Responses of Leaf Expansion, Plant Transpiration and Leaf Senescence of Different Soybean (Glycine max. (L.) Merr.) Genotypes to Soil Water Deficit

The responses of eco-physiological processes such as leaf expansion, plant transpiration and senescence to soil water deficit have been reported to be genotype-dependent in different crops. To study such responses in soybean (Glycine max. (L.) Merr.), a 2-year (2017 and 2021) outdoor pot experiment was carried out on the Heliaphen automated phenotyping platform at INRAE in Toulouse (France). Six soybean cultivars (Sultana-MG 000, ES Pallador-MG I, Isidor-MG I, Santana-MG I/II, Blancas-MG II and Ecudor-MG II) belonging to four maturity groups (MG) commonly grown in Europe were subjected to progressive soil water deficit from the reproductive stage R1 for 17 and 23 days in 2017 and 2021, respectively. The fraction of transpirable soil water (FTSW) was used as an indicator of soil water deficit. Non-linear regression was used to calculate FTSWt, that is, the FTSW threshold for which the rate of the eco-physiological process in stressed plants starts to diverge from a reference value. According to FTSWt, the three eco-physiological processes showed significant differences in sensitivity to water deficit: leaf expansion exhibits the highest sensitivity and the widest range (FTSWt: 0.44-0.93), followed by plant transpiration (FTSWt: 0.17-0.56), with leaf senescence showing the narrowest range (FTSWt: 0.05-0.16). Among six cultivars, regarding leaf expansion, Cvs Santana (FTSWt = 0.48 in 2017; FTSWt = 0.44 in 2021), Blancas (FTSWt = 0.51 in 2017; FTSWt = 0.48 in 2021) and Ecudor (FTSWt = 0.46 in 2017; FTSWt = 0.52 in 2021) in late MGs (I/II to II) exhibited higher tolerance to soil drying. Conversely, the cv. Sultana in the earliest MG (000) showed the highest sensitivity (FTSWt = 0.91 in 2017; FTSWt = 0.93 in 2021) to water deficit. However, concerning the FTSWt values for plant transpiration (0.17-0.56 in 2017; 0.19-0.31 in 2021) and senescence (0.05-0.16 in 2017; 0.06-0.16 in 2021), their range did not demonstrate a correlated trend with the MG. In addition, a negative linear correlation was observed between values of FTSWt of normalised leaf expansion at the whole-plant level (NLE) and specific leaf area (SLA) measured on irrigated plants for both years. This suggests that genotypes with high values of SLA could be associated with higher tolerance of leaf expansion to soil water deficit. Such a non-destructive phenotyping method under outdoor conditions could bring new information to variety testing process and provide paths for integrating genotypic variability into crop growth models used for simulating soybean eco-physiological responses to water deficit across the plant, field and even regional scales.