Application of water-saving treatments reveals different adaptation strategies in three Iranian melon genotypes

Melon is a major horticultural crop in Iran, where it is grown in semi-arid areas in irrigated fields. Concerns about future water security stimulate the development of water-saving irrigation practices, which requires a better understanding of melon physiology under reduced water supply. To this end, one variety of muskmelon, "Samsoury", and two varieties of the Dudaim group, "Birjand" and "Kerman", were grown in pots in a greenhouse with three water management treatinents, consisting in refilling soil water to field capacity at different intervals: three days (T3), six days (T6) and nine days (T9). Leaf water status declined when longer refilling intervals were applied, but all genotypes maintained the same leaf turgor pressure irrespective of the treatment. Turgor was maintained both through osmotic adjustment (OA) and change in cell wall elasticity. Samsoury showed the highest OA in T3 arid T6, while variation in cell wall elasticity was more evident in Birjand and Kerman. Samsoury showed also the highest photosynthesis rate (A(n)) and stomatal conductance (g(s)) in T3 and T6, but the three genotypes exhibited similar values in T9. In the Dudaim-types A(n) and g(s) declined linearly as soon as water potential started to decrease, while Samsoury was able to keep gas-exchange parameters stable down to a water potential value of -11.5 bars, to steeply decrease thereafter. Finally, Samsoury had the least reduction in biomass allocated to epigeic part, and allocated a higher proportion of biomass to roots in response to water restriction. From all the results, it is concluded that Samsoury performs better when water-saving treatments allow low to moderate water stress, thanks to its anisohydric-like behaviour, consisting in a looser coupling of stomatal closure with water potential, which ensures higher carbon accumulation under fluctuating water supply. The Dudaim-types showed instead a rather more conservative behaviour, typical of isohydric plants, which privilege water loss avoidance even at low water restriction levels, at the cost of carbon assimilation arid growth.