The fifteenth- and twentieth-century colonization of the Basin of Mexico by the Great-tailed Grackle (Quiscalus mexicanus)

Historical evidence indicates that Great‐tailed Grackles colonized the Basin of Mexico from the Gulf Coast lowlands in the fifteenth century. They were probably assisted by an intentional introduction, but colonization succeeded because of anthropogenic habitat alterations over the previous two centuries. During the Colonial period, grackles withdrew from the Basin, only to recolonize it in recent decades. This withdrawal was also due probably to changes in land use, including drainage of much of the water from the Basin's lakes.     

Transmission characteristics of Spiroplasma citri and its effect on leafhopper vectors from the Circulifer tenellus complex

Several leafhopper variants of the Circulifer tenellus complex were collected in “citrus stubborn” affected areas in Israel. Two of these variants transmitted the Spiroplasma citri to Matthiola incana after being injected with the disease agent. The variant from Atriplex halimus was designated Circulifer tenellus-A (CTA) and the variant from Portulaca oleracea was designated Circulifer tenellus-? (CTP). Transmission characteristics were determined for both leafhoppers. A high rate of transmission (43.3%) was obtained by single CTA leafhoppers that were injected with the Amiad S. citri isolate from the Upper Galilee, compared with 7% transmission obtained with the CTP leafhoppers. The Gilgal S. citri isolate from the Jordan Valley, was not transmitted by either. Injection was more effective than acquisition access feeding to render the leafhopper infective for both CTA and CTP. The minimum acquisition access period needed for the CTA variant to transmit the Amiad isolate was 1 h. Longer AAPs did not necessarily result in a higher rate of transmission. The minimum incubation period was 6 days and the maximum was 32 days. The LP₅₀ calculated from the logarithmic curve y = 45.74Ln(x)–53.68 was 9.64 days. The minimum inoculation access period (IAP) was lh. The same transmission parameters for the CTP variant could not be determined, as no transmission was obtained even when groups of five-six insects were placed on a single plant.     

The effect of grass maturing and root decay on N2O production in soil

The N₂O flux from the surface of grass-covered pots was only significant following grass maturing. Removal of the above-ground plant material resulted in an immediate and long-lasting increase in N₂O production in the soil. The results suggest that easily available organic matter from the roots stimulates the denitrification when the plants are damaged. Grass cutting might therefore result in a marked nitrogen loss through denitrification. The quantitative effect was equal in soil with and without succinate added. The size of the anaerobic zone around the roots is therefore sufficient to allow for denitrification activity mediated by increased organic matter availability because of plant cutting.     

A Comparative Study on the Effects of H2S and SO2 Fumigation on the Growth and Accumulation of Sulphate and Sulphydryl Compounds in Trifolium pratense L., Glycine max Merr. and Phaseolus vulgaris L.

Maas, F. M., De Kok, L. J., Peters, J. L. and Kuiper, P. J.C. 1987. A comparative study on the effects of H₂S and SO₂ fumigationon the growth and accumulation of sulphate and sulphydryl compoundsin Trifolium pratense L., Glycine max Merr. and Phaseolus vulgarisL.—J. exp. Bot. 38: 1459-1469. The effects of 0—25 mm³ dm³ H₂S and SO₂ on growth andsulphur content of shoots of Trifolium pratense, Glycine maxand Phaseolus vulgaris were studied. After 2 weeks of fumigationthe yield of T. pralense was reduced by 32% by H₂S, but notaffected by SO₂. Yield of G. max was not affected by H₂S, butreduced by 20% by SO₂, whereas that of P. vulgaris was increasedby 11% by H₂S and not affected by SO₂. Increases in sulphydrylcontent were already observed after 24 h of exposure to H₂Sand SO₂ in all plants. The increase was greatest in T. pratenseand smallest in P. vulgaris and, except for T. pratense, alwaysgreater in the H₂S-exposed than the SO₂-exposed plants. Oneday of exposure resulted in an increase in sulphate contentonly in the SO₂-fumigated plants, with the highest accumulationin T. pratense and the lowest in P. vulgaris. After 2 weeksan increase in sulphate content was also observed in the H₂S-exposedplants. This increase was also highest in T. pratense and lowestin P. vulgaris. Transpiration rate was not affected by a 24 h exposure to H₂Sor SO₂ and was highest in T. pratense, intermediate in G. maxand lowest in P. vulgaris. The order of theoretical rates of deposition of H₂S and SO₂correlated with the observed increases in sulphydryl contentduring the first 24 h of exposure in both H₂S and SO₂-fumigatedplants and with the increase in sulphate content in the SO₂-exposedplants. The increases in sulphydryl content were only 8% ofthe theoretical H₂S and SO₂-deposition fluxes, whereas sulphateaccumulation accounted for at least 57% of the theoretical SO₂-depositionflux. Key words: Air pollution, clover, French bean, Glutathione, Soybean, sulphur metabolism.