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JXB Advance Access originally published online on September 24, 2004
Journal of Experimental Botany 2004 55(408):2599-2605; doi:10.1093/jxb/erh263
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Journal of Experimental Botany, Vol. 55, No. 408, © Society for Experimental Biology 2004; all rights reserved

RESEARCH PAPER

Apparent respiratory discrimination is correlated with growth rate in the shoot apex of sunflower (Helianthus annuus)

T. W. Ocheltree* and J. D. Marshall

College of Natural Resources, University of Idaho, Moscow, Idaho, ID 83844-1133 USA

* Present address and to whom correspondence should be sent: Department of Forest Science, Oregon State University, 321 Richardson Hall, Corvallis, OR 97331, USA. E-mail: troy.ocheltree{at}oregonstate.edu

The literature offers no consensus as to whether the {delta}13C of respired CO2 is identical to that of the respiratory substrate, perhaps because of differences in measurement technique and growth conditions. To address this issue, the {delta}13C of respired CO2 from growing sunflower shoot apices was measured and compared with that of soluble carbohydrates extracted from the respiring tissues. Shoot apices were studied because any influence of growth and biosynthesis was expected to be maximally expressed in these rapidly growing tissues. The two most probable substrates, starch and soluble sugars, were similar in {delta}13C (P=0.46). The {delta}13C of respired CO2 was enriched in 13C compared with these putative substrates (P<0.0001). This apparent enrichment ranged from 2.2{per thousand}–5.7{per thousand}, and decreased with relative growth rate (P<0.0001). The respiratory enrichment was counterbalanced by a depletion in the tissue constructed from the residual carbohydrates. The depletion varied from 2.2{per thousand} to 3.0{per thousand} relative to soluble carbohydrates (P<0.05), as predicted from mass-balance arguments. These results support the idea that respired CO2 is enriched relative to its substrates. Variation in growth rates may help to explain the variable amounts of respiratory discrimination described in the literature.

Key words: Discrimination, Helianthus annuus, respiration, stable carbon isotopes


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