LIGHT EFFECTS ON RHIZOME MORPHOGENESIS IN NUTSEDGES (CYPERUS SPP) - IMPLICATIONS FOR CONTROL BY SOIL SOLARIZATION

Soil solarization is a process of soil disinfestation that involves so lar heating of moist soil covered with transparent polyethylene film. This nonchemical approach to controlling soilborne pests is being inve stigated as an alternative to methyl bromide fumigation. Summer solari zation controlled annual weed species and suppressed purple nutsedge. Although some nutsedge tubers sprouted despite the solarization treatm ent, the resulting shoots were almost always trapped under the clear s olarization film. Conversely, in rows that were mulched with black fil m, nutsedge rhizomes punctured the film so that leaf expansion occurre d above the film. In controlled pot experiments conducted in darkness, yellow nutsedge rhizomes readily penetrated 19- and 30-mu m clear fil ms as effectively as opaque films. Thicker clear films and bubble film reduced nutsedge penetration. In the greenhouse and laboratory, nutse dge penetration of transparent polyethylene film was inversely related to irradiance levels when the film was in direct contact with the soi l. However, when there was a 5- to 10-mm space between the soil and th e film, the lowest irradiance level (30 mu mol m(-2) s(-1)) was as eff ective as 320 mu mol m(-2) s(-1) in reducing penetration by purple nut sedge. The film penetration by nutsedge rhizomes appears to be linked to a light-dependent morphological change from rhizome growth to leaf development, which occurs before film penetration with clear mulch and after film penetration with opaque mulch. The alternate sprouting and foliar scorching of nutsedge shoots trapped under clear films could p otentially be exploited to deplete nutsedge tubers that occur at soil depths that do not develop lethal temperatures under soil solarization .