Photosynthetic decline in flag leaves of two field-grown spring wheat cultivars with different senescence properties

Photosynthetic characteristics were compared between two field-grown spring wheat (Triticum aestivum L.) cultivars, Ningmai 8 (NM8) and its half-father sibling Ningmai 9 (NM9), to investigate the differences in photosynthetic decline of flag leaves between these two cultivars with different senescent appearance after emergence through senescence. Maximum photosynthetic activities for these two cultivars were observed at around 10 days after emergence (DAE). Photosynthetic functionally, therefore, 10 DAE was taken to define the point at which leaf senescence was initiated. During the photosynthetic rapid decline occurring after 27 DAE, as compared with NM8, NM9 showed a significantly higher Chl content, a higher Chl a/b ratio, light-saturated photosynthetic rate (PCa = 360) and light- and CO2-saturated photosynthetic rate (Pmax), carboxylation efficiency (CE), but a significantly lower dark respiration rate (RD) and CO2 compensation point (Ccomp). Thylakoid membranes in NM9 also showed a slightly higher electron transport activity of PS II, but similar absorption spectra properties measured at room temperature as compared to NM8. The slower photosynthetic decline may contribute some to its higher grain yield in NM9. Due to the slower decrease of Chl, CE and PCa = 360, and the lower RD in NM9, flag leaf senescence was delayed in comparison to the earlier senescent NM8. Compared with its half-father sibling NM8, NM9 had a later onset and a slower rate of flag leaf senescence, which may be partly responsible for its higher grain yield.