Root protein and vegetative storage protein are key organic nutrients for alfalfa shoot regrowth

Organic reserves in below-ground storage organs are generally recognized as indicators of regrowth and persistence potential of forages. The objective of this experiment was to examine the causal relationships between regrowth potential of alfalfa (Medicago sativa L.) and root organic reserves, i.e., nonstructural carbohydrates or N reserves as N, soluble protein, and vegetative storage protein (VSP). Variations in reserve level induced by cultivar differences (cv. Lodi or Europe), length of the previous regrowth period (30 or 45 d), or by competitive position for light within a dense canopy were studied during 35 d of regrowth. Field grown plants were harvested at weekly intervals, and separated as dominant, intermediate, and suppressed plants. Taproot starch and N contents were influenced by the length of the previous regrowth period, but not by the position of the plant within the canopy. In contrast, soluble protein and VSP concentrations increased when the previous regrowth lengthened from 30 to 45 d and were greater in dominant plants within the canopy. Starch, N, soluble protein, and VSP contents were greater in taproots of Lodi plants. Shoot regrowth was linearly related to taproot soluble protein and VSP contents on the day of defoliation, whereas no relationships was detected between shoot regrowth and initial starch or N contents. These results suggest that root protein and VSP are key organic nutrient for alfalfa shoot regrowth after harvest.