Effects of tree pruning in a closed citrus orchard on some parameters of photochemical reactions and fruit quality

Abstract: 【Objective】This study examined the effects of different tree pruning treatments on leaf photosynthesis, chlorophyll fluorescence and fruit quality in a closed citrus orchard in order to optimize tree canopy structure and improve the microenvironment in the orchard and productivity.【Methods】In the experiment, 15-year-old trees in a closed Valencia orange orchard were pruned into different canopy shapes, e.g.open center shape, hedgerow shape and trunk shape, and the effects of these different pruning treatments on photosynthetic active radiation (PAR) , photosynthetic pigments, fast chlorophyll fluorescence, fruit yield and quality were studied.A digital plant canopy imager (CI-110, CID Bio-Science, Inc.Camas, USA) was used in this experiment, which consisted of an auto-leveling image capturing probe, PAR sensors and a built-in USB interface for an optional palm-top computer installed with CI-110 software.PARs in different treatments were recorded.The un-pruned trees in the closed orchard were used as the control (CK) , and the signals of PF (prompt fluorescence) and DF (delayed fluorescence) in attached leaves in the treatments were collected.The kinetics of prompt chlorophyll a fluorescence (PF) at 820 nm (MR) was simultaneously recorded by a multifunctional plant efficiency analyzer, M-PEA (Hansatech Instrument Ltd., UK) , which was programmed by a special computer software that controlled its operation, data recording and downloading.A well-developed leaf from the middle level of each tree was chosen for the above analyses throughout the whole experiment.【Results】The PAR, photosynthetic pigment contents and fast chlorophyll fluorescence parameters in plants under different treatments obtained in June to September were significantly different.PAR decreased significantly with time.From June to August, the photosynthetic pigments content showed an increasing trend but decreased in September.Chla/b constantly decreased throughout the experiment.After different pruning treatments, leaf Chl a, Chl b, Chl a+b and Car contents increased significantly compared with CK, indicating that the treatments promoted the accumulation of photosynthetic pigments and photosynthesis in closed orchards.By measuring and analyzing the dynamic changes in leaf chlorophyll fluorescence in different treatments, it was found that the values of Fm, FJ, FI and Fo all decreased during the experiment, and the values in CK plants was the highest.The activity parameters per unit PSII reaction center (ABS/RC, DIo/RC and TRo/RC) and the activity parameters per unit cross sectional area (ABS/CS, TRo/CS and DIo/CS) all decreased with time, but RC/CS displayed an opposite trend.Among all the treatments, the values of ABS/RC, DIo/RC, TRo/RC, ETo/RC, ABS/CS, DIo/CS and TRo/CS in CK were highest, while those of ETo/CS and RC/CS followed a pattern of open creased.The values of j Po and j Do had little difference between CK and various trunk shapes, but the values of Yo and j Eo in CK were significantly smaller than in the other treatments.The Wk, VJ and Mo in all the treatments were highest in June;VI reached a maximum value in August, but Sm and N fell to minimum in August.At the same period, VJ, Wk, VI and Mo were the highest and Sm and N were the lowest in CK and the electron transfer ability of leaf photosynthetic system was highest in the open center shape, followed by hedgerow shape and trunk shape, and CK had the lowest value.From July to August, the REo/RC, REo/CS, j Ro, y Ro and d Ro in all the treatments decreased, and the PSI related fluorescence parameters in CK were lower than in the other three treatments.PIabs, PIcs and PItotal in the open center shape and hedgerow shape all increased with time, and were significantly higher than in CK.Yield and fruit quality of open center shape were the best, followed by hedgerow shape and trunk shape, and worst in CK.【Conclusion】Tree pruning could improve the micro environment in orchard with increased PAR, and thus significantly improved photosynthetic performance, plant productivity and fruit quality.Among the three pruning treatments, the open center shape was the best, followed by hedgerow shape and trunk shape.