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Effects of Shading on Winter Wheat Yield, Spike Characteristics, and Carbohydrate Allocation¹

Light intensity has variable effects on plant morphology, carbohydrate allocation, and yield. This study tested whether shading ‘Vona’ winter wheat (Triticum aestivum L.) altered allocation and spike characteristics that contribute to final grain yield under field conditions (Nunn clay loam, Aridic Argiustolls). The shaded treatment consisted of 50% ambient light reduction from 1 week before booting through the first week of grain fill (6 weeks total). Plants harvested were partitioned into stem, leaf blades by position on culm, and spike components. At maturity, spikes were divided into individual spikelets by position on the rachis. Carbon-14-labeled tracer was applied 24 h prior to each harvest. Shading significantly decreased spike weights for the later samplings, but had little effect on leaf or stem weights. Despite high spike sink activity and increasing spike size, when carbohydrates were limiting, photosynthate partitioning to spikes was not increased. Greater tiller mortality resulted in fewer culms per plant (and fewer spikes per plant) in shaded treatments. Shading altered spike morphology by decreasing the number and weight of kernels per spikelet and, therefore, total spikelet and spike weight; the number of grain-bearing spikelets was unaffected. Spikelets were decreased most at the lower central portion of the spike, contrasting with other studies that found decreases in the upper half or basal portion of the spike. The combined effects of shading resulted in 32% lower yield, due mostly to lower spike density, but also to decreased kernel number and size.

Key Words: Triticum aestivum L. • Partitioning • Tiller density • Carbon-14 labeling • Sink strength • Sink activity • Photoasimilate

² Ecologist, USDA-ARS, Natural Resource Ecology Lab., Colorado State Univ.; agronomist, USDA-ARS, Agricultural Engineering Res. Ctr., Colorado State Univ.; and soil scientist (retired), USDA-ARS, Plant Sciences Building, Colorado State Univ., Fort Collins, CO 80523, respectively.

Received for publication June 25, 1986.