Developmental gradients in wheat leaves. Response of leaf segments in different genotypes cultured in vitro

Leaves of 21 different wheat genotypes (Triticum aestivum L. cultivars and Triticum species) were tested for response in tissue culture. Cultures could be established from all genotypes in the presence of 2,4-D in the nutrient medium, but only from immature basal leaf regions. Corresponding to the basipetal developmental gradient characteristic of the leaves, there appeared to be a gradient in sensitivity towards 2,4-D. The most basal, highly meristematic regions were comparatively sensitive. Callus growth could be initiated from cells which had apparently left the mitotic cell cycle and which were in the process of differentiation particularly when the 2,4-D concentration was increased to 30 mg l(-1) , a concentration so far considered as «unphysiological» in tissue culture. However, no evidence was found for any reaction in differentiated, mature cells. Most cultures derived from the immature leaf segments could not be induced to form shoots, but were capable of root regeneration. Plant regeneration could only be achieved in 6 genotypes and in these there again appeared to be a gradient in response along the leaves. The ratio of plant regeneration to root regeneration was highest in expiants taken from regions that were temporally and spatially closest to the apical meristem. The ratio dropped more or less drastically, depending on the genotype, when cultures were derived from slightly older explants. The implications of the differential responses in wheat leaves for theoretical and applied research is discussed.