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Sorghum genotypes vary in their response to higher sowing density, but the traits explaining these variations are unknown. In the present study, a 3D-imaging based approach identified the phenotypic traits responsible for the genetic variation in sorghum's response to high sowing density. Twenty sorghum genotypes, some varying in their response to density, were grown and 3D-images were collected weekly between weeks 4–6. From these images, 80 phenotypic traits, including 33 architectural and 47 multispectral, were extracted. The within-genotype means of these 80 traits, and two indicators of the sowing density response (Biomass ratio (Br) and Transpiration ratio (Tr)), measured in a previous study with 13 common genotypes, were used in a Spearman correlation analysis. Seventeen and four traits were strongly correlated with Br and Tr, respectively. The majority of these traits, predominantly architectural, strongly suggest that, under high sowing density, a fuller light interception, having more leaf area in the lower canopy, lead to a larger Br, while more vertically aligned leaves favour larger Tr values, which related to higher water use efficiency in another study. Furthermore, a Principal Component Analysis (PCA) indicated traits contributing to better photosynthesis could be used to estimate Br. Similarly, a combination of traits relating to leaf angle were good indicators of the genetic variation in Tr values. These results provide insights about the strategies some sorghum genotypes have developed to thrive under higher sowing density and that could be used as biomarkers for the breeding of density-resistant cultivars.

IntroductionPearlmillet is themain subsistence crop for smallholder farmers systemswhere it is grown at low plant density. Intensifying pearl millet cultivation could boost productivity although it may have trade-offs. Increasing planting density would indeed increase the leaf area and the related water budget, whereas a denser canopy could create a more favorable canopymicroclimate to the benefit of the water use efficiency (WUE) of the crops. The first aim of this work was to test the yield response of popular pearlmillet varieties to an increased density and to assess possible genotypic variation in this response. The second aim was to measure the water use and the WUE of the crop in different densities.MethodTo this end we designed several field and lysimetric experiments To increase the robustness of the results, these trials were carried out in India and Senegal, using two independent sets of genotypes adapted to both sites.ResultsIn the field, the higher sowing density significantly increased yield in all genotypes when trials were carried out in high evaporative demand conditions. There was no genotype x density interaction in these trials, suggesting no genotypic variation in the response to density increase. The high-density treatment also decreased the vapor pressure deficit (VPD) in the canopies, both in the field and in the lysimeter experiments. In the lysimeter trials, although the higher density treatment increased water use, the resulting increase in biomass was proportionally higher, hence increasingWUE of the crops in all genotypes under high density. The increase in yield under high density was closely related to the increase in WUE, although this link was more tight in the high- than in the low evaporative demand seasons. This confirmed a strong environmental effect on the response to density of all genotypes tested.DiscussionAlthough they did not open a scope for breeding density tolerant cultivars, these results highlight the possibility to improve pearl millet yield by increasing the density, targeting specifically areas facing high evaporative demand.