The Growth and Development of the Wheat Apex: The Effects of Photoperiod on Spikelet Production and Sucrose Concentration in the Apex

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Abstract
Spring wheat (Triticum aestivum cv. Warimba) plants were grown in a controlled environment (20°C) in two photoperiods (8 or 16 h). In the first instance, plants were maintained in each of the photoperiods from germination onwards at the same irradiance (375 μE m−2 s−1). In the second case, all plants were grown in a long photoperiod until 4 days after double-ridge initiation when half the plants were transferred to a short photoperiod with double the irradiance (16 h photoperiod at 225 or 8 h at 475 μE −2 s−1). The rates of growth and development of the apices were promoted by the longer photoperiod in both experiments. Shoot dry weight gain was proportional to the total light energy received per day whereas the dry weight of the shoot apex increased with increasing photoperiod even when the total daily irradiance was constant. The principal soluble carbohydrate present in the shoot apex was sucrose, although low concentrations of glucose and fructose were found in the apices of long photoperiod plants late in development. Sucrose concentration was invariably greater in the slow-growing apices of short photoperiod plants, but rose to approach this level in the long photoperiod plants when the terminal spikelet had been initiated.