Phenotypes and the onset of competition in spring barley stands of one genotype: daylength and density effects on tillering

Understanding the genotype–environment interactions requires a distinction between two effects of the environment on plants of a given genotype: the achievement of growth throughout their whole life and the prior determination of their growth potentials. Spring barley seeds from the same seed lot were sown at the end of the winter at three altitudes: 320 m a.s.l. (end of February), 880 m (beginning of April) and 1120 m (beginning of May), and at the end of May at the lowest site, in sparse and dense stands with and without nitrogen fertilization. This experimental design in natural conditions gave differences in initial daylengths (12, 14 and more than 15 h) that must modify the growth potentials, and different levels of competition, acting on growth achievement. The tillering patterns were measured on individual plants. The tillers in the axils of the first and the second leaf were lacking on many plants growing in the long day treatments in the sparse stands, and this increased up to between 70 and 90% in the dense stands. This seems to be due to far-red signals on seedlings emerging in the conditions which induce the floral development, before any competition can occur. Afterwards, the cessation of tillering as a result of the onset of competition, was delayed markedly during stem elongation in the stands established with long daylengths. Such phenotypes showed strongly reduced growth potentials. © 2000 Elsevier Science B.V. All rights reserved.