In crop studies, shading is a common approach for reducing source activity (i.e. assimilates produced by photosynthesis); however, if source reduction is not directly proportional to the degree of light attenuation, estimations could be flawed. We analysed the effects of shading at three levels on light transmittance, air and canopy temperatures, and actual photosynthetic activity at different times of day, and on grain weight in a modern genotype of bread wheat (Triticum aestivum L.). The treatments were imposed 10 days after anthesis and comprised three levels of shading with meshes: 80%, 50% and 10% transmittance during midday. The decrease in midday photosynthetic activity (electron transport rate, ETR; and CO₂ assimilation, An) in the shading treatments compared with control plants did not fit a 1 : 1 relationship with light transmittance of the meshes; for example, the reduction in ETR and An was <50% under the mesh with 50% light transmittance. The decrease in light transmittance and photosynthesis caused by the meshes was higher during the morning and afternoon than midday. In the grain-filling period, midday ETR showed a progressive, age-dependent decrease in control plants compared with shading treatments. Initial differences in photosynthetic activity between treatments were attenuated throughout the grain-filling period. The diurnal course of air temperature showed a progressive decrease from control to shade treatments, mainly during midday. This pattern was confirmed by infrared thermometry of canopy temperature, with severe shade showing the lowest temperature. Greenness measurements at 19 days after anthesis showed delayed senescence in shaded plants, possibly due to the decrease in temperature under shade. Grain weight per ear was reduced in shading treatments (mainly with 50% and 10% light transmittance). The decrease in grain weight per ear plotted against the decrease in photosynthesis (evaluated through ETR or An) did not fit a 1 : 1 relationship. This could be explained in part by an observed increase in dry matter retranslocation from the stem. In summary, estimations of source reduction on the basis of light attenuation could be biased, because the photosynthesis decrease is less than proportional, and photosynthesis differences are attenuated throughout grain filling.

在作物研究中，遮光是降低源活性（即光合作用产生的同化物）的一种常用方法。但是，如果光源的减少与光衰减的程度不成正比，则估计可能会出错。我们分析了三种水平的遮光对一天中不同时间的透光率，空气和冠层温度以及实际光合作用活性以及现代小麦基因型（Triticum aestivum L.）的籽粒重量的影响。花后10天进行处理，包括三个阶段的网格阴影：正午时的透射率分别为80％，50％和10％。午间光合活性的降低（电子传输速率，ETR；和CO ₂同化，An）与对照植物相比，在遮光处理中与网的透光率不符合1：1关系; 例如，ETR和An的减少在具有50％透光率的网孔下为＜50％。网格引起的透光率和光合作用的下降在上午和下午比中午更高。在灌浆期，与遮光处理相比，对照植物中的中午ETR显示出逐渐的，年龄依赖性的下降。在整个籽粒充实期，处理之间光合活性的初始差异被减弱。气温的昼夜过程显示，从控制到遮阴处理的逐渐减少，主要是在午间。通过冠层温度的红外测温法确认了这种模式，严重的阴影显示了最低的温度。花后19天的绿度测量结果表明，遮荫植物的衰老延迟了，这可能是由于遮荫下温度下降所致。在遮光处理中，每只耳朵的谷物重量减少了（主要是透光率分别为50％和10％）。每只耳朵的谷物重量减少与光合作用的减少作图（通过ETR或一）不符合1：1的关系。这部分可以通过观察到干物质从茎中重新转运的增加来部分解释。总而言之，基于光衰减的光源减少量估计值可能会产生偏差，因为光合作用的下降小于比例，并且整个谷物填充过程中光合作用的差异都会减弱。