Limitation of kernel yield by the size of conducting tissue in winter wheat varieties

Abstract

Twenty-six winter wheat varieties of different earliness and stem height were cultivated in field trials during two growing seasons. At maturity, the number and dry weight of kernels on the main stem were measured. The size of translocating tissue was evaluated by measuring the number of vascular bundles and the cross-sectional area of vascular bundles and phloem of the peduncle about 2 cm below the ear. The number of vascular bundles was not correlated with the cross-sectional area of one bundle. A correlation coefficient of 0.64 was established for relationships between phloem cross-sectional area and total number of kernels per ear. There was a significant correlation between total kernel weight per ear and bundle (0.51) or phloem (0.58) cross-sectional area per peduncle. However, when the calculation was performed for those varieties that with a similar phloem area produced greatest kernel dry weight per ear, the correlation coefficient was 0.92. For those varieties there was extremely good agreement in the coefficients of regression of total kernel dry weight per ear (W e, g ear −1) on phloem cross-sectional area (A p, mm 2), namely 36 and 38 for 1985 and 1986, respectively. Calculated rates of translocation during the kernel-filling period varied from 3.93 to 12.3 g cm −2 h −1. The values are in keeping with other published data on rates of translocation in sieve tubes. It is concluded that under conditions favourable for both a high rate of assimilate production and high sink capacity, the number and size of vascular bundles, and especially the phloem cross-sectional area, limit wheat productivity.