Studies on the nature of the incompatibility in a cucurbitaceous graft

Musk melon (M), cucumber (C) and Cucurbita ficifolia (F) could succesfully be grafted in all single combinations, except for M/F which required foliage on the stock to survive. Defoliation of this stock caused the plant to wilt and die, generally in 4-5 days; necrosis started in the stock: a rapid and specific collapse of its sieve tubes occurred before any visible symptom was evident in the melon scion. With stock foliage present, a good union between xylem and phloem of the partners was formed.

Growth and other phenomena reacted strongly to the number of leaves on a stock. The effect of these leaves depended largely on light intensity. M/F plants could recover from advanced stages of incompatibility, by renewed contact with leaves of the stock species.

From these and experiments with double grafting it was concluded that the stock leaves provided the stock with some specific substance (enzymic or hormonal) enabling the stock phloem to function normally.

Shortage or complete absence of this substance might have influenced some enzymic process in the complex of sieve tube and companion cell, upsetting metabolism as shown by local accumulation of starch. The stock leaves depended on root vitality, which in turn was determined by interaction with the scion. The growth-regulating activity of the stock leaves thus proved to be connected with the nature of the interactions between M and F.