Assimilate partitioning between the flesh and the rind is responsible for purple spot in loquat fruit.
This work reports on the relationship between purple spot and the balance of sugars between flesh and rind tissues in loquat trees (Eriobotrya japonica Lindl., cv. 'Algerie') in Alicante, Spain, as affected by thinning, night temperature, and exposure to sunlight. In Spain, 15% of annual loquat fruit production is affected by purple spot. From fruit set to colour break, the total sugar concentration (TSC) in the flesh of fruit from non-thinned trees increased by 170%, compared with an increase of 803% in fruit from trees thinned to one fruit per panicle. Thinning had a similar effect on TSC in the rind. In the flesh, sucrose and glucose concentrations increased up 7.5- and 24-fold, respectively, after thinning; whereas fructose and sorbitol levels increased 2.5- and 1.2-fold, respectively. In the rind, sucrose (×14.5), glucose (×38), fructose (×3.6) and sorbitol (×1.3) also increased. At colour break, the gradient of TSC between the flesh and the rind increased from 143 mg g-1 DW to 417 mg g-1 DW due to thinning, and correlated with the incidence of purple spot. Maintaining night temperatures above 15°C, or wrapping the fruit, reduced the gradient of TSC from 396 mg g-1 DW to 279 mg g-1 DW and from 363 DW to 296 mg g-1 DW, respectively, and reduced spotting. There was a significant relationship (R2=0.91) between the incidence of purple spot and the gradient of TSC between the flesh and the rind in the three experiments.