The carbon and nitrogen physiology of Achillea millefolium L. (Yarrow)

Achillea millefolium (L.) in New Zealand has become an increasing problem on arable and horticultural land where selective control remains difficult to achieve. Patterns of carbon and nitrogen accumulation and use were investigated from 1987 until 1991 to determine the physiological basis of the ability of A. millefolium to grow and persist in agricultural habitats. This provided a crucial first step in formulating effective control strategies. Plants were either established from clonal rhizome material and grown individually or collected from naturally occurring populations in Canterbury, New Zealand. The growth and carbon (C) and nitrogen (N) economy of individual container-grown plants was studied from December 1987 until January 1990. After June 1988 rhizomes were the dominant component of plant biomass. The following summer, despite an increase in the mass of flower stems, the relative allocation to seed formation had declined. In the final summer both the absolute weight and the relative allocation of biomass invested in sexual structures also fell. These results indicate a shift from a ruderal to a more competitive ecological strategy coincident with the shift from a disturbed to a more stable habitat. Nonstructural C was stored primarily as fructan, which was principally located in rhizomes and roots. High concentrations of fructan accumulated in these tissues during the autumn and winter in 1988 and throughout 1989. The maintenance of rhizome weight and the fructan pool during winter in 1988, and the recommencement of rhizome expansion the following spring, appeared to be supported by root rather than rhizome-held reserves. Achillea millefolium consequently maintained a high vegetative reproductive potential irrespective of the time of year. The individual plants showed considerable flexibility in the efficiency with which they used N. Throughout the experiment, tissue N was low in comparison with cultivated and other wild species. Despite containing relatively low N concentrations, rhizomes acted as winter N storage organs. Cool season N losses were reduced by the apparent translocation of N from old aerial shoots to rhizomes in autumn. Rhizome N content subsequently declined with the onset of spring growth. Plant growth from December 1987 to March 1988 and from September 1988 to March 1989 was primarily reliant on N taken up from the soil during the growing season. From September 1989 to January 1990, however, when soil N had become depleted there was no net increase in plant N content and spring growth was supported by the increasingly efficient use of existing plant N. The results from individual container-grown plants were compared with observations of plants from naturally occurring field populations of A. millefolium, Cirsium arvellse (L.) Scop and Elytrigia repens (Nevski.). Plants were collected on four occasions, March, August and October 1990 and January 1991 from nine different sites. Nonstructural carbohydrates accumulated primarily in the subterranean organs of each of the three species. Fructan was the principal form of nonstructural carbohydrate stored by A. millefolium in the field but concentrations were generally lower than those measured in container grown plants. The seasonal pattern of growth and development of A. millefolium at the undisturbed site differed from those of plants from disturbed sites but closely resembled that of the container grown plants. Differences between sites in terms of leaf morphology in A. millefolium, flowering in A. millefolium and C. arvense, and carbohydrate and N storage in all three species, indicated that variations between field and container grown plants may primarily be attributed to grazing and cultivation effects. Responses of individual A. millefolium, C. arvense and E. repens plants to defoliation were also investigated. Unlike grazing tolerant species reported in the literature, A. millefolium responded to defoliation by allocating an increased proportion of resources to roots. This was primarily achieved at the expense of sexual reproduction and did not retard leaf regrowth. Defoliated plants underwent a prolonged period of resource reallocation, during which aerial regrowth was primarily supported by C and N stored in roots. Rhizome reserves were apparently mobilized to support regrowth but the utilization of root reserves and the contribution of current photosynthesis in new leaves allowed rhizome reserves to be conserved and the regenerative capacity of rhizome buds to be retained. In E. repens and C. arvense current photosynthate from new leaves appeared to be the major source of photosynthate for regrowth after a single defoliation. In E. repens, N uptake was not interrupted and rhizome and root reserves were conserved. With further defoliations, reserves in E. repens, principally from rhizomes, were utilized and there was a decline in plant vegetative reproductive potential. Unlike the other species, sexual reproduction in C. arvense was only slightly depressed by defoliation. The results suggest that adaptations and responses of the three species to defoliation have been at least partially shaped by grazing. When A. millefolium was supplied with 1,2,5,10 or 20 molm⁻³ of external N, plants showed characteristics of plants adapted to fertile habitats (competitive and ruderal species) and plants from infertile habitats (stress tolerant species). The amount of N taken up increased with rising external N concentration through increasing specific absorption rate rather than increasing root biomass. A relatively high proportion of total nonstructural carbohydrate (TNC) was allocated to roots and high TNC concentrations were maintained irrespective of external N concentration. Achillea millefolium also demonstrated a high degree of morphological plasticity in terms of organ size, number, development and time to flowering. Achillea millefolium could therefore be classified as a competitive-stress tolerant-ruderal. Increased N uptake influenced growth primarily through changes in leaf canopy development and duration. The maintenance of high chlorophyll concentrations and a relatively constant leaf thickness over the range of external N concentrations from 1 to 10 mgg⁻¹ suggested that the photosynthetic capacity of leaves on a unit area basis may reach a maximum at relatively low external N concentrations. This would allow photosynthesis to exceed the immediate demands of growth and photosynthate to accumulate in below ground organs despite the N-induced restriction of the leaf canopy. The results of the current study demonstrated that while having a highly generalised ecological strategy, A. millefolium is well adapted to the disturbed and variable habitats that characterise agricultural environments. Effective control strategies for A. millefolium infestations on arable land may however, be designed which take advantage of the responses of A. millefolium to a combination of rhizome fragmentation, external N supply and defoliation.