Photosynthetic traits in Australian wheat varieties released between 1958 and 2007

We investigated crop and leaf photosynthetic traits in a set of Australian wheats released between 1958 and 2007. Environmental variation generated a range in pre-anthesis radiation use efficiency between 1.54 and 2.68gMJ−1. For these contrasting conditions, radiation use efficiency increased linearly with year of cultivar release at a rate of 0.012gMJ−1 per year.The fraction of incident radiation at different depths in the canopy profile increased linearly with year of cultivar release.

Leaf greenness increased with year of cultivar release but the magnitude of this effect varied in the canopy profile; SPAD increased 0.24unitsyr−1 between 0 and 0.2m, 0.44unitsyr−1 between 0.2 and 0.3m, 0.22unitsyr−1 between 0.3 and flag leaf, and 0.11unitsyr−1 for the flag leaf. There was a sharp extinction of nitrogen concentration with canopy depth relative to the extinction of radiation in older varieties that shifted to a flatter nitrogen-radiation extinction in newer varieties.

Increased radiation use efficiency in newer varieties was associated with the relaxation in the coupling between the extinction of nitrogen and radiation in the profile which was in turn partially related to the improved nitrogen status of modern varieties, as quantified with the nitrogen nutrition index.Stomata density (range: 38–56mm−2), stomata length (0.043–0.055μm), stomata width (0.024–0.032μm), light-saturated photosynthesis (9.3–19.6μmolm−2s−1) and respiration (1.6–7.3μmolm−2s−1) measured in the flag leaf at anthesis were unrelated to year of cultivar release.Shifts in crop-level traits associated with breeding and selection for yield were a likely source of improvement in pre-anthesis radiation use efficiency.