Ex vitro photosynthetic activity in plantlets of tissue-cultured Douglas-fir.

Plantlets of Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) had lower rates of net CO(2) uptake and higher rates of root respiration than greenhouse-grown seedlings. They also had a lower photosynthetic efficiency at high photosynthetic photon flux density, and exhibited significant depression of CO(2) uptake under water stress. The ability of plantlets to utilize high irradiance and to photosynthesize under conditions of water stress was enhanced by the presence of many roots. Carbon dioxide uptake rate increased with increasing root number, and decreased as the needle surface area/root surface area ratio decreased. High root number, low needle surface area/root surface area ratio, and elongated roots were associated with low apparent rates of shoot and root respiration.