WHY ARE SOME RAIN TREES YELLOW-LEAVED?

Abstract

The exact mechanism of yellow leaf development in the yellow-leaved form of the rain tree is unknown. In this study, it was discovered that yellow-leaved trees first produce green leaves which progressively turn completely yellow. This probably results from an early onset of senescence coupled with a delay in leaf abscission. Successful wedge grafts of yellow-leaved scions onto green-leaved rootstocks, and retention of the yellow-leaved condition in the scions (yellow shoot tips, green then yellow leaf colour, higher photosynthetic rates) indicated that the trigger of the mechanism is isolated in the shoots and mineral deficiencies do not contribute to the yellowing effect despite receiving the same xylem sap (thus presumably the same root chemical signals) from the green-leaved rootstock which bears dark green leaves simultaneously. This mechanism is more complicated than expected, as a higher leaf N content, higher photosynthetic rates coupled with lower chlorophyll content in the green leaves of the yellow-leaved trees compared to those of the dark green leaves of the green-leaved trees, are contrary to expectations, although the yellow leaves of the yellow-leaved trees have lower photosynthetic rates, as expected. More work is necessary to work out this interesting phenomenon.