Nitrogen is taken up by most plant species in the form of nitrate and ammonium. The objective of this study was to investigate the effect of different nitrogen forms on the growth of watermelon seedlings. Plants were grown in hydroponic culture with five nitrate (NO3−)/ammonium (NH4+) ratios (100/0, 75/25, 50/50, 25/75, 0/100). When the proportion of NH4+ was increased, the leaf number, leaf area, shoot height, net photosynthesis, biomass, and root growth were significantly decreased. Higher concentrations of nitrogen (N) and phosphorus (P) were observed when plants were supplied with mixed NO3− and NH4+ compared to NO3− or NH4+ alone, whereas the concentrations of potassium (K), calcium (Ca), and magnesium (Mg) were decreased with increasing NH4+. The microelements concentrations were generally increased with more NH4+ added. In addition, plants fed with higher NO3−/NH4+ ratios resulted in more minerals accumulation.

In this study, seven watermelon (Citrullus lanatus) cultivars were tested in solution culture experiment with limiting and ample phosphorus (P) supply to evaluate P uptake and utilization of watermelon under low P stress. Different genotypes showed considerable diversity in terms of biomass accumulation, P uptake, P utilization, root morphological parameters and photosynthetic parameters under low P stress. At low P supply, genotype XN8 and ZCHY were clearly superior to other genotypes in terms of total dry matter yield. The genotype ZJ has the highest P uptake ability, while the genotype XN8 has the highest P efficiency ratio and P utilization efficiency among the seven genotypes under low P stress. The P uptake ability of these genotypes was related significantly and positively to root morphological parameters and photosynthesis parameters under low P stress, the P utilization efficiency showed significant and positive correlation with total dry matter. Results showed existence of genetic differences among watermelon genotypes with regard to P absorption and utilization. The seven genotypes were classified into four groups: efficient responsive (ER), inefficient responsive (IER), efficient non-responsive (ENR) and inefficient nonresponsive (IENR) according to P utilization efficiency under low P stress and dry matter at high P supply. XN8 was identified as ER, and ZCHY was identified as ENR, which may be valuable resources for watermelon production in different soil with low P stress.

Watermelons were grafted on bottle gourd and pumpkin, effects of grafting on phosphorus up- take and utilization of watermelon at early stage under low phosphorus stress were studied by hydroponics. The results showed that grafting can enhance dry matter, leaf area and plant height of watermelon at early stage. And root morphological parameters and photosynthetic rate of grafted watermelon were superior to watermelon. Phosphorus uptake and utilization of grafted watermelon were higher than watermelon under low Phosphorus stress. Different rootstocks performed differently, watermelon grafted on pumpkin performed better than watermelon grafted on bottle gourd. So grafting can enhance phosphorus uptake and uti- lization of watermelon at early stage under low phosphorus stress.

Effects of low phosphorus（P） stress on iron（Fe）,manganese（Mn）,calcium（Ca） and magnesium（Mg） uptake in watermelon seedlings of seven cultivars were studied in this paper by the method of plant solution culture.It has been demonstrated that low P stress can affect the Fe,Mn,Ca and Mg uptake in the seven watermelon cultivars.Low P stress can reduce Fe uptake of watermelon,but the effects of low P stress on Mn,Ca and Mg uptake were different among cultivars.There were significant and positive correlation between Ca and Mg uptake under low P stress.The differences of Fe,Mn,Ca and Mg content among the seven cultivars were studied with the relative content of elements as screening parameters.Our results showed that Zaojia and Zaochunhongyu were identified as the Fe high efficient uptake genotypes,Zaojia was identified as the Mn high efficient uptake genotype,Lifang was identified as the Ca and Mg high efficient uptake genotype under low P stress.