Reasonable drip irrigation frequency improving watermelon yield and quality under regulated deficit irrigation in plastic greenhouse

In the northwest of China, the area of facility watermelon cultivation continuously expands with popularization. In order to explore an efficient water-saving irrigation system of watermelon growth in plastic greenhouses, the experiment was conducted to investigate the influence of different mulched drip irrigation frequency on growth, yield, quality, and water use efficiency of watermelons in plastic greenhouses during each growth stage under regulated deficient irrigation. According to preliminary study, a diameter of 20 cm evaporating dish was used to measure water surface evaporation so as to control irrigation water. Three different levels of irrigation frequency were designed including high frequency (once every 2 days), intermediate frequency (once every 4 days), and low frequency (once every 6 days). Each treatment was kept at the same total irrigation amount with 0.4 evaporation amount measured by dish (Ep) during the seedling period, 0.8Ep during the flowering fruit-bearing period, 1.6Ep at the fruit enlargement period, and 0.8Ep at the mature period, respectively. The experiment had measured plant morphological index, photosynthetic indexes, fruit quality index, fruit yield, and water use efficiency. The results showed that irrigation with intermediate frequency at seedlings stage, high frequency at flowering fruit-bearing stage, intermediate frequency at fruit enlargement period, and low frequency at the mature stage (T4 treatment) were the best treatment in all of the nine treatments. In the seedling, flowering, fruit enlargement, and mature periods, this treatment displayed the best growth potential in plant height and stem diameter with a relatively high level of chlorophyll content, net photosynthetic rate, transpiration rate, stomatal conductance, and leaf water use efficiency. This high ability of photosynthesis assimilation in turn contributed to the fruit enlargement and had more advantages against adversity stress early spring. The results of plant height and stem diameter showed that from the stage of seedling to flowering fruit-bearing, the high-frequency irrigation promoted plant height, while intermediate frequency and low frequency irrigation benefited the increase of stem diameter, and because of the different irrigation water during different stages, there were an obvious compensation effect between plant height and stem diameter, in the aspect of quality. T4 treatment had the thinnest peel, which was only 8.30 mm, larger edible part, higher content of total vitamin C, and the highest total center soluble solid content, total edge soluble solid content, as well as soluble protein, which reached 12.85%, 10.80%, 0.35 mg/g respectively. In addition, the fruit sweetness of the T4 treatment was high and had a better quality. In the aspect of fruit production, this treatment also had higher fruit rate, single fruit weight, the highest total fruit production 63.72 t/hm2, and the highest water use efficiency 303.64 kg/(hm2·mm). Therefore, considering the growth potential, yield, quality, and irrigation water use efficiency, the T4 treatment is the best irrigation frequency plan for the whole watermelon growth period, that is, once every 4 days irrigation at seedling stage, once every 2 days irrigation at flowering fruit-bearing stage, once every 4 days irrigation at fruit enlargement stage, and once every 6 days irrigation at the mature stage. T4 is the best treatment in all the nine treatments and the best irrigation frequency scheme for watermelon irrigation in greenhouses under regulated deficit irrigation in early spring of the northwest region. The research could provide valuble information for high efficiency and water-saving irrigation facilities cultivation in the spring of northwest in greenhouse watermelon. ©, 2014, Chinese Society of Agricultural Engineering. All right reserved.