Alfalfa root carbohydrates and regrowth potential in response to fall harvests

The adverse effect of harvesting alfalfa (Medicago sativa L.) during a critical fall test period on the persistence and the following spring regrowth has been historically attributed to a reduction in the levels of root organic reserves, especially total nonstructural carbohydrates (TNC). Recent reports also pointed out the role of specific carbon (C) reserve components in winter survival. This study assessed the effect of the timing of a fall harvest on the regrowth potential in relation to quantitative changes in C reserves in alfalfa roots during fall and winter. The experiment was conducted under simulated winter conditions in an unheated greenhouse with two alfalfa cultivars (AC Caribou and WL 225) subjected to four fall harvest treatments: two summer harvests (control) and a third harvest taken in the fall at 400, 500, or 600 growing degree days (GDD) after the second harvest. Shoot regrowth was reduced by a fall harvest, especially when plants were harvested at 400 or 500 GDD. Root soluble sugar concentrations (sucrose, raffinose, and stachyose) measured during the overwintering period were higher in plants harvested in the fall than in those harvested only twice; these concentrations were also higher in the more hardy cultivars AC Caribou compared with WL 225. A fall harvest did not consistently affect root TNC concentrations, but decreased markedly the total amount of TNC in roots, especially in plants harvested at 400 GDD. In both cultivars, shoot regrowth in spring was correlated positively to the total amounts of root starch (r = 0.54 and 0.61) and TNC (r = 0.55 and 0.56), but not correlated to their concentrations. Our results suggest that the total amount of C organic reserves in alfalfa roots rather than their concentrations can be determinant factors of shoot regrowth.