Effect Of Rootstock On Absorption And Utilization Efficiency Of Potassium In Watermelon

China is the larggest country for watermelon(Citrullus lanatus (Thunb.) Matsum. and Nakai) production. Potassium is very important for the growth and development of plant and fruit in watermelon cultivation. Fruit quality is negtively affected by low potassium. However, the shortage of soil potassium and potassium fertilizer in our country had become an important factor to limit the production of watermelon. Improving the potsssium efficiency is one of the most important approaches to solve these problems. The effects of different rootstocks on plant growth and fruit quality of watermelon was analyzed in this paper, and the potassium uptake and utilization for different rootstocks of watermelon grow under different potassium conditions were compared, then the effectiveness of grafting to improve potassium efficiency in watermelon was studied and conducted. The main results are as follows:1. Two mini-watermelon cultivars’Zaochunhongyu’and’Wanfulai’were grafted onto two different rootstocks’Nabizhen’(Lagenaria siceraria Standl) and’Hongdun’ (Citrullus lanatus sp.) respectively, and the ungrafted plants were as control. The effect of different rootstocks on plant growth and fruit quality of watermelon were investigated. The results showed that the wild watermelon’Hongdun’performed well with the characteristics of strong growth, superior quality and strong resistance.2. The growth and the potassium absorption and utilization for three types of rootstocks (Citrullus lanatus sp., Cucurbita spp, Lagenaria siceraria Standl) of watermelon were studied. The results showed that there were significant differences in potassium content, biomass yield and potassium utilization between the rootstocks under three potassium levels (0,0.75 and 3 mM K). Under low potassium levels (0.75 mM K), the Citrullus lanatus sp. had smaller reduction of leaf area, dry matter, and absorbed potassium than other types of rootstocks. Significant growth inhibition and reduction in absorbed potassium were observed in Cucurbita spp, but the potassium use efficiency value of Cucurbita spp was the highest among all the rootstocks. The plant growth index of Lagenaria siceraria Standl decreased significantly, and the potassium use efficiency value was lower than other two types of rootstocks. In conclusion,’Hongdun’,’Jingxin NO.4’and Lagenaria siceraria Standl were considered to be the high potassium absorption rootstock, the high potassium utilization rootstock, and the low potassium absorption and utilization rootstock, respectively.3. Green house experiment was carried out to determine growth and potassium absorption and utilization of watermelon plant’Zaochunhongyu’(Citrullus lanatus (thumb.) Matsum. and Nakai) either ungrafted of grafted onto three commercial rootstocks:’Hongdun’(Citrullus lanatus sp),’Jingxin NO.4’(Cucurbita spp) and ’Nabizhen’(Lagenaria siceraria Standl) grown in hydroponics. Plants were supplied with nutrient solutions having two levels of potassium (6.0 or 0.6 mM K). The low potassium nutrient solution had the same basic composition plus the decrease of K2SO4. Significant depression of shoot, root biomass production, leaf area, net photosynthetic rate, and leaf macro-(N, K) and microelements (Cu) under low potassium level (0.6 mM K) was observed in both grafted and ungrafted plants. Decreasing the concentration of potassium concentration in the nutrient solution significantly enhanced soluble sugar and chlorophyll content of leaves, and the potassium use efficiency in low potassium treatment in comparison to the control. Under low potassium level, the reduction of shoot biomass growth and the potassium content per plant were significantly lower in plants grafted onto Cucurbita spp and Citrullus lanatus sp in comparison to those grafted onto the Lagenaria siceraria Standl and the ungrafted plants. Moreover, the high reduction in net photosynthetic rate was more severe in ungrafted plants and grafting combinations Lagenaria siceraria Standl in comparison with the grafting combinations Citrullus lanatus sp and Cucurbita spp. The K、N、P、Ca、Mg concentration in xylem sap were significantly higher in plants grafted onto Citrullus lanatus sp and Cucurbita spp in comparison to that of Lagenaria siceraria Standl and ungrafted plants. The decrease in leaf potassium concentration of plants grafted onto Lagenaria siceraria Standl and ungrafted plants grow under low potassium level (0.6 mM K) was significantly compared to plants grafted onto Citrullus lanatus sp and Cucurbita spp. The volume of xylem sap of plants grafted onto Citrullus lanatus sp and Cucurbita spp decreased under low potassium conditions. These results support the hypothesis that grafting with Citrullus lanatus sp and Cucurbita spp can enhance the growth parameters of watermelon by the stronger absorption and uptake of nutrients under low potassium level, thus could improve low potassium tolerance of watermelon.