Fernando Miguez

Plant breeding progress is often based on assessments of genetic gain, a measure of year‐on‐year improvement of newly developed varieties for important traits of interest. Herein, we present data from the USDA‐coordinated hard winter wheat ( Triticum aestivum L.) Northern Regional Performance Nursery (NRPN) collected from 1959 to 2021 for grain yield, grain volume weight, days‐to‐heading, and plant height and use it to estimate absolute and relative rates of genetic gain (loss) of the control cultivar "Kharkof" for winter wheats adapted to the Northern Great Plains of North America. Regression analyses revealed significant relative grain yield increases of 0.67% Kharkof year −1 or 39.2 kg ha −1 year −1 for the NRPN over the 63‐year period. From 2008 to 2021, however, negative linear relationships were observed for relative grain yield for all NRPN entries and the five highest yielding NRPN lines each year, with genetic grain yield losses estimated at −0.95 and −0.79% Kharkof year −1 , respectively. Surprisingly, genetic progress for grain yield in the NRPN was limited to just 8.9 kg ha −1 year −1 over the past 13 years. Moreover, linear‐plateau models significantly ( p < 0.0001) fit all relative and absolute yield datasets from 1959 to 2021, with yield plateaus beginning by 2008 for all groups. These results support that winter wheats adapted to the Northern Great Plains have reached an upper yield plateau and could be nearing a yield potential ceiling due to global warming or other combined abiotic and biotic environmental stressors that are straining the biophysical limits of the crop.

Aging in perennial plants is traditionally observed in terms of changes in end-of-season biomass; however, the driving phenological and physiological changes are poorly understood. We found that 3-year-old (mature) stands of the perennial grass Miscanthus×giganteus had 19–30% lower Anet than 1-year-old M.×giganteus (juvenile) stands; 10–34% lower maximum carboxylation rates of Rubisco and 34% lower light-saturated Anet (Asat). These changes could be related to nitrogen (N) limitations, as mature plants were larger and had 14–34% lower leaf N on an area basis (Na) than juveniles. However, N fertilization restored Na to juvenile levels but compensated only 50% of the observed decline in leaf photosynthesis with age. Comparison of leaf photosynthesis per unit of leaf N (PNUE) showed that mature stands had at least 26% lower PNUE than juvenile stands across all N fertilization rates, suggesting that other factors, besides N, may be limiting photosynthesis in mature stands. We hypothesize that sink limitations in mature stands could be causing feedback inhibition of photosynthesis which is associated with the age-related decline in photosynthesis.