Abstract:

Walnut is a tree species that exudation occurs frequently. Walnut exudation is not only an important biological phenomenon, also related to the effect of pruning and the success of grafting. Therefore, it has received more attention over years and a series of researches have been conducted. Different from most other tree species that exudation occurs normally in spring, walnut exudation can happen in spring, autumn, winter and even in growth season of summer. It was found that spring exudation was generated from root pressure but winter exudation was the result of stem or shoot pressure. The circulation of freezing and melting in winter could increase the level of soluble substance in shoots or stems and enhance their pressure intensity and lead to the occurrence of exudation. Besides, winter exudation was closely related to the restoration of xylem embolism. The constituent of winter and spring sap showed different. In this paper, the available researches were summarized and some important results were reviewed to anticipate to give a more comprehensive understanding of walnut exudation and to provide theoretical instruction for walnut cropping practices of grafting and pruning. The further research and some important concerns were discussed also in this paper.

Key words: Juglans regia, exudation, root pressure, stem pressure