Environmental regulation of carbon isotope composition and crassulacean acid metabolism in three plant communities along a water availability gradient

Expression of crassulacean acid metabolism (CAM) is characterized by extreme variability within and between taxa and its sensitivity to environmental variation. In this study, we determined seasonal fluctuations in CAM photosynthesis with measurements of nocturnal tissue acidification and carbon isotopic composition (δ13C) of bulk tissue and extracted sugars in three plant communities along a precipitation gradient (500, 700, and 1,000 mm year−1) on the Yucatan Peninsula. We also related the degree of CAM to light habitat and relative abundance of species in the three sites. For all species, the greatest tissue acid accumulation occurred during the rainy season. In the 500 mm site, tissue acidification was greater for the species growing at 30% of daily total photon flux density (PFD) than species growing at 80% PFD. Whereas in the two wetter sites, the species growing at 80% total PFD had greater tissue acidification. All species had values of bulk tissue δ13C less negative than −20‰, indicating strong CAM activity. The bulk tissue δ13C values in plants from the 500 mm site were 2‰ less negative than in plants from the wetter sites, and the only species growing in the three communities, Acanthocereus tetragonus (Cactaceae), showed a significant negative relationship between both bulk tissue and sugar δ13C values and annual rainfall, consistent with greater CO2 assimilation through the CAM pathway with decreasing water availability. Overall, variation in the use of CAM photosynthesis was related to water and light availability and CAM appeared to be more ecologically important in the tropical dry forests than in the coastal dune

Photosynthesis and Optimal Light Microhabitats for a Rare Cactus, Mammillaria gaumeri, in Two Tropical Ecosystems

Mammillaria gaumeri, an endemic cactus from Yucatan, is threatened due to human-mediated habitat fragmentation but knowledge regarding factors that would inform management plans is limited, including information on optimal microhabitats and growth. We studied seasonal patterns of microclimate, net CO2 uptake and growth of adult individuals during the dry, rainy and northwind (late fall to winter) seasons in separate populations located in a coastal dune and in a dry forest. To provide an assay of primary productivity, we measured the production of new axillary buds (areoles) from the apex. We found that patterns of net CO2 uptake in the three seasons were consistent with obligate crassulacean acid metabolism. The reduction in soil water potential and the increase in water vapor pressure deficit led to a reduction in 24-h net CO2 uptake from 18.0 to 1.3 mmol CO2/m2, after 30 d of drought. Dry-weight gain was maximal during the rainy season and there was no growth during the dry season, thus seasonal patterns of net CO2 uptake and growth for M. gaumeri reflected soil and atmospheric water contents. Annual dry-weight gain increased linearly with plant size and with total daily photosynthetic photon flux density (PPFD) up to 80 percent of ambient. Maximal growth was in exposed microhabitats receiving 60-80 percent of total ambient PPFD. We hypothesize that low-magnitude disturbances, resulting in more exposed microhabitats, might be beneficial for growth and conservation of adult individuals of this rare species

Microambientes de luz, crecimiento y fotosíntesis de la pitahaya (Hylocereus Undatus) en un agrosistema de Yucatán, México

Con el fin de incrementar el crecimiento de los tallos de una cactácea en las primeras etapas de una plantación comercial se estudió el efecto del microambiente de luz sobre la fisiología de Hylocereus undatus (Haworth) Britton & Rose en Yucatán, México. Se colocaron esquejes de tallos en cuatro tratamientos de luz para evaluar su crecimiento durante 55 semanas. La elongaciónde los tallos fue 67% mayor con 36 a 48% de flujo de fotonespara fotosíntesis (FFF) diario incidente que en tallos con 25% y90% de FFF. Durante el periodo de lluvias la acidez tisular (unamedida de la actividad fotosintética de plantas con metabolismoácido) fue 44% mayor en los individuos bajo 36 a 48% del FFF diario incidente, que en aquellos con menor o mayor radiación. Durante el periodo de nortes (previo a la sequía con eventos esporádicos de fuertes vientos y poca lluvia) la acidez fue similar bajo 36%, 48% y 90% del FFF diario incidente, pero 44% menor en el tratamiento con menor radiación. La eficiencia cuántica máxima indicó que plantas expuestas sufrían fotoinhibición durante el periodo de nortes. El periodo de lluvias presenta condiciones ambientales óptimas para la fotosíntesis de H. undatus, por las bajas temperaturas del aire y del déficit de presión de vapor por la noche