Effects of shading on morphology, physiology and grain yield of winter wheat

In a field experiment, winter wheat (Triticum aestivum L.) cultivars Yangmai 158 (YM 158, shading tolerant) and Yangmai 11 (YM 11, shading-sensitive) were subjected to shading between jointing and maturity. Three shading treatments were applied, i.e. 92% (S1), 85% (S2) and 77% (S3) of full radiation (S0, control). Compared with S0, the observed grain yield increased in the S1 and S2 treatments of YM 158 but not in S1 of YM 11. The yield loss of YM 11 was 2.3% and 6.7% in S2 and S3, respectively, and 5.9% in S3 of YM 158, which was much less than the corresponding reduction in radiation. Under the shading treatments applied, leaf area index, length of the peduncle internode, area of the upper leaves and content of pigments increased, which favoured efficient light capture. Shading modified light quality in the canopy as indicated by increases of diffuse- and blue light fractions and a reduction of the red light fraction. Shading also altered light-use efficiency as exemplified by reductions in the chlorophyll a/b ratio and the rate of non-photochemical quenching (NPQ), and by increases in the electron transport rate between PSII and PSI (ETR) and of the quantum yield of PSII (ΦPSII), concomitant with no significant change in the maximum photochemical efficiency of photosystem II under dark-adapted conditions (Fv/Fm). By contrast, photosynthetic carbon-use (Pn) in the flag leaf of both cultivars was reduced in the S3 treatment only. The lower leaves were found to be more tolerant to low radiation than the flag leaf, as in most cases Pn of the third and the penultimate leaves were found to increase under shading treatments. Shading increased the redistribution of dry matter from vegetative organs into grains. The responses of the morphological and physiological traits to shading are discussed in relation to the variations of the resulting grain yield in the contrasting wheat cultivars.