Abstract
Photosynthetic characteristics of south-facing single full-leaf in one-year branch of Platanus acerifolia L. with branch and stem pruning (BSP) or under natural conditions (NG) were determined using Li-6400 portable photo-meter. The results show that diurnal variations in net photosynthetic rate (P_n) of P. acerifolia single full-leaf under BSP and NG follows a di-peak curve. The first peak appears at 11:00, the second at 15:00, while the minimum peak occurs at 13:00 (when light intensity is highest). This indicates a remarkable midday depression. The leaves under both conditions suffer from photo-inhibition. Pn [μmol(CO_2)·m~(-2)·s~(-1)] of the first peak, second peak and the minimum peak of P. acerifolia leaf under BSP are 12.16, 9.21 and 6.47, which are respectively 27.06%, 69.93% and 159.84% higher than those under NG. This implies that BSP improves photosynthesis, while at the same time reduces photo-inhibition. Best-fit response curves for P_n to photosynthetic photon flux density (PPFD) in P.acerifolia leaf under BSP and NG are expressed as P_n=12.30 (1-1.135 3 e~(-0.030 2PPFD/12.30)) (R~2=0.989 1~(**)) and Pn = 10.92 (1-1.130 7e~(-0.030 5PPFD/10.92)) (R~2=0.984 9~(**)) respectively. The values of max P_n (P_(max)), light compensate point, light saturation point and PPFD range of P. acerifolia leaf under BSP are 12.30 μmol(CO_2)·m~(-2)·s~(-1), 51.67 μmol·m~(-2)·s~(-1), 1 927.32 μmol·m~(-2)·s~(-1) and 1 875.65 μmol·m~(-2)·s~(-1) respectively; which are 12.64%, 17.33%, 13.85% and 13.76% higher than those under NG, respectively. This suggests that BSPtreatment could increase utilization of light energy in P. acerifolia. The best-fit response curves for P_n to CO_2 concentration (Ci) of P. acerifolia leaf under BSP and NG are expressed as P_n =11.96(1-1.471 6e~(-0.028 7Ci/11.96)) (R~2=0.982 4~(**)) and P_n = 10.70(1-1.465 7e~(-0.029 2Ci/10.70)) (R~2 =0.981 0~(**)) respectively. P_(max), CO_2 compensate point, CO_2 saturation point and Ci range under BSP are respectively 11.96 μmol(CO_2)·m~(-2)·s~(-1), 67.54 μmol·mol~(-1), 872.02 μmol·mol~(-1), and 804.48 μmol·mol~(-1); which are 11.78%, ?27.44%, 0.39% and 3.73% higher than those under NG, respectively. This indicates that BSP treatment increases CO_2 assimilation ability of P. acerifolia under low CO_2 concentration condition.
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