Selection for yield over five decades favored an isohydric and phenological adaptations to early-season drought in Australian wheat

Background and Aims: Climate change in the Mediterranean-climate region of Australia is reducing growing season rainfall and delaying first autumn rain or the onset of ‘autumn break’. We tested the hypothesis that selection for yield and agronomic traits has favored adaptation to early season drought in Australian wheat (Triticum aestivum L.). Methods: Ten wheat varieties released between 1958 and 2012 were grown in a glasshouse. After sowing in dry soil, the equivalent of 25 mm rainfall was supplied, with no subsequent watering provided for 32 days to induce an early season drought treatment (ESD) while a well-watered treatment (WW) was planted on a wet soil that was water-saturated 48 h before sowing. We measured soil and plant water status, gas exchange, shoot and root traits at the end of drought (32 days after sowing) and at anthesis, and grain yield per plant at maturity. Results: Grain yield increased with year of release at 0.43% yr^–1 under well-watered conditions and at 0.35% yr^–1 under drought. The improved yield under drought was associated with a shorter time to flowering, and a change from isohydric behavior (maintained Ψ_leaf, reduced g_s, leaf photosynthesis and transpiration rates in response to drought) in older varieties to anisohydric behavior (decreased Ψ_leaf and increased g_s, leaf photosynthesis and transpiration in response to drought) in newer varieties that reduced leaf area and maintained higher g_s, and higher photosynthesis per unit leaf area. Conclusions: Direct selection for yield and agronomic traits between 1958 and 2012 has improved adaptation to early-season drought. Our collection of varieties is an interesting model to probe for variation in drought tolerance.