Physiological Responses to Soil Moisture Stress in Two Amazonian Gap- Invader Species

Abstract

1. Disturbed habitats in Amazonia, like active pastures and abandoned agricultural areas, are characterized by significant diurnal and seasonal drought stress that may restrict successful colonization by many plant taxa. Some species, however, show a remarkable capacity to withstand these water-limited conditions. We investigated, under controlled conditions, the bulk tissue-water relations and the gas-exchange characteristics in response to soil moisture stress of Solanum crinitum Lam. and Vismia guianensis (Aubl.) Choisy, two co-occurring gap-invading small trees kown to dominate, disturbed, drought-prone habitats in the eastern Brazilian Amazon region. 2. Bulk tissue-water relations of these two species were very different. Vismia guianensis had significantly higher bulk tissue elastic modulus (more rigid tissues) and lower bulk tissue osmotic potential at full hydration and at turgor loss than S. crinitum. 3. The elastic and osmotic properties could either enhance turgor maintenance or increase the ability of a plant to extract soil water. In turn, this may help explain their ability to succeed in drought-prone habitats. With more elastic tissue properties, S. crinitum 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 gradients from the leaves to the soil with little tissue water loss, thus enhancing 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 water-use efficiency (A/g) actually dropped when water was limiting for this species. In contrast, V guianensis maintained high A/g under both watering regimes and was always more water-use efficient than S. crinitum. Moreover, S. crinitum had two- to fourfold higher dark respiration than V. guianensis. We believe that it is this combination of traits that leads to lower growth in S. crinitum when compared to V. guianensis in the field. 5. We suggest that the different gas-exchange responses and tissue water relations properties represent two different strategies for dealing with water deficits. Solanum crinitum, appears to be more able to cope with short-term water stress and is regarded to be a successful stress tolerator. In contrast, V. guianensis, appears to be able to cope better with chronic, long-term water stress, what we call a successful stress-avoider