Adaptations of central Amazon tree species to prolonged flooding:: Root morphology and leaf longevity

Varzeas are species-rich forest communities of the Central Amazon floodplains, inhabited by highly adapted tree species that can withstand long flooding periods. The leaf shedding behaviour and morphological traits that may contribute to adaptation to low oxygen levels were studied at the Ilha de Marchantaria on the lower Solimoes-Amazonas river, Brazil, and in greenhouse experiments with cuttings of six tree species typical of the Amazon floodplain. Comparison of deciduousness in situ revealed that four of the species under investigation, Salix martiana, Tabernaemontana juruana, Laetia corymbulosa and Pouteria glomerata, are able to maintain their leaf system during the aquatic period. Adventitious roots were formed by S. martiana and T. juruana, but anatomical characteristics differed between the species. Whereas S. martiana developed lysigenous aerenchyma in its roots during aerobic and anaerobic growth, only small intercellular spaces of schizogenous origin were formed in the root cortex of T juruana. Similar to the latter species, such spaces were constitutively formed in the deciduous species Crateva benthami and Vitex cymosa. Suberin deposits were observed in tangential and radial cell walls of the hypodermis of roots from T juruana, L. corymbuloso and P. glomerata. Suberin deposits were less pronounced in roots of S. martiana and absent in V. cymosa and C. benthami. The data show that different, almost contrasting, survival mechanisms have evolved in roots of plants with similar life forms in the same habitat. The results further suggest that the morphological traits of the investigated trees are causally linked with the in situ leaf shedding behaviour.