Photosynthetic Physiological Basis of Forage Mass Stability in a Progeny of Rhizome-Rooted ‘Qingshui’ Medicago sativa L.

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, Gs 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; Ci, Gs and Tr values of RSA−03 were higher than those of RSA−01. At five years of age, Pn, Gs, and WUE values, and Sta content of RSA−03 were higher than those of CK1; Ci value and Suc content of RSA−03 were higher than those of CK2; Car content and Gs value of RSA−03 were higher than those of other progenies. The forage mass; Chl(a/b) ratio; Pn, Gs, 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, Gs and NSC were direct factors affecting forage mass at age one year, and both Pn and SP affected forage mass indirectly through Gs; Gs and Chl(a+b) were direct factors affecting forage mass at age five years, and SS affected forage mass indirectly through Gs. Interestingly, Chlb, Chl(a/b), Pn, Tr, Gs, Ci, 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.