Fig. 5

The response of chlorophyll fluorescence (see Table 1 for full descriptions) to stepwise changes in photosynthetic photon flux density (--- PPFD) in two modern bread wheat cultivars—T. aestivum ‘Paragon W07/08’ (black circle) and T. aestivum ‘Pavon76’ (inverted black triangle)—and landrace T. aestivum ‘CS94’ (white circle). After a dark adaptation period of 1 h, PPFD was increased to 500 µmol m⁻² s⁻¹ for 15 min. Subsequently, PPFD was decreased to 100 µmol m⁻² s⁻¹ for 10 min and then increased to 1000 µmol m⁻² s⁻¹ for 10 min. From measurements of maximal (F_m) and minimal (F_o) fluorescence the following parameters can be calculated: operating efficiency in the light (a—F_v’/F_m’), PSII quantum yield of PSII (b—F_q’/F_m’), maximum non-photochemical quenching (c—NPQ) and fraction of open PSII reaction centres (d—qL). Measurements were taken every minute and error bars indicate standard error (n = 4)