodes, stem diameter, average internode length, leaf–stem ratio, leaf area, Pn (net photosynthetic rate), Tr (transpiration rate), upper leaf SP (soluble protein), Suc (sucrose) content, middle stem Sta (starch) content, middle stem ZT (zeatin) and IAA (indole-3-acetic acid). Further analysis showed that Tr, IAA and LA played a direct role in plant height, with Tr contributing the most to plant height, followed by IAA. Finally, we found that the starch content of the middle stem had a significant impact on plant height through principal component analysis. These results provide new insights into the formation and genetic improvement of plant height traits in leguminous forages such as alfalfa.

Rhizome-rooted alfalfa (Medicago sativa L.) is an excellent forage for establishing grazing and ecological grasslands, requiring a high and stable yield. Studying the genetic and physiological basis of stable expression of biomass traits is essential for improving production performance in rhizome-rooted alfalfa. We analyzed forage mass and photosynthetic physiological indices of the improved progenies (RSA−01, RSA−02, and RSA−03), parental “Qingshui” (CK₁), and “WL168” (CK₂) at ages one and five years and their relationships, then revealed heterotic stability. Moreover, we explored the effects of interannual dynamics and genetic differences on tested indices. The results revealed compared with the forage mass of CK₁, RSA−03 at ages one and five years increased by 22.17% and 19.72%, respectively, while RSA−01 and RSA−02 varied from 1.40% to 8.65%, indicating obvious heterosis in forage mass of RSA−03. At one year of age, G_s value, Car content and SS content of RSA−03 were higher than those of CK₁; SS content of RSA−03 were higher than those of CK₂ and RSA−02; C_i, G_s and T_r values of RSA−03 were higher than those of RSA−01. At five years of age, P_n, G_s, and WUE values, and Sta content of RSA−03 were higher than those of CK₁; C_i value and Suc content of RSA−03 were higher than those of CK₂; Car content and G_s value of RSA−03 were higher than those of other progenies. The forage mass; Chl(a/b) ratio; P_n, G_s, and WUE values; Suc content of RSA−03 at age five years were higher than those at age one year by 9.99%–44.24%. Through path analysis, G_s and NSC were direct factors affecting forage mass at age one year, and both P_n and SP affected forage mass indirectly through G_s; G_s and Chl(a+b) were direct factors affecting forage mass at age five years, and SS affected forage mass indirectly through G_s. Interestingly, Chlb, Chl(a/b), P_n, T_r, G_s, C_i, Suc, SP, and SS were more influenced by age than genetics, while the opposite was true for Car and Sta. Accordingly, RSA−03 showed obvious and stable heterosis in forage mass and photosynthetic physiology, recommending the establishment of grazing pastures and ecological vegetation.