A study on the sun-scald, a physiological disorder, of grapes

Abstract

1. Experiments were carried out in 1957 and 1959 to study physiologically the cause of so-called sunscald disease of grapes which used to deuelop on the fruit at the pit-hardening stage, especially in the glasshouse growing. 2. This disorder usually occured on the sides of berries directly exposed to the sun-light; first appearing as a slight, brownish, scalded spot in the pulp tissue through the rind, and then gradually growing into a hollow after the collapse of the tissue. At times, the whole berry rapidly became soft and brownish as if boiled, and dropped from the branch after shrinkage. The anatomical observation of the affected tissue showed that the protoplasma of the cells degenerated necrotically and coagulated. 3. By wrapping the berries borne on the vine in vinyl bags, newspapers, and parchment papers of various colors, respectively, it was confirmed that the plots with black or white vinyl bags (having the highest daily fruit temperature) suffered from the disorder more than any other plots, regardless of the quality of light passed into them. 4. When treated in an incubator with high temperature, such as with 40°C air temperature (35°C fruit temperature) for 3.5 hrs., with 42°C (36.6_??_37.5°C) for 2 hrs., with 45°C (38.8°C) for 1.5 hrs., with 48°C (39.6_??_40°C) for 1hr., even normal berries had an attack of the disorder similar to that which developed on the vines naturally. 5. The respiratory rate of berries affected by high temperature and by heavy disorder was observed respectively in vitro, and the result was, that the higher the temperature of berries or the heavier the disorder, the more increased was the respiratory rate. In any case of extreme high temperature or very heavy disorder, however, the respiratory rate suddenly fell with an abnormal respiratory quotient. 6. Finally, as the result of chemical analysis, the disordered berries were found to contain less reducing sugar and tartaric acid than the normal ones, and that they accumulated alcohol and acetaldehyde in their affected portions. Thus, it is noticeable that with an artificial addition of acetaide hyde the tissue of normal berries becomes soft and brownish, accompanied by a reduction of the respiratory rate.