PHOTOSYNTHETIC RESPONSES TO DROUGHT CONDITIONS IN THREE COASTAL DUNE PLANTS IN RELATION TO THEIR ZONAL DISTRIBUTION

摘要

Species-specific responses of three coastal dune plants to drought conditions were examined to investigate the causal relationship between drought tolerance and plant distribution in coastal dune regions. Measurements of net photosynthetic rates were undertaken in two meteorologically contrasting years, 1989 and 1990; the summer of 1990 was characterised by long rainless periods, whereas the meteorological conditions of the summer months in 1989 did not differ significantly from the average values. The reductions of net photosynthetic rates caused by the 1990 summer aridity differed widely among the three species. Net photosynthetic rate (Pn) of Ischaemum anthephoroides, a C-4 plant, distributed in hind dunes, fell to only 33-43% of the values in the same season in 1989. By contrast, those of Carex kobomugi and Calystegia soldanella, which were generally growing in more seaward sites, were 67-87% and 76-97%, respectively. In July 1990, Pn of Ischaemum anthephoroides was significantly lower than that of the other two species. Ischaemum anthephoroides showed the most pronounced reduction of leaf water potential, which was as low as -2.5 MPa, while the leaf water potentials of Calystegia soldanella and Carer kobomugi remained around -1.5 and -2.0 MPa, respectively, in the 1990 summer period. Such a low tolerance to drought of I. anthephoroides is attributed to the shallow distribution of the root system. These findings are considered to be among reasons why I. anthephoroides is restricted to the inland area, where drought conditions are milder than in seaward areas.