Quantification of photoperiodic effects on growth of Phleum pratense.

Background And Aims: Accurate quantifications of plant responses to photoperiod are useful for physiological studies, in growth modelling and in other studies of environmental effects. The objective of the current work was a mathematical description of photoperiodic influence on plant morphological traits, using functions with few and common parameters related to key plant characteristics and typical response patterns.

Methods: Two latitudinal cultivars of timothy (Phleum pratense) were studied in a climate chamber experiment at 9, 12, 15, 18, 21 and 24 h photoperiods. Seedling growth was recorded by measurements of main tiller leaf tip heights every other day from the 5-6 leaf stage onwards, and as plant size and dry weight at days 37, 46, 62 and 70, i.e. at the end of experiment. The plant responses to photoperiod were described by the term PPR = (eci(PP-PPc))/(1 + e(ci+di)(PP-PPc)), where PP = photoperiod in h, PPc = photoperiod of maximum response, c = characteristic coefficient of main response interval, d = sensitivity coefficient characterizing course of function beyond the main response interval. PPR was tested on experimental data for different growth characteristics (i), e.g. size of individual leaves (Yi), identified by their sequential numbers on the main tiller (LN) using the function: Yi = Ybi + aiLN + biLNalphai (PRR). The growth course was described by the same function, replacing LN with day number of treatment exposure.

Key Results And Conclusions: The functions described with high precision (r2 > 0.97) the effect of photoperiod on growth as expressed by several plant characteristics, such as leaf area development, top and root DM production, as well as cultivar differences. Green leaf area was more sensitive to photoperiod than above-ground DM production. The southern cultivar 'Grindstad' was more sensitive than the northern one 'Engmo'. The functional relationships suggest mechanisms for plants' daylength responses and latitudinal adaptation.