Lolium multiflorum Lam. plants were used to investigate whether phytochrome-mediated tillering responses, anticipatory to competition: (a), are potentially disturbed by the twilight drop in the red to far-red ratio (R:FR) of sunlight (caused by atmospheric factors), and (b), occur in densely-sown (> 280 plants m−2) grass canopies, where competition is quickly established.
Isolated plants grown under sunlight received natural low R: FR during twilight, but supplementary R provided simultaneously did not increase tillering. When a wide range of R: FR was provided at the end of natural or fluorescent light photoperiods, tillering was reduced only by very low R: FR (lower than natural twilight R: FR). Leaf sheath length followed a similar pattern of response.
Single plants were grown in pots placed at various densities. High densities reduced the R:FR at plant bases and the number of tillers per plant before changes in dry weight and leaf number were found.
Both very low R:FR provided at the end of the photoperiod, and increasing plant densities, caused more erectophile shoots.
In densely-sown canopies of Lolium multiflorum both R:FR signals caused by neighbours, and tillering responses occur well before strong competition is established. Tillering is not obviously affected by the twilight drop in R: FR.