| This study evaluated and improved a culture technique used for screening plants for tolerance to low P conditions, investigated differences in response to P availability in germplasms of white clover (Trifolium repens L.), subterranean clover (Trifolium subterraneum L.), and alfalfa (Medicago sativa L.), assessed definitions commonly used to determine nutrient efficiency, and presents criteria for assessing nutrient efficiency.; Sand-alumina medium was a simple, inexpensive, and effective method of providing a range of P concentrations ( (P) {dollar}sb{lcub}rm S{rcub}{dollar}) for plant growth. Loading alumina with varying solutions of P resulted in steady-state P concentrations in sand-alumina medium. A shorter P-loading period and autoclaving, decreased (P) {dollar}sb{lcub}rm S{rcub}{dollar}. X-ray diffractometry and scanning electron microscopy indicated alumina was boehmite and pseudoboehmite (AlOOH), which appeared to be substantially altered by P-loading solutions to form a highly insoluble precipitate containing Al, K, and P on alumina granule surfaces. Potassium concentration in the desorption solution appeared to control (P) {dollar}sb{lcub}rm S{rcub}{dollar}.; The response to available P was assessed in seventeen purportedly P efficient and inefficient germplasms of white clover, subterranean clover, and alfalfa, using the sand-alumina culture technique and two low P soils. There were significant differences in yield, P accumulation and many root parameters, among germplasms of all three species. These differences were associated with larger root systems, but did not necessarily indicate greater efficiency of P uptake or utilization.; Two germplasms of alfalfa and white clover were used to evaluate various criteria identified in the literature as measures of nutrient efficiency. Germplasms differed in maximum shoot and total dry mass and in solution P requirement to achieve maximum yield, but not in P concentration of tissue, internal P utilization, or P uptake per unit of fine roots. Differences may have resulted from factors other than P efficiency. To reduce the confounding effects that other factors have on nutrient efficiency, equivalent yields of lines must be demonstrated where nutrients are not limiting. Differences in nutrient efficiency can then be related to differences in response curves at low levels of nutrient availability. Mechanisms that enable enhanced nutrient efficiency may then be identified less ambiguously using this approach. (Abstract shortened with permission of author.)... |
