Thermal time as a non-destructive method of estimating tuber initiation in potatoes

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Abstract
Water stress during the period when tubers are initiated may reduce the final number of tubers (MacKerron & Jefferies, 1986) and increase the incidence of common scab (Streptomyces scabies) in susceptible cultivars (Lapwood & Hering, 1970). It is important, therefore, for the maintenance of a large number of tubers and for the limitation of scab, to be able to estimate the time of tuber initiation. Tuber initiation has been defined differently by a number of workers (e.g. Sale, 1979; Sands, Hackett & Nix, 1979; O'Brien et al. 1983; MacKerron & Jefferies, 1986) based on either the number or proportion of stolon apices that have reached a certain size (usually twice the diameter of their stolons). These are definitions of tuber growth rather than development and require destructive sampling at frequent intervals for recognition of the event. Temperature has a major influence on the rates at which plant organs develop so that the time taken to reach a given developmental stage may be related to thermal time, defined as the time-integral of temperature (Jones, 1983). In potato the physiological age of tubers (Wurr, 1978; Allen et al. 1979; O'Brien et al. 1983), sprout development and extension (MacKerron, 1984), and the initiation of leaf primordia (Kirk, Davies & Marshall, 1985) have been related to thermal time. In a model of the development and bulking of potatoes Sands et al. (1979) used a single non-linear function to describe the effects of time and temperature on several development processes including tuber initiation. That function did not adequately describe the time taken to tuber initiation and Sands et al. (1979) did not then use tuber initiation as an important stage in their model.