Contrasting physiological responses to excess heat and irradiance in two tropical savanna sedges

Research Article SPECIAL ISSUE: Stress and Survival in Tropical Environments Contrasting physiological responses to excess heat and irradiance in two tropical savanna sedges C. John-Bejai, A. D. Farrell*, F. M. Cooper and M. P. Oatham Department of Life Sciences, The University of the West Indies, St. Augustine, Trinidad and Tobago Received: 18 July 2013; Accepted: 12 September 2013; Published: 7 November 2013 Citation: John-Bejai C, Farrell AD, Cooper FM, Oatham MP. 2013. Contrasting physiological responses to excess heat and irradiance in two tropical savanna sedges. AoB PLANTS 5: plt051; doi:10.1093/aobpla/plt051 Abstract. Tropical hyperseasonal savannas provide a rare example of a tropical climax community dominated by graminoid species. Species living in such savannas are frequently exposed to excess heat and light, in addition to drought and waterlogging, and must possess traits to avoid or tolerate these stress factors. Here we examine the contrasting heat and light stress adaptations of two dominant savanna sedges: Lagenocarpus guianensis, which is restricted to the sheltered forest edge, and Lagenocarpus rigidus, which extends from the forest edge to the open savanna. An ecotone extending from the forest edge to the open savanna was used to assess differences in a range of physiological traits (efficiency of photosystem II, cell membrane thermostability, stomatal conductance, leaf surface reflectance and canopy temperature depression) and a range of leaf functional traits (length : width ratio, specific leaf area and degree of folding). Lagenocarpus guianensis showed significantly less canopy temperature depression than L. rigidus, which may explain why this species was restricted to the forest edge. The range of leaf temperatures measured was within the thermal tolerance of L. guianensis and allowed photosystem II to function normally, at least within the cool forest edge. The ability of L. rigidus to extend into the open savanna was associated with an ability to decouple leaf temperature from ambient temperature combined with enhanced cell membrane thermostability. The high degree of canopy temperature depression seen in L. rigidus was not explained by enhanced stomatal conductance or leaf reflectance, but was consistent with a capacity to increase specific leaf area and reduce leaf length: width ratio in the open savanna. Plasticity in leaf functional traits and in cell membrane thermostability are key factors in the ability of this savanna sedge to survive abiotic stress. Keywords: Canopy temperature depression; cell membrane thermostability; environmental gradient; heat stress; leaf functional traits; leaf reflectance; light stress; tropical savanna. Introduction As the effects of global climate change become more apparent, there is increasing interest in understanding how plants will respond to a warmer environment (Reynolds et al. 2001; Larkindale et al. 2005; Allakhverdiev et al. 2008; Mittler et al. 2012). By understanding the responses of species that have evolved in environments frequently exposed to heat stress, we will be better placed to manage the transition to a warmer environment in both wild and cultivated systems. Vegetation communities within the Aripo Savannas Environmentally Sensitive Area, in the northeast of Trinidad, * Corresponding author’s e-mail address: Published by Oxford University Press on behalf of the Annals of Botany Company. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/ licenses/by/3.0/), which permits unrestricted reuse, distribution, and reproduction in any medium, provided the original work is properly cited. AoB PLANTS www.aobplants.oxfordjournals.org & The Authors 2013 1 John-Bejai et al. — Physiological responses to heat and light in two tropical savanna sedges are segregated into two distinct habitats: open savanna and seasonally flooded marsh forest (Beard 1953; Richardson 1963). The edaphic savanna provides a rare opportunity to study species traits in a tropical climax community dominated by graminoid and herbaceous species. In particular, it offers an opportunity to better understand the ecophysiology of sedges, an economically important but understudied group (Barrett 2013), which is dominant in this savanna. The abiotic character of the open savanna poses a challenge to plant species, as illustrated by the markedly lower floral diversity of these areas as compared with adjacent marsh forest (Richardson 1963). Resistance to environmental extremes is a dominant selective force in adverse habitats such as this and species capable of persisting will be expected to display adaptive responses to their environment. Resident species must cope with the challenges imposed by edaphic factors, namely poor soil drainage, low fertility and an acidic pH (Richardson 1963). The influence of soil drainage on species composition is observed to be greatest in the open savannas as the shallow clay pan depths that characterize these areas result in more pronounced periods of inundation and subsequent drought (Richardson 1963). Coupled with the challenges imposed by soil drainage, open savanna species will inevitably be exposed to periods of high irradiance and high temperature. The influence of these two factors on the traits of the open savanna species has yet to be determined. In light of predicted climate change, the adaptive responses of species to high temperature and high irradiance have garnered considerable attention in recent years (Reynolds et al. 2001; Larkindale et al. 2005; Allakhverdiev et al. 2008; Mittler et al. 2012). Understanding the response of graminoid species is of particular interest given their importance as agricultural crops (Reynolds et al. 2001). Within natural systems, the adaptive response of species to spatial heterogeneity in temperature can be used as a proxy for temporal change in temperature (Rice and Emery 2003; Davis et al. 2005; Guerin et al. 2012). The Aripo Savannas provide an opportunity to study such responses in a relatively intact climax ecosystem. Furthermore, successful management and restoration of such environments during climate change necessitates that adaptive responses of resident species to microclimate be understood (Rice and Emery 2003; Harris et al. 2006; Guerin et al. 2012). A botanical survey of the savanna (Cooper and Oatham 2012) found differences in distribution between two widespread and related sedges: Lagenocarpus guianensis and Lagenocarpus rigidus, the former limited to the margins of marsh forest and the latter occurring in open savanna as well as at the forest margin. The capacity of L. rigidus to inhabit areas of open savanna was taken as indicative of the presence of stress adaptations that are not shared 2 with L. guianensis. The measurements conducted in this study aim to compare these two species with respect to the presence of traits related to high irradianc (...truncated)