Salinity induced fruit hypodermis thickening alters the texture of tomato (Solanum lycopersicum Mill) fruits

Irrigation of tomato (Solanum lycopersicum Mill) plants with brackish (including saline) water improves fruit taste and reduces yields. Salinity additionally leads to toughening of tomato fruit skin, the causative mechanism for which is unknown. The aim of this study is to characterize the effects of salinity on tomato fruit skin texture. Tomato (cherry and full size) plants were irrigated with fresh water (control, ED=1.01 dSm^-1) and saline water (up to 12.61 dSm^-1). Organoleptic assessment was compared to the force needed to puncture the cherry tomato fruit skin and to specific skin dry weight. The skin structure of cherry and full-size tomatoes was characterized by light microscopy. Chemical characterization of cell walls building the skin of cherry tomatoes was conducted by Raman micro-spectroscopy. Skin penetration force was found to be linearly correlated with its specific weight and with consumers' perceived thickness. Fruit histology revealed a linear correlation between thickness of the sub-epidermis and salinity of the irrigation water. The tougher tomato skin obtained under conditions of salinity is attributed to increased number of hypodermal cell layers rather than to changes in cell wall composition.