Quantifying changes in tree physiology after amelioration to reduce sunburn on apples

[摘要] ENGLISH ABSTRACT: Research was done in response to initial findings that combined calcium (Ca) and boron (B) reduced sunburn damage in apples. With the mode of action for this amelioration being unclear, current knowledge of this new sunburn-reducing strategy needed to be increased. With other strategies that are already able to eradicate or reduce sunburn incidence, like kaolin-film sprays or shade nets respectively, another alternative now exists in combined Ca and B. The motivation behind this sunburn-reducing strategy lies in its ease of use, economic viability due to low infrastructural expenses as well as its sustainability.Continued research endeavours into this field are especially motivated by the changing climate, which confronts the deciduous fruit industry with increased temperatures. This could further exacerbate sunburn incidence which has already been up to 50% in some orchards. Future water restrictions, with more severe droughts and sporadic rainfall can give rise to water stress in the orchard, which increases sunburn.In this study 'Cripps' Pink' and 'Granny Smith' apples were studied in addition to 'Golden Delicious' apples and alternative combined Ca and B formulations were evaluated to compare the success of the initial study on 'Golden Delicious' with other formulations and cultivars. In the first season, 'Cripps' Pink' apples on Welgevallen Experimental Farm, Stellenbosch, South Africa, were sprayed with weekly foliar applications of Ca and B for six weeks from 40 days after full bloom. Mineral analysis was done on the peels two weeks after the final application, as well as at harvest. Treatments containing Calcinit™, a calcium nitrate product, gave the highest B concentrations in the peel and no significant differences were observed for Ca concentrations. The Spraybor® Calcinit™ treatment had a significantly higher amount of B in the peel than the Spraybor® Manni-Plex® Ca treatment as well as the control, at harvest. This, however, did not reflect in a significant reduction in sunburn incidence at harvest. In an attempt to exacerbate sunburn damage, the 'Cripps' Pink' apples were harvested relatively late, which gave rise to pink colour development of the bicolour cultivar. Masking of sunburn, especially of the milder sunburn classes like class 1 and class 2 sunburn occurred.In the second season, a separate trial was done on 'Golden Delicious' and 'Granny Smith' apples in Grabouw, Elgin Valley, South Africa. The 'Cripps' Pink' trial was repeated with alternative Ca and B formulations. In light of the first season's results, the 'Cripps' Pink apples were harvested earlier to prevent excessive masking of sunburn. Significant differences were observed between the control and all Ca B foliar treatments, which all brought about a significant reduction in class 1 sunburn incidence. Significant differences were also recorded between treatments in 'Golden Delicious' apples. The 0.5 g.L-1 Spraybor® Calsol® treatment was the only treatment that had significantly lower (5.7 %) Class 1 fruit than the control (16.4 %) and Manni-Plex Cal-Zn Manni-Plex B treatment (17.4 %). The same treatment also had a significantly lower Class 2 sunburn incidence than the control. The poor performance of the Manni-Plex® Cal-Zn Manni-Plex® B treatment was unexpected and was perhaps due to the alteration of the original (2011/13) Manni-Plex® Ca formulation during 2015.Another objective of this study was to elucidate the mode of action of Ca and/or B ameliorating sunburn incidence, by looking at cell wall thicknesses below the peel. It was hypothesised that thicker cell walls due to Ca B applications resulted in sunburn reduction. Initial observations showed a trend of increased cell wall thicknesses when Ca and B formulations were applied, specifically especially with the Spraybor® Calcinit™ treatment. Cell walls were also thicker in the peel of sunburnt fruit than fruit with no sunburn.Additionally, the final objective of this study was to determine how these foliar applications affected fruit physiology like chlorophyll fluorescence and fruit surface temperature. Even though there were significant differences between treatments with respect to all physiological parameters quantified in this study at some point, none of the effects could be related directly to a reduction in sunburn incidence; neither could the physiological changes as a result of the treatments explain the mode of action of the significant reduction of sunburn incidence observed.Even though the efficacy of Ca and B applications could be established on 'Cripps' Pink' apples, a clear mode of action could not be established for this sunburn ameliorating strategy on a physiological level. More research therefore needs to be done into a possible biochemical and/or metabolic reaction caused by the combination of Ca and B in reducing sunburn on apples.

[发布机构] Stellenbosch University