Chlorophyll a Fluorescence and Photosynthetic and Growth Responses of Pinus radiata to Phosphorus Deficiency, Drought Stress, and High CO2

Abstract

Needles from phosphorus deficient seedlings of Pinus radiata D. Don grown for 8 weeks at either 330 or 660 microliters CO2 per liter displayed chlorophyll a fluorescence induction kinetics characteristics of structural changes within the thylakoid chloroplast membrane, i.e. constant yield fluorescence (Fo) was increased and induced fluorescence ([FP-FI]Fo) was reduced. The effect was greatest in the undroughted plants grown at 660 .mu.l CO2 L-1. By week 22 at 330 .mu.l CO2 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 .mu.l CO2 L-1. The light saturated rate of photosynthesis of needles with adequate P was higher at 660 .mu.l CO2 L-1 than at 330 .mu.l CO2 L-1, whereas photosynthesis of P deficient plants showed no increase when grown at the higher CO2 concentration. The average growth increase due to CO2 enrichment was 14% in P deficient plants and 32% when P was adequate. In drought stressed plants grown at 330 .mu.l CO2 L-1, there was a reduction in the maximal rate of quenching of fluorescence (RQ) 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 .mu.l CO2 L-1 this response was eliminated showing that CO2 enrichment improved the ability of the seedlings to acclimate to drought stress. The average growth increase with CO2 enrichment was 37% in drought stressed plants and 19% in unstreassed plants.