Diurnal changes in gas exchange and carbon partitioning in needles of fast- and slow-growing families of loblolly pine (Pinus taeda).

We investigated diurnal and seasonal changes in carbon acquisition and partitioning of recently assimilated carbon in fast- and slow-growing families of loblolly pine (Pinus taeda L.) to determine whether fast-growing families exhibited greater carbon gain at the leaf level. Since planting on a xeric infertile site in Scotland County, NC, USA in 1993, five Atlantic Coastal Plain (ACP) and five "Lost Pines" Texas (TX) families have been grown with either optimal nutrition or without fertilization (control). In 1998 and 1999, gas exchange parameters were monitored bimonthly in four families and needles were analyzed bimonthly for starch and soluble sugar concentrations. Although diurnal and seasonal effects on net photosynthesis (A(net)) and maximum rate of light-saturated photosynthesis (A(max)) were significant, few family or treatment differences in gas exchange characteristics were observed. The A(net) peaked at different times during the day over the season, and A(max) was generally highest in May. Instantaneous water-use efficiency (WUE(i)), derived from gas exchange parameters, did not differ among families, whereas foliage stable isotope composition (delta(13)C) values suggested that TX families exhibited lower WUE than more mesic ACP families. Although there were no diurnal effects on foliar starch concentrations, needles exhibited pronounced seasonal changes in absolute concentrations of total nonstructural carbohydrates (TNC), starch and soluble sugars, and in partitioning of TNC to starch and sugars, mirroring seasonal changes in photosynthesis and shoot and root growth. In all families, foliar starch concentrations peaked in May and decreased to a minimum in winter, whereas reducing sugar concentrations were highest in winter. Some family and treatment differences in partitioning of recently assimilated carbon in needles were observed, with the two TX families exhibiting higher concentrations of TNC and starch and enhanced starch partitioning compared with the ACP families. We conclude that growth differences among the four families are not a function of differences in carbon acquisition or partitioning at the leaf level.

Carbon and phosphorus partitioning in Pinus serotina seedlings growing under hypoxic and low-phosphorus conditions.

Ten-week-old pond pine (Pinus serotina Michx.) seedlings were grown in solution culture at 5 or 100 microM P and under aerobic or hypoxic solution conditions. After 6 and 10 weeks in the treatments, changes in relative growth rate (RGR), P acquisition and allocation, and carbohydrate partitioning were determined by analyzing tissue for total P, soluble sugars and starch. Six weeks of low-P growth conditions decreased seedling dry weight and the ratio of shoot dry weight to root dry weight (S/R) by 39 and 51%, respectively, in comparison to seedlings from the aerobic, high-P (control) treatment. Mean RGRs of shoots in the low-P treatment were reduced by 33%, whereas root growth was unaffected. After 10 weeks of low-P growth conditions, however, both shoot and root RGRs were significantly reduced, and plants had lower S/R ratios than in any other treatment. Slowed shoot growth was accompanied by starch and nonstructural carbohydrate accumulation in needles, indicating that needle growth was not limited by carbohydrate supply. Six weeks of low-P growth conditions decreased total seedling P by 75%, reflecting a 97% reduction in the net uptake rate (NUR). Shoot NUR as a fraction of seedling NUR was also greatly reduced in the low-P treatment, indicating that low-P growth conditions affected P translocation to the shoot more than P accumulation by roots. In contrast, 6 weeks of hypoxic growth conditions decreased total dry weight of seedlings in the high-P treatment by 41% relative to their aerobic counterparts. Root growth was affected more than shoot growth, however, and S/R ratios increased. After 10 weeks, S/R ratios doubled, primarily because of the reduction in root RGR. Nevertheless, roots of hypoxic seedlings contained a higher percentage of total seedling P than their aerobic counterparts. Net P acquisition per seedling decreased by more than 50% under hypoxic growth conditions, as a result of reductions in both root RGR and seedling NUR. Starch accumulation in shoots of hypoxic seedlings reflected reductions both in root growth and in transport of carbohydrates to nonwoody roots. Carbohydrate availability did not appear to be limiting growth of hypoxic woody roots, which are well-aerated internally, but it may have limited metabolic processes in nonwoody roots of seedlings from the high-P treatment.

Effects of anaerobic growth conditions on phosphorus tissue concentrations and absorption rates of southern pine seedlings.

A non-circulating, continuously flowing solution culture was used to examine the long- and short-term effects of anaerobic growth conditions on phosphorus uptake in 12-week-old seedlings of three pine species. Sand pine (Pinus clausa (Engelm.) Sarg.) and a drought-hardy loblolly pine (P. taeda L.) had the largest reductions in biomass after 8 weeks in anaerobic solution, whereas the more flood-tolerant pond pine (P. serotina Michx.) and wet-site loblolly pine seedlings were least affected. Anaerobic growth conditions reduced P concentrations in the shoot and increased P concentrations in the root. Short-term (32)P-uptake experiments were conducted with intact seedlings to determine absorption rates (influx) of inorganic phosphate (Pi) by aerobically and anaerobically grown pine seedlings. Influx of Pi was weakly correlated with shoot fresh weight, root fresh weight, root P and shoot P per gram fresh weight root, suggesting that internal P levels and biomass influenced Pi absorption. The highest Pi absorption rates were found in anaerobically grown seedlings under anaerobic (32)P-uptake conditions. Of these, sand pine had the highest absorption rate in 50 microM KH(2)PO(4) (0.96 micromol Pi g(-1) FW root h(-1)), and the wet-site loblolly pine exhibited the lowest rate (0.24 micromol Pi g(-1) FW root h(-1)). Aerobically grown seedlings had similar Pi absorption rates that were not significantly affected by O(2) concentration in the (32)P-uptake solution.