NEMATOLOGICAL ENVIRONMENT AND SELECTION PRESSURE ON SIX SASRI RESEARCH FARMS
Current estimates indicate that the loss in yield of sugarcane from root-feeding nematodes amounts to approximately R250 million per year. Tolerant varieties provide an inexpensive and long-lasting option to reduce crop loss from nematodes. Currently, genotypes are selected on the SASRI research farms without direct attention being given to their tolerance or susceptibility to nematodes. Not much is known about the nematode communities that exist on the research farms and whether these nematode communities exert sufficient selection pressure on the genotypes in the selection programme. The objectives of this work were (i) to characterise the plant breeding selection sites according to their nematode communities, and (ii) to measure the effect that nematodes have on the yield of varieties at plant breeding selection sites and thus identify the selection pressure for nematode tolerance. To achieve this, the 2nd stage single lines at six research farms were surveyed for nematode populations. Results showed that Empangeni, Gingindlovu and Kearsney had similar nematode profiles, whereas other research farms exhibited differences in nematode communities. For example, Bruyns Hill and Glenside had higher numbers of Helicotylenchus, whereas Pongola had higher numbers of Hemicycliophora and Rotylenchulus. The common variety at all sites, NCo376, exhibited a similar nematode community to other sugarcane clones at a particular site, but a substantially different community between sites. Yield data from single line trials showed that nematode communities influenced selection at four of the six sites. Two variety x nematicide trials were planted to assess the loss due to nematodes. At Empangeni, the yield responses (which reflect crop loss) in the plant crop ranged from 10 to 27%. At Gingindlovu, the responses in the plant crop ranged from 44 to 52%.
Les nématodes parasites des racines associés Au riz en Basse-Casamance (Sénégal) et en Gambie
Au cours d’une enquete SUT les Nematodes radicicoles parasites du riz au Senegal et en Gambie, 82 echantillons ont ete etudies, dont 6’7 en Basse-Casamance (Senegal) et 15 en Gambie. Les trois types de rizieres existant en Basse-Casamance ont presente entre eux des differences sensibles tant dans les peuplements que dans les populations de Nematodes parasites du riz. Les rizieres de mangrove ainsi que certaines rizieres moyennes en sont pratiquement exemptes, probablement a cause des phenomenes de sulfatoreduction dont elles sont le siege. Les rizieres moyennes hebergent un petit nombre d’especes a la fois frequentes et abondantes alors qu’elles sont sensiblement plus nombreuses dans les rizieres hautes. En Basse-Casamance, les especes frequentes et abondantes sont : Tylenchorhynchus mashoodi, Hirschmanniella spinicaudata, Criconemoides palustris, Xiphinema bergeri, Heterodera spp. Pratylenchus SP., Paratylenchus sp. et Trichodorus spp. alors qu’en Gambie une seule repond a ces criteres : Hirschmanniella oryzae.
Inhibition of Root Growth and Polyamine Metabolism in Sunflower (Helianthus annuus) Seedlings Under Cadmium and Copper Stress
Although sunflower is usually regarded as a highly tolerant crop, impairment of root growth at initial stages of plant development may result in poor crop establishment and higher susceptibility to pathogen attack. In order to evaluate if Cd2+ and Cu2+ may impact on sunflower germination and initial root development, a pot experiment under controlled conditions was carried out. Possible involvement of polyamine metabolism in sunflower response to these stressors was also investigated. Although Cd2+ and Cu2+ treatments affect neither seed germination nor radicle emergence, sunflower seedlings grown in the presence of these heavy metals showed significant inhibition of root growth, being this inhibition greater for Cd2+. Both metals caused significant increases in proline contents at the highest concentrations tested (0.5 and 1 mM), and these increments were more pronounced for Cd2+ treatments, especially between days 3 and 10. Metals also increased putrescine (Put) contents at all concentrations assayed from the seventh day onward, causing no variations on this polyamine time-course pattern. Spermine and spermidine contents, however, were increased only by 1 mM Cd2+. Arginine decarboxylase seems to have been the enzyme responsible for Put increases under both metal treatments. This work demonstrates that initial root growth of sunflower seedlings may be significantly impaired in Cd2+ or Cu2+ contaminated soils. It also shows that polyamines are key biological compounds, which are probably involved in signaling pathways triggered under stress environmental conditions.