Yield improvement in temperate maize is attributable to greater stress tolerance

A retrospective analysis of the physiological basis of genetic yield improv ement may provide an understanding of yield potential and may indicate aven ues for future yield improvement. Rate of yield improvement of maize (Zea m ays L,) hybrids in Ontario, Canada has been approximate to 1.5% yr(-1) duri ng the last five decades. Comparison of short-season hybrids representing y ield improvement from the late 1950s to the late 1980s showed that genetic yield improvement was 2.5% per year and that most of the genetic yield impr ovement could be attributed to increased stress tolerance. Differences in s tress tolerance between older and more recent hybrids have been shown for h igh plant population density, weed interference, low night temperatures dur ing the grain-filling period, low soil moisture, low soil N, and a number o f herbicides. Yield improvement is the result of more efficient capture and use of resources, and the improved efficiency in resource capture and use of newer hybrids is frequently only evident under stress. Improved resource capture has resulted from increased interception of seasonal incident radi ation and greater uptake of nutrients and water. The improved resource capt ure is associated with increased leaf longevity, a more active root system, and a higher ratio of assimilate supply by the leaf canopy (source) and as similate demand by the grain (sink) during the grain-filling period. Improv ements of resource use under optimum conditions have been small, as leaf ph otosynthesis, leaf-angle distribution of the canopy, grain chemical composi tion, and the proportion of dry matter allocated to the grain at maturity ( i.e,, harvest index) have remained virtually constant. Genetic improvement of maize has been accompanied by a decrease in plant-to-plant variability. Results of our studies indicate that increased stress tolerance is associat ed with lower plant-to-plant variability and that increased plant-to-plant variability results in lower stress tolerance.