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 susceptihle 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 hoth cultivars were significantly shorter than uninfected controls. At 55 d, infected plants of hoth cultivars had significantly less shoot and root hiomass, 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 hoth 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 hoth cultivars, a strong correlation was ohserved between the rate of photosynthesis and stomatal conductance during photosynthetic induction, hut 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-, analysis and also from determination of ^'C isotope discrimination were consistent with a stomatal limitation to photosynthesis in the leaves of Sfriga-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 he twice that of uninfected plants. A possihle role of ABA in determining host response to infection hy S. hermonthica is discussed.
SUMMARYBiophysical parameters controlling leaf expansion in hybrid poplar (Populus nigra L. x P. deltoides Bartr. ex Marsh = P. X euramericana) were measured for first-year cuttings growing under controlled conditions, with or without ozone fumigation. As individual leaves expanded, leaf water potential remained constant whilst solute potential declined and turgor (P) increased from approximately O' S MPa to over 1-0 MPa as leaf size increased. Yield turgor (Y) measured by stress-relaxation of leaf discs within psychrometers, increased from approximately 0-2 MPa to over 1-0 MPa during the same period. As the value of Y approached that of P, the effective turgor for grovrth (P-Y) declined and became zero as leaf growth ceased.Cell wall extensibility (measured as percentage plasticity by the ' Instron' technique) was related to leaf growth rate such that highest values of plasticity corresponded to maximum rates of growth. As growth ceased, wall extensibility declined to very low values.Fumigation with 50 nl 1"' ozone for 16 h d"^ had no effect on any of the measured parameters with the result that growth and mature leaf size were unaffected by comparison with control plants receiving charcoal-filtered air. Additional episodes of 120 nl ]"' ozone on three consecutive days during the period of maximum leaf growth similarly had no effect on expansion or final leaf size.
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