SUMMARYThe competitive ability of a tetraploid, and its morphologically indistinguishable progenitor diploid Dactylis glomerata L., from Galicia (Spain), was tested in a two-year study, using isolated plants and the same individuals grown at two densities (16 and 36 plants per pot, respectively). Inverse linear regression equations were used to quantify the response of the different ploidy levels (hereafter cytotypes) to density. For the diploids, inter-cytotype competition coefficients were significantly higher than intra-cytotype coefficients. The substitution rate mcreased from 1-8 m the first year to 3-9 m the second year, when most of the diploids failed to flower and some died. The competitive superiority of the tetraploids was associated with higher mean tiller weight, particularly in the mixed cultures. The tetraploids also had heavier seeds and faster leaf production in early spring, and they flowered earlier than the diploids. Several of these differences between cytotypes appear to be due to differences in ploidy level and may play a major role in the establishment of the tetraploids in Galician pastures, where competition is high.

Diurnal variation in ion content of the solution bathing roots of two plants growing together in sand culture was analysed for three pairs of grass-legume species (Lolium multiflorum and Trifolium pratense; Zea mays and Glycine hispida; Avena sativa and Vicia sativa) and their monospecific controls. Biomass and nitrogen content of plants were determined.Ion concentration (NO~-, NO{, NH~, and K § and pH of root solutions were measured for LoliumTrifolium plant pairs and controls at 6 hours intervals over 36 h, starting at 8 am within a circadian cycle. Root solutions were regularly depleted in NO; by the grasses (Lolium-Lolium control) throughout the cycle. For associations involving the legume (Lolium-Trifolium and Trifolium-Trifolium), NO; depletion was followed by NO; enrichment at night, from late afternoon to early morning; the enrichment was more marked for the Lolium-Trifolium association. Solutions which did not contain NO2 ions, were enriched by trace amounts of NH2-ions, largely depleted in K § and alkalanized for all associations throughout the cycle.Repeating the experiment with the three pairs of species at the vegetative phase of development confirmed the previous results: NO; enrichment during the night for associations with legumes. When the experiment was repeated with older plants which had almost completed their flowering stage, depletion only was observed and no NO; enrichment.These data suggest that NO~-enrichment results from N excretion from active nodulated roots of the legume, accounting for the increase in both biomass and nitrogen content of the companion grass in grass-legume association. The quantitative importance and periodicity of nitrogen excretion as well as the origin of nitrate enrichment are discussed.

Five genotypes of Trifolium repens and Lolium perenne were collected as neighbouring pairs along a fertility gradient in a natural pasture. After vegetative multiplication, the 25 possible combinations of Lolium genotype x Trifolium genotype were planted in the greenhouse in order to investigate competition between the genotypes. The comparison of the five combinations whose individual components had been neighbours with the combinations of genotypes that had not coexisted before disclosed no difference in total biomass production over 7 months. However, the yield of Trifolium increased when grown with the Lolium genotype which had been its natural neighbour, while the latter showed a decrease in yield. This neighbour specificity existed even when carryover effects from the sampling site had been eliminated (preconditioning period of 18 months) and when native Rhizobium strains were not present (inoculation with a non-native strain). The complex pattern of neighbour specificity with time indicated the importance of environmental conditions for its outcome. These results are a further confirmation of positive effects on the growth of Trifolium repens when grown together with its natural neighbour. They are discussed in the context of coexistence and coevolution.

We performed an extensive experiment to compare male fertility, flowering period, seed set and germination rate between triploid and tetraploid cross-progeny of Dactylis glomerata L. originating trom reciprocal diploid (2.\) X tetraploid (4.t) crosses. Four hundred and forty-five triploid and tetraploid individuals, constituting the cross progeny of 2x-Ax reciprocal crosses between diploids from nine distinct subspecies, and tetraploids trom tive subspecies, were tested for male fertility and phenology under homogeneous conditions. X significant negative correlation was observed between the parental genetic distance and the frequency of sterile panicles (Sl^) in 3.\-progeny, suggesting that selection may occur against fertilit\' in triploid progeny from interploid crosses involving closely related subspecies. Genetic distance between the parental subspecies was not significantly correlated with the frequency of fertile panicles {V'Y), non-spreadmg panicles (NT, non-dehiscent anthers with pollen inside), and ST panicles in the Ax progeny, nor with the frequency of ¥T and NT panicles in the 3.\: progeny. .A.mong the 4.v progeny individuals that produced panicles, more than 60",, were fully fertile, but 24",, possessed only sterile panicles. In the triploids, 63 "" of the reproductive plants were male-sterile, but 31 ",, had variable proportions of fertile panicles and 6"o possessed only fertile panicles. Male fertility in the progeny was signiticantly aftected by individual ploidy level, parental diploid subspecies identity, and by the interaction between these 2 factors in the case of the fertile progeny plants. The frequency of vegetative plants was higher in 4.Y progeny than in 3.\-progeny, more particularly when the seed parent was tetraploid. However, when the seed parent was diploid, the frequencies of vegetative plants were similar in 3.v and 4A progeny. Pollen viability was significantly higher in anthers from fertile panicles than in anthers from non-spreadmg panicles; viability of pollen from non-spreading panicles was, in turn, significantly higher than in sterile panicles. Also, seed set was higher in panicles with fertile anthers than in the two other types. However, seeds from plants with fertile, non-spreading and sterile anthers showed no differences m germination rate. Therefore, in Dcictylis glomerata, tetraploid cross progeny arising from 2ri gametes possess higher fertility, on average, than triploid cross progeny involving normal gametes. In addition to lower male fertility (non-spreadmg or atrophied anthers), the triploids also showed signiticantly lower pollen fertility in anthers and lower seed set in open pollinating conditions, probably because disturbance in meiosis, due to odd chromosome associations, also occurs in eggs. Moreover, triploids were observed to reproduce preferentiallv with individuals possessing the same ploidy levels as their seed parent. These results suggest that the production of 4.x-individuals, derived from 2ii gametes m interploid crosses, is likely to be the main ...

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