UNFRUITFULNESS OF YOUNG LITCHI TREES IN RELATION TO THEIR PECULIAR ROOT GROWTH BEHAVIOR - AN OVERVIEW

Some elite cultivars of litchi, such as 'Nuomici' and 'Guiwei', are shy- or even non-bearing during their young tree stage, mainly due to their very poor fruit set. Girdling before fruit set usually yields good results. It was found that girdling almost eliminated an early peak of root growth emerging during the period prior to physiological fruit drop. Such an additional early peak of root growth was found absent in young trees of some generous bearers like cv. 'Huaizhi', which tend to bear fruit at an early age compared to 'Nuomici' and 'Guiwei'. It was also not found in full-grown bearing trees of cv. Nuomici. There was a flushing peak on the non-girdled young 'Nuomici' trees following the early root growth peak naturally emerging in late April/early May. This was not seen in girdled young trees of this cultivar, where such root growth peak was greatly reduced or eliminated. Improved fruit retention by girdling was found associated with the higher gibberellin level and the lower abscisic acid level, as well as with the higher level of carbohydrates in the fruit on girdled trees. An experiment with label ¹⁴C showed that downstream translocation of carbohydrates was almost or completely blocked by spiral or closed girdling respectively, causing the inhibition of root growth.