Relationships Between Nitrogen and Water Concentration in Shoot Tissue ofMolinia caeruleaDuring Shoot Development

Plants ofMolinia caeruleawere supplied with either a low (0.2mol m^-3) or high (10 mol m^-3) supply of nitrogen over two growingseasons. A total of 14 destructive plant harvests were made:when plants were in an over-wintering state prior to the secondseason; immediately following bud burst; and on 12 further occasionsthroughout the second season. The relationships between shootnitrogen concentration on a dry mass basis, shoot water contentand plant developmental stage were investigated. Shoot nitrogenconcentration on a dry mass basis fell as the growing seasonprogressed. In contrast, the concentration of nitrogen in tissuewater after bud burst showed only a slight reduction. The concentrationof nitrogen both on a dry mass basis and in tissue water wasgreater for plants receiving the higher supply of nitrogen.Shoot water content was highest immediately following bud burstthen declined as the season progressed, with plants receivingthe low nitrogen supply having slightly greater shoot watercontents. It was concluded that the decline in shoot nitrogenconcentration ofM. caeruleaon a dry mass basis as the mass increasedwas mainly explained by changes in shoot water content. Theobserved increase in the rate of decline of both shoot nitrogenconcentration and water content with increased shoot mass coincidedwith the cessation of leaf tissue production and was thereforedue to a switch from the production of leaves to other tissues.Copyright1999 Annals of Botany Company Molinia caerulea(L.), purple moor grass, nitrogen, water content, shoot development.     

The Effects of Nitrogen Supply and Defoliation on the Seasonal Internal Cycling of Nitrogen in Molinia caerulea

Plants of Molinia caerulea were grown in pots for two seasonsat two levels of nitrogen (N) supply and two levels of defoliation.All N supplied was enriched with ¹⁵N in the first season andwas at natural abundance in the second season. This allowedthe contribution of remobilization from overwintering storesto be discriminated from current root uptake in supplying Nfor new shoot growth in the second season. The effects of Nsupply and defoliation upon the internal cycling of N in M.caerulea were quantified. N was remobilized from both roots and basal internodes to supportnew shoot, especially leaf, growth in spring. Roots suppliedmore N than basal internodes. Since the remobilization mainlyoccurred before the onset of root N uptake, internal cyclingwas important for the earliest period of shoot growth. An increasedN supply increased the amount of N remobilized to new shootgrowth, however, the proportion of N remobilized from overwinteringstores was independent of N supply. Defoliation increased theamount of N remobilized from the roots, and had no effect onthe ¹⁵N content of basal internodes of plants receiving a lowsupply of N. Remobilization of N from leaves of undefoliatedplants occurred later in the season. Remobilization from leavessupplied flowers in plants receiving a low N supply and bothflowers and new basal internodes in plants receiving a higherN supply. Key words: Molinia caerulea, internal cycling, nitrogen, defoliation     

Nitrogen Supply Affects the Remobilization of Nitrogen for the Regrowth of Defoliated Lolium perenne L.

Nitrogen remobilization from roots and pseudostems during regrowthof Lolium perenne L. was studied in miniswards grown with contrastinglevels of (NH₄)₂SO₄ in solution culture. Growth with a highN supply (5.0 mol m^–3) increased theweight of leaf laminae recovered at each of five weekly clippings,and decreased the proportion of photosynthate used for rootgrowth. Clipped plants growing in a steady-state were suppliedwith ¹⁵N for 48 h and the recovery of labelled N in laminaemeasured after five weekly cuts. Recovery of labelled N in thelaminae from the second clipping onwards was derived only fromremobilization of N from roots and pseudostem. Miniswards grownwith low N (0.5 mol m^–3) relied moreupon remobilization of N for lamina growth than did high N plants.Thus after 14 d 20% of lamina N was labelled in low N plantsbut only 3% was labelled in the high N treatment. Thereafter,N remobilization declined until at the final clipping after35 d, labelled N represented only 4% and 1 % of the lamina Nin the low and high N plants. When plants were not clipped beforethe labelling period, they took up more ¹⁵N if grown with highN than cut plants. Thereafter, the remobilization of N followeda similar pattern as in the cut plants. Exponential models were used to calculate the rate of N transferfrom roots and pseudostem to laminae. When grown with low N,the half-life of remobilization was 1.56 weeks. High N miniswardshad an initial rapid remobilization with a half-life of 0.66weeks, and a slower phase with a half-life of 2.98 weeks. Key words: Lolium perenne L., nitrogen supply, regrowth, remobilization, internal cycling     

Measurement of xylem sap amino acid concentrations in conjunction with whole tree transpiration estimates spring N remobilization by cherry (Prunus avium L.) trees

Prunus avium trees were grown in sand culture for one vegetative season with contrasting N supplies, in order to precondition their N storage capacities. During the spring of the second year a constant amount of ¹⁵N was supplied to all the trees, and the recovery of unlabelled N in the new biomass production was used as a direct measure of N remobilization. Destructive harvests were taken during spring to determine the pattern of N remobilization and uptake. Measurements of both xylem sap amino acid profiles and whole tree transpiration rates were taken, to determine whether specific amino acids are translocated as a consequence of N remobilization and if remobilization can be quantified by calculating the flux of these amino acids in the xylem. Whereas remobilization started immediately after bud burst, N derived from uptake by root appeared in the leaves only 3 weeks later. The tree internal N status affected both the amount of N remobilization and its dynamics. The concentration of xylem sap amino acids peaked shortly after bud burst, concurrently with the period of fastest remobilization. Few amino acids and amides (Gln, Asn and Asp) were responsible for most of N translocated through the xylem; however, their relative concentration varied over spring, demonstrating that the transport of remobilized N occurred mainly with Gln whereas transport of N taken up from roots occurred mainly with Asn. Coupling measurements of amino acid N in the xylem sap with transpiration values was well correlated with the recovery of unlabelled N in the new biomass production. These results are discussed in relation to the possibility of measuring the spring remobilization of N in field‐grown trees by calculating the flux of N translocation in the xylem.     

Imaging the Vascular Continuity of Prunus avium Petioles During Leaf Senescence Using Nuclear Magnetic Resonance Spectroscopy

Nuclear magnetic resonance (NMR) micro-imaging was used to studythe distribution of mobile protons in the petiole and stem ofPrunus avium throughout the period of senescence of an attachedleaf. The base of the NMR micro-imaging probe was modified toallow access to the coil from below, enabling a branch of theintact tree to be inserted into the probe which, in turn, wasinserted into the base of the NMR cryomagnet. A multi-sliceimage was used to determine the final orientation of the petiolein the probe. NMR spin echo imaging parameters were investigatedby using variable relaxation delays (D1) ranging from 0·1to 4·0 s. Transverse slices of the petiole with a D1ranging between 1·0 to 4·0 s produced bright imagescorresponding to the cortex and leaving the xylem vessels asa dark horseshoe-shaped area. Reducing D1 to 0·25 s or0·1 s produced images with more detail, differentiatingthe phloem and xylem vessels from the surrounding cortex. A longitudinal slice, 0·5 mm thick, through the stemand subtending petiole was imaged repeatedly over 6 d, duringwhich the leaf senesced and abscinded. The images showed detailsof the vascular strands in the stem with a section continuouswith the petiole. During leaf senescence the development ofthe abscission zone in the petiole was observed. The petioleremained hydrated throughout leaf senescence, demonstratingthe usefulness of NMR imaging to probe the vascular integrityof the intact plant. Key words: Spin echo pulsed gradient, cherry, petiole, vascular integrity     

The Fate of Sulphate within the Capsular Polysaccharide of the Unicellular Red Alga Rhodella maculata

³⁵SO^2–₄ was used in pulse-chase experiments to study themetabolism of sulphate in the capsular polysaccharides of R.maculata. Radioactivity reached an equilibrium between the chiefintracellular pools during an 8 h pulse and there was no lossof radioactivity from the cells during an extended chase ofup to 14 d, until cultures reached stationary growth phase.Any loss then found was due to dissociation of sulphated capsularmaterial from the cells. The failure of biochemical assays todetect extracellular sulphatase or sulphotransferase activitiessupports the conclusion from pulse-chase experiments that thereis no turnover of sulphate in the capsular polysaccharide ofR. maculata. Key words: sulphated polysaccharides, sulphate turnover, Rhodella