FLURIDONE REVEALS ROOT ELONGATION DIFFERENCES AMONG ALFALFA GERMPLASMS

Alfalfa (Medicago sativa L.) is a deep-rooted perennial that may be ab le to remove nitrate from subsoil beneath the rooting zone of annual c rops, Alfalfa cultivars with rapid and deep root elongation would be b eneficial for reducing future nitrate contamination of ground water su pplies, The objectives of this research were to (i) develop a protocol using the herbicide fluridone (1-methyl-3-phenyl-5-[3-(trifluromethyl ) phenyl]-4(1H)-pyridinone) as a marker for indicating alfalfa root pe netration in soil, and (ii) determine the variability in rate of geotr opic root elongation within and among diverse alfalfa germplasms, Flur idone caused rapid, distinctive bleaching of alfalfa leaves at a rate of 25 mg a.i. kg(-1) soil, without being fatal. When transplanted afte r onset of fluridone symptoms, alfalfa recovered and resumed normal gr owth and flowering. Alfalfa populations did not differ in sensitivity to fluridone. Decomposition of roots from a fluridone-treated plant di d not cause symptom development in other plants. Individual plants wit hin alfalfa germplasms differed by 56 d in onset of symptoms, indicati ng a similar difference in root activity at 80 cm depth in soil. Mean geotropic root elongation rate for 12 diverse germplasms ranged from 1 .12 to 1.40 cm d(-1). Populations selected for unique root morphologic al traits were significantly faster in geotropic root elongation rate than their nonselected parental populations. Our method permits in sit u identification of plants with rapid geotropic root elongation rates within heterogeneous populations and has proven useful in germplasm se lection.