Sorghum is the fifth most important cereal crop world-wide and feeds millions of people in the Sahel. However, it often faces early-stage water deficit due to false onsets of rainy seasons resulting in production decrease. Therefore, developing early drought tolerant material becomes a necessity but requires a good knowledge of adaptation mechanisms, which remains to be elucidated. The present study aimed at assessing the effects of early drought stress on ten elite sorghum varieties tested over two years (2018–2019) at the National Agronomic Research Centre (CNRA) of Bambey (Senegal, West Africa). Two different water regimes (well-watered and drought stress) were applied during the dry season. Water stress was applied by withholding irrigation 25 days after sowing for one month, followed by optimal irrigation until maturity. Soil moisture measurements were performed and allowed to follow the level of stress (down to a fraction of transpirable soil water (FTSW) of 0.30 at the end of stress). An agro-physio-morphological monitoring was carried out during the experiment. Results showed highly significant effects of early drought stress in sorghum plants growth by decreasing leaf appearance, biomass, height but also yield set up. The combined analysis of variance revealed highly significant differences (p ≤ 0.01) between varieties in the different environments for most characters. Under water deficit, the variability was less strong on leaf appearance and plant height at the end of stress. The adaptation responses were related to the capacity of varieties to grow up fast and complete their cycle rather, increase the dead leaves weight, reduce photosynthesis rate, stomatal conductance, leaf transpiration and increase the roots length density. However, varieties V1, V2, V8 and V9 showed promising behavior under stress and could be suitable for further application in West Africa for sorghum breeding and farming.

Background: The benefits of leaf stay-green for maintaining grain filling in sorghum under drought was largely demonstrated. However, its role in the stability of a dual production (grain, stem sugar) in tall sweet sorghum remains to be elucidated. This study aimed to analyze the impact of a post-anthesis drought on sugar accumulation along stem internodes in sweet and tall West-African sorghum with variable leaf stay-green and grain yield abilities. Methods: Four accessions with similar phenology were studied in two consecutive years in the field at the National Agronomic Research Centre (CNRA) of Bambey (Senegal, West Africa) under two post-anthesis water treatments (irrigated, non-irrigated). Plant morphology, stem sugar related traits, grain production, and plant leaf area (PLA) variation were assessed. Carbohydrate contents (sucrose, hexoses, starch) were determined during grain filling in the whole stem juice and at three internode levels: bottom, median, top. Analysis of variance was performed to test post-anthesis water treatment, accession, organ, year effects and their interactions on the studied traits. Results: Panicle dry weight (PDW) was not affected by drought, but strongly varied among years and accessions. The PDW/PLA ratio was negatively correlated with the variation of sucrose and hexoses at the three internodes levels. This carbohydrates reduction was mainly influenced by the PDW. The bigger the panicle the higher the carbohydrates remobilization from the stem to panicle for grain filling. This was mainly shown on accessions G3 and G11 which exhibited low stay green ability. However, G10 with low PDW/PLA ratio and showing higher stay green ability, exhibited a low reduction of total soluble sugars and sucrose and inversely higher increase of hexoses mainly at the median internode. Conclusions: This ability to better maintain green leaf area and high hexoses in the stem under post-flowering drought could be an osmoregulation mechanism to adapt to drought. Therefore, stay-green is an important trait to consider for sweet sorghum breeding and particularly in the objective of developing dual purpose varieties in drought prone environments.

Understanding sorghum response to water stress at different developmental stages is important for developing sorghum varieties with improved tolerance to drought. This study set out to finely characterize key agro-physiological responses and water use of sorghum subjected to different drought scenarios. A greenhouse experiment was conducted using a DroughtSpotter facility that enables real-time quantification of water use by gravimetric tracking. Four different water treatments were assessed: (i) early vegetative drought stress (DS1), (ii) DS1 plus recovery (DS1R), (iii) late vegetative drought stress (DS2), and (iv) well-watered (WW). Plant pheno-morphology and yield data were recorded. Leaf fluorescence and photosynthesis were continuously recorded throughout the experiment. Our results showed that green leaf number and plant leaf area were more affected by DS2 than DS1 and delayed plant flowering. Nevertheless, plants in DS2 were taller and produced higher stem and leaf biomass compared to DS1. No significant difference was recorded in grain yield between DS1 and DS2 but were outperformed by DS1R and WW. The transpiration and photosynthetic rates were shown to decrease at the same time as the stomatal conductance. This can therefore be assimilated to a stomatal down-regulation limiting CO₂ uptake. However, the increase in intercellular CO₂ concentration is likely to indicate the presence of CO₂ in the substomatal cavity that was not conveyed to the carboxylation sites. This suggests a non-stomatal limitation of the photosynthesis. Moreover, the plants recovered quite well from DS1, and this was more prominent for physiological parameters than morphological ones. Globally, water use efficiency (WUE) for DS2 was higher compared to WW and DS1 treatments, confirming the growing point differentiation as a critical stage where drought stress should be avoided to ensure yield and better WUE. Adaptation responses were related to the reduction of transpiration through plant leaf area reduction, the reduction of stomatal conductance, and the increase of intercellular CO₂ limiting photosynthesis. Further studies focusing on the biomarkers of stress and transcriptomic analyses are needed to provide further insight into the drought adaptation mechanisms of this line.