Predictability of wheat growth and yield in light-limited conditions

In seeking better predictions of grain yield under light-limited conditions, shading was applied to field-grown winter wheat cv. Slejpner during each of five consecutive phases (canopy expansion, ear expansion, pre-flowering, grain expansion and grain filling). Absolute measures were taken of solar radiation and its effects on growth in three seasons, at a site where water and nutrient supplies were not limiting. Replicate mobile shades automatically occluded 0·80 of incident light when mean total solar radiation exceeded 250 J/m 2 per s. Mean effects over seasons of shading on incident total solar radiation were −296, −139, −78, −157 and −357 MJ/m 2 for the five phases respectively, and corresponding effects on shoot dry weight were −236, −184, −58, −122 and −105 g/m 2 . Estimated efficiency of radiation use after flowering was 1·2 g/MJ unshaded, tending to increase with shading. Shading in all phases reduced grain dry matter yield: mean effects over seasons were −106, −64, −61, −93 and −281 g/m 2 for the five consecutive shading periods. Shading from GS31–39 increased mean maximum area of the two top leaves from 2700 to 3100 mm 2 per leaf but, with fewer stems, canopy size remained unaffected. This and the next shading, from GS39–55, reduced specific leaf weight from 42 g/m 2 by 4 and 3 g/m 2 respectively, but effects on shoot dry weight were largely due to stem and ear. By flowering, stem weights, and especially their reserves of water-soluble carbohydrates, had partially recovered. Effects on yield of shading from GS31–39 were explained by a reduction in grains/m 2 of 3070 from 26109. Shading from GS39–55 reduced grains/m 2 by 4211 due to fewer grains per ear, whilst mean weight per grain increased in compensation. Shading from GS55–61 decreased grains/ear by 2·5. Shading from GS61–71 decreased ear growth and reduced stem weight, and at harvest resulted in 4·3 less grains/ear. Effects of the final shading from GS71–87 were fully explained by a reduction in mean dry weight/grain of 10·3 mg. Except for shading from GS71–87, source- and sink-based explanations of grain yield both proved feasible, within the precision of the measurements. Constraints to accurate comparison of source- and sink-based approaches are identified, and the implications for yield forecasting are discussed.