SHIFTS IN PHOTOSYNTHETIC RESPONSES TO IRRADIANCE AND TEMPERATURE IN VIOLA SPECIES

The steady-state photosynthetic irradiance and temperature responses of Viola hondoensis and V. keiskei (Violaceae) growing in proximity in their native environment and under three levels of artificial shade of relative irradiance (RI) of 7, 20 and 50 % were investigated. The maximum photosynthetic rates and temperature optima under natural conditions were much higher in V. keiskei than in V. hondoensis, except when in leaves of V. keiskei chlorophyl was degraded due to autumn frosts. When gown under artificial shade, both species had higher temperature optima as RI decreased. Since leaves under the three RI conditions experienced a similar leaf temperature, it was concluded that the shifts in the temperature responses with RI resulted from acclimation to the varying irradiance. The higher temperature optima observed in V. keiskei under natural conditions were probably due to the fact that V. keiskei grew in more shady microhabitats than V. hondoensis.