STRIGA HERMONTHICA REDUCES PHOTOSYNTHESIS IN SORGHUM - THE IMPORTANCEOF STOMATAL LIMITATIONS AND A POTENTIAL ROLE FOR ABA

We report the effects of the root hemiparasite Striga hermonthica (Del ,) Benth, on the growth and photosynthesis of two cultivars of sorghum : CSH-1, a susceptible variety, and Ochuti, which shows some tolerance to S, hermonthica in the field, Within 4 d of parasite attachment to the host roots, infected plants of both cultivars were significantly s horter than uninfected controls, At 55 d, infected plants of both cult ivars had significantly less shoot and root biomass, and significantly smaller leaf areas than uninfected controls, The dry weight of S, her monthica attached to host roots was insufficient at this stage to expl ain the decreased growth in terms of a competing sink for carbon and n itrogen, Leaf chlorophyll and nitrogen per unit area were greater in i nfected plants of both cultivars compared with control plants, However , whereas photosynthesis and transpiration in young leaves of infected CSH-1 plants declined with time when compared with controls, the rate s in infected Ochuti plants were similar to those in uninfected contro ls throughout the time course of observation, In both cultivars, a str ong correlation was observed between the rate of photosynthesis and st omatal conductance during photosynthetic induction, but infection resu lted in a much slower induction than in controls, In CSH-1 plants, bot h steady-state photosynthesis and stomatal conductance were lower than in controls, whereas in leaves of Ochuti steady-state photosynthesis and stomatal conductance eventually reached the same values as in the control leaves, Results from A/C-i analysis and also from determinatio n of C-13 isotope discrimination were consistent with a stomatal limit ation to photosynthesis in the leaves of Striga-infected plants, The c oncentration of the plant growth regulator abscisic acid (ABA) was mea sured in the xylem sap of infected CSH-1 plants only, and was found to be twice that of uninfected plants, A possible role of ABA in determi ning host response to infection by S, hermonthica is discussed.