Development of grafted salt-tolerant rockmelon (Cucumis melo L.) using various salinity sources to improve plant performance

Nowadays, there is an increase in demand for high fruit quality of rockmelon for local and export market. To improve fruit quality, salt addition in nutrient solution is a viable approach that can be implemented. However, excess and continuous supply deleteriously affects the rockmelon’s growth and yield. One of the potential strategies to utilize varying sources of salinity for fruit quality improvement without causing growth and yield reduction is by growing grafted rockmelon using salt-tolerant Cucurbitaceae rootstock. Therefore, the objectives of this research are to determine a suitable rockmelon scion age to be grafted on cucurbit rootstocks, to identify the salinity tolerance levels of grafted rockmelon and to evaluate the growth, physiological process, yield and fruit quality of salt-tolerant grafted rockmelon under varying salinity sources. Three scion ages (8, 13 and 18 days after sowing; DAS) of rockmelon (cv. Glamour) were grafted onto two types of Cucurbitaceae viz. bottle gourd (Lagenaria siceraria) and bitter gourd (Momordica charantia). These factorial combinations were arranged in a Randomized Complete Block Design (RCBD) with three replications. Rockmelon scion ages at 13 and 18 DAS were found to be suitable for both cucurbit rootstocks, as they exhibited higher scion height, leaf number, leaf and stem dry weight compared to 8 DAS. Thus, 13 DAS scion age of rockmelon which served as the fastest scion produced was selected to be used for grafting in the subsequent study. The following experiment involved grafting of 13 DAS rockmelon scion age onto three cucurbit rootstocks (selfgrafted, bottle gourd, bitter gourd) and subjected to four levels of NaCl salinity (0, 25, 50, 75 mM). These factorial combinations were arranged in a RCBD with three replications. Among graft combinations, rockmelon/ bottle gourd was classified as the least salt-sensitive due to its capability to reduce toxic ions (Na+ and Cl-) accumulation in the leaf, acquired moderate plant water status among others while having the least significant relationships in growth reductions under saline conditions. Furthermore, NaCl levels at 75 mM were found to be detrimental on most of the growth parameters, leaf gas exchanges and relative water content, which further causes ionic stress that negatively affected nutrient availability in rockmelon. Therefore, graft combination of rockmelon/ bottle gourd and salinity level at 50 mM were selected to be tested in the last experiment. In the last experiment, rockmelon/ bottle gourd was subjected to four types of salinity sources (Basic nutrient solution; BNS, NaCl (50 mM) + BNS, KNO3 (50 mM) + BNS, high strength nutrient solution; NS). These treatments were arranged in a RCBD with four replications. It is revealed that, salinity induced by NaCl (50 mM) salt and high strength NS is capable to increase fruit quality components without interfering the physiological process. When treated with KNO3 (50 mM) salt, most of the growth and fruit quality parameters were sustained. However, the physiological process such as photosynthetic pigments was significantly impaired. Yield component such as fruit weight were decreased regardless of salinity sources by NaCl (28.8%), KNO3 (28.26%) and high strength NS (27.72%) applications as compared to BNS treatment. Overall, 13 DAS scion age is recommended to be grafted with bottle gourd as a salttolerant rootstock for rockmelon. Taking into account the cost saving factor, incorporation of NaCl (50 mM) in nutrient solution is a feasible approach on rockmelon/ bottle gourd due to high fruit quality results whilst sustaining all the physiological process.

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