Manipulation of fruit dry matter accumulation and fruit size in ‘Scifresh’ apple through alteration of the carbon supply, and its relationship with apoplastic sugar composition

Abstract

SummaryThe supply of carbon to fruit on whole apple trees of ‘Scifresh’ grafted onto M.9 rootstock was altered by adjustment of the crop load [43% removal at 60 d or 90 d after full bloom (DAFB)], by three periods of 75% shading (from 60 – 90, 90 – 120, or 60 – 120 DAFB), or by a combined treatment of fruit removal at 60 DAFB and shading from 60 – 120 DAFB. Altering the timing and duration of shading, and crop load adjustment, resulted in changes in fruit dry matter concentration (DMC), with a range at harvest from 14.0 – 17.5%. Thinning to reduce the crop load enhanced fruit DMC, with greater effects on both fruit DMC and mean fruit fresh weight the earlier it was done. Shading reduced fruit DMC, with greater reductions the longer the shade was left over the trees. All treatments had greater effects on fruit DMC and fruit quality (flesh firmness and soluble solids concentration) at harvest than on mean fruit fresh weight. This trial also provided fruit samples for measurements of both the carbohydrate content and apoplastic composition of cortical tissues. Starch contents and total apoplastic sugar concentrations were increased by reductions in crop load and were decreased by shading, with starch content being the most responsive. Apoplastic sugar concentrations increased during fruit development. The relative order of abundance of sugars in the apoplast and in the symplast plus vacuole was similar, although differences in the ratios of sugars were evident, with sorbitol accounting for ≤ 10% of the total sugar content in the apoplast and ≤ 3% in the symplast plus vacuole. This work has revealed the possibility of using apoplastic sugar concentration as an indicator of the carbohydrate supply to apple fruit.