Relationships among rainfall, leaf hydrenchyma, and Crassulacean acid metabolism in Pyrrosia lanceolata (L.) Fraw. (Polypodiaceae) in central Taiwan

Leaf succulence is common among drought-adapted plants, including many tropical and subtropical epiphytic species. A prominent anatomical feature of many such succulent leaves is a clear, water-storing tissue often referred to as “hydrenchyma” (water-storage parenchyma). Functionally, hydrenchyma appears to store water for use by the leaf during drought. Although this has been confirmed in several laboratory studies, field studies linking the amount of hydrenchyma in plants with availability of water in their environment are lacking. In this study, the relative amount of leaf hydrenchyma in one of the most widely distributed epiphytes in Taiwan, Pyrrosia lanceolata, was measured in plants growing along a gradient of annual mean precipitation from 2048 to 3688 mm. In addition, because Pyrrosia lanceolata is a Crassulacean acid metabolism (CAM) plant, the amount of CAM activity was also examined in plants along the gradient. At each of seven sites along the precipitation gradient, leaves were collected, and, using thin mid-leaf slices, the relative areas of the leaf cross-sections occupied by hydrenchyma were determined. CAM, measured as diel changes in leaf acidity, was measured in plants from each site in the field, after 3 days of water-saturation in the greenhouse, and also after 14 days without water in the greenhouse. Regressions of relative hydrenchyma with ten environmental variables in the dry season revealed that the amount of hydrenchyma was significantly and positively correlated with monthly mean number of rainless days, monthly mean number of days with daily mean temperature over 30 °C, and monthly mean temperature. During the wet season, relative hydrenchyma area correlated only with the amount of cloud cover, and the correlation was negative. All plants at all sites exhibited CAM acid fluctuations in the field, under water-saturated conditions, and after desiccation. The largest nocturnal acid accumulations were found when plants were well-hydrated in the field and in the greenhouse, although evidence of drought-induced elevations of CAM was found at the drier sites. The results of this study indicate that the amount of leaf hydrenchyma was greatest in areas with warmer, drier environments. Also, drought-induced elevation of CAM activity occurred in plants from drier sites. This may help to explain the wide range of environments inhabited by this epiphytic fern in Taiwan.