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Using carbon isotope discrimination to predict water-use efficiency

K.T. Mubick and G.D. Farquhar

Research School of Biological Sciences Australian National University, GPO Box 475, Canberra, ACT 2601

The process of photosynthetic CO2 uptake "costs" transpiration water. For a plant, transpiration efficiency, the ratio of dry matter accumulation to water lost is, in part, a function of pi/pa, the ratio of the CO2 pressure inside leaves, pi, to that around the leaves, pa. Transpiration efficiency, W, of leaves and whole plants is negatively related to pi/pa. For a crop, water-use efficiency, which includes all sources of water loss in addition to transpiration, will also be related to pi/pa if there is no genetic variation in parameters such as early growth when the soil evaporation component of evapotranspiration is relatively large, or crop architecture which can create significant aerodynamic resistances to heat and vapour transfer.

There is also a relationship (positive, in this case) between carbon isotope discrimination and pi/pa in C3 species. Figure 1 shows the relationship between carbon isotope discrimination, Δ, and pi/pa in leaves of different peanut genotypes. At any instant in time, different genotypes tended to have different values of pi/pa and of Δ. The genetic control of Δ is strong in leaf or stem material of peanut, cowpea, barley, and wheat (2). Genotypic ranking of A tends to be maintained in these species when they have been grown at different sites with different water availability. The broad sense heritabilities (proportion of total variance of Δ which can be ascribed to genotype, rather than to environment or to interactions between the two) ranges between 60% and 90%. Discrimination may be a good predictor of yield in wheat (1).

Because transpiration efficiency is related (negatively) to pi/pa and carbon isotope discrimination is related (positively) to pi/pa, we expect a negative relationship between W and Δ. This has been shown to be true in the following C3 species: wheat, peanut, barley, cotton, and tomato (2). Figure 2 shows a plot of the negative relationship between W and Δ in pot-grown plants of barley cultivars. This has also been shown in field-grown peanut cultivars (3,4). The results to date show promise for the use of A as a predictor of water-use efficiency.

1. Condon, A.G., Richards, R.A., and Farquhar, G.D. 1987. Crop Science 27, 996-1001.

2. Farquhar, G.D., Ehleringer, J.R., and Hubick, K.T. 1989. Annual Review Plant Physiology Plant Molecular Biology 40, 503-537.

3. Wright, G.C., Hubick, K.T., and Farquhar, G.D. 1988. Australian Journal of Plant Physiology 15, 815-925. ibid. Proc. 5th Aust. Agron. Conf. Perth.

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