Yield improvement in temperate maize is attributable to greater stress tolerance

A retrospective analysis of the physiological basis of genetic yield improvement may provide an understanding of yield potential and may indicate avenues for future yield improvement. Rate of yield improvement of maize (Zea mays L,) hybrids in Ontario, Canada has been approximate to 1.5% yr(-1) during the last five decades. Comparison of short-season hybrids representing yield 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 improvement could be attributed to increased stress tolerance. Differences in stress tolerance between older and more recent hybrids have been shown for high plant population density, weed interference, low night temperatures during the grain-filling period, low soil moisture, low soil N, and a number of 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 radiation and greater uptake of nutrients and water. The improved resource capture is associated with increased leaf longevity, a more active root system, and a higher ratio of assimilate supply by the leaf canopy (source) and assimilate demand by the grain (sink) during the grain-filling period. Improvements of resource use under optimum conditions have been small, as leaf photosynthesis, leaf-angle distribution of the canopy, grain chemical composition, 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 associated with lower plant-to-plant variability and that increased plant-to-plant variability results in lower stress tolerance.