Establishment and growth of shoots following low and high cutting of lucerne in relation to the pattern of nutrient uptake

Abstract

Lucerne plants (Medicogo sativa cv. Hunter River) were either frequently or infrequently cut down and subsequent differences in shoot regrowth were compared in two experiments. The first experiment demonstrated that differences in final shoot weights arose from differences developed during the first 7 days of regrowth. High level cutting (15 cm) increased the shoot yield of frequently but not of infrequently cut plants. Net uptake of both nitrogen and phosphorus was related to the growth rate of shoots until commencement of flowering, when uptake ceased for c. 15 days even though both roots and shoots continued to gain weight. Towards the end of flowering uptake of nitrogen and phosphorus recommenced and accumulation of both nutrients occurred in the tap-root and lateral roots. The relative nitrogen and phosphorus content of leaves on crown shoots was highest on day 7 and the same for frequently and infrequently cut plants. Leaves on crown and stubble shoots 7 days after high level cutting had a significantly lower relative nitrogen and phosphorus content than leaves on plants cut low. Thereafter the relative nitrogen and phosphorus content of a11 leaves declined with the greatest decline occurring after the commencement of flowering. In the second experiment early morphogenesis of the shoot population was investigated. Establishment of shoots was completed between 3 and 5 days after cutting. Higher shoot weights on infrequently cut plants were accounted for by a larger number of small shoots at the time of cutting. Relative growth rates of shoots did not appear to be influenced by prior cutting frequency. The relative nitrogen content of buds and shoot apices was low at cutting but doubled within 2 or 3 days and then declined after day 7. These results are discussed in relation to the role of 'plant factors' in shoot regrowth of lucerne.