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Newly Discovered Soluble Proteins in the Leaves of Sour Orange Trees: Do They Facilitate the Trees' Response to Atmospheric CO2 Enrichment?
Volume 6, Number 4: 22 January 2003

The large, positive, long-term, physiological responses of sour orange trees to atmospheric CO2 enrichment are well documented.  When continuously exposed to a 75% increase in the air's CO2 concentration (an increase of 300 ppm above the local background value of 400 ppm), the trees exhibit an approximate 80% increase in wood biomass and fruit production (Idso and Kimball, 2001), an 80% increase in water use efficiency (Leavitt et al., 2003), and 2 to 15% increases in the vitamin C concentration of the juice of their fruit (Idso et al., 2002).  Although even larger growth responses have been observed in other trees - primarily coniferous species, which Saxe et al. (1998) purport to exhibit a mean growth increase of 130% in response to "close to a doubling" of the air's CO2 content - it is difficult to imagine how these tremendous increases in productivity can be maintained by simply making more carbon dioxide available to the trees.  Hence, in an attempt to dig a little deeper into this conundrum, the three of us set out about ten years ago upon a long and tedious journey to look for other less obvious phenomena that might possibly facilitate these amazing responses.

The first leg of our journey led to the discovery that, although spring branch growth typically begins on the very same day of the year in both the ambient and CO2-enriched orange trees, the initial rate of new branch biomass production is much greater in the CO2-enriched trees.  In the study that first reported this discovery (Idso et al., 2000), for example, we found that three weeks after the start of spring branch growth, each new branch on the CO2-enriched trees was more than four times more massive than its counterpart on the ambient-treatment trees; and on a per-tree basis, there was over six times more new-branch biomass on the trees growing in the 700-ppm CO2 treatment than in the 400-ppm CO2 treatment.  Then, just as dramatically as this CO2-induced ultra-enhancement of new spring branch growth began, it ended; and ten weeks into the growing season, the CO2-induced stimulation of new-branch biomass had leveled out at a value commensurate with the long-term enhancement of wood biomass and fruit production, i.e., approximately 80%.

This tremendous initial stimulation of new spring branch and leaf growth in the CO2-enriched sour orange trees, which quickly establishes a greatly increased capacity for further augmented growth, is probably what enables the trees in the high-CO2 air to maintain such a high level of wood and fruit production throughout the remainder of the growing season.  Likewise, the elevated season-long productivity of the CO2-enriched trees is probably what enables them to grow so rapidly in the spring.

Others have both thought and observed much the same thing.  In a study of the tropical tree species Luehea seemannii, for example, Lovelock et al. (1999) found that an increase in the atmosphere's CO2 concentration enhanced net photosynthesis rates throughout the long tropical growing season, and that the carbohydrates thereby produced were stored in terminal branch tissues, from whence they speculated they could be withdrawn early in the following year to facilitate greater first-flush leaf growth.  Similarly, Olszyk et al. (2001) found that an increase in the air's CO2 concentration increased the bud lengths of Pinus ponderosa seedlings at the end of the growing season, leading to a strong but transitory stimulation of the growth of the seedlings' terminal buds the following spring.

But where do CO2-enriched sour orange trees get the extra nitrogen (N) that is needed to support the hugely-enhanced production of new branch and leaf tissue (six-fold more per tree) that occurs at the beginning of each growing season?  In an experiment that may hold the answer to this question, Idso et al. (2001) measured the concentrations of three soluble leaf proteins having molecular masses of 33, 31 and 21 kDa at weekly intervals for a period of one full year in the foliage of the CO2-enriched and ambient-treatment trees.  During the central portion of the year, the abundances of the three proteins were generally lower in the leaves of the CO2-enriched trees than they were in the leaves of the ambient-treatment trees; but in the latter part of the year and continuing for a short while into the next year, this relationship was reversed, as the leaf concentrations of the three proteins in the CO2-enriched trees surpassed those of the proteins in the trees growing in ambient air.  Then in the spring, when new growth began to appear, the concentrations of the three proteins in the foliage of the CO2-enriched trees plummeted, possibly as a result of giving up the nitrogen they had stored over winter to supply the needs of the newly developing branches and leaves.

The idea sounds intriguing; but is there any other evidence to support it?  Actually, there is; in fact, there are a number of observations that argue for its validity.

First of all, Idso et al. (2001) found that the N-terminal amino acid sequence of the 21-kDa protein has homology with sporamin B, an implicated vegetative storage protein (VSP) that can comprise 60-80% of the total soluble protein found in sweet potato tubers.  They also determined that it shares low sequence homology with trifoliatin, a soluble leaf protein from trifoliate orange that shares 62% amino acid similarity with sporamin B and can comprise up to 65% of total leaf protein.  In addition, they say that "immunoelectron microscopy demonstrated the presence of the proteins within amorphous material in the vacuoles of mesophyll cells, where VSPs are commonly located."

Perhaps the most telling evidence of all was the fact that - starting at about day-of-the-year 225 - the CO2-enriched trees experienced a period of leaf senescence and abscission that was not observed in the ambient-treatment trees.  "This phenomenon," Idso et al. (2001) report, "peaked at about day 300, when the CO2-enriched trees shed leaves at a rate 2.5-2.7 times greater than the normal background rate of the ambient-treatment trees, whereupon the enriched-tree leaf-fall rate diminished, returning to normal by the end of the year."

According to the hypothesis formulated by Idso et al. (2001), nitrogen reabsorbed from these second-year leaves during the process of senescence became available for storage in first-year leaves of the CO2-enriched trees, going initially into the 21-kDa protein starting at about day 225, into the 31-kDa protein starting at about day 265, and into the 33-kDa protein starting at about day 335, according to the trends defined by their weekly measurements of leaf protein concentrations.  Then, when new branches and leaves began appearing in the spring, the stored nitrogen was remobilized from the prior first-year leaves (which now became second-year leaves) to supply the needs of the developing branch and leaf tissues, whereupon the concentrations of the three putative storage proteins in the now-second-year CO2-enriched leaves rapidly dropped to levels that were once again less than those observed in similar-age ambient-treatment leaves, and they remained at that reduced level until the second-year leaves gave up even more nitrogen prior to their senescence in the fall.

Whether this scenario is correct in all its particulars remains to be confirmed by others. That it is important that such be done is suggested by the fact that the 31- and 33-kDa proteins were detected only in citrus trees (but in all eight varieties tested) in a survey of a large selection of C3 and C4 plants that included both herbaceous and woody species (excluding coniferous trees), plus the possibility that the newly discovered proteins could be genetically exploited to perhaps enhance the responses of other plants to atmospheric CO2 enrichment.

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.

Idso, S.B. and Kimball, B.A.  2001.  CO2 enrichment of sour orange trees: 13 years and counting.  Environmental and Experimental Botany 46: 147-153.

Idso, S.B., Kimball, B.A., Shaw, P.E., Widmer, W., Vanderslice, J.T., Higgs, D.J., Montanari, A. and Clark, W.D.  2002.  The effect of elevated atmospheric CO2 on the vitamin C concentration of (sour) orange juice.  Agriculture, Ecosystems and Environment 90: 1-7.

Leavitt, S.W., Idso, S.B., Kimball, B.A., Burns, J.M., Sinha, A. and Stott, L.  2003.  The effect of long-term atmospheric CO2 enrichment on the intrinsic water-use efficiency of sour orange trees.  Chemosphere 50: 217-222.

Lovelock, C.E., Virgo, A., Popp, M. and Winter, K.  1999.  Effects of elevated CO2 concentrations on photosynthesis, growth and reproduction of branches of the tropical canopy tree species, Luchea seemannii Tr. & Planch.  Plant, Cell and Environment 22: 49-59.

Olszyk, D.M., Johnson, M.G., Phillips, D.L., Seidler, R.J., Tingey, D.T. and Watrud, L.S.  2002.  Interactive effects of CO2 and O3 on a ponderosa pine plant/litter/soil mesocosm.  Environmental Pollution 115: 447-462.

Saxe, H., Ellsworth, D.S. and Heath, J.  1998.  Tree and forest functioning in an enriched CO2 atmosphere.  New Phytologist 139: 395-436.