Abstract
While the majority of plants use the typical C3 carbon metabolic pathway, ~6% of angiosperms have adapted to carbon limitation as a result of water stress by employing a modified form of photosynthesis known as Crassulacean acid metabolism (CAM). CAM plants concentrate carbon in the cells by temporally separating atmospheric carbon acquisition from fixation into carbohydrates. CAM has been studied for decades, but the evolutionary progression from C3 to CAM remains obscure. In order to better understand the morphological and physiological characteristics associated with CAM photosynthesis, phenotypic variation was assessed in Yucca aloifolia, a CAM species, Yucca filamentosa, a C3 species, and Yucca gloriosa, a hybrid species derived from these two yuccas exhibiting intermediate C3-CAM characteristics. Gas exchange, titratable leaf acidity, and leaf anatomical traits of all three species were assayed in a common garden under well-watered and drought-stressed conditions. Yucca gloriosa showed intermediate phenotypes for nearly all traits measured, including the ability to acquire carbon at night. Using the variation found among individuals of all three species, correlations between traits were assessed to better understand how leaf anatomy and CAM physiology are related. Yucca gloriosa may be constrained by a number of traits which prevent it from using CAM to as high a degree as Y. aloifolia. The intermediate nature of Y. gloriosa makes it a promising system in which to study the evolution of CAM.
Highlights
The daily fixation of atmospheric carbon dioxide is a defining trait of green plants, and is arguably the basis for the major part of terrestrial biodiversity
In order to better understand the morphological and physiological characteristics associated with Crassulacean acid metabolism (CAM) photosynthesis, phenotypic variation was assessed in Yucca aloifolia, a CAM species, Yucca filamentosa, a C3 species, and Yucca gloriosa, a hybrid species derived from these two yuccas exhibiting intermediate C3–CAM characteristics
Hybrid indices indicate Y. gloriosa samples used in this study were largely later generational hybrids segregating for alleles from each parent
Summary
The daily fixation of atmospheric carbon dioxide is a defining trait of green plants, and is arguably the basis for the major part of terrestrial biodiversity. Plants experience a number of stresses that make the photosynthetic machinery less than optimal, including drought, shade, and high temperatures. As they cannot relocate to avoid the stress, plants have instead modified their photosynthetic pathways in ways to circumvent limitations caused by abiotic stress. One such modification, Crassulacean acid metabolism (CAM), has evolved a number of times independently across angiosperms in response to carbon limitation due to water stress. For C3 plants during times of water stress, stomata close to prevent water loss, which leads to a depletion of internal CO2 and initiation of photorespiration.
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