Drought resistance and soil water extraction of a perennial C4 grass: contributions of root and rhizome traits

Previously, we showed that genotypic differences in soil water extraction were associated with drought response, but we did not study underground root and rhizome characteristics. In this study, we demonstrate a similar relationship between drought resistance and soil water extraction but investigate the role of underground organs. Eighteen bermudagrass genotypes (Cynodon spp.) from four climatic zones were assessed under continuous drought at two locations with contrasting soils and climates. The criterion for drought resistance was the duration required to reach 50% green cover (GC(50)) after water was withheld. GC(50), physiological traits, rhizome dry matter (RhDM), root length density (RLD) and average root diameter (ARD) were determined in both locations; water extraction was measured in one location. Large genotypic variation for drought resistance was observed in both locations, with GC(50) being 187-277 days in a clay soil and 15-27 days in a sandy soil. Drought-resistant genotypes had greater soil water extraction and a higher water uptake rate. GC(50) was correlated with relative water content (r = 0.76), canopy temperature differential (r = -0.94) and photosynthetic rate (r = 0.87) measured during drought; RhDM (r = 0.78 to ~0.93) before and after drought; and ARD after drought (r = 0.82 to ~0.94); GC(50) was not correlated with RLD. Ecotypes collected from the Australian Mediterranean zone had superior drought resistance and were characterised by a large rhizome network. This is the first comprehensive study with perennial C-4 grasses describing the association between water extraction, root distribution, rhizomes and drought resistance.