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Variation in carbon isotope discrimination among diverse sorghum hybrids

K.T. Hubick1, G.L. Hammer2, G.D. Farquhar1, L.J. Wade3, S. von Caemmerer1 and S. Henderson1

1 R.S.B.S., Aust. Nat. Univ., GPO Box 475, Canberra, ACT 2601
Qld. Dept. Prim. Ind., GPO Box 46, Brisbane, Qld. 4001
Qld. Dept. Prim. Ind., PO Box 81, Emerald, Qld. 4720

There are two naturally occurring stable isotopes of carbon. Their abundance ratio, 13C/12C, is different in plants from the atmosphere because 13CO2 and 12CO2 are photosynthetically assimilated at different rates (1). For plants with the C3 pathway, variation in carbon isotope discrimination (A) within a species has been correlated with transpiration efficiency and crop yield (1). Within a C4 species, such as sorghum, variation in A has not been reported. Models predict that in C4 photosynthesis, A should depend on leakiness of the bundle sheath cells and/or the ratio of assimilation rate to stomatal conductance (2). The former should be related to radiation use efficiency (RUE) and the latter to transpiration efficiency (TE). Variation among sorghum genotypes for RUE and TE has been reported (3,4). In this study we examined diverse sorghum hybrids for variation in A.


Twelve hybrids of sorghum were grown in two dryland field experiments planted 3 February 1988 at Emerald in central Queensland. Time of flowering was recorded in each plot and grain yield was determined from a 16 m2 quadrat in each plot. Samples of leaf tissue were collected from each plot shortly before harvest, finely ground, and measured for A by mass spectrometry.

Results and discussion

Hybrids differed significantly in 4. This is the first finding of significant variation in A among genotypes in one C4 species.

A correlated negatively with grain yield (Fig. 1). Hybrid Texas 610SR had a high value for A (4.83) and a low grain yield (2400). This was consistent with its low RUE (3), which could be related to high

leakiness. However, it is also consistent with its high TE (4), which could be related to low conductance. Hybrid ATx623/RTx430 was not included in this study. Further investigation is required to understand this variation as it has direct consequences in plant


Fig. 1 Grain yield vs carbon breeding. Isotope discrimination for 12 sorghum hybrids.

1. Farquhar, G.D. et al. 198g. Ann. Rev. Plant Physiol. Plant Mol. Biol. 40:503-537

2. Farquhar, G.D. 1983. Aust. J. Plant Physiol. 10:205-226

3. Hammer, G.L. et al. 1989. Proc. Aust. Agron. Conf., Perth

4. Donatelli, M. et al. 1989. Proc. Aust. Agron. Conf., Perth

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