To arrive at a better understanding of variation in specific leaf mass (SLM, leaf weight per unit leaf area), we investigated the chemical composition and anatomical structure of the leaves of 14 grass species varying in potential relative growth rate. Expressed on a dry weight basis, the fast‐growing grass species with low SLM contained relatively more minerals and organic N‐compounds, whereas slow‐growing species with high SLM contained more (hemi)cellulose and lignin. However, when expressed per unit leaf area, organic N‐compounds, (hemi)cellulose, total structural carbohydrates and organic acids increased with increasing SLM. For the 14 grasses, no trend with SLM was found for the leaf volume per unit leaf area. Leaf density was positively correlated with SLM. Variation in density was not caused by variation in the proportion of intercellular spaces. The proportion of the total volume occupied by mesophyll and veins did not differ either. A high SLM was caused, at least partly, by a high proportion of non‐veinal sclerenchymatic cells per cross‐section. The epidermal cell area was negatively correlated with SLM. We conclude that the differences in SLM and in the relative growth rate (RGR) between fast‐ and slow‐growing grass species are based partly on variation in anatomical differentiation and partly on chemical differences within cell types.

Previous experiments have shown that the anatomy and chemical composition of leaves of inherently fast‐ and slow‐growing grass species, grown at non‐limiting nitrogen supply, differ systematically. The present experiment was carried out to investigate whether these differences persist when the plants are grown at an intermediate or a very low nitrogen supply. To this end, the inherently fast‐growing Poa annua L. and Poa trivialis L., and the inherently slow‐growing Poa compressa L. and Poa pratensis (L.) Schreb. were grown hydroponically at three levels of nitrate supply: at optimum (RGRmax) and at relative addition rates of 100 and 50 mmol N (mol N)−1 d−1 (RAR100 and RAR50), respectively. As expected, at the lowest N supply, the potentially fast‐growing species grew at the same rate as the inherently slow‐growing ones. Similarly, the differences in leaf area ratio (LAR, leaf area:total dry mass), specific leaf area (SLA, leaf arear:leaf dry mass) and leaf mass ratio (LMR, leaf dry mass:total dry mass) disappeared. Under optimal conditions, the fast‐growing species differed from the slow‐growing ones in that they had a higher N concentration. There were no significant differences in C concentration. With decreasing N supply, the total N concentration decreased and the differences between the species disappeared. The total C concentration increased for the fast‐growing species and decreased for the slow‐growing ones, i.e. the small, but insignificant, difference in C concentration between the species at RGRmax increased with decreasing N supply. The chemical composition of the leaves at low N supply, analysed in more detail by pyrolysis–mass spectrometry, showed an increase in the relative amounts of guaiacyl lignin, cellulose and hemicellulose, whereas those of syringyl lignin and protein decreased. The anatomy and morphology of the leaves of the four grass species differing in RGRmax were analysed by image‐processing analysis. The proportion of the total volume occupied by mesophyll plus intercellular spaces and epidermis did not correlate with the amount of leaf mass per unit leaf area (specific leaf mass, SLM) at different N supply. The higher SLM at low N supply was caused partly by a high proportion of non‐veinal sclerenchymatic cells per cross‐section and partly by the smaller volume of epidermal cells. We conclude that the decrease in relative growth rate (and increase in SLM) at decreasing N supply is partly due to chemical and anatomical changes. The differences between the fast‐ and slow‐growing grass species at an optimum nutrient supply diminished when plants were growing at a limiting nitrogen supply.

Under iron-limited conditions, Pseudomonas putida WCS358 produces a siderophore, pseudobactin 358, which is essential for the plant growth-stimulating abiity of this strain. Cells of strain WCS358, provided that they have been grown under Fe3+ limitation, take up 55Fe3+ from the 5sFe3 -labeled pseudobactin 358 complex with Km and V,, values of 0.23 ,uM and 0.14 nmol/mg of cell dry weight per min, respectively. Uptake experiments with cells treated with variods metabolic inhibitors showed that this Fe3+ uptake process was dependent on the proton motive force. Furthermore, strain WCS358 was shown to be able to take up Fe3+ complexed to the siderophore of another plant-beneficial P. fluorescens strain, WCS374. The tested pathogenic rhizobacteria and rhizofungi were neither able to grow on Fe3+-defitcient medium in the presence of pseudobactin 358 nor able to take up "5Fe3+ from 55Fe3 -pseudobactin 358. The same applies for three cyanide-producing Pseudomonas strains which are supposed to be representatives of the minor pathogens. These results indicate that the extraordinary ability of strain WCS358 to compete efficiently for Fe3+ is based on the fact that the pathogenic and deleterious rhizosphere microorganisms, in contrast to strain WCS358 itself, are not able to take up Fe3+ from Fe3+-pseudobactin 358 complexes.Frequent cultivation of monocultures on the sarne field is a practical demand of modern agriculture. However, frequent cultivation of, e.g;, potato in the same field results in yield decreases of up to 30% (11,12,22). The causal agents of these yield decreases are assumed to be deleterious, cyanide-producing Pseudomonas spp. (1, 23). The rhizosphere also harbors various pathogenic microorganisms which influence the potato yield, e.g., bacteria like Erwinia carotovora, which can cause rotting of the potato tubers, and fungi like Verticillium spp., which may cause wilting of the potato plants (12,23).Bacterization of seed potatoes with certain fluorescent Pseudomonas spp. has a beneficial effect on potato yield (2, 11). These plant-beneficial Pseudomonas strains have been selected after screening of large numbers of fluorescent, root-colonizing Pseudomonas spp. on antibiosis activity against a series of rhizosphere microorganisms (10). For some Pseudomonas spp. this antibiosis activity is primarily based on the production of antibiotic compounds (5), while for other Pseudomonas spp., like Pseudomonas putida WCS358, antibiosis is based on successful competition for Fe3" by strain WCS358 in comparison with that by the pathogenic or deleterious microorganisms (9,22,23). Under Fe3" limitation, the beneficial Pseudomonas cells produce powerful fluorescent siderophores (7,17,18,25), Fe3+-chelating compounds, which are part of high-affinity Fe3+ uptake systems. Recently, it has been demonstrated that these beneficial Pseudomonas strains actually produce these siderophores in the rhizosphere (3,23). Also the ability of the beneficial Pseudomonas strain to produce siderophores was shown to be a prerequisite for the...

Microbial growth in the rhizosphere is affected by the release of organic material from roots, so differences in carbon budgets between plants may affect their rhizosphere biology. This was tested by sampling populations of bacteria and bacteriophagous fauna from the rhizosphere of Lolium perenne, Festuca arundinacea, Poa annua, and Poa pratensis, under conditions of high and low nitrate availability. Concentrations of soluble phenolics and lignin varied considerably between the species but were not related to differences in rhizosphere biology. L. perenne and F. arundinacea supported fewer bacteria than the Poa species. There was no significant rhizosphere effect on the groups of protozoa. The major indicators of rhizosphere productivity were the bacterial-feeding nematodes (mainly Acrobeloides spp.), and there was a large positive effect of added nitrate. Nematode biomass was significantly lower in the rhizosphere of the slow-growing P. pratensis compared with the fast-growing P. annua, indicating that the differential allocation of carbon has affects on rhizosphere biology. A large rhizosphere effect on enchytraeid worms was also observed, and their potential importance in the rhizosphere is discussed.

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