Wilting Point is An Overlooked Drought Tolerance Trait in Rice

The soil moisture tension (SMT) at wilting point is by convention estimated at 15 bar (pF 4.2) and is thought to vary little among species and crop varieties. However, as rice is particularly drought sensitive, its wilting point may differ from this value and show genetic diversity. We phenotyped the critical SMT at which transpiration ceases as a genotypic trait for the PRAY-indica diversity panel (300 accessions). A dry-down approach in pots was used during tillering stage in greenhouse and phytotron experiments. Soil evaporation was suppressed and weight loss attributed to plant transpiration alone. Once weight was constant and plants terminally stressed, gravimetric soil moisture content was measured and converted to SMT by pF curve. Both experiments had three replicates as randomized blocks. Observed critical SMT varied significantly (P<0.0001) between 3 and 5 bar among genotypes, translating into a 14% difference in water extracted between these extrema in the paddy soil used. Dryland-adapted cultivars extracted more water than flooding-adapted ones. Broad-sense heritability (H 2 ) was between 0.61 and 0.72. A genome-wide association study (GWAS) identified 6 quantitative trait loci (QTL), co-localizing with 4 functionally plausible candidate genes, including two root nitrate reductases. In conclusion, the conventional wilting point does not apply to indica rice, which is much more sensitive. This has consequences for the calculation of the fraction of transpirable soil water (FTSW) parameter used in crop models. The observed genotypic differences and QTLs indicate that this new trait may useful in for breeding for drought tolerance.