Physiological responses of rosewoods <i>Dalbergia cochinchinensis</i> and <i>D. oliveri</i> under drought and heat stresses

Abstract

<i>Dalbergia cochinchinensis</i> and <i>D. oliveri</i> are classified as vulnerable and endangered, respectively, in the IUCN Red List and under continued threat from deforestation and illegal harvesting for rosewood. Despite emerging efforts to conserve and restore these species, little is known of their responses to drought and heat stress, which are expected to increase in the Greater Mekong Subregion where the species co-occur and are endemic. In this study of isolated and combined drought and heat effects, we found that <i>D. oliveri</i> had an earlier stomatal closure and more constant midday water potential in response to increasing drought level, suggesting that <i>D. oliveri</i> is relatively isohydric while <i>D. cochinchinensis</i> is relatively anisohydric. Heat shock and drought had synergistic effects on stomatal closure. Our results indicate contrasting relationships in water relations, photosynthetic pigment levels, and total soluble sugars. An increase in chlorophyll a was observed in <i>D. cochinchinensis</i> during drought, and a concomitant increase in carotenoid content likely afforded protection against photo-oxidation. These physiological changes correlated with higher total soluble sugars in <i>D. cochinchinensis</i>. By contrast, <i>D. oliveri</i> avoided drought by reducing chlorophyll content and compromising productivity. Anisohydry and drought tolerance in <i>D. cochinchinensis</i> are adaptations which fit well with its ecological niche as a pioneering species with faster growth in young trees. We believe this understanding of the stress responses of both species will be crucial to their effective regeneration and conservation in degraded habitats and in the face of climate change.