Defining and understanding internal brown fleck in tubers of potato (Solanum tuberosum L.)

Harper, Stephen Mark (2006). Defining and understanding internal brown fleck in tubers of potato (Solanum tuberosum L.) PhD Thesis, School of Land, Crop and Food Sciences, The University of Queensland.

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Author Harper, Stephen Mark
Thesis Title Defining and understanding internal brown fleck in tubers of potato (Solanum tuberosum L.)
School, Centre or Institute School of Land, Crop and Food Sciences
Institution The University of Queensland
Publication date 2006
Thesis type PhD Thesis
Supervisor P. Blamey
Total pages 215
Collection year 2006
Language eng
Subjects L
Abstract/Summary There are numerous internal browning disorders of potato (Solanum tuberosum L.) tubers that seriously affect the marketing of this important staple throughout the world. A search of the scientific literature (Chapter 2) revealed that these disorders have generally not been well defined nor are they well understood. In Australia, the term, brown fleck (BF), has been widely used to describe the presence of any internal browning. This disorder, or at least one form thereof, is especially serious in spring (i.e. grown over winter) crops in Queensland. As with most internal disorders of potato tubers, improper definition and poor evaluation of possible causes has hindered the development of a successful field management strategy. This study was initiated (i) to clearly describe the initiation and development of BF, both at the microscopic and macroscopic levels and (ii) to evaluate its possible causes, thereby developing ways that may reduce its impact in commercial crops. A cytological study on potato (cv. Sebago) tubers was conducted to evaluate the initiation and development of BF lesions (Chapter 3). Two forms of BF were observed. The first was initiated early in tuber development (77 days after planting (DAP)). This form of BF resulted in the death of individual phloem sieve cells (PSC) in the tuber medulla and spread to adjacent PSC and healthy parenchyma cells (PC). Symptom development accelerated through to maturity (112 DAP). A further form of BF developed late in maturity (100-112 DAP) where starch amyloplasts of healthy PC browned but occurred independently of the presence of BF in proximal PSC. This study demonstrated that two discrete forms of BF exist and that these forms may have different causes that possibly require different management strategies for their control. Many published studies have shown high variability in using a visual rating to estimate the incidence of internal disorders, including BF, and evaluate possible treatment effects. To overcome this problem of high variability, a computer-based digital imaging technique was developed (Chapter 3) that quantifies the area of tuber tissue visibly affected with BF and allows the calculation of previously unused BF indices and more accurate statistical evaluation of treatment effects. Published scientific information has implicated high temperature in the development of browning disorders of potato tubers. To study this aspect, a series of glasshouse experiments was conducted to evaluate the effects of air and soil temperature on the incidence of BF (Chapter 4). A low incidence of BF was observed at a continuous day/night temperature (DNT) of 18/13oC but was substantially increased at 23/18oC and 28/23oC DNT during tuber development and especially from 91 DAP (Experiment 4.1). Experiment 4.2 showed that the incidence of BF increased with increased duration of exposure (0, 1, 2 and 4 d) to a 28/23oC DNT regime. It is noteworthy that BF incidence at 28/13oC DNT was not different from that at 18/13oC DNT suggesting that elevated day temperature did not increase BF incidence (Experiment 4.3). However, the incidence of BF (total area affected with BF plant-1) at 28/23oC DNT was six-fold greater than that at 28/13oC DNT (552.3 and 92.2 mm2 plant-1 respectively). This clearly demonstrated that BF development was increased by elevated night temperature. Experiment 4.4 evaluated the relative effects of air (18/13oC and 23/18oC DNT) and soil temperature (18oC, 23oC and 28oC) on BF incidence. This increased with increased soil and air temperature. Increasing soil temperature at a 23/18oC air DNT did not significantly increase the number of BF lesions per tuber but did increase the average lesion size and consequently the area affected with BF. The overall conclusion from the series of experiments on temperature effects was that (i) elevated night temperature and (ii) elevated soil temperature increased the incidence of BF. However, the presence of BF, albeit low, in the continuous 18/13oC DNT treatments suggested that factors other than high temperature induce BF or affect lesion development. Indeed, within the literature, the plant’s nutritional status has been implicated in affecting the development of internal disorders of potato tubers (Chapter 2). Consequently, a series of field and glasshouse experiments was conducted to evaluate the effects of calcium (Ca) (up to 127 kg ha-1) and boron (B) (up to 5.0 kg ha-1) on BF incidence. In this study (Chapter 5), the application of Ca or B did not significantly reduce the incidence of BF. The previous studies on temperature effects (Chapter 4) had shown that the incidence of BF is positively correlated with foliage mass. A series of experiments was conducted, therefore, to evaluate the role of assimilate partitioning in the development of BF. Given that BF development was found to be directly related to elevated night temperature and foliage dry mass, and initially occurred in the PSC alone, it was hypothesised that high foliar respiration of assimilate reserves may induce an assimilate deficit in the tubers resulting in PSC death. Two glasshouse experiments evaluated the effects of weekly foliar sucrose supplementation (0-4%), from 56-107 DAP, (Experiment 6.1) and CO2 enrichment of the atmosphere (Experiment 6.2) (1080 ìmol mol-1 for 7 d from 94 DAP) on the incidence of BF. Foliar sucrose application increased tuber fresh yield but did not significantly affect BF incidence. Enrichment with CO2 consistently increased the incidence of BF above that in untreated control plants, suggesting that reduced assimilate supply does not cause the initial death of PSC. Three field experiments (Experiments 6.3, 6.4 and 6.5) evaluated the effects of foliage removal, shading and covering of plants with plastic at night (to increase temperature) on BF incidence. Removal of 25-100% of the foliage at 91 DAP consistently reduced BF incidence in mature tubers (115 DAP), but neither shading nor covering with plastic had significant effects. A final study on the phenology of BF development and its relationship to specific weather and yield variables was conducted (Chapter 7). The incidence of BF was positively correlated with leaf dry mass, night temperature and soil temperature, but not to day temperature, relative humidity or solar radiation, thus supporting the previous experimental results. This study has provided improved descriptions of BF, both microscopically and macroscopically, and developed a computer-based procedure to measure its incidence. Factors that increased BF in tubers of cv. Sebago include high air temperature, especially at night, and high soil temperature. Experimental evidence has also been provided that shows a positive relationship between BF and yields of foliage and tubers. There was no influence on BF incidence of Ca or B applications to the soil studied. The evidence presented suggests that the incidence of BF is related to increased night-time assimilate loading to the tubers causing cell damage. Further research is needed, however, to confirm this conclusion, specifically to determine the initial cause of PSC death. At the practical level, the experimental results suggest that BF may be best managed by potato growers monitoring their crop from flowering onwards for the first incidence of BF in tubers. Short term weather outlooks can then be used to predict the likelihood of conditions favourable for the development of BF (especially periods of elevated night temperature). If crops are near to maturity, tubers would be harvested should this seem likely. Furthermore, it is suggested that crops should be grown under conditions to reduce vigorous foliar development, this being achieved, for example, by ensuring that irrigation and nutrition are not excessive. Further research is required to manage plant foliage to reduce BF incidence without decreasing tuber yield.

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Created: Fri, 21 Nov 2008, 14:57:19 EST