Comparison of light- and temperature-based index models for potato (Solanum tuberosum L.) growth and development

Robust and simple methods of normalizing for the weather of particular growing seasons enable improved in-season decisions, such as supplemental fertilizer timing. Common methods involve expressing the progress of the season in terms of degree-days or cumulative incident solar radiation, in contrast to days after planting or emergence. This study compared a newly introduced temperature-derived variable, “relative thermal index” (RTI) based on a modified beta distribution model, with growing degree day (GDD), physiological day (Pday), and cumulative photosynthetically active radiation (PAR), for their ability in normalizing for seasonal differences in growth and development of well-managed commercial potato fields of cvs Russet Burbank and Russet Norkotah in humid, temperate central Wisconsin. The proposed RTI was superior to all other investigated variables in reducing the standard deviation of crop mass among field-years through full canopy development, after which PAR became the more powerful normalization. Values of 15, 27, and 30 RTI corresponded to tuber initiation onset, canopy full interception of light, and tuber-bulking onset, respectively, for both varieties; RTI values were more consistent across the two varieties for these stages than were other variables. Presumably RTI was superior to other thermal time indices because it more appropriately reflected the dependence of potato growth rate on relatively cool temperatures.