PHOTOSYNTHETIC CHARACTERISTICS OF A GIANT ALPINE PLANT, RHEUM-NOBILE HOOK-F-ET-THOMS AND OF SOME OTHER ALPINE SPECIES MEASURED AT 4300-M, IN THE EASTERN HIMALAYA, NEPAL

The photosynthetic characteristics of a giant alpine plant, Rheum nobi le Hook. f. et Thoms. and of some other alpine species were studied in situ at 4300 m, in the Eastern Himalaya, Nepal, during the summer mon soon season. Although rainy and overcast weather was predominant, the daytime photon flux density (400-700 nm) ranged from 300 to 500 mumol quanta m-2 s-1. Under such conditions, the temperature of leaves of R. nobile ranged from 10 to 14-degrees-C, and the rate of photosynthetic CO2 exchange ranged from 10 to 16 mumol CO2 m-2 s-1. The ratios of th e maximum rate of photosynthetic CO2 fixation to leaf nitrogen content (defined as instantaneous nitrogen-use efficiency, NUE) for the Himal ayan forbs that were examined in situ were similar to the NUE values r eported for lowland herbaceous species examined under lowland conditio ns. In contrast to the common belief, theoretical calculations indicat e that the decrease in the rate of photosynthesis due to low atmospher ic pressure is small. These Himalayan forbs appeared to overcome this small disadvantage by increasing stomatal conductance. Suppression of photosynthesis caused by blockage of stomata by raindrops appeared to be avoided by either of two mechanisms: plants had large hypostomatous , leaves that expanded horizontally or they had obliquely oriented amp histomatous leaves without bundle sheath extensions. All these observa tions indicate that the gas-exchange characteristics of alpine forbs i n the Eastern Himalaya are adapted to the local wet and humid monsoon conditions and thus photosynthetic rates attained during the monsoon p eriod are similar to those of lowland plants.