Reserve mobilization during regrowth after cutting of forage species: quantification and physiological mechanisms in ryegrass and lucerne

Time course of remobilisation of reserves was studied during the regrowth of Loli11m perem1e and Medicago sativa by 15N labelling, carbohydrate analyses and by measuring time course of several enzymes activities. When photosynthesis decreased during early regrowth, and consequently reduced availability of carbon skeletons within the plant, both nitrogen (N) uptake (lolium perene) and N2 fixatlon (Medicago saliva) decreased. Nilrogenase, as well as nitrate reductase activities, decreased. Reduction of external N assimilation was compensated for by intensive mobilization of endogenous N in roots of lucerne and in stubble of ryegrass. In the same manner, some of the carbon (C) reserves of organs remaining after defoliation were translocated to regrowing shoals. Activities of hydrolytic enzymes such as fructan-exo-hydrolase or proteinase therefore increased after defoliation in all remaining organs which acted as source organs. The second period of regrowth corresponded with an increase in carbohydrate synthesis, and was associated with (i) reconstitution of N and C reserves concomitant with a decrease of hydrolytic activities, (ii) increases of mineral N uptake and N2 fixation associated with higher enzyme activities.