Causes and mechanisms of red drupelet reversion in blackberries

Red drupelet reversion (RDR) is a physiological disorder of blackberries, whereby individual or groups of drupelets that are black at harvest revert to red, usually after the fruit has been harvested and placed into cool storage. RDR reduces the visual and physical quality of the fruit and is considered a major physiological disorder of commercial blackberries. This thesis examined the physiochemical changes that occur during RDR development and investigated pre and postharvest factors associated with the development of the disorder. The physiochemical properties of drupelets that were affected and unaffected by RDR were examined. The total anthocyanin concentration in black, partially red, and fully red drupelets was 1841 mg kg\(^{-1}\), 1064 mg kg\(^{-1}\) and 769 mg kg\(^{-1}\) by fresh weight respectively. Anthocyanins containing acylated or disaccharide sugar moieties were more stable than anthocyanins with non-acylated and monosaccharide sugar moieties. The pH of partially red (3.05) and fully red drupelets (3.01) was lower than that of black drupelets (3.32). The firmness, measured by penetrometer, of partially red (1.90 N) and fully red drupelets (1.77 N) was lower than fully black drupelets (2.39 N). Electrolyte leakage over 24 h was higher from partially red (84.8 %) and fully red drupelets (90.0 %) than fully black drupelets (64.9 %). Examination by light and electron microscopy showed consistent cell disruption, separation and loss of integrity in the upper mesocarp of affected drupelets. The physiochemical symptoms associated with the development of RDR were consistent with mechanical injury, causing cell decompartmentalisation and subsequent anthocyanin degradation. The effects of handling fruit and climatic factors at harvest on RDR incidence and severity were investigated during 10 harvests in 2017. Fruit that were handled during harvest had at least one drupelet develop RDR in 85 % of samples, while only 6 % of fruit that were not handled had any drupelets that developed the disorder. The incidence and severity of RDR was significantly higher when fruit skin temperatures exceeded 23 °C during harvest, and these conditions were also associated with reduced skin firmness of drupelets that were affected and unaffected by RDR. The degree of colour change following controlled, repeatable impact damage at a range of temperatures and subsequent storage conditions was measured by colourimeter. Impact injury caused a significant colour difference (ΔE) relative to the control fruit in 95 % of fruit. As temperature during impact and the subsequent rate of temperature change increased, the severity of colour change worsened. The effects of nitrogen (N) application rate on RDR, fruit quality, and yield were investigated in a two-year trial. A high N application rate of 212 kg ha\(^{-1}\) produced fruit with significantly higher incidence and severity of RDR than medium (106 kg ha\(^{-1}\)) and low N (53 kg ha\(^{-1}\)) rates. The high N treatment increased yield through increasing the number of harvestable fruit in year one, and both the number of harvestable fruit and fruit mass in year two. Firmness and physiochemical fruit quality were not affected by N treatment. The findings establish the major underlying physiochemical changes associated with RDR in blackberries and demonstrate the effects of abiotic factors associated with the development of the disorder in commercial settings. Future research directions and potential management techniques for reducing the incidence of RDR in commercial settings are also discussed