The Effect of Grain Number per Ear (Sink Size) on Source Activity and its Water-Relations in Wheat

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Abstract
Blum, A., Mayer, J. and Golan, G. 1988. The effect of grain number per ear (sink size) on source activity and its water-relations in wheat.–J. exp. Bot. 39: 106–114. Work was done to evaluate the nature of sink-source relationships in wheat (Triticum aestivum L.), when the strength of the sink was modified by the removal of half of the grain from the ear at about anthesis. The main hypothesis was that sink-source relationship would be modified by water stress and that a weaker sink would improve the drought resistance of the source. Two experiments were performed. The first experiment evaluated the effect of de-graining in two wheat varieties grown in the field. The second experiment (in the greenhouse) evaluated the effect of de-graining in plants subjected to water stress after anthesis by immersing the root system in a solution of polyethylene glycol (6000), as compared with non-stressed controls. In both experiments measurements were performed after de-graining to provide data on leaf gas exchange, leaf water potential, osmotic adjustment of leaves and ears (greenhouse), the percent of stem weight loss as an index of stem reserve mobilization, final root weight (greenhouse) and ear weight components. De-graining caused a decrease in flag leaf stomatal conductance, carbon exchange rate (CER) and transpiration and an increase in flag leaf water potential. These effects were stronger with water stress. De-graining did not affect osmotic adjustment in the flag leaf but induced better adjustment in glumes and awns. De-graining decreased the percent of stem weight loss and increased final root weight, especially under drought stress. A weaker sink was, therefore, considered to improve plant drought resistance in terms of the maintenance of higher leaf water potential, a larger root, a better osmotic adjustment in the ear and, possibly, increased flag leaf longevity. The ‘cost’ of this improved drought resistance was in reduced flag leaf CER and reduced stem (and root?) reserve mobilization.