Response of shoot growth, photosynthetic capacity, flowering, and fruiting of potted 'Nagami' kumquat to different regulated deficit irrigation

The objective of this study was to determine the level of regulated deficit irrigation (RDI) for the potted 'Nagami' kumquat culture. Three treatments, control, RDI 1, and RDI 2, representing 75, 55, and 35% of available soil water content, respectively, were implemented for two weeks after the elongation of the newly-sprouted shoots (the first flush) was nearly terminated. Leaf water potential, leaf photosynthetic activity, shoot vegetative growth, flowering, and fruiting were monitored to evaluate the effects of the RDI treatments. After water withholding for two weeks (stage B), trees treated with RDI 2 decreased considerably in their leaf water potential, and exhibited the highest integrated leaf water potential index, which indicated that their leaves had suffered from the highest level of stress. The net CO2 assimilation rate (P-n) and available fluorescence (Fv)/maximum fluorescence (Fm) and Fv'/Fm' ratios also decreased substantially in RDI 2-treated trees. However, the stressed trees exhibited compensatory growth, and all growth parameters returned to normal following re-watering. RDI 2 treatment increased flowering potential of the trees, showing the highest average flowering ratio of 50%, which differed substantially from that of the control and RDI 1. No significant differences were observed between the responses of RDI 1 and control, indicating that both treatments were not severe enough to induce a visible physiological effect in trees. By contrast, RDI 2 treatment efficiently changed the shoot growth phase and increased the number of flowers on the shoots, which may be exploited as an efficient approach for kumquat flowering control.