Photosynthesis and transpiration by young Larix kaempferi trees: C3 responses to light and temperature

The effects of photon flux density and temperature on net photosynthesis and transpiration rates of mature and immature leaves of three-year-old Japanese larch Larix kaempferi (Lamb.) Sarg. trees were determined with an infrared, differential open gas analysis system. Net photosynthetic response to increasing photon flux densities was similar for different foliage positions and stage of maturity. Light compensation was between 25 and 50 μmol m^?2 s^?1. Rates of photosynthesis increased rapidly at photon flux densities above the compensation level and became saturated between 800 and 1000 μmol m^?2 s^?1. Transpiration rates at constant temperature likewise increased with increasing photon flux density, and leveled off between 800 and 1000 μmol m^?2 s^?1. Photosynthetic response to temperature was determined in saturating light and was similar for all foliage positions; it increased steadily from low temperatures to an optimum range betweeen 15 and 21°C and then decreased rapidly above 21°C. Transpiration rate, however, increased continuously with rising temperature up to the experimental maximum. CO₂ compensation concentrations for mature foliage varied between 58 and 59 μl l^?1; however, foliage borne at the apex of the terminal leader compensated at 75 μl l^?1. None of these data support the claim that Japanese larch possesses C₄ photosynthetic characteristics.