Water transfer at graft union of tomato plants grafted onto Solanum rootstocks

Sudden wilting of grafted plants after a long-term normal growing is a characteristic of a typical delayed graft-incompatibility. Although hydraulic conductance is low at the root of plants grafted onto dwarfing rootstocks, it is unclear whether hydraulic conductance is low at the graft union in delayed graft-incompatible combinations of scions and rootstocks. Hence, growth, stem thickening, water potential, hydraulic resistance and xylem morphology at the graft union of tomato plants grafted onto tomato, eggplant, and torvum (Solanum torvum) rootstocks were compared. Indices of stem thickening, absolute value of water potential and curving of xylem vessels immediately above graft interface were the greatest in torvum rootstock, less in eggplant rootstock and the least in tomato rootstock. The index of hydraulic resistance was larger in Solanum rootstocks than in tomato rootstock. It is concluded that increased hydraulic resistance at the graft union is a cause of delayed graft-incompatibility.