The potential of Sorghum landraces to overcome P and water limitation

Crop production in semi-arid regions is often affected by nutrient (N and P) and water deficiency. Hence, crop selection and cropping sequences are mainly influenced by the water supply during the rainy season, which underlies severe annual fluctuations. Under these conditions sorghum cultivation is common practice in smallholder farming systems due to its high potential to cope with water scarcity.

To examine the adaptation potential of sorghum (Sorghum bicolor L. Moench) to water and P stress, we measured transpiration and N uptake of five different sorghum lines (two Indian sorghum landraces, two African landraces and an Indian elite line) under the impact of organic (cowpea root residues) and mineral N-15 inputs. The plants were cultivated in either a P depleted (100 mg P kg^-1 soil) or P enriched (320 mg P kg^-1) Alfisol with a well-watered (WW) or water-stressed (WS) treatment. The experiment was carried out in the lysimetric phenotyping system (ICRISAT, India).

Cowpea labelling was carried out by injecting liquid N-15-label into the plant stem on a weekly basis over the growth period to ensure a homogeneous N-15 distribution in all parts of the plant. Mineral N-15-label was applied after soil saturation on the soil surface approximately two weeks after sorghum sowing to ensure no leakage of the tracer. The sorghum growth period was from middle of September 2018 till beginning of February 2019.

Under WW conditions, the sorghum lines showed different transpiration rates irrespectively of the P supply, whereas biomass and yield production was affected positively by P supply and organic residues. All sorghum lines had reduced transpiration rates, biomass and yield production under WS conditions. However, the African landraces were less susceptible to water stress than the Indian lines and could still produce yield and biomass. Furthermore, N delivery from cowpea residues could be proven in all treatments, while an efficient water supply had a positive impact on the N uptake from residues.

Overall an efficient P supply had only a positive influence on sorghum biomass and yield in interactions with a sufficient water supply or crop residues.

We can conclude that yield and biomass production of sorghum is not only dependent on transpiration rates. The potential to overcome water stress is enhanced for landraces and most properly belowground traits can explain the variation. Furthermore, we could demonstrate that all sorghum lines used biopores at both P levels to cover part of their N demand from cowpea residues. 

In this experiment, African landraces showed improved drought adaptation mechanisms compared to the bread elite line. Further soil and plant analysis will unravel the underlying traits such as improved mycorrhization, root morphology or nutrient uptake.