Potato tuber greening : mechanisms of resistance and risk factors

Potato tuber greening is an economically major defect for the potato industries. Greening occurs when potato tubers are exposed to light, resulting in accumulation of chlorophyll in the tuber peripheral cell layers, and can occur at any stage throughout the potato supply chain, in the field, in storage, or in retail stores. The propensity for tuber greening is influenced by genetic, cultural, physiological and environmental factors. This thesis aimed to extend our knowledge on tuber greening by elucidating the physiological and genetic factors associated with greening propensity. I also examined risk factors associated with greening in the field and post-harvest, including nitrogen fertilisation, varietal effect, tuber physiological age and lighting conditions. Despite prior research, there remains a lack of fundamental understanding of the physiological and genetic factors associated with greening resistance. Results show that potato varieties varied in their resistance to tuber greening, and that varietal resistance was associated with suberin deposition in tuber periderm, and to a lesser extent, the number of periderm cell layers and other pigments in the tuber periderm. We propose that periderm suberin provides a protective barrier against light-induced pigments accumulation in the tuber peripheral cell layers. Tuber greening resistance and periderm suberin content was further influenced by tuber maturity, with more mature tubers having both greater suberin content and greening resistance. Greening in tubers was associated with an upregulation of genes involved in chlorophyll biosynthesis, particularly, Glutamyl-tRNA reductase 1 (HEMA1), Magnesium-chelatase subunit H (CHLH) and Magnesium-protoporphyrin IX monomethyl ester cyclase (CRD), and a downregulation of genes involved in chlorophyll degradation, including Chlorophyll (ide) b reductase NYC1 (NYC), Pheophytinase (PPH), and Pheophorbide A oxygenase (PAO), which we hypothesise are key transcriptional regulatory genes in tuber greening. Comparison of a resistant and a susceptible variety indicated resistance may be associated with reduced upregulation of candidate chlorophyll biosynthesis genes and downregulation of degradation genes, following light treatment. This study showed that increased tuber nitrogen content was associated with an increase in tuber greening propensity likely associated with more efficient photosynthesis. However, it was also shown that the rates of nitrogen fertiliser application to potato plants during growth was not necessarily associated with changes in tuber nitrogen content, the response being influenced by variety. Specific light wavelengths were shown to vary in their capacity to induce tuber greening, with blue light being the most stimulatory. Using this information, a survey of commercial stores identified differential risks of tuber greening based on lighting conditions. This thesis provides the first evidence of a physiological mechanism for resistance to tuber greening, indicating a central role of periderm suberisation. It opens new research possibilities to enhance resistance to greening through genetic manipulation of suberin biosynthesis and key chlorophyll metabolism genes. It also improves our knowledge on the agronomic and post-harvest factors affecting greening, which provides information for the potato industry and retail stores to assist in reduction of tuber greening incidence and the associated economic losses.