The effects of arbuscular mycorrhizal fungi and deficit irrigation on the yield and sugar content of watermelons (Citrullus lanatus)

Abstract

Many studies have demonstrated that arbuscular mycorrhizal fungi (AMF) and deficit irrigation (DI) have positive effects on the fruit yield or quality. This study aims to test whether the watermelon sugar content and yield can be improved by appropriate amounts of AMF and DI methods and to clarify the underlying physiological mechanism. Diploid and triploid watermelon cultivars and their pumpkin rootstock-grafted seedlings were treated with AMF, DI and DI + AMF in a randomised complete block design with five replications. The results showed that DI significantly reduced the relative water content (RWC), PN, alkaline α-galactosidase activity, but increased the insoluble acid invertase (IAI), sucrose synthase (SuSy) and sucrose phosphate synthase (SPS) activities compared with the well-watered (WW) treatment, which led to a decrease in the fruit yield and an increase in the fruit sugar content. Although the AMF improved the RWC, PN and alkaline α-galactosidase, IAI, SuSy and SPS activities in all the watermelon lines under both the DI and WW conditions, the improvement magnitude of these parameters was more pronounced in the pumpkin-root watermelon lines than the corresponding own-root watermelon lines, especially under the DI condition. The integrated application of AMF and DI increased the fruit yield to a level similar to the WW value in the pumpkin-root watermelon lines and sugar content to an optimal level in all the watermelon lines.

Effect of Mycorrhizal Inoculation on Melon Plants under Deficit Irrigation Regimes

“…Moreover, we found a significant linear correlation (R 2 = 0.991 ***) between RWC and stomatal conductance and between marketable yield and RWC (R 2 = 0.988 ***) and stomatal conductance (R 2 = 0.999 ***) confirming that the lower marketable yield recorded with deficit irrigation could be the result of a lower water and nutrient uptake from the soil and CO 2 assimilation. It is well-known that photosynthesis is very sensitive to water shortages and that many crops suffer a reduction in their net photosynthetic rate when RWC drops down [34,55]. The negative effect of water deficit on melon yield could be also related to the reduction in nutrient solubility determined by the limitation of the soil water content, which reduces the uptake of nutrients by plants, thus limiting plant growth and resulting in a decrease in plant and fruit biomass accumulation [56].…”

“…The inoculated plants also produced fruit with a higher value of SSC/TA (an important index of fruit flavor) than control plants with severe deficit irrigation, showing that these fruits had a better balance between fruit sweetness and acidity [27]. It was supposed that well-watered plants are less influenced by mycorrhizal inoculation [34], thus explaining the low differences in quality parameters recorded when plants received a sufficient amount of water, but we confirmed that AMF can increase the sugar content of melon fruit, especially under water deficit conditions [65,109,113].…”

“…Many commercial microbial biostimulants containing AMF are widely available in the market, and their application to agricultural fields is rising [32]. Several experiments performed in different environmental conditions and on many crops have shown that when indigenous fungi are absent or in low levels in the rhizosphere soil inoculation with AMF can be a feasible and rewarding way to benefit plant growth and yield [27,[33][34][35].…”