Partitioning of Carbohydrates and Development of Tissues in the Graft Union of Peaches Grafted on Prunus tomentosa Thunb. Rootstock.

Carbohydrate partitioning and graft union development of peaches grafted onto Prunus tomentosa Thunb. were compared with the one on Prunus persica (L.) Batsch. The low total dry matter confirmed the dwarfing effects of P. tomentosa. The distribution (%) of dry matter was lower in the new leaves and shoots compared with that on P. persica, but the root dry matter distribution tended to be higher for the former rootstocks. That the roots are a sink for carbohydrates was demonstrated by the translocation of labeled 13C, which was found mainly in the small roots of dwarfing rootstocks. The carbohydrate content showed that the main sugar present is sorbitol which followed a similar pattern in both rootstocks. The carbohydrate contents in peaches on dwarfing rootstocks were lower in the new shoots and tended to be higher in the roots ; the starch content in roots, branches, and leaves of dwarfing rootstock was higher than that in the peach stock. During the first months the graft union on both rootstocks was characterized by the development of similar callus and cambial cells, with connective vascular cambium, but in the dwarfing rootstock necrotic cells were observed in the callus and in the new xylem of scion. The graft union zone of dwarfing rootstocks, two and four years after budding, showed necrotic points in the xylem of rootstocks which were rarely observed in vigorous rootstocks. Thinner vessels were observed in the intergraft zone on dwarfing rootstocks. The xylem elements orientation lost a normal pattern, and vessels in swirling shape were observed in the intergraft zone on both rootstocks. These observations indicate that necrosis in the graft union was brought about by some substances produced by the scion or rootstock, which accumulated because of a disfunctional vascular system. Hence, the poor scion-rootstock relationship affected growth.