Striga hermonthica reduces photosynthesis in sorghum: The importance of 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 shorter than uninfected controls, At 55 d, infected plants of both cultivars had significantly less shoot and root biomass, and significantly smaller leaf areas than uninfected controls, The dry weight of S, hermonthica attached to host roots was insufficient at this stage to explain the decreased growth in terms of a competing sink for carbon and nitrogen, Leaf chlorophyll and nitrogen per unit area were greater in infected 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 rates in infected Ochuti plants were similar to those in uninfected controls throughout the time course of observation, In both cultivars, a strong correlation was observed between the rate of photosynthesis and stomatal conductance during photosynthetic induction, but infection resulted in a much slower induction than in controls, In CSH-1 plants, both 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 determination of C-13 isotope discrimination were consistent with a stomatal limitation to photosynthesis in the leaves of Striga-infected plants, The concentration of the plant growth regulator abscisic acid (ABA) was measured 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 determining host response to infection by S, hermonthica is discussed.