PHYSIOLOGICAL-RESPONSES TO SOIL-MOISTURE STRESS IN 2 AMAZONIAN GAP-INVADER SPECIES

1. Disturbed habitats in Amazonia, like active pastures and abandoned agricultural areas, are characterized by significant diurnal and seaso nal drought stress that may restrict successful colonization by many p lant taxa. Some species, however, show a remarkable capacity to withst and these water-limited conditions. We investigated, under controlled conditions, the bulk tissue-water relations and the gas-exchange chara cteristics in response to soil moisture stress of Solanum crinitum Lam . and Vismia guianensis (Aubl.) Choisy, two co-occurring gap-invading small trees known to dominate, disturbed, drought-prone habitats in th e eastern Brazilian Amazon region. 2. Bulk tissue-water relations of t hese two species were very different. Vismia guianensis had significan tly higher bulk tissue elastic modulus (more rigid tissues) and lower bulk tissue osmotic potential at full hydration and at turgor loss tha n S. crinitum. 3. The elastic and osmotic properties could either enha nce turgor maintenance or increase the ability of a plant to extract s oil water. In turn, this may help explain their ability to succeed in drought-prone habitats. With more elastic tissue properties, S. crinit um is able to maintain turgor and thus leaf gas exchange over greater changes in tissue water content, whereas having relatively more rigid tissues would aid V. guianensis in developing water potential gradient s from the leaves to the soil with little tissue water loss, thus enha ncing water uptake from drier soils. 4. Photosynthetic capacity in S. crinitum was less affected by soil moisture stress than V. guianensis. Furthermore, because S. crinitum maintained stomatal conductance (g), but photosynthesis (A) declined under moisture stress, intrinsic wate r-use efficiency (A/g) actually dropped when water was limiting for th is species. In contrast, V. guianensis maintained high A/g under both watering regimes and was always more water-use efficient than S. crini tum. Moreover, S. crinitum had two- to fourfold higher dark respiratio n than V. guianensis. We believe that it is this combination of traits that leads to lower growth in S. crinitum when compared to V. guianen sis in the field. 5. We suggest that the different gas-exchange respon ses and tissue water relations properties represent two different stra tegies for dealing with water deficits. Solanum crinitum, appears to b e more able to cope with short-term water stress and is regarded to be a successful stress tolerator. In contrast, V. guianesis, appears to be able to cope better with chronic, long-term water stress, what we c all a successful stress-avoider.