Raimo F., et al., Potato Research
Internal brown spot (IBS) is a physiological disorder of potato tubers consisting in the appearance of punctiform and/or enlarged rust-coloured necrosis in parenchymal tissues. The IBS disorder leads to important economic impact and yield depreciation since quality, sensory and processing features of tubers may be highly compromised.
Nowadays, the causes of IBS are still debated; however, the combination of several genetic, nutritional and environmental factors seems to have a relevant role in modulation of symptoms.
In the current work, IBS occurrences together with skin roughness were monitored in potato tubers of different cultivars harvested in 2 years of field trials under different field conditions (environment, irrigation rate).
All three IBS-susceptible cultivars used, Luminella, Majestic and Ricciona di Napoli, showed significant IBS incidence.
Importantly, incidence and severity increased with tuber size.
Ricciona di Napoli was the cultivar most affected by both IBS and skin roughness. Findings showed a strong positive correlation between IBS and skin roughness, suggesting that suboptimal growth conditions may have affected both physiological disorders.
The association between IBS and potato skin roughness may provide new insights for the implementation of non-destructive IBS detection in potatoes.
Results of the field trials revealed the importance of genotype, environmental conditions, soil properties and irrigation rate on the development and expression of IBS symptoms. The importance of the genotype on IBS occurrence was also recognised by Wolcott and Ellis (1959). In the current study, among the three IBS-susceptible cultivars used, Ricciona di Napoli showed the highest IBS incidence, with levels, on average, never below 20% in both years. Furthermore, IBS disease occurred in all experimental sites characterised by loose soils that could favour this physiological disorder of potato tubers (Larson and Albert 1945). IBS is a real problem in one of the most suitable Italian areas for potato cultivation (ware and processing raw), with a great risk of affecting significantly the local economy. In fact, the appearance of browning disorder within tubers produces loss of quality and depreciation and causes significant yield losses.
In the current study, it was observed that IBS incidence and severity increased with tuber size, in agreement with previous reports (Hiller and Thornton 2008). In view of this, all the agrotechniques that are focused on tuber size limitation might form a useful strategy for a significant containment of the disorder; for instance, the adoption of reduced irrigation rates, especially when IBS-susceptible cultivars are cropped in environmentally facilitating areas. As a matter of fact, this work showed that the field plots characterised by no irrigation (FT4 in 2015) or reduced external irrigation supply (FT5 in 2016) had lower IBS and skin roughness occurrence. Besides, a reduced NPK soil content proved to be associated with low IBS and skin defects, while Ca did not show any effects. In a previous study, Sterrett et al. (2006) found that high levels of P in the substrate were associated with internal necrosis incidence, while Ca content was not correlated in this way. In our trials, the presence of tubers with defects was negatively related to soil organic C levels likely due to the overall beneficial physical and biological effects that organic matter has on soil-plant-microbiota interaction (Pane et al. 2015).
As for IBS disorder, Ricciona di Napoli was again the most sensitive cultivar to skin roughness occurrence with around 50% of tubers affected by the defect, regardless of the cultivation site and the tuber size. Skin roughness incidence increased with increasing water supply. Furthermore, it is noteworthy that FT1 plot with a high clay soil texture showed higher IBS and skin roughness so that these defects could be associated with a strengthened soil capacity to retain water. These findings agree with Ginzberg et al. (2012), who reported that maintaining soil humidity at levels close to the field capacity may be favourable for potato skin roughness occurrence.
One of the main findings of this work was the positive correlation between skin roughness occurrence and IBS symptoms, as demonstrated by the Spearman’s rho correlation coefficients (rS). Smooth skin tubers (russeting index = 0) were marginally (17%) affected by barely visible internal necrosis, whereas tubers with fully rough skin (russeting index = 2) showed high IBS severity (up to 90%).
The high correlation between IBS and skin roughness provides evidence for the coexistence of internal and external symptoms in the tubers: both defects are likely developed under the same conditions. So far, the physiological significance of these findings is not conclusive and hence one cannot say if the mechanisms and conditions triggering both defects are common. From a physiological point of view, this is still an unanswered question. The literature (Yencho et al. 2008; Ginzberg et al. 2012) indicates that both IBS and roughness of smooth-skinned cultivars may be associated with nutritional and/or environmental stress-induced modifications of the Ca flux in potatoes. The preferential translocation of Ca via xylem is promoted by the enhanced transpiration activity of the canopy and is accentuated by triggering environmental conditions. This Ca pathway weakens parenchymal cells of the tubers, known as the plant organs most prone to be affected by Ca deficiency.
A Ca concentration decrease in the flesh tissues may cause the cell collapse in the tuber medulla, affecting therefore membrane functionality and cell-wall structural integrity, especially in the bulking growth stage of tubers and under environmental stress factors, such as high temperatures (Ozgen et al. 2006; Gunter and Palta 2008; Yencho et al. 2008). Recently, Pentangelo et al. (2017) observed an acceleration in IBS symptom development during the final part of the cropping cycle (June–July) when the maximum temperatures exceed 30 °C. In the current study, this threshold was exceeded from mid-June, the period corresponding to the maximum growth rate of tubers.
On the other side, under heat stress conditions, an intensification of tuber skin suberisation occurs so roughness may be a secondary outcome of plant response (Ginzberg et al. 2009). Ca could interfere in the formation of the phellem by promoting the adhesion of the suberised cells and preventing their normal flaking (Peters and Wiltshire 2006; Ginzberg et al. 2012). A decreasing Ca concentration gradient from skin to parenchyma has been observed (Subramanian et al. 2011) and was probably sustained by the preferential Ca uptake from the tuber sphere through periderm (Palta 1996), and by a secondary functional root system (Kratzke and Palta 1985). Consistent with a possible role of the Ca ion in russeting development, Ginzberg et al. (2012) found higher calcium concentrations in the russetted skin areas of the tuber than in smooth ones, while an external supply of Ca fertiliser reduced the defect in the harvested potatoes. Nevertheless, the hypothesis needs to be further verified, and specific studies determining Ca concentrations in the cultivated soil and in the tissues at different stages and conditions of tuber formation are still necessary to clarify the aetiology of potato IBS.
The presented findings about the association between IBS and potato skin roughness might provide a useful modelling tool for the development of non-destructive detection strategies for IBS-affected potatoes.
Picture Hollow heart and internal browning by AHDB