Changes in the non-photochemical quenching of chlorophyll fluorescence during aging of wheat flag leaves

Non-photochemical quenching of chlorophyll fluorescence (q(N)) and its thre e components (q(Nf), q(Nm), and q(Ns)) in the flag leaves of wheat grown in the field were studied by a fluorometer PAM-2000 on clear days. The diurna l variation patterns of q(N) in just fully extended (JFEL) and aging leaves (AL) were similar, but q(N), declined markedly in JFEL while it remained a t a relatively high level in AL under strong sunlight at noon. Furthermore, at midday q(Nf) was higher than q(Ns) in JFEL, but much lower in AL. The r esults show the relative contributions of different mechanisms in:preventin g the photosynthetic apparatus from photodamage change during leaf developm ent.