Is Mutual Shading a Decisive Factor for Differences in Overall Canopy Specific Leaf Area of Winter Wheat Crops?.

In crop growth simulators, the daily increase of leaf area is often derived from the product of leaf mass increase and specific leaf area (SLA), while the SLA of already formed and expanded leaves is assumed as conserved. The SLA is influenced by many factors and remarkable efforts have been made to isolate the most important ones, aiming to obtain a robust empirical prediction. For a single leaf, SLA is negatively correlated to light intensity. Thus, mutual shading was discussed as a key factor for overall canopy (cSLA) modelling. Our aim was to investigate whether cSLA is primarily a function of growth conditions or light environment. Therefore, winter wheat (Triticum aestivum L. variety Dekan) was grown under different irrigation and nitrogen fertilization (N) treatments. Across all treatments, cSLA was positively correlated to LAI with an increase of approximately 15.8 (cm2/g) per LAI unit, but the relation was only significant for the phase after one node stage. The observed interaction between cSLA and LAI did not differ significantly between years and treatments. This impact of LAI was confirmed by an additional, independent and previously published dataset including different N treatments. We also analysed the influence of drought stress on SLA distribution within the canopy. Independently from the presence or absence of drought, the SLA increased from the top to the lower leaf-layers. Drought mainly reduced leaf area (L) in the higher leaf-layers, whereas SLA was reduced especially in the lower leaf-layers and the SLA of the flag leaves was not affected. Therefore, the lower cSLA under drought stress seems to be primarily an effect of differences in light environment within the canopy.