EFFECTS OF DE- AND REHYDRATION IN DESICCATION-TOLERANT LIVERWORTS: A CYTOLOGICAL AND PHYSIOLOGICAL STUDY

Liverworts are probably the earliest-diverging group of green land plants, but in contrast to mosses and vascular plants, there has been only limited information on the physiology of desiccation tolerance and none on the cytological consequences of de-and rehydration in this group. We explore the physiological and cytological effects of drying and rewetting in several desiccation-tolerant liverworts and examine parallels with and differences from vascular plants and mosses. Observations were made of water-stress responses and recovery using infrared gas analysis and modulated chlorophyll fluorescence, and structural and ultrastructural changes were observed with light and electron microscopy. Dehydration elicits profound cytological changes: fragmentation of the vacuole, rounding of the chloroplasts and mitochondria with thylakoids, and cristae becoming rearranged but remaining undamaged. These changes closely parallel those seen in mosses and desiccation-tolerant vascular plants, as does recovery of the chlorophyll-fluorescence parameter F-v/F-m, which shows half-recovery within minutes or an hour or two but returns to normal, unstressed values only after 24-48 h. As in mosses, de- and rehydration are associated with the de- and repolymerization of the cortical microtubule cytoskeleton. Particularly interesting and described here for the first time is the behavior of the oil bodies, membrane-bound organelles unique to liverworts. In the dry state, these remain substantially unchanged, but after rewetting, they become flattened and regain their normal morphology only after 48 h. When subjected to unnaturally fast rate of drying, the oil bodies, together with other organelles, disintegrate on rewetting. Our findings-that the major changes associated with de- and rehydration in liverworts closely parallel those in mosses and, indeed, vascular plants-imply that the basic physiology and cytology of desiccation tolerance is ancestral in land plants. The unexpected discovery that the oil bodies, while retaining their original volume during drying, collapse during rehydration suggests a shifting of soluble carbohydrates or other moieties into the cytosol and provides the first experimental evidence that the oil bodies may have a crucial role in desiccation tolerance.