Effects of nutrient solution electrical conductivity (EC) and drip irrigation frequency on growth and quality of watermelon grown in coco coir substrate in solar greenhouse

Abstract: 【Objective】The study aimed to explore a kind of optimal supply mode of nutrient solution and provide a guidance basis for high efficiency and standardized management of the nutrient supplement for watermelon plants grown in coconut-coir substrate through investigating the effects of nutrient solution electrical conductivity (EC) and drip irrigation frequency on plant growth and fruit weight and quality.【Methods】The experiments were designed with 4 EC levels (E1, E2, E3, and E4 representing the EC of nutrient solution at 0.15, 0.25, 0.35 and 0.45 mS · cm^-1 , respectively) and 2 irrigation rate levels(R1 and R2 representing the irrigation rate of nutrient solution at 2 times·d-1 and 4 times·d^-1 , respectively), complete stochastic region design of two factors was applied with 8 groups of treatment. The regulation of EC values of each treatment level was realized by diluting the A and B mother liquor. Because of the difference of the plant growth and the weather, average daily irrigation volume for each plant was controlled within 0.8-1.5 L, which was divided into several equal parts according to the irrigation rates. And the nutrient solution was applied at the following times: R1 at 8:30 and 14:30, R2 at 8:30, 11:30, 14: 30 and 17:30.【Results】The different EC values of nutrient solution and drip irrigation frequency significantly affected the growth of watermelon plants and the formation of fruit yield and quality in the greenhouse. With the increase of EC value of nutrient solution, the shoot growth performance (including plant height, stem diameter, leaf area), and root morphology (including total length, surface area, volume and diameter of root), and the plant biomass of the watermelon during vegetative growth period increased first and then decreased. The low irrigation frequency (2 times·d^-1 ) significantly increased the stem diameter, leaf area and root morphological parameters of watermelon plants at the rosette stage, promoting the improvement of root-shoot ratio, 26.52% higher than that of the high irrigation frequency (4 times· d^-1 ), which was conducive to the cultivation of strong seedlings. In addition, compared with the low irrigation frequency, the high irrigation frequency significantly increased the plant height of watermelon in vine elongating stage by 6.75%, and obviously increased the stem diameter, total root surface area and root volume and biomass of watermelon in fruit setting stage by 5.29%, 13.33%, 10.90%, 7.71%, 11.26%, and 7.19%, respectively. At the same time, the interaction of the nutrient solution EC value and drip irrigation frequency on watermelon plant height reached a significant level (p＜0.05), and the comprehensive effect of the E2R1 combination was the highest, which was 2.94 cm 24.53 cm and 154.86 cm at the rosette stage, vine elongating stage and fruit setting stage, respectively. Furthermore, at rosette stage and/or fruit setting stage, this interaction on stem diameter, total root length, root surface area, root- shoot ratio and biomassreached extremely significant level (p＜0.01), and the E2R1 was the optimum combination. In the formation stage of fruit yield and quality, with the increase of EC value, the fruit weight and organic acid of watermelon respectively decreased from 2.49 kg and 0.67% to 1.96 kg and 0.58% , while the soluble solids, soluble sugar and sugar/ acid ratio respectively increased from 10.07%, 5.89% and 8.87 to 10.39%, 6.47% and 11.19%. Meanwhile, compared with the high irrigation frequency, the low irrigation frequency significantly reduced the fruit weight and organic acid by 6.41% and 11.94, and significantly increased the soluble solids, soluble sugar and sugar/ acid ratio by 2.08%, 6.20% and 11.94%, respectively. Therefore, the low irrigation frequency was beneficial to improving the fruit quality, and the high irrigation frequency was beneficial to increasing the fruit yield. In addition, the interaction of experimental factors only had an extremely significant effect on the fruit weight (p<0.01), and the treatment of E1R2 and E2R2 had the largest fruit weight, which were 2.52 kg and 2.65 kg, respectively. In the different combinations, the fruit quality of E4R1 treatment was optimum, including the lowest organic acid content, which was 0.55%, and the highest soluble solids, soluble sugar and sugar/acid ratio, which were 10.62%, 6.73% and 12.18%, respectively.【Conclusion】In the condition of greenhouse cultivation, the combinations of 0.25 mS·cm^-1 EC and 2 times·d^-1 irrigation rate during rosette stage, 0.25 mS · cm^-1 EC and 4 times · d^-1 irrigation rate during vine stage, 0.15-0.25 mS· cm^-1 EC and 4 times· d^-1 irrigation rate during fruit swelling stage, and 0.45 mS· cm^-1 EC and 2 times· d-1 irrigation rate during maturation stage, would ensure the watermelon plants grown in coconut-coir substrate to grow well and produce quality fruits with high yield.