The Responses of Net Photosysthesis to Light and Temperature inSpartina townsendii(sensu lato), a C4Species from a Cool Temperate Climate

Abstract

Carbon dioxide and water vapour exchanges for single attached leaves of the temperate C₄ grass Spartina townsendii were measured under controlled environment conditions in an open gas-exchange system. The responses of net photosynthesis, stomatal resistance, and residual resistance to leaf temperature and photon flux density are described. The light and temperature responses of net photosynthesis in S. townsendii are compared to information on these responses in both temperate C₃ grasses and sub-tropical C₄ grasses. Adaptation of photosynthesis in this C₄ species to a cool temperate climate is indicated both by the light and temperature responses of net photo-synthesis. Unlike the C₄ grasses examined previously, significant rates of net photosynthesis can be detected at leaf temperatures below 10°C. Rates of net photosynthesis equal or exceed those reported for temperate C₃ grasses at all of the temperature (5–40°C) and photon flax density (13–2500μmol m⁻² s⁻¹) conditions examined. Maximum rates of net photosynthesis in S. townsendii are almost double those reported for C₃ herbage grasses. Unlike temperate C₃ grasses, the major limitation to net photosynthesis at low leaf temperatures (10°C and below) is the stomatal resistance, showing that the low residual resistance characteristic of C₄ species is maintained in S. townsendii even at low leaf temperatures.