SUMMARYThe probability of the parasitic angiosperm Striga hermonthica achieving a successful union with its sorghum host is in part dependent on the concentration of ammonium nitrate in the growth medium. Germination rates were higher with 1 mM than with 3 mM ammonium nitrate. Ammonium nitrate did not influence either the stability of active components of host root exudate, or their perception by the parasite's seed. High concentrations of ammonium nitrate did not significantly reduce the leakage of potassium ions from sorghum roots (although patterns of exudation may differ between groups of compounds). Thus, it appears that ammonium nitrate reduced either the production of stimulatory compounds, or their specific leakage from host roots. Subsequent attachment of the parasite radicle and early growth of the plant were also lower at higher concentration of ammonium nitrate, and possible reasons for these observations are discussed.

The effects of nitrogen availability on growth and photosynthesis were followed in plants of sunflower (Helianthus annuus L., var. CATISSOL-01) grown in the greenhouse under natural photoperiod. The sunflower plants were grown in vermiculite under two contrasting nitrogen supply, with nitrogen supplied as ammonium nitrate. Higher nitrogen concentration resulted in higher shoot dry matter production per plant and the effect was apparent from 29 days after sowing (DAS). The difference in dry matter production was mainly attributed to the effect of nitrogen on leaf production and on individual leaf dry matter. The specific leaf weight (SLW) was not affected by the nitrogen supply. The photosynthetic CO 2 assimilation (A) of the target leaves was remarkably improved by high nitrogen nutrition. However, irrespective of nitrogen supply, the decline in photosynthetic CO 2 assimilation occurred before the end of leaf growth. Although nitrogen did not change significantly stomatal conductance (gs), high-N grown plants had lower intercellular CO 2 concentration (C i ) when compared with low-N grown plants. Transpiration rate (E) was increased in high-N grown plants only at the beginning of leaf growth. However, this not resulted in lower intrinsic water use efficiency (WUE).

In mature and young leaves of sunflower (Helianthus annuus L. cv. Catissol-01) plants grown in the greenhouse, photosynthetic rate, stomatal conductance, and transpiration rate declined during water stress independently of leaf age and recovered after 24-h rehydration. The intercellular CO 2 concentration, chlorophyll (Chl) content, and photochemical activity were not affected by water stress. However, non-photochemical quenching increased in mature stressed leaves. Rehydration recovered the levels of non-photochemical quenching and increased the F v /F m in young leaves. Drought did not alter the total Chl content. However, the accumulation of proline under drought was dependent on leaf age: higher content of proline was found in young leaves. After 24 h of rehydration the content of proline returned to the same contents as in control plants.Additional key words: chlorophyll; drought; intercellular CO 2 concentration; leaf water potential; non-photochemical quenching; photochemical activity; rehydration; stomatal conductance to water vapour; transpiration rate. ---The leaf water status and carbon uptake is under stomata control and stomata closure is one of the earliest responses to drought, resulting in protection of the plants against harmful dehydration but also inevitably results in reduction of CO 2 uptake for photosynthesis. The stomata control is important only under mild stress (Cornic 2000). However, metabolic inhibition of photosynthesis also takes place at mild water stress (Tezara et al. 1999) and it becomes more important as the water stress intensifies. Most of the studies in this area were done on mature leaves and the few available data comparing the effects of water stress on mature and young leaves indicate that the photosynthetic responses to water stress are strongly dependent on leaf age (David et al. 1998). In addition, there are indications that water stress accelerates leaf senescence (Olsson 1995).The accumulation of osmolyte compounds in the cells as a result of water stress is often associated with a possible mechanism to tolerate the harmful effect of water shortage. The contribution of sugars as an osmotic solute in expanded and partly expanded sunflower leaves was studied by Jones and Turner (1980). They found that contents of sugars did not change in fully expanded leaves. In opposition, the contents of soluble sugars in partly expanded leaves were reduced. In addition to sugars, some plants also accumulate other low molecular mass compounds, such as proline (Gzik 1996, Bajji et al. 2001. These osmolyte accumulations in plant cells might contribute, via lowering the cell osmotic potential, to maintaining several physiological processes, such as photosynthesis, stomatal conductance, and leaf expansion even under stressed conditions. The present experiment was designed to study the responses of photosynthesis to water stress in fully expanded and expanding leaves of sunflower plants. Additionally, the change in proline content and the ability of plants to recover from the water stres...

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The influence of Striga hermonthica on sorghum growth and photosynthesis is dependent on the provenance of the parasite seed. Inter-provenance differences are correlated with emergence time of the parasite above ground, as well as with the mass (but not number) of Striga hermonthica 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 nitrogen 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, nitrogen may influence provenances through processes which occur post-germination. Potential explanations for inter-provenance difference on these later processes are discussed. Influence de I'azote sur la croissance et la photosynthese du sorgho infectepar Striga hermonthica de differentes provenancesL'influence de Striga hermonthica sur la croissance et la photosynthdse du sorgho est dependante de la provenance du parasite. Les differences entre provenances sont correlees avec la date de levee du parasite ainsi que par la masse (mais pas le nombre) de plantes de Striga supportees par la plante hote. L'influence prepond^rante d'une fixation precoce sur la croissance et la photosynthese de la plante hote a ete montree en inoculant S. hermonthica a des plantes hotes agees de 3 et 19 jours. On observait des interactions significatives entre la fourniture d"azote et I'origine du parasite en ce qui concerne la relation entre (1) la biomasse de l'hote et la biomasse du parasite et (2) l'effet sur la photosynthfese 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 I'orgine du parasite par le biais de processus qui surviennent apres germination. Les explications possibles des differences entre provenances en ce qui concerne ces derniers processus sont dis-cut6es. Einflufi der StickstoffdUngung aufWachshim undPhotosynthese der Sorghum-Hirse hei Befall mit Striga hermonthica verschiedener Herktinft Die Beeinflussung des Wachstums und der Photosynthese der Sorghum-Hirse durch Striga hermonthica hangt von der Herkunft des Parasiten ab. Der unterschiedliche EinfluB ist sowohl mit dem Zeitpunkt des Austreibens des Parasiten als auch mit der Masse, jedoch nicht der Dichte der Striga-hermonthica-Pflanzen 290 /. Cechin and M. C Press korreliert, Der stSrkere Einflufi eines friihen Befalls der Wirtspflanze auf Wachstutn und Photosynthese wurde dadurch erfaBt, daB der Parasit sowohl auf 3 Tage als auch auf 19 Tage alte WirtspOanzen inokuliert wurde. Die Wechselwirkung zwischen der Stickstoffdungung und der Herkunft sowohl auf die Beziehung zwischen de...