EFFECT OF SEASONAL WATER AVAILABILITY ON PHENOLOGY AND THE ANNUAL SHOOT CARBOHYDRATE CYCLE OF TROPICAL FOREST SHRUBS

1. The effect of seasonal water availability on phenology and shoot to tal non-structural carbohydrate (TNC) of three Psychotria species was investigated by irrigating two large plots of mature tropical forest i n Panama during the dry season for five consecutive years. This study reports results for the fifth and final year of irrigation. 2. Irrigat ion did not affect total production of leaves, inflorescences or infru ctescenes but did advance the timing of leaf production in all Psychot ria, and advanced the timing of inflorescence production in P. furcata and P. marginata and infructescence production in P. furcata. Irrigat ion also reduced shoot TNC levels. 3. Shoot TNC levels in all three sp ecies of Psychotria showed similar depressions at the end of the dry s eason and maximum levels near the end of the wet season. Depletion of shoot TNC occurred just before or during peak leaf production but did not coincide with production of inflorescences or infructescences. The refore, we hypothesize that shoot TNC supported part of the cost of ne w leaf production but production of inflorescences and infructescences was most likely supported by current photosynthate. 4. Tropical plant s generally show less seasonal variation in TNC than plants from more variable climates but stored carbon still plays an important role in t he growth of Psychotria, although its direct role in reproduction appe ars limited.