Photothermal time (PTT) is a product between growing degree-days (GDD) and day length (hours) for each day. PTT = GDD × DL[1] It can be used to quantify environment,[2] as well as the timing of developmental stages of plants.[3]
References
^.Ravi Kumar, S.; Hammer, Graeme L.; Broad, Ian; Harland, Peter; McLean, Greg (March 2009). "Modelling environmental effects on phenology and canopy development of diverse sorghum genotypes". Field Crops Research. 111 (1–2): 157–165. doi:10.1016/j.fcr.2008.11.010. ISSN0378-4290.