Studies on Development of Windbreak Trees in the Tropics

1. Effects of gibberellin on seedling growth in some windbreak trees

Abstract

Recently in Okinawa, Casuarina equisetifolia has been mainly used as windbreak tree as it grows fast. However, in terms of other characteristics for windbreak trees, such as wind resistance, salt breeze tolerance, usefulness for wood production etc., Garainia subelliptica or Calophyllum inophyllum seem to be more suitable, but they grow slowly. This study was focussed on the effects of gibberellin on seedling growth in four kinds of windbreak trees. Using one-year-old seedlings of Casuarina equisetifolia which grows faster and of Podocarpus macrophyllus, Garainia subelliptica and Calophyllum inophyllum which grow more slowly, studies were carried out in Miyako Is., Okinawa, from Aug. 1, 1990 to Jan. 2, 1991. Forty seedlings of each tree species were used for this study and half of them were treated with a gibberellin tape placed at about 3 cm below their growing points. Throughout this period, the height of seedlings was measured at intervals of two weeks. It was shown that the difference in growth between the control and the treated seedlings was remarkable in Calophyllum inophyllum. The growth ratio, represented by the percentage of the length increment divided by the height at the beginning of the study, was 61.8% for the treated seedlings, or seven times as much as that for the control. Also in Garainia subelliptica, the difference was significant and the growth ratio of the control was 32.2%, which was twofold the value of the control. The results showed that the treatment by gibberellin tape exerted a significant effect on the acceleration of seedling growth in two kinds of trees which are suitable for windbreak but naturally grow slowly.