Ecophysiological mechanisms used by Aster kantoensis, an endangered species, to withstand high light and heat stresses of its gravelly floodplain habitat

Aster kantocensis Kitam., an endangered plant species of the family Compositae, is a local endemic to the gravelly floodplains of a few rivers in central Japan. The successful growth of A. kantoensis is mainly restricted to sparsely vegetated sites where, due to lack of continuous vegetation, high radiant energy input results in stressful conditions with excessive light and heat. To reveal the ecophysiological characteristics which enable the species to cope with such environmental stresses, we measured leaf temperature, shoot architecture and photosynthetic and transpirational responses together with the microclimate of the natural habitat. Even under sunny summer conditions, the leaf temperature of A. kantoensis was much lower (35-39 degrees C) than the soil surface temperature (max. 60 degrees C). The relationship between leaf position (height from the ground) and leaf temperature showed that the caulescent rosette form of A. kantoensis helps avoid leaf overheating. Moreover, in situ gas exchange measurements revealed that the high transpirational capacity (as high as 10 mmol H2O m(-1) s(-1)) was effective in controlling leaf temperature. as long as the soil water supply was not severely limited. Since it has effective mechanisms to avoid the multiple stresses indigenous to its gravelly floodplain habitat, A. kantoensis can maintain a high photosynthetic rate (up to 30 mu mol CO2 m(-1) s(-1)) without any midday depression under sunny summer conditions. (C) 2000 Annals of Botany Company.