Effects of exposure to below-freezing temperatures, soil moisture content and nitrogen application on phyllochron in cool-season grasses

Accumulated temperature may provide an indicator of the phenology of cool-season grass to assist in the timing of management operations but its value is highly dependent on a reliable measure of phyllochron, i.e. time between the elongation of successive leaves. Field and controlled environment studies with Italian ryegrass (IRG, Lolium multiflorum Lam.) and tall fescue (TF, Festuca arundinacea Schreb.) measured leaf-appearance responses to accumulated temperature with varying conditions of exposure to low temperatures, soil moisture content and nitrogen application. In a controlled environment, soil volumetric water contents below 20% (equivalent to moisture potentials greater than -0.1 MPa) or more frequent exposure to below-freezing air temperatures increased the phyllochron values on the main tiller of IRG and TF. There was no evidence of an interaction between soil moisture content and cold exposure on phyllochron values. Nitrogen application resulted in only small reduction in phyllochron values. In field studies over 4 years the phyllochron values in IRG during the months of January to March were 184, 180, 180 and 167 accumulated degrees C above 0 degrees C leaf(-1), more than double the mean value measured in a controlled environment. A greater understanding of the impact of variable low temperatures and of soil moisture potential on the phyllochron is necessary before accumulated temperature can be used to indicate changes in development stages in grasses in different environments.