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The Shuttling of Nitrogen from One-Year-Old to Current-Year Foliage in CO2-Enriched Atmospheres
Volume 11, Number 26: 25 June 2008

In a paper recently published in Tree Physiology, Maier et al. (2008) describe the effects of a nitrogen fertilizer application on upper-canopy needle morphology and gas exchange in approximately 20-meter-tall loblolly pine (Pinus taeda L.) trees previously exposed to elevated atmospheric CO2 concentrations (200 ppm above ambient) for nine years at the Duke Forest FACE facility in Orange County, North Carolina, USA. This work revealed that during the tenth year of exposure to elevated CO2, there was a strong enhancement (greater than 50%) of light-saturated net photosynthesis per unit leaf area across all age classes of needles, but that the stimulation was 28% greater for current year foliage than for one-year-old foliage. In addition, they report that current-year foliage incorporated the added nitrogen into photosynthetic components that increased the photosynthetic capacity of the current-year foliage, but that the one-year-old foliage tended to simply store extra nitrogen, which subsequently served as "an important source of nitrogen for the development of current-year foliage" via "efficient retranslocation of nitrogen from senescing one-year-old foliage to developing foliage."

These findings sounded eerily familiar to us, as we had observed a similar phenomenon several years earlier in sour orange tree (Citrus aurantium L.) foliage in an open-top chamber experiment we conducted at Phoenix, Arizona (Idso et al., 2001), where half of the trees we studied had been grown from the seedling stage for the prior six years in air that was continuously enriched with an extra 300 ppm of CO2. In the seventh year of that study, we identified three putative vegetative storage proteins located within amorphous material in the vacuoles of leaf mesophyll cells that was rerouted, "starting at about day 25 of the new year, into developing foliage on the new branch buds of the CO2-enriched trees." We speculated that this phenomenon may have been "the key that allows the CO2-enriched trees to temporarily stockpile the unusually large pool of nitrogen that is needed to support the large CO2-induced increase in new-branch growth that is observed in the spring," citing the work of Idso et al. (2000), who had previously found that 24 days after new-branch emergence in the spring, "the new branches of the CO2-enriched trees were, on average, 4.4 times more massive than the new branches of the trees growing in ambient air," and that "the total new-branch tissue produced on the CO2-enriched trees to that point in time was over six times greater than that produced on the ambient-treatment trees."

If there is a common mechanism that links our results with those of Maier et al., it could well revolve around the hypothesized vacuolar storage proteins we identified in the sour orange tree foliage. In this regard, we detected immunologically-related proteins in a variety of other citrus species, but not in 20 different grasses, shrubs and trees growing in the Biosphere 2 facility near Oracle, Arizona. Nevertheless, this possibility is deserving of further study; for if found to have merit, Idso et al. (2001) further speculated that the proteins in question "could possibly be genetically exploited to enhance the responses of other plant species to atmospheric CO2 enrichment," which could prove to be a valuable property, indeed, of agriculturally-important plants in a high-CO2 world of the future.

Sherwood, Keith and Craig Idso

References
Idso, C.D., Idso, S.B., Kimball, B.A., Park, H.-S., Hoober, J.K. and Balling Jr., R.C. 2000. Ultra-enhanced spring branch growth in CO2-enriched trees: can it alter the phase of the atmosphere's seasonal CO2 cycle? Environmental and Experimental Botany 43: 91-100.

Idso, K.E., Hoober, J.K., Idso, S.B., Wall, G.W. and Kimball, B.A. 2001. Atmospheric CO2 enrichment influences the synthesis and mobilization of putative vacuolar storage proteins in sour orange tree leaves. Environmental and Experimental Botany 48: 199-211.

Maier, C.A., Palmroth, S. and Ward, E. 2008. Short-term effects of fertilization on photosynthesis and leaf morphology of field-grown loblolly pine following long-term exposure to elevated CO2 concentration. Tree Physiology 28: 597-606.