Measuring the Water Status of Watermelon Fruits by Psychrometer and 1H Nuclear Magnetic Resonance Imaging

A gradient in sugar levels in watermelon (Citrullus lanatus (Thunb.) Matsum. et Nakai) fruits implies an osmotic pressure imbalance. The implied osmotic pressure gradient runs opposite to the direction of physical growth, which suggests counterintuitive water movement within the fruit. We conducted experiments to identify the factors that cause and maintain this pressure gradient by psychrometry and 1H nuclear magnetic resonance (NMR) imaging. We made simultaneous psychrometer and 1H NMR measurements of hydroponically grown watermelon fruits immediately after harvesting, and made microscopic observations at the center of the fruits. Psychrometry showed a large water potential gradient from the skin to the center. 1H NMR measurements of blocks of tissue showed no effect of cutting on measurements. The water status showed the same trend by both psychrometry and 1H NMR imaging. In support of this trend, NMR imaging and microscopic examination revealed the disappearance of xylem at the center of the fruits. 1H NMR images of segments of tissue could be combined into continuous detailed images. There is a clear water status gradient within watermelon fruits. NMR and microscope observations revealed possible mechanisms for this gradient in the form of septa and the disappearance of xylem tissue in watermelon fruits.