Sugar beet development under dynamic shade environments in temperate conditions

In Wallonia (Belgium) almost half of the arable land undergoes a four-year crop rotation. Winter crops often follow spring crops within the rotation scheme. This is a challenging context to implement silvoarable agroforestry (AF) systems, in terms of species choice, plot design, and tree management, since the periods of crop resource capture clearly differ. AF is defined here as the deliberate introduction of trees in the cropped area. Combining spring crops with trees induces an important overlap of the growing period of both plants which has an impact on one of the primary resources for both plants: light. In this study, we monitored an important spring crop for the region, sugar beet (Beta vulgaris L.). We quantified the impact of the shade environment on sugar beet morphology, growth dynamic, productivity and quality. We used artificial shade to isolate the impact of shade from other possible interactions in agroforestry systems. The field experiment was conducted over two consecutive years (2015 and 2016) on the experimental farm of Gembloux Agro-Bio Tech, Belgium. We installed the shade structures so as to reproduce a North-South and East-West tree line orientation. The experiment simulated canopy shade of late-flushing hybrid walnut by overlapping military camouflage netting. In 2015, the North-South orientation induced two distinct shade conditions: periodic shade (PS) and continuous shade (CS). In 2016, the East-West orientation created two periodic shade treatments, one during the morning (PSam) and one in the afternoon (PSpm). In both experimental years, shading was imposed from mid-June until harvest, resulting in 132 days of shade in 2015, and 140 days in 2016 on a growing season of 192 (2015) and 188 (2016) days in total. Sugar beet under shade tended to produce longer petioles. In 2015, at the first sampling date, we observed a higher specific leaf area and single leaf area under the CS and PS treatment, while there were no differences in 2016. All the shade treatments significantly changed the dry matter partitioning between the sugar beets compartments. Under the shade treatments, the quantity of biomass allocated to the leaves was significantly reduced as compared to the proportion of biomass for the petioles. Likewise, quantity of root dry matter formed per gram of shoot dry matter was reduced under shade. Thus, at harvest, all the shade treatments significantly reduced the final root dry matter and sugar yield. Furthermore, sugar beet quality, and more specifically sugar extractability, was affected by shading but to a lesser extent than for the final root dry matter and sugar yield.