Maize yield is associated with abscisic acid and water potential under reduced soil water supply but with indoleacetic acid in genotypic renewal

Irrigation and breeding are important practices for improving yield and water use efficiency of maize (Zea mays L.) in arid regions. However, the physiological mechanisms of yield under varying water supplies and genotypes remain unclear. Here, we examine the different physiological mechanisms underlying maize yield responses to varying soil water supplies and three genotypes (MC670, ZD958, and ZD2#) cultivated in northwestern China over the past five decades. The declining water supply significantly reduced maize leaf hydraulic transport, stomatal conductance (g_s), net photosynthetic rate (A), yield, kernel number, biomass, and evapotranspiration (ET). Conversely, it led to an increase in abscisic acid (ABA), hydrogen peroxide, intrinsic water use efficiency, and water productivity. Interestingly, there was no significant impact on indoleacetic acid (IAA), thousand kernel weight, or harvest index (HI). Breeding efforts increased leaf IAA levels, biomass, thousand kernel weight, yield, HI, and water productivity without altering physiological traits or ET. The superior yield of MC670 could be attributed to a simultaneous enhancement in both kernel number and thousand kernel weight, while ZD958 exhibited greater yield stability. ABA and hydraulic traits (predawn leaf water potential, leaf water potential, and whole-plant hydraulic conductance) coordinated g_s under reduced soil water supply, while ABA and predawn leaf water potential regulated yield by modulating g_s to affect both A and ET. Breeding for yield gains was associated with IAA-induced enhancements in biomass and HI, independent of key physiological traits (e.g., g_s and A) and ET. The observed increase in water productivity primarily stemmed from notable yield improvements rather than alterations in ET. Hence, the selection of high-yielding genotypes under water-limited and well-watered conditions requires consideration of water-related physiological traits and IAA levels, respectively.