Physiological characterization of the yellow-skinned ‘Koukou’ apples (Malus × domestica Borkh.) retaining green color with low brix values, and approaches for decreasing their production

Abstract

Background of the study - Following postharvest storage some 'Koukou' apples (Malus x domestica Borkh.) can show unfavorable taste characteristics, in particular low brix values. These apples cannot be distinguished from their normal counterparts based on appearance. Empirically, we inferred that the inclusion of somewhat greener fruits (hereinafter referred to as green 'Koukou' apples in this report) at harvest was a source of this problem. Objective - We aimed to elucidate the reasons behind the development of green 'Koukou' apples, and to show the approaches to reduce such fruit. Methods - We compared the quality of normal and green 'Koukou' apples after harvest during cold storage for 135 days. We determined the positions within the tree canopy in which these green 'Koukou' apples frequently tend to develop. Results - We also found that post-harvest cold storage improved the skin color of green 'Koukou' apples to a point, in which it was difficult to visually distinguish from normal fruit. However, the eating quality of green 'Koukou' apples was inferior to that of normal apples. Additionally, the sucrose content was lower in green 'Koukou' apples than in normal apples. We observed that green 'Koukou' apples were unevenly distributed among the inner and strong branches of the canopy and were more frequently found on the longer bourses and/or bearing shoots. Conclusions - We demonstrated that green 'Koukou' apples were not merely immature fruits but were a kind of physiological disorder retaining green color with low brix values. We suggest that it is important to make the tree vigor low to prevent the production of green 'Koukou' apples. Additionally, the thinning of greener fruit on the strong shoots before harvest may be a viable strategy to reduce the incidence of green fruit. By adopting these approaches, green 'Koukou' apples may be reduced in the marketplace.