Contribution of stored N to new growth
On the basis of N budgets it was shown that the initial growth of the alpine herb Bistorta bistortoides was supported solely by remobilized N, and that uptake of N only occurred after growth had started (Jaeger & Monson, 1992). By contrast, new leaves of R. acetosa, an evergreen herb, and of the deciduous grass M. caerulea contained c. 40–50% of N derived from current uptake as early as mid March when grown in sand culture, indicating that N remobilization and uptake can occur simultaneously even in early spring (Bausenwein et al., 2001; Thornton & Bausenwein, 2000). The present study showed that F. rubra and A. capillaris behaved similarly to R. acetosa by utilizing both internal and external N by mid March. We observed significant differences between A. capillaris and F. rubra in the percentage of remobilized N in the new growth over the entire observation period, with A. capillaris having consistently higher values. As the two species differed in the timing of their growth, the contribution of remobilization in relation to d. wt gave a more meaningful basis for comparison. Under these circumstances, A. capillaris and F. rubra have similar contributions of remobilization. The temporal differentiation in shoot development between A. capillaris and F. rubra has been recognized before (Grime et al., 1988), and, as our data show, can be expanded to root growth.
Origin of remobilized N
Net balances of labelled N show that in F. rubra and A. capillaris the turnover of old leaves and the concomitant release of N out of the senescing tissues were responsible for the recovery of labelled N in new above-ground plant material. The root system was not a winter storage compartment of N contributing to above-ground growth in spring. Instead, there was a slight increase in the content of labelled N in the roots of both species towards the end of the experiment. We know from both F. rubra and L. perenne seedlings (Schulte auf’m Erley et al., 2000) and our own data from more short-term labelling experiments with F. rubra and A. capillaris (unpublished) that a proportion of the N remobilized from senescing leaves can be incorporated into the roots.
The sequential use of leaf nitrogen through translocation of N from older leaves to young expanding leaves is well-known for a variety of species, for example the sedge Eriophorum vaginatum (Jonasson & Chapin, 1985), seedlings of L. perenne and F. rubra (Schulte auf’m Erley et al., 2000) or vegetative Triticum aestivum plants (Vouillot & Devienne-Barret, 1999). For overwintering plants, data are more limited and – to our knowledge – not available for temperate grasses. It has been reported that Eriophorum vaginatum, a sedge occurring in tundra ecosystems, stores N in overwintering leaves (Chapin et al., 1980), however, these leaves are also long-lived and continue elongation in the following spring. Also, it is known that evergreen trees such as Eucalyptus globulus or Pinus radiata use N from their foliage for leaf growth in spring (Nambiar & Fife, 1991; Wendler et al., 1995). In Picea sitchensis, 15N labelling has shown that overwintering needles provided nearly all, and the root system none, of the N derived from remobilization in young needles (Millard & Proe, 1993).
Many experiments determining N remobilization in grasses to new leaf material and its site of origin involve defoliation; in several cases it had been shown that both roots and remaining stubble remobilize N (Ourry et al., 1988; Bakken et al., 1998). In wheatgrass, Agropyron dasystachyum, new growth was supported equally by N remobilized out of old leaves and of roots (Li et al., 1992). In that experiment, all leaves had been removed from the plant at the start of the labelling period with 15N, with the chase period starting once plants had reached a 2-leaf stage, when N accumulation in the leaves might not have been at its maximum. These experimental findings suggest that N remobilization from the root to the new shoot occurs when the N pool above-ground is insufficient.
The fact that N derived from old leaves is important for spring growth can explain certain field observations: autumn cuts adversely affected spring growth in various grass species including F. rubra and A. capillaris (Grant & Hunter, 1968). Later cuts, leaving the plants insufficient time for regrowth before winter were more detrimental than earlier cuts, and might be due to the lack of a substantial pool of leaf N that could contribute to early growth.
Winter N storage in grasses had been assigned to the root system on the basis of increases of %N and of N content in the root system in winter. There are two problems with this. First, %N (in relation to d. wt) fluctuations of roots are not necessarily an indication of storage as other compositional changes of the root will influence its d. wt. Second, increases in the N content of the root system in winter might be related to root growth, which according to Weinmann (1948), continued, when, above-ground, growth, had ceased; Stuckey (1941) reported significant production root systems starting in October. Garwood (1967) measured the appearance of new roots of L. perenne, Dactylis glomerata and Phleum pratense, which were produced in increasing numbers through late winter to early spring. Allocation of N to the roots before and during winter is more likely to be associated with tissue production than the formation of storage pools.
A factor limiting the potential of grass roots as winter storage organs might be their longevity. Long-term seasonal storage requires structures with a sufficient lifetime to allow for filling in autumn and withdrawal in spring. In a recent investigation, using minirhizotrons for direct observations of specific root cohorts of A. capillaris accessions identical to those used in our experiment, the longevity of roots was short with a medium life-span of 38 d (J. A. Baddeley, unpublished). In L. perenne only 84% or 38% of the roots survived after 21 d, depending on the climatic conditions of the study site (Watson et al., 2000). Even though these observations were on root cohorts initiated in summer, and it is known for both grasses and trees that summer roots have a shorter life-span than roots initiated towards the end of the year (Garwood, 1967; Eissenstat & Yanai, 1997), they give an indication that parts of the root system of temperate grasses might be too short-lived for winter storage. Species-specific differences exist for root longevity, and these had been attributed to variations in tissue-density and root diameter, with low tissue-density (as observed in species with high relative growth rate from fertile environments) and small root diameter resulting in shorter life-spans (Ryser, 1996; Eissenstat & Yanai, 1997). The deciduous grass M. caerulea, which remobilizes N for spring growth predominantly out of its roots (Thornton & Millard, 1993), has a root system dominated by main axes with big diameters and high physical strength (Jefferies, 1915). By contrast, both F. rubra and A. capillaris produced root systems with few main axes and many fine laterals, and so were probably not equipped with sufficiently long-lived roots suitable as storage organs.
Integration of tillers
Until 17 April, the labelled N decline in old leaves nearly matched the labelled N increase in new leaves, whereas in later harvests a net import of labelled N into the reproductive tiller occurred. Not all the N of old leaves would have been available for retranslocation as some N was retained in the dead leaves. We were not able to sample completely the dead leaves because affiliation to an individual tiller was often not clear. In his review on nutrient resorption from senescing leaves, Aerts (1996) gives 59% of the initial leaf N as average resorption efficiency for a wide range of graminoids. Even though resorption efficiencies can be as high as 75% (Festuca pratensis; Hauck et al., 1997) or 86% (Triticum aestivum; Williams, 1955), the amount of labelled N available from old leaves of OFTs would have been insufficient to account for the labelled N recovered in the new leaves. Moreover, growth of new tillers was also supported by remobilized N. In the case of F. rubra, approx. 2–3 new tillers were associated with a OFT. In the absence of any net losses of labelled N from the root we conclude that N is translocated between tillers, with those becoming reproductive acting as the sink and those remaining vegetative as the source of (labelled) N. This is more apparent for F. rubra as the measurements did not adequately cover the reproductive development of A. capillaris. The movement of N out of vegetative tillers can be concluded from the decrease in their N content while their d. wt remained constant. Our sampling of OVTs at later harvests was biased towards vigorous tillers, neglecting those that were very small or (nearly) dead. We might therefore have underestimated the source potential of the average OVT. In clonal vegetative plants, movements of N are predominantly from older to ontogenetically younger tillers, though in Schizachyrium scoparium (bunchgrass) the reverse movement of 15N had also been detected (Welker et al., 1991). Flowering might alter this pattern. In Hordeum vulgare, flower development was correlated with the death of some secondary tillers despite the fact that they had established their own root system, indicating a hierarchical integration of tillers in favour of reproductive ones (Anderson-Taylor & Marshall, 1983).
To conclude, undefoliated A. capillaris and F. rubra plants are largely independent of N uptake from the soil in early spring, because N requirements for above-ground growth are met by N remobilization from the old leaves. In late spring, N from the soil is an important N source; however, flower development can result in further internal N translocations, with vegetative tillers providing N to the flowering tillers.