Light effects on wax begonia: Photosynthesis, growth respiration, and maintenance respiration

The effect of increasing photosynthetic photon flux (PPF) on photosynthesis and respiration in wax begonia (Begonia semperflorens-cultorum) was examined by measuring CO2 exchange rate (CER) of plants continuously for a period of 25 days under four different PPF treatments (5.3, 9.5, 14.4, and 19.4 mol(.)m(2.)d(-1)) in a whole-plant gas exchange system. Net-photosynthesis (P-n) and dark respiration (R-d) in plants increased linearly with increasing PPF. Plants grown at 5.3 or 9.5 mol(.)m(2.)d(-1) respired more than they photosynthesized during the initial growth period, which resulted in a negative daily carbon gain (DCG). It appears that the cost of acclimation to low PPF for plants grown at 5.3 and 9.5 mol(.)m(2.)d(-1) was high as the percentage of maintenance (R-m) to total respiration (R-T) at harvest was 87 and 83 %, respectively, while it was only 70% at 14.4 and 19.4 mol(.)m(2.)d(-1). Carbon use efficiency (CUE) of plants was higher at 14.4 or 19.4 mol(.)m(2.)d(-1) than at 5.3 or 9.5 mol(.)m(2.)d(-1), due to the lower ratio of R-m to R-T in plants. At harvest, crop dry weight (DWCROP) increased linearly with increasing PPF. At the end of the experiment, plants grown at high PPF had an increased photosynthetic capacity [because of increased total leaf area (LA) and radiation capture], DCG, and growth respiration. These data indicate that low PPF not only decreases the photosynthetic rate but also increases the importance of maintenance respiration in the carbon balance of the plants, thereby further reducing the growth rate of plants.