Mineral concentrations and cytokinin activity in the xylem exudate of watermelon grafted to squash (Cucurbita maxima × C. moschata 'Shintosa') were compared with those of watermelon grafted to bottle gourd (Lagenaria siceraria Standl. 'Sakigake') and non-grafted watermelon in relation to fruit quality. In addition, the effect of fruit number on mineral concentrations and cytokinin activity in the xylem exudate was also investigated.
1. Watermelon plants grafted to squash (W/squash) showed more vigorous growth than watermelon plants grafted to bottle gourd (W/bottle gourd) and non-grafted watermelon. The node order of first pistillate flower was lowered by grafting to bottle gourd. The flesh of the mature fruit was firmer and the textural quality was lower in W/squash than they were in W/bottle gourd and non-grafted watermelon plants. Hence, grafting to squash plants lowered melon quality in comparison to melons from the W/bottle gourd graft combination or to those from the non-grafted watermelon plants.
2. Concentrations of NO₃-N, P, K, Ca and Mg in the xylem exudate collected at 3 different growth stages: at anthesis, 14 days after pollination and fruit mature, were determined. The mineral absorption rate of W/squash was faster throughout the growth period in comparison with the rate of W/bottle gourd; whereas, the mineral absorption rate of W/bottle gourd was slower as the fruit matured.
3. The effect of crop load on mineral concentrations in exudates of both W/bottle gourd and W/squash at 14 days after pollination was undetectable. But as the fruit matured, the mineral concentrations in the exudate of W/bottle gourd seemed to decrease more so in the 2-fruited treatment than those in the 1-fruited one; there was no significant difference in mineral contents between the 1- and 2-fruited treatments of W/squash.
4. The cytokinin activity, mainly ribosylzeatin, in xylem exudate collected at 14 days after pollination was significantly higher in W/squash than in W/bottle gourd scion/stock combination and in the non-grafted watermelon. Two-fruited treatment decreased the cytokinin activity in both rootstocks. These results suggest that the root cytokinin is rapidly affected by the assimilate supply translocated from the shoot.
5. It was concluded that the functions of W/squash roots, for example, mineral uptake and cytokinin synthesis, were considered more active and tolerable to heavier crop load than those of W/bottle gourd and non-grafted watermelon roots.