| dc.description.abstract | The influence of nitrogen supply on relationships of relative growth rate (RGR) to leaf physiology, structural and non-structural carbon partitioning, and nitrogen- and water-use efficiencies were examined in loblolly pine (Pinus taeda L.) clonal lines differing in growth potential. Nine 18-month-old loblolly pine clones were grown in a climate-controlled greenhouse for 20 weeks under two contrasting nitrogen (N) regimes (50 and 250 ppm) and a growth analysis was carried out. Higher nitrogen increased plant RGR and largely resulted in proportional shifts in biomass from roots and stems to needles. The RGR of plants receiving higher nitrogen was increased primarily through increased leaf area ratio (LAR), which was increased through higher leaf mass fraction (LMF) and not through changes in needle morphology. Although concentrations of needle glucose in plants receiving 250 ppm N were 22 percent higher than plants receiving lower N, total non-structural carbohydrate concentrations in needles of plants receiving 50 ppm N were nearly double that of clones receiving 250 pm N, primarily due to starch accumulation of the nitrogen-deficient plants. Plants receiving 250 ppm N also had 39 and 18 percent lower starch in the coarse and fine roots, respectively. Plants receiving higher nitrogen were also more water-use efficient, but had lower photosynthetic nitrogen-use efficiency. LAR, net assimilation rate (NAR), specific leaf area (SLA), and LMF were all positively correlated with RGR, but the main influence on RGR differences among clones was LAR. In addition, leaf-level rates of photosynthesis and respiration were positively correlated with RGR; however, faster-growing clones did not exhibit greater carbon economy at the leaf level. Both instantaneous water-use efficiency (A/E) and δ13C were positively correlated with RGR and photosynthetic nitrogen-use efficiency was negatively correlated with RGR. The identification of physiological and morphological traits underpinning differences in RGR among clones and how these traits are affected by nitrogen supply provides new information on trait correlations within species and parallels broader patterns observed among species. | en |
