Fruit cracking in muskmelon: Fruit growth and biomechanical properties in different irrigation levels

Muskmelon cracking during ripening is a physiological disorder that causes severe economic losses. Cracking is related with fruit growth, cuticular membrane (CM), biomechanics and cell wall. Nevertheless, the mechanism of cracking induced by irrigation strategy remains unclear. Therefore, we conducted an experiment in muskmelon grown under three irrigation levels: full irrigation (T1, 100% ET), mild deficit irrigation (T2, 75% ET of T1), and severe deficit irrigation (T3, 50% ET of T1). The results showed that fruit mass of T2 and T3 at maturity were decreased by 11.06% and 19.67%, while fruit cracking rate decreased by 53.13% and 84.38% compared to T1, respectively. Irrigation modified fruit growth rate during fruit expanding (Stage I) and maturation stage (Stage II), with a higher growing rate observed under high irrigation levels. Furthermore, fruits under T2 and T3 had a significantly less deformable (lower strain rate) cuticle than that under T1, resulting in higher CM per unit fruit surface area. The mechanical properties showed different dynamic mode during fruit growth. The pericarp break force and pericarp toughness increased at Stage I and then decreased at Stage II, while pericarp brittleness displayed an increasing trend before harvest. We concluded that deficit irrigation alleviated the fruit cracking by adjusting fruit growth rate, and increasing the pericarp break force and toughness at Stage II. However, both of T1 and T3 resulted in accelerated cell wall decomposition during fruit ripening. Therefore, mild deficit irrigation is an appropriate practice strategy that can greatly mitigate fruit cracking while slightly reducing fruit growth.