Why do cherries split?
Dr. Moritz Knoche of the University of Hannover, in north Germany, explained his theory at the International Fruit Tree Association’s conference in Tasmania.
The skin of a cherry has an extremely thin cuticular membrane that transports substances into and out of the fruit. The surface of the cherry has stomata that are functional during the first two or three weeks of fruit development, but not during the major part of fruit development, when some are open and some are closed. Different cultivars have different stomata densities. The surface also has microscopic cracks that are not visible to the naked eye, but are large enough that water can flow through into the cherry.
A cherry has three growth stages, Knoche explained. During the first, it is small and green. During the second, the seed develops, and at the end of that stage (about 45 days after full bloom), the pit hardens. During stage three, there’s an explosive increase in fruit mass, with the surface of the cherry increasing at a rate of one square centimeter (a sixth of a square inch) per day, which he noted is a large rate of growth in relation to the small size of the fruit.
As the surface expands, the cuticular membrane becomes thinner because the amount of skin that the cherry has during fruit development doesn’t change. The same amount of skin is distributed over the enlarging fruit surface, and the thickness of the cuticle decreases by about 60 percent by the time the fruit is mature. Expansion of the cherry puts a strain on the cuticle, and as the strain increases, the number of microscopic cracks also increases, providing openings for more water uptake.
When the fruit surface is wet, the cell walls soften and the number of microscopic cracks further increases. High humidity can also lead to more cracking.
Knoche estimated that 30 to 50 percent of water uptake occurs along the stem-fruit juncture, either through a leaky stem or skin cracks, and the rest through the fruit surface.
The amount of water taken up depends on the fruit surface area (meaning the size of the cherry), its permeability, and the driving force (gradient in water potential). Although water can move either in or out of the cherry skin, about 14 times more water goes in than moves out via transpiration.
Knoche said knowing how the skin cracks will be useful in finding mechanisms to reduce splitting. Strategies might be to reduce the strain on the cuticle, keep the fruit surface dry, and reduce its permeability.
Applying salts is one way to reduce the skin’s permeability. In experiments, Knoche found that ferric chloride applications reduced cracking from 82 percent to zero, but resulted in bad discoloration of the fruit surface. He is exploring other potential solutions.