Carbon isotope ratios of mature leaves from the C3 angiosperm root hemiparasites Striga hermonthica (Del.) Benth (-26.7%o) and S. asiatica (L.) Kuntze (-25.6%o) were more negative than their C4 host, sorghum (Sorghum bicolor [L.] Moench cv CSHI), (-13.5%o Although environmental factors other than those affecting Ci may influence carbon isotope ratios, the magnitude of these is thought to be small (16), and experimental evidence is accruing to support the model (Eq. 1) (5,6,8,10,11,17,27).Striga hermonthica and Striga asiatica are root hemiparasitic angiosperms possessing the C3 pathway of carbon fixation. Their preferred host is the C4 plant, sorghum, from which they obtain a large proportion of their water and inorganic solutes. Although transfer of '4C-labeled metabolites has been demonstrated (15,25), the extent of the C flux is unknown. In Striga the carbon isotope composition will not only reflect its own photosynthetic and environmental characteristics, but also those ofthe host (23).In this paper we report the carbon isotope composition of the

Gas exchange characteristics are reported for both members of the sorghum-Striga host-parasite association. Both Striga hermonthica (Del.) Benth and Striga asiatica (L.) Kuntze had transpiration rates considerably in excess of those of sorghum (Sorghum bicolor (L.) Moench, cv CSH1). Stomatal conductance in both Striga spp. showed little response to periods of darkness and moderate water stress. Low rtes of net CO2 fixation and high rates of dark respiration led to no net daily (24 hours) C gan, and Striga would appear to be reliant on its host for photosynthate. Infection of sorghum plants with either S. hermoxthica or S. asiatica reduced host photosynthetic capacity. Infected sorghum plants were also more prone to water stress, but reduced rates of CO2 fixation could not be accounted for in terms of lower stomatal conductance. Lower stomatal conductances were associated with an increase in water use efficiency (WUE) in uninfected sorghum; however, Strigainfected sorghum plants had lower WUE than those of uninfected plants.We suggest that Striga exerts a specific effect on processes affecting C acquisition in sorghum leaves. The water relations of S. hennonthica and S. asiatica are not characteristic of plants growing in semiarid environments and are more likely to reflect the nature of the parasitic life-style. Despite transfer of water and solutes from host to parasite, the reduction in C fixation observed in infected sorghum plants appears to be the major determinant of growth reductions observed in sorghum supportng Striga Parasitic angiosperms rely on one or more neighboring plants (the host) either partially or totally for the acquisition of water, inorganic and organic solutes. Transfer is achieved via the haustorium, which may be located either above ground (e.g. green leafy mistletoes) or below ground (e.g. Striga) and links the Striga hermonthica and Striga asiatica (Scrophulariaceae) are parasitic on members of the Gramineae. They are the most serious cause of yield reduction in sorghum in the rainfed semiarid tropics (3). Their low photosynthetic activity compared to autotrophic Scrophulariaceae (12,19) coupled with the reduction in productivity of host plants (1), suggests a dependence on host photosynthate. In addition, Striga has only a vestigial root system, making independent uptake of water and inorganic solutes unlikely.In comparison with autotrophic Scrophulariaceae, Striga has a high Tn and poor stomatal control (19) and may have mechanisms in common with the mistletoes for the withdrawal of host transport fluids.In this paper we compare the C and H20 relations of sorghum and Striga, and consider: (a) the extent to which Striga is reliant on host photosynthate, (b) the effects of parasitism on host C acquisition, (c) the extent to which Striga employs a high Tn for the diversion of host water and solutes, and (d) the consequences of an additional water sink on the water status of the host. MATERIALS AND METHODSPlant Material and Growth Conditions. Sorghum (Sorghum bicolor (L.) M...

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CO assimilation in relation to light intensity and the relationship between leaf nitrogen and phosphorus concentrations and CO assimilation in 14 species of ecologically important Zimbabwean trees were examined. Eight of the species are members of the Fabaceae (Leguminosae). In the majority of Zimbabwean climax woodlands, the dominant trees are non-nodulating members of the sub-family Caesalpinioideae. The species examined have higher light saturation points (>700 μmol m s) than woody species from temperate areas; one species, Acacia nigrescens, did not reach saturation at photon fluxes greater than 1500 μmol m sec. Higher leaf nitrogen content was found to correlate positively with higher CO assimilation rates (r=0.85; P≦0.0003); there was no correlation between leaf phosphorus content and CO uptake rates. There were no significant differences between sites in terms of leaf nitrogen or phosphorus content, but the mean photosynthetic rate at one of the sites (Chizedzi) was lower. Taxa from the nodulating legumes were found to have higher leaf nitrogen contents (309.1±SD 22 mmol m) than those of the non-nodulating species (239±33); the lowest nitrogen contents were found in nonleguminous trees (179±42), with the exception of Ziziphus mucronata. This species may form an association with an N-fixing actinomycete.

The current study examined the variability in the pathogenicity of populations of Drechslera teres f. teres and D. teres f. maculata (the net and spot forms of D. teres) from Ireland and northern Europe. A population of progeny isolates from a mating of net and spot forms was also examined. Significant variation in virulence was found both between and among net form and spot form isolates (p<0.001). In the Irish population, significant differences were found between the net and spot forms, with the spot form isolates more virulent (p<0.05). Progeny isolates were significantly more virulent than net form or spot form populations (p<0.001). Significant differences were found in cultivar reactions, with cv. Botnia most susceptible to both forms of the pathogen (p<0.001). Cultivar Boreal 94145, although quantitatively resistant, was found to be very susceptible to both forms of the pathogen and to progeny isolates. Cultivars CI 5791, CI 2330 and CI 9819 were all less susceptible to infection by both forms, but were more susceptible to spot form isolates. Significant correlations were found between whole plants and detached leaf experiments for the net form isolates only (p<0.001). This study illustrates the importance of including both net form and spot form isolates in resistance studies and the need for a clearer understanding for the genetic basis of resistance to the net and spot forms. It also highlights the limitations of using a detached leaf assay for screening of net blotch of barley.