ALBERT

Notes: The facultative hemiparasitic angiosperm Rhinanthus minor was grown on 11 different host species, and in the absence of a host. The height of R. minor parasitising the legume Trifolium pratense exceeded that of unattached plants by more than an order of magnitude, with performance on grasses and non-legume dicotlyedonous hosts between these two extremes. Light saturated rates of photosynthesis in R. minor on different hosts ranged from 1. 5 to 22. 5 μmol g−1 dry weight min−1, and were positively correlated with growth. Foliar nitrogen concentrations in the parasite exceeded those of the hosts. The former were positively related to light saturated rates of photosynthesis, and inversely related to photosynthetic nitrogen use efficiency. There was no relationship between leaf nitrogen concentration and water use efficiency in R. minor. The data are discussed in relation to studies of nitrogen and water use in mistletoes.

Notes: Long-term ecosystem-level experiments, in which the environment is manipulated in a controlled manner, are important tools to predict the responses of ecosystem functioning and composition to future global change. We present the results of a meta-analysis performed on the results of long-term ecosystem-level experiments near Toolik Lake, Alaska, and Abisko, Sweden. We quantified aboveground biomass responses of different arctic and subarctic ecosystems to experimental fertilization, warming and shading. We not only analysed the general patterns but also the differences in responsiveness between sites and regions. Aboveground plant biomass showed a broad similarity of responses in both locations, and also showed some important differences. In both locations, aboveground plant biomass, particularly the biomass of deciduous and graminoid plants, responded most strongly to nutrient addition. The biomass of mosses and lichens decreased in both locations as the biomass of vascular plants increased. An important difference between the two regions was the smaller positive aboveground biomass response of deciduous shrubs in Abisko as compared with Toolik Lake. Whereas in Toolik Lake Betula nana increased its dominance and replaced many of the other plant types, in Abisko all vascular plant types increased in abundance without major shifts in relative abundance. The differences between the responses of the dominant vegetation types of the Toolik Lake region, i.e. tussock tundra systems, and that of the Abisko region, i.e. heath systems, may have important implications for ecosystem development under expected patterns of global change. However, there were also large site-specific differences within each region. Several potential mechanistic explanations for the differences between sites and regions are discussed. The response patterns show the need for analyses of joint data sets from many regions and sites, in order to uncover common responses to changes in climate across large arctic regions from regional or local responses.

Notes: Summary Three cultivars of maize (TMV-1, Katumani and Staha) were grown in the absence and presence of the root hemiparasite Striga asiatica in the field in eastern Tanzania. Infested Katumani and TMV-1 plants yielded 22% and 25% less grain than uninfested plants; however, the grain yield of Staha was similar in both infested and uninfested plants. Measurements of gas exchange and the quantum yield of photosystem II (ΦPSII) revealed that photosynthesis and stomatal conductance were lower in infested plants of TMV-1 and Katumani than in control plants, but were not as severely affected in S. asiatica-infested plants of the cultivar Staha. Chlorophyll fluorescence measurements, particularly the Fv/Fm ratio, showed that S. asiatica predisposed TMV-1 to greater photoinhibition than control plants after exposure to irradiances of 2000 µmol quanta m−2 s−1 in the field during the day. The lowering of the Fv/Fm ratio resulted from a decrease in Fm rather than in Fo. In contrast to TMV-1, infested Staha plants were not more photoinhibited than control plants by mid-afternoon. We suggest that the ability of Staha to yield well in the presence of S. asiatica may result, in part, from the ability of this cultivar to maintain high rates of photosynthesis in the field, thus limiting the extent of photoinhibition.

Notes: The influence of Striga hermonthica on sorghum growth and photosynthesis is dependent on the provenance of the parasite seed. Inter-prove-nance differences are correlated with emergence time of the parasite above ground, as well as with the mass (but not number) of Striga her-monthica plants supported by the host. The greater effects of early attachment on host growth and photosynthesis were demonstrated by inoculating S. hermonthica plants to both 3-day-old and 19-day-old hosts. There were statistically significant interactions between ni-trogen supply and provenance on the relationship between (1) host biomass and parasite biomass and (2) photosynthetic impairment and parasite biomass. There was no statistically significant interaction between nitrogen supply and provenance on the relationship between host biomass and parasite emergence time. Thus, ni-trogen may influence provenances through processes which occur post-germination. Poten-tial explanations for inter-provenance difference on these later processes are discussed. Influence de l'azote sur la croissance et la photo-synthèse du sorgho infecté par Striga hermonthica de différentes provenances L'influence de Striga hermonthica sur la croissance et la photosynthèse du sorgho est dépendante de la provenance du parasite. Les différences entre provenances sont corrélées avec la date de levée du parasite ainsi que par la masse (mais pas le nombre) de plantes de Striga supportees par la plante hôte. L'influence prépondérante d'une fixation précoce sur la croissance et la photosynthèse de la plante hôte a été montrée en inoculant S. hermonthicaà des plantes hôtes âgées de 3 et 19 jours. On observait des interactions significatives entre la fourniture d'azote et l'origine du parasite en ce qui concerne la relation entre (1) la biomasse de l'hôte et la biomasse du parasite et (2) l'effet sur la photo-synthfese et la biomasse du parasie. L'interaction ne s'observait pas en ce qui concerne la relation entre la biomasse de l'hote et la date de levee du parasite. L'azote pourrait done interagir avec l'orgine du parasite par le biais de processus qui surviennent apres germination. Les explications possibles des différences entre provenances en ce qui concerne ces derniers processus sont discutées. Einfluß der Stickstoffdüngung auf Wachshim und Photosynthese der Sorghum-Hirse hei Befall mit Striga hermonthica verschiedener Herktinft Die Beeinflussung des Wachstums und der Photosynthese der Sorghum-Hirse durch Striga hermonthica hängt von der Herkunft des Parasiten ab. Der unterschiedliche Einfluß ist sowohl mit dem Zeitpunkt des Austreibens des Parasiten als auch mit der Masse, jedoch nicht der Dichte der Striga-hermonthica-Pflanzen korreliert, Der stärkere Einfluß eines frühen Befalls der Wirtspflanze auf Wachstutn und Photosynthese wurde dadurch erfaßt, daß der Parasit sowohl auf 3 Tage als auch auf 19 Tage alte Wirtspflanzen inokuliert wurde. Die Wechselwirkung zwischen der Stickstoffdüngung und der Herkunft sowohl auf die Beziehung zwischen der Biomasse der Wirtspflanze und der des Parasiten als auch auf die Beziehung zwischen der Photosyntheseleistung und der Biomasse des Parasiten war statistisch signifikant. Die Beziehung zwischen der Biomasse der Wirtspflanze und dem Zeitpunkt des Austreibens des Parasiten wurde durch die Stickstoffdüngung und die Herkunft nicht signifikant beeinflußt. Die Stickstoffdüngung beeinflußt also die Parasitierung mehr durch Prozesse, die nach dem Austreiben ablaufen, als die Keimung und die fruhen Entwick-lungsstadien. Mogliche Erklarungen für die Wirkungsunterschiede je nach Herkunft auf die letztgenannten Prozesse werden diskutiert.

Notes: Summary The genus Striga contains some of the most noxious parasitic plants, which have a devastating impact on cereal production in Africa; of most importance are Striga hermonthica and Striga asiatica. Complete resistance to infection by Striga species does not exist in cultivated cereals. Of great interest is the possibility that wild relatives of cereals may provide a genetic basis for resistance or tolerance to infection and may be of enormous value for the development of resistant crops. A wild relative of cultivated sorghum, Sorghum arundinaceum, demonstrated tolerance to infection by S. asiatica, with little impact of S. asiatica on host growth or grain production compared with the detrimental impact of the parasite on cultivated sorghum. Infection by S.hermonthica, however, had a significant influence on host performance for both wild and cultivated sorghum. Differences in host:parasite responses may be explained by the timing of parasite attachment and differences in host specificity for these two Striga species.

Notes: Decimation of cereal growth and yields by hemiparasitic Striga species cannot be accounted for entirely by the removal of host-plant resources. The production of toxic compounds by the parasite has been suggested. An investigation of three species of the economically important Striga and the related Alectra vogelii has now resulted in the isolation of eight iridoid glucosides (mussaenosidic acid, mussaenoside, gardoside methyl ester, bartsioside, isoaucubin, melittoside, aucubin and eurostoside), two caffeoyl phenylethyl glycosides (calceolarioside  A and verbascoside) as well as shikimic acid and trigonelline, all identified by NMR spectroscopy. The iridoids are potent cytotoxins and probably represent an anti-herbivore defence system common to Scrophulariaceae (sensu lato). This has the potential to explain differences in tolerance apparent for contrasting host taxa and cultivars. The nature of the iridoids present also provides additional validation of the recent transferral of parasitic Scrophulariaceae (s.l.) to Orobanchaceae.

Notes: The survival of dipterocarp seedlings in the understorey of south-east Asian rain forests is limited by their ability to maintain a positive carbon balance. Photosynthesis during sunflecks is an important component of carbon gain. Field measurements demonstrated that Shorea leprosula seedlings in a rain forest understorey received a high proportion of daily photon flux density at temperatures supra-optimal for photosynthesis (72% at ≥30 °C, 14% at ≥35 °C). To investigate the effect of high temperatures on photosynthesis during sunflecks, gas exchange and chlorophyll fluorescence measurements were made on seedlings grown in controlled environment conditions either, under uniform, saturating irradiance (approximately 539 µmol m−2 s−1) or, shade/fleck sequences (approximately 30 µmol m−2 s−1/approximately 525 µmol m−2 s−1) at two temperatures, 28 or 38 °C. The rate of light-saturated photosynthesis, under uniform irradiance, was inhibited by 40% at 38 °C compared with 28 °C. However, during the shade/fleck sequence, photosynthesis was inhibited by 59% at 38 °C compared with 28 °C. The greater inhibition of photosynthesis during the shade/fleck sequence, when compared with uniform irradiance, was driven by the lower efficiency of dynamic photosynthesis combined with lower steady-state rates of photosynthesis. These results suggest that, contrary to current dogma, sunfleck activity may not always result in significant carbon gain. This has important consequences for seedling regeneration processes in tropical forests as well as for leaves in other canopy positions where sunflecks make an important contribution to total photon flux density.

Notes: The survivorship of dipterocarp seedlings in the deeply shaded understorey of South-east Asian rain forests is limited by their ability to maintain a positive carbon balance. Photosynthesis during sunflecks is an important component of carbon gain. To investigate the effect of elevated CO2 upon photosynthesis and growth under sunflecks, seedlings of Shorealeprosula were grown in controlled environment conditions at ambient or elevated CO2. Equal total daily photon flux density (PFD) (∼7·7 mol m−2 d−1) was supplied as either uniform irradiance (∼170 µmol m−2 s−1) or shade/fleck sequences (∼30 µmol m−2 s−1/∼525 µmol m−2 s−1). Photosynthesis and growth were enhanced by elevated CO2 treatments but lower under flecked irradiance treatments. Acclimation of photosynthetic capacity occurred in response to elevated CO2 but not flecked irradiance. Importantly, the relative enhancement effects of elevated CO2 were greater under sunflecks (growth 60%, carbon gain 89%) compared with uniform irradiance (growth 25%, carbon gain 59%). This was driven by two factors: (1) greater efficiency of dynamic photosynthesis (photosynthetic induction gain and loss, post-irradiance gas exchange); and (2) photosynthetic enhancement being greatest at very low PFD. This allowed improved carbon gain during both clusters of lightflecks (73%) and intervening periods of deep shade (99%). The relatively greater enhancement of growth and photosynthesis at elevated CO2 under sunflecks has important potential consequences for seedling regeneration processes and hence forest structure and composition.

Notes: Orobanche species are holoparasites which are very efficient sinks for host-derived solutes. Here, we report the use of direct measurements of xylem sap solute concentrations and water fluxes, together with a modelling procedure to calculate element fluxes within an association between Orobanche cernua and its tobacco host. Infection of tobacco by the parasite markedly influenced carbon acquisition and partitioning; net fixation of carbon was 20% higher in infected tobacco compared with controls. Orobanche cernua caused a 84% increase in net carbon flux moving downward from the tobacco shoot and 73% of this carbon was intercepted by the parasite, almost entirely through the phloem (〉99%). Further, the parasite also exerted a large impact on the nitrogen relations of the plant, notably nitrate uptake was stimulated and the amino acid content of xylem sap was lower. The parasite also relied heavily on host phloem for the supply of other resources, with only 5 to 15% of N, and 16% of K, 23% of Na, 63% of Mg and 13% of S being derived from the xylem. Thus, we provide quantitative information on the phloem dependency of the parasite and show that host carbon and nitrogen metabolism is stimulated as a consequence of infection.