Closed: Minimisation of sugar yield due to frost

Main Objectives

Freezing temperatures can damage sugar beet roots, often leading to losses in sugar and causing problems during processing. However, it is not clear what temperature regimes cause damage leading to losses, or why under other conditions at freezing temperatures the beet apparently escape damage. In addition, after roots have been obviously damaged by frost, it is not clear what subsequent conditions promote losses in sugar concentration, and how much of this sugar is lost to regrowth, translocation, respiration, dilution by tissue water, etc. The susceptibility to frost damage appears to be highly variable from field to field and even between plants within a field. Sources of variation may be different air temperatures and duration of low temperatures, acclimation due to prior cold exposure and stress history of the crop. It is known that in certain species, the process of 'cold-hardening' involves the accumulation of solutes such as glycine betaine and sucrose. Cold-tolerant species (e.g. strawberry) can allow tissue water to 'supercool' below 0°C in the absence of ice nucleation, delaying the formation of apoplastic ice crystals that can grow and eventually damage cells (Ref 2). Sugar beet probably employs all of these processes to some extent, but there are few data available. Varieties, too, may differ in their intrinsic biological capacity to tolerate cold temperatures , and because varieties are not pure genetic stocks, there may be important genetic differences between plants within a seed lot. Evidence of these varietal and individual plant effects was seen in the Broom's Barn Rust/Frost trial of 2010/11. There is evidence to suggest that the size of the top, or the amount of foliage covering the crowns may afford some protection against frost. Use of fungicides that promote the longevity of the canopy may therefore contribute to minimisation of frost damage.
The effect of cold temperatures on beet roots have been studied previously, and important information has been learned from this work. It is known that root tissues freeze at temperatures around -2.5 to -3°C. However, some of these experiments were done on topped roots placed in storage, not on intact plants in soil, and important questions remain. Comparing roots grown under different conditions, differences in the extent of damage and the apparent ability to recover has been shown (Ref 8). The freezing response in root tissue has been shown by measuring isotherms, or by visual inspection of the typical 'glassy' appearance. However, neither technique unequivocally demonstrates cell death or viability; flooding of intercellular spaces or the formation of apoplastic ice crystals do not necessarily imply that tissues are irreversibly damaged. Other assays, such as the use of vital stains and membrane leakage tests could contribute to diagnostics for membrane integrity and cell viability.