Comparative Desiccation Tolerance of Three Desert Pteridophytes: Response to Long-term Desiccation

Abstract

There are over 30 pteridophyte species in Big Bend National Park, Texas. While most are poikilohydrous, sharing some degree of physiological desiccation tolerance, individual species have distinctive distributions along an aridity gradient in the park. We examined three species with respect to the influence of extended desiccation on photosynthetic recovery over 24 h and desiccation-induced membrane damage. The low-elevation species Selaginella lepidophylla was least sensitive to extended desiccation and had the fastest photosynthetic recovery and the least membrane damage. The high-elevation species Cheilanthes tomentosa was most sensitive to extended desiccation and had the slowest photosynthetic recovery and the greatest membrane damage. The mid-elevation species Notholaena sinuata var. cochisensis was intermediate in response in all cases. These results document significant interspecific physiological differences that may influence the distributions of these desiccationtolerant pteridophytes along an aridity gradient in west Texas. INTRODUCTION Many nonvascular and vascular plants are capable of surviving almost total dehydration in their native habitats. These desiccation-tolerant species and their physiological and morphological adaptations have been the subjects of much research (Bewley, 1979). While desiccation tolerance is widely distributed, there may be a considerable degree of variation in apparent desiccation tolerance within broad taxonomic groups. Variation in desiccation tolerance has been frequently reported in lichens and bryophytes, and less often in intertidal algae (see Bewley and Krochko, 1982 for review and numerous references). Desiccation tolerance is an integrated physiological characteristic that may comprise many specific features and such variation may be expressed as differing sensitivities to frequency, duration, and rate of desiccation and differing rates and degrees of physiological recovery upon wetting. In virtually all cases, there is very good correlation between desiccation tolerance features and the water status of the habitats where particular species, or ecological races, are found. In contrast to these nonvascular groups, desiccation-tolerant pteridophytes have been infrequently studied from a comparative perspective. While desiccation tolerance of individual species has been studied (Oppenheimer and Halevy, 1962; Stuart, 1968; Nobel, 1978; Eickmeier, 1979), few physiologically based comparative studies are available (Eickmeier, 1980). Most often such studies are of a survey nature and only document drought-tolerance limits in a way that makes species comparisons difficult (Gaff, 1977; Gaff and Latz, 1978). This lack of detailed comparative studies is surprising since there are many pteridophytes native to arid desert regions of the southwestern U.S. that have distinct distributions along elevationally based aridity gradients. For example, Hevly (1963) examined the distributions and adaptations of cheilanthoid ferns along a 2400-m elevation gradient in Arizona. While species distributions did overlap to some degree, ferns in the genus Cheilanthes were found at the highest mean elevation, followed by species of the genus Pellaea, with members of the genus Notholaena being found at the lowest mean elevation. Hevly attempted to correlate morphological and anatomical features of the sporophytes and spore germination requirements with the observed distributions of the species. Aside from the fact that the species were apogamous, thereby eliminating the need of water for sexual reproduction, no clear ecological explanation was evident and Hevly (1963, p. 174) suggested that a comparative physiological study might be useful