Agave Adaptation to Aridity

摘要: To show features of Agave taxa adapting to arid habitats, comparative studies at three taxonomic levels in the genus are presented. There is a brief review physiology and some aspects recent evolution discussed. Comparisons among species groups within several traits differentiating desert from related taxa. Related Deserticolae group examined over transect Baja California, revealing patterns linking leaf shape climate. In comparison leaves A. desert] Engelm. along an elevational gradient, high intrapopulation variation obscures differences between sites. Results summarized as hypotheses be tested. introduction It fortunate that, coinciding with publication Gentry's monograph (19821, there has been proliferation research on physiology. Integration these two lines offers insight into forces shaping evolution, promises provide valuable foundation for refining systematics this group. paper I have focused those morphology ecology respect radiation habitats. Ultimately it would desirable generate set "rules" summarizing adaptive responses individual organs specific components natural selection. develop ideas concerning significance morphological features, necessary. established that normally exhibit Crassulacean acid metabolism (CAM, nocturnal carbon fixation (Ehrler, 1969; Nobel, 19761. At seedling stage after extended periods moisture surplus, however, diurnal dioxide uptake can observed iJordan 1979; 'Hartsock By assuming most biomass production occurs CAM mode, inferences drawn relationships temperature productivity. With atmospheric exchange occurring mainly night, optimum should reflect compromise maximizing minimizing water vapor loss. given stomata! resistance, rate loss will positive function because saturation pressure increases heating, thereby increasing concentration gradient plant atmosphere. The necessary relationship influx less certain. Metabolic activity diffusion through mesophyll air space increase temper ature, whereas solubility aqueous solutions cell wall decreases higher temperatures (Nobel 19741. Nobel Hartsock 119781 found internal resistance deserti sensitive temperature, rapidly above 16 °C. They attributed behavior changes biochemical involved rather than simple processes leaf. if resistances largely determined by reactions, they subject change Temperature acclimation Smith, 1983) seems reasonable optimal also he flexible evolutionary time. contrast, cooler invariably minimize transpiration. Stomata! closure during day characteristic CAM; therefore controlled radiant convective heat environment. logically assumed result selection, Calvin cycle enzymes tend maximum when potential favorable stomatal opening. deserti, (19781 was hardly affected daytime 20° 37 °C, but decreased below range. 40 Desert Plants 7