GROWTH RATE AND SURFACE WETNESS ARE CRITICAL FACTORS IN MICROCRACKING, LOSS OF WATER VAPOUR AND RUSSETING OF THE APPLE FRUIT SURFACE

Abstract

Russeting is an economically critical fruit surface disorder in horticultural crops like apple globally . If cuticle of primary fruit surface is failed mechanically and develops cracks , periderm is formed underneath the cracks and replaces the cuticle. This periderm becomes visible on the surface and the surface turns brown. If the cracks are repaired in time then there is no need for periderm formation. Fruit surface growth rate is the primary cause of microcracking in the fruit surface and regarded as a symptom of russeting. Surface wetness is said to increase microcracking and eventual russeting of the fruit surface . The objectives were to quantify ; micro cracking of the cuticle , fruit surface permeability and fruit surface russeting . Selected portion of the fruit surface was exposed to CPPU solution (20 mil-I) or liquid water for two days and 12 days respectively and sampled four times after the termination of the treatment. Fruit sampled were analysed for microcracking , permeability and russeting . Microcracking of the fruit surface was quantifed using acridine orange infiltration . Permeability was quantified by water loss using excised epidermal segments (ES). Calibrated photographs of the russeted fruit were taken and russeted area quantified by image analysis using Cell P. " At 21 DAFB, AO infiltrated area of the CPPU treated and moisture treated surface of the fruit were respectively 4-fold and 8-fold more than that of the untreated surface . There was no effect of CPPU and moisture on micro cracking of the fruit surface at 44, 73 and 100 DAFB. Water loss through the excised ES of CPPU treated and moisture treated surface of the fruit were higher than that of the untreated surface at 21 DAFB. Permeability of CPPU treated and moisture treated surface of the fruit were respectively 1.5-fold and 4-fold higher when compared to the untreated surface at 21 DAFB . The effect decreased when CPPU and moisture were applied at 44, 73 and 100 DAFB. The permeability of CPPU treated and the moisture treated fruit surface were more pronounce in early fruit development with increased AO infiltrated area, but the effect decreased in later stages of development and became similar to the untreated surface as AO infiltrated area decreased with time. There was 10 % and 37 % russeting when the fruit surface was treated with CPPU and moisture respectively at 21 DAFB . There was no russeting (0 %) in the untreated surface of the fruit. Surface moisture had a more severe effect than the effect of increase growth rate. However, surface moisture alone is not enough to induce cracks if fruit surface growth rate is low. Therefore, both factors are critical in rnicrocracking , loss of water vapour and russeting of the apple fruit surface.