Effects of simulated mechanical pruning models on shoot growth and fruit setting of Guiwei litchi

Abstract:【Objective】In order to develop an alternative pruning practice to replace the expensive, labor-intensive manual pruning, the optimal mechanical pruning models of Litchi chinensis cv. Guiwei were explored.【Methods】Guiwei trees grown in Longan Germplasm Resource Nursery in Guangdong Province were selected as materials with a single tree as one replicate. The trees were pruned in two different seasons and with four retraction pruning models. The prunings were carried out at flowering stage in March and post-harvest stage in mid-July. The four retraction pruning models included hedging one side of canopy (HP), hedging the top of the canopy (TP), hedging the top and two sides of the canopy (HTP), alternate branch pruning (ABP). The traditional pruning was used as control (CK). The branch diameter at pruning edge was less than 2 cm in HP, TP and HTP. Each treatment had three replicates. The effects of mechanical pruning methods on shoot growth and leaf quality in the current year, and yield in two successive years were compared.【Results】These pruning models resulted in different shoot growth, leaf development and fruit setting. The HP treatment was carried out at the flowering stage, and biological response was investigated. The canopy width decreased significantly by 67.0-98.0 cm in December, and the yield in the current year was 50% lower than that of the CK, the yield in the next year was not different from that of the CK. In December, the canopy height of the TP treatment made at the flowering stage was decreased by 30.0-54.0 cm, similar to the canopy reduction of the TP treatment post harvest, resulting in a significant yield reduction in the current year. The yield of the ABP treatment at the flowering stage was stable in the current year and the next year, but both were lower than that of the CK. In the pruning experiment at flowering stage, the growth rate of sprouted shoots from pruning edge during the period of May to June was lower than those from July to October in autumn. From May to June, the growth rate of shoots in the TP and HP treatment were significantly lower than that of the ABP treatment. The branch diameter at pruning edge of the ABP treatment was larger than those of the TP and HP, and had much more numbers of sprouted shoots and faster growth rate of shoots. This result indicated that the ABP treatment was not suitable for mechanized pruning because of the additional thinning work. From May to June, the HP and TP treatment had no significant effect on the chlorophyll content of the top leaves of sprouted shoots. The Top leaves of the ABP treatment had the lowest chlorophyll content, while the HTP treatment significantly increased the maximum chlorophyll content of the top leaves compared with the CK. Among the four treatments, the HTP showed the lowest dry matter and total soluble sugar content in mature leaves of the sprouted shoots. This suggested that the consumption of nutrients of the HTP was large. The starch content in mature leaves of the sprouted shoots from trees treated with HP was the highest, the total soluble sugar content was higher. The starch and dry matter of top leaves on sprouted shoots of the TP treatment were significantly higher than those of the other treatments, and shoots of the TP treatment grew vigorously, which was disadvantageous for fruit setting. Pruning at flowering stage might affect the fruit development of Guiwei litchi. Severe fruit drop, slow fruit development and fruit with larger seed were observed in the trees treated with TP. The HP treatment did not affect the fruit setting on the unpruned side of the same tree. Other pruning models did not affect fruit quality and development. Regarding to post harvest pruning, the shoot growth rate of the TP was faster than that of the HP. The branches of the TP could mature timely in early November and the yield was higher in the next year.【Conclusion】The HP treatment at flowering stage was the optimal retraction pruning mode for the litchi cultivar Guiwei since the treatment neither inhibited the fruit bearing on the unpruned side in the current year, nor promoted the expansion of litchi canopy. The TP treatment would be the suitable retraction pruning model after harvest because this could improve pruning efficiency and save a lot of labor.