0. RESPIRATION’S LINK TO TEMPERATURE.
“Respiration is the process by which carbon compounds are oxidized to provide the metabolic energy and substrates needed for growth and maintenance of all living cells” (Klotz et al., 2008). Harvested roots are heterotrophic – they cannot produce their own energy given they are not actively photosynthesising – and thus respiration of stored compounds provides all metabolic energy and substrates.
Respiration control by temperature
In plants, respiration is believed to be controlled by the availability of respiratory substrates, cellular energy status, or total respiratory capacity (Klotz et al., 2008). Klotz et al. (2008) also note that respiration pathways of sugar beet are relatively simple, “since respiration occurs from a single substrate (sucrose), and occurs primarily by the linear progression of sucrolysis, glycolysis and the TCA cycle with limited participation by the oxidative pentose phosphate pathway”. Klotz et al. (2008) here cites Barbour and Wang (1961) – an early work on metabolism in sugar beets that went a long way in describing the major pathways.
With regard the role of temperature in controlling respiration, Vallarino and Osorio (2019, p217) states: “In general, in all plant tissues, an increase in temperature increases the rate of their metabolism, and hence the consumption of compounds such as sugars and stored TCA cycle acids that serve as metabolic substrates. Modification in organic acid metabolism in response to temperature probably results from the impact of temperature on the reaction rates of glycolysis, TCA cycle respiration, fermentation, and gluconeogenesis by modifying enzyme activities and kinetic properties of the transport systems involved.”
The broad general knowledge of respiration in plants and the relative simplicity of the sugar beet respiration pathways does not mean it is fully described, especially for sugar beets under various stress conditions. In what was one of the most comprehensive studies of respiration control in sugar beet up to this point, Klotz et al. (2008) were not able to show that any of the suspected factors were both regulating respiration and temperature dependent, at least not over a 13 day period with the two control temperatures of 1°C and 10°C. Indeed, an answer to the question of what was regulating this process remained allusive; “The lack of regulation of respiration by respiratory capacity, ADP availability or energy status suggests that a respiratory substrate other than ADP restricts respiration in stored sugarbeet roots. Possible substrates that may be limiting include molecular oxygen, NAD+ and reduced carbon compounds whose availability can be limited by a restriction in sucrose cleavage, glycolysis or the TCA cycle.” Other studies have focused on temperature, accumulated temperature, and disease stresses (Klotz and Finger, 2004), wounding stress (Klotz et al., 2006), and dehydration stresses (Lafta and Fugate, 2009; Lafta et al., 2020), yet none could conclude that they had described the mechanisms regulating respiration.
A subsequent study by the same group of researchers with collaborators focused on the above mentioned glycolysis. They were able to draw strong links between the availability of respiratory substrates, glycolysis and respiration rates in sugar beets (Megguer et al., 2017). Temperature was not a factor in this analysis, but glycolysis has been shown to be temperature dependent in other plants.1
Barbour, R. D. and C. H. Wang (1961). “Carbohydrate Metabolism of Sugar Beets I. Respiratory Catabolism of Mono and Disaccharides.” Journal of the American Society of Sugar Beet Technologists 11: 436-442.
Klotz, K. L. and F. L. Finger (2004). “Impact of temperature, length of storage and postharvest disease on sucrose catabolism in sugarbeet.” Postharvest Biology and Technology 34(1): 1-9.
Klotz, K. L., et al. (2006). “Wounding increases glycolytic but not soluble sucrolytic activities in stored sugarbeet root.” Postharvest Biology and Technology 41(1): 48-55.
Klotz, K. L., et al. (2008). “Respiration in postharvest sugarbeet roots is not limited by respiratory capacity or adenylates.” Journal of Plant Physiology 165: 1500-1520.
Lafta, A. M. and K. K. Fugate (2009). “Dehydration accelerates respiration in postharvest sugarbeet roots.” Postharvest Biology and Technology 54(1): 32-37.
Lafta, A. M., et al. (2020). “Dehydration during Storage affects Carbohydrate Metabolism and the Accumulation of Non-sucrose Carbohydrates in Postharvest Sugarbeet Roots.” Journal of Agriculture and Food Research.
Megguer, C. A., et al. (2017). “Glycolysis Is Dynamic and Relates Closely to Respiration Rate in Stored Sugarbeet Roots.” Frontiers in plant science 8(861).
Vallarino, J. and S. Osorio (2019). Organic Acids. Postharvest Physiology and Biochemistry of Fruits and Vegetables. E. M. Yahia and A. Carrillo-Lopez. Duxford, UK, Woodhead Publishing.
1 Direct extract from: English, W. (2020). Long Term Storage of Sugar Beets and the Role of Temperature. Introductory paper at the Faculty of Landscape Architecture, Horticulture and Crop Production Science. Alnarp, Sweden, Faculty of Landscape Architecture, Horticulture and Crop Production Science, Swedish University of Agricultural Science. 2020:14.