Photosynthetic responses to low temperature in Betula pubescens and Betula tortuosa

Abstract

The acclimation of the net flux density of CO2 to temperature, and the effects of freezing‐stress on the functional integrity of photosynthesis were compared for Betula tortuosa and Betula pubescens. Single expanded leaves of B. tortuosa had a higher rate of net photosynthesis and a greater capacity to acclimate to a low growth temperature regime than had leaves of B. pubescens. At low temperatures, stomatal conductances were higher for B. tortuosa than for B. pubescens. In neither species were the stomatal conductances determining the decrease of the rate of net photosynthesis at low temperatures. There was a marked difference between leaves of the two species in their ability to withstand freezing, as revealed by measurements of changes in temperature‐induced variable fluorescence. The chloroplast thylakoids of B. tortuosa were better able to maintain their functional integrity at sub‐zero temperatures than were those of B. pubescens.