Morphological and physiological adjustments to waterlogging and drought in seedlings of Amazonian floodplain trees.

Plants in central Amazonian floodplains are subjected to waterlogging or complete submersion for 50-270 days every year. Most trees have growth reductions, photosynthetic activity can be reduced for some weeks to months, and leaf fall increases during the high-water period, but leaf flush, flowering and fruiting also occur in waterlogged plants. Whether flooding can trigger the changes in phenology, growth and metabolism of the plants has not yet been established. The aim of this study was to analyse the extent to which waterlogging was directly responsible for morphological, phenological and physiological changes in floodplain seedlings. In two flooding experiments performed at different times of the year, the effects of waterlogging, submersion and drought were tested in seedlings of six species with different growth strategies. One experiment was performed in the period of highest precipitation and rising river levels, and a second experiment in the period of highest river levels and the onset of the period of lowest precipitation. All results were comparable in the two experiments, and the morphological, phenological and physiological responses were linked to the treatments. Height growth and new leaf production were not severely affected in the waterlogged seedlings. All waterlogged plants produced adventitious roots, lenticels and stem hypertrophy. Submersion and drought caused a state of rest, but soon after the water had receded, leaves resprouted. Five to 12 weeks after the end of submersion, the seedlings reached the height of the control plants, showing a high ability to compensate the period of rest induced by submergence. Only the three deciduous species subjected to waterlogging showed a different phenological behaviour in the two experiments, perhaps related to genetically fixed phenological rhythms which are synchronous to those of adult trees in the field.