Needles from phosphorus deficient seedlings of Pinus radiata D. Don grown for 8 weeks at either 330 or 660 microliters CO(2) per liter displayed chlorophyll a fluorescence induction kinetics characteristic of structural changes within the thylakoid chloroplast membrane, i.e. constant yield fluorescence (F(O)) was increased and induced fluorescence ([F(P)-F(I)]/F(O)) was reduced. The effect was greatest in the undroughted plants grown at 660 mul CO(2) L(-1). By week 22 at 330 mul CO(2) L(-1) acclimation to P deficiency had occurred as shown by the similarity in the fluorescence characteristics and maximum rates of photosynthesis of the needles from the two P treatments. However, acclimation did not occur in the plants grown at 660 mul CO(2) L(-1). The light saturated rate of photosynthesis of needles with adequate P was higher at 660 mul CO(2) L(-1) than at 330 mul CO(2) L(-1), whereas photosynthesis of P deficient plants showed no increase when grown at the higher CO(2) concentration. The average growth increase due to CO(2) enrichment was 14% in P deficient plants and 32% when P was adequate. In drought stressed plants grown at 330 mul CO(2) L(-1), there was a reduction in the maximal rate of quenching of fluorescence (R(Q)) after the major peak. Constant yield fluorescence was unaffected but induced fluorescence was lower. These results indicate that electron flow subsequent to photosystem II was affected by drought stress. At 660 mul CO(2) L(-1) this response was eliminated showing that CO(2) enrichment improved the ability of the seedlings to acclimate to drought stress. The average growth increase with CO(2) enrichment was 37% in drought stressed plants and 19% in unstressed plants.