CARBOHYDRATE PARTITIONING AND CHANGES IN WATER RELATIONS OF GROWING APPLE FRUIT

In the growing apple sugars accumulate steadily while starch shows a transitory increase during fruit development. Carbohydrate partitioning between starch and sugars can be altered by changing the C-assimilate supply to growing fruit, which is achieved by adjusting the number of fruit per cluster at the start of fruit development. In the larger fruit, those allowed to develop singly, the concentration of osmotically active components was on average 14% higher than in smaller fruit grown in a cluster. Fructose and sucrose accumulation was increased by 9 % and 20 % respectively in the larger fruit. Starch concentration peaked in August and was on average 62 % higher in larger apples. In contrast, there was no accumulation of glucose and at the time when starch levels were highest the concentration of glucose in the single larger fruit was 27 % lower than in the smaller fruit. The results indicate that an increase in C-assimilate supply to growing fruit favours the formation of starch with glucose as a preferred precursor. It is suggested that the transitory formation of starch plays a part in maintaining turgor pressure in the storage cells of apple fruit by partitioning an increased supply of sugars into insoluble carbohydrate. Measurements of parameters showing water relations of growing apples support this assumption. Water potential estimated in apples declined during growth (−1.8 to −12.8 bar) and osmotic potential also dropped (−11.8 to −20.3 bar). However, values of turgor pressure based on these data remained fairly constant (8.2 ±1.5 bar) indicating that the starch synthesis which occurs during sugar accumulation in the growing apple fruit has an osmoregulatory function. © 1990, Gustav Fischer Verlag, Stuttgart. All rights reserved.