Changes in chlorophyll a fluorescence induction kinetics in cereals infested with Russian wheat aphid (Homoptera: Aphididae)

Plant entries that previously had tested resistant or susceptible to Russian wheat aphid, Diuraphis noxia (Mordvilko), were used to evaluate the effect of aphid feeding on leaf chlorophyll content and in vivo chlorophyll fluorescence induction kinetics. D. noxia feeding for 96 h caused significant reductions in chlorophyll a, chlorophyll b, and total chlorophyll content in susceptible wheat ('Pavon', 'TAM W-101') and barley ('Wintermalt'). Total chlorophyll content and constituent chlorophyll a and b levels were not significantly affected by D. noxia in resistant wheat (PI 366616, PI 372129) or barley (PI 366450) and in neither resistant (PI 386156) nor susceptible ('Beagle 82') triticale. D. noxia infestation resulted in significant alterations of the primary fluorescence induction transients. When compared with noninfected control plants, infested susceptible wheat and barley showed significantly increased nonvariable fluorescence (Fo) and decreased maximal fluorescence (Fm) and variable fluorescence (F,). In contrast, values for these parameters did not differ significantly among the infested and noninfested controls of resistant plants. When compared with control plants, the photochemical efficiency of photosystem II (Fv/Fm) and the half-rise time from F0 to Fm (t(1/2)) were reduced significantly in the infested susceptible entries but remained relatively unchanged for the infested resistant cultivars. No significant differences between infested and noninfected plants were observed in the measured chlorophyll fluorescence parameters for either the resistant or the susceptible triticale, which implied a much different resistance mechanism(s) to D. noxia. The results of this investigation suggest that D. noxia damage goes beyond the simple removal of photosynthates from the plant. The substantial decrease in Fv/Fm following aphid infestation for the susceptible wheat and barley indicated a significant decrease in the capacity and efficiency of the primary photochemistry of photosystem II.