Morphological and physiological leaf adaptations to seasonal and diurnal abiotic stress for two barrier island sand dune species.

摘要

The creation and maintenance of artificial dune systems using native sand dune species will be an important strategy in future coastal management plans, functioning to provide protection to beachfront property while maintaining natural habitat and sustaining ecosystem processes. Information regarding the leaf morphological and physiological adaptations of native dune vegetation to seasonal and diurnal abiotic stress will be essential in artificial dune construction to maintain optimal growth conditions for native species. The aim of this dissertation research was to explore these adaptations for two common sand dune species during the growth season on a North Carolina barrier island.;The first two studies (Chapter III and IV) focus on leaf orientation properties (i.e. leaf angle and leaf azimuth) in the clonal dune perennial Hydrocotyle bonariensis and its role in regulating the sunlight incidence of both leaf surfaces, its influence on developing leaf anatomy, and its effect on leaf temperature and leaf photosynthetic gas exchange. Results showed no daily variations in leaf orientation but did reveal a seasonal increase in leaf inclination and corresponding reductions in midday sunlight incidence, as well as a seasonal increase in leaf azimuth that tracked the progressively more southern solar azimuth and maximized sunlight incidence in the morning and/or evening. In addition, leaf cross-sections revealed multiple adaxial palisade mesophyll layers and abaxial spongy mesophyll, corresponding to predictions based on the total amount, as well as the ratio between, sunlight incidence of both leaf surfaces. Control inclined leaves had greater leaf photosynthesis, transpiration, and stomata conductance and lower leaf temperatures compared to experimental leaves held in a horizontal orientation, indicating the importance of leaf inclination in maintaining optimal leaf temperatures and facilitating leaf photosynthetic gas exchange. Furthermore, results from an experiment impeding gas exchange (thus blocking evaporative cooling) indicated an important role of transpiration in reducing leaf temperatures in H. bonariensis in the sand dune habitat.;The last two studies (Chapter V and VI) investigate the influence of leaf structure, leaf orientation, and the abiotic environment on leaf physiology for Hydrocotyle bonariensis and the common perennial shrub Iva imbricata in the sand dune habitat. Leaf structure was significantly different between species and could be predicted according to each species position along the sand dune abiotic stress gradient. In addition, seasonal and diurnal leaf temperatures, leaf photosynthetic gas exchange, and photoinhibition risk significantly differed between species. Specifically, photoinhibition occurred during the growth season in H. bonariensis leaves, most pronounced at midday, while there was relatively no photoinhibition in I. imbricata, and leaf photosynthesis and water use efficiency was greater while leaf temperature, transpiration, and stomata conductance were lower in I. imbricata leaves compared to H. bonariensis . Multiple stepwise regressions revealed that leaf orientation properties, the presence of stomata on abaxial and adaxial leaf surfaces, leaf water content, vapor pressure deficit, and air temperature were important predictors of leaf physiology in H. bonariensis, while leaf size, presence of stomata on both leaf surfaces, leaf water content, vapor pressure deficit, and incident sunlight of the habitat were the best predictors of leaf physiology in I. imbricata. Furthermore, seasonal and diurnal patterns suggested that leaf orientation properties and rapid transpiration are critical in reducing photoinhibition and facilitating photosynthetic gas exchange in H. bonariensis while reduced leaf size, increased transpiration, and presence of water-storage cells in I. imbricata alleviates photoinhibition and maintains optimal conditions for leaf photosynthetic gas exchange.;In conclusion, this study is one of few that examine the relationship between leaf structure and orientation, leaf physiology, and the abiotic environment in coastal sand dune species and provides additional evidence for the importance of leaf structure and orientation in regulating leaf-level microclimate and promoting leaf photosynthesis. Results from this study will provide essential information to coastal management strategies using native vegetation to create and maintain sand dune systems. (Abstract shortened by UMI.)