The Effects of Photoperiod and Mineral Nutrient Supply on Growth and Primordia Production at the Stem Apex of Barley Seedlings

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Abstract
Growth and development of the shoot apex in seedlings of three barley cultivars was examined in two daylengths (8, 16 h) and at two mineral nutrient levels (× 1, × 0.1). Production of primordia was greater at the higher nutrient level and in the longer days. The rate of production varied with cultivar but in all cases the plastochron shortened with transition to spike formation. Early flowering (cv. Clipper) was associated with a high rate of primordial production and early transition to spike formation, late flowering (cv. Proctor) with a low rate of production and a longer vegetative phase. The cultivar Akka showed intermediate characteristics. The volume of the apical dome increased linearly with increasing numbers of primordia, the rate of increase varying with cultivar and treatment. Enlargement of the dome was due mainly to increase in cell number. The transition of the apex to produce spikelet primordia occurred with widely differing volumes of the apical dome, thus invalidating the hypothesis that transition is dependent upon attainment of a critical dome size. Although both the rate of production of primordia and enlargement of the dome were markedly affected by photoperiod, both were unaffected when the photoperiodic treatment was given directly to the shoot apex. It is considered that the fate of a primordium once initiated is determined by competition for available metabolites between it, other primordia and the apical dome.