WATER RELATIONS IN HOT AND COLD FRUIT

Abstract

When the sun shines on a fruit in the field, it warms it differentially, so that the shady side remains cooler. For a bulky fruit like a melon, the temperature difference between the sunny and shady side can be some tens of degrees. What are the likely consequences of this for the water relations of the fruit ? It is plausible to predict that, as the sunny side warms up, water will distil from there to the cooler tissues on the shady side. This could cause physical stress. For instance it would be likely to cause a gradient of turgor pressure across the fruit. Growth in cell size is driven by turgor, so how could the fruit remain spherical under such conditions? Such a change could be opposed by existence of a higher molar concentration of solute on the sunny side. This could be achieved by a higher weight concentration of sugar, or by an increase in monosaccharide as opposed to disaccharide. We have investigated how harvested fruit respond to a temperature gradient, and the solute composition of fruits grown under sunny conditions. The relevance of the results to plant adaptation to temperature stress and to eating quality will be discussed.