Fig. 1. Variation in the response of A to a change in Ci. The plotted curves are derived from leaf gas-exchange measurements on wheat genotypes reported in Condon et al. (1990). Data points at ambient Ca are plotted for three ‘genotypes’. These data points indicate that lower values of Ci (and of Ci/Ca), and thus greater values of A/T, may be achieved through either greater photosynthetic capacity (indicated by a steeper initial slope of the solid lines originating near the origin) or by lower stomatal conductance (indicated by a smaller slope in the dashed lines originating at Ca). Changes in Ci/Ca can also be achieved through changes in both capacity and conductance (adapted from Condon and Hall, 1997. Reprinted with permission from Elsevier.).
Fig. 2. (A). Relationship between Ci/Ca measured on flag leaves and 13C measured in dry matter of growing ears of wheat enclosed in the flag leaf sheath, for 13 genotypes. The growing ear is a major sink for C assimilated by the flag leaf. The solid line is a linear regression fitted to the data points. The dashed line is the relationship 13C=4.4+23.6Ci/Ca (equation 8) (adapted from Condon et al., 1990 with permission from CSIRO Publishing.). (B). Relationship between plant water-use efficiency (g dry matter produced kg–1 water transpired) and 13C of plant dry matter, for 16 wheat varieties grown without water limitation in large pots sealed at the soil surface to eliminate soil evaporation. The solid line is a linear regression fitted to the data points (adapted from Condon et al., 1990 with permission from CSIRO Publishing.).
Fig. 3. Relationships between measures of crop productivity and 13C for recombinant inbred lines of bread wheat grown under rainfed conditions at Condobolin, eastern Australia, in 1992 and 1993. Closed symbols are for 1992 and open symbols are for 1993. The lines were F6 (1992) and F7 (1993) progeny of crosses between parents with low and high values of 13C. The values of 13C plotted are genotype means for leaf material of F5 lines sampled early in the 1991 season when there was no soil water deficit. Least-squares linear regressions are shown for statistically-significant relationships (P <0.05, n=30) (adapted from Condon and Hall, 1997. Reprinted with permission from Elsevier.).
Fig. 4. (A) The relationship between above-ground biomass production to anthesis and leaf 13C, and (B) the relationship between leaf 13C and stomatal conductance, for eight wheat genotypes grown at Moombooldool, eastern Australia in 1985. Values of 13C are average values for leaves from well-watered plants sampled well before anthesis. Values of stomatal conductance are the average of data obtained at five sampling times before anthesis. Least-squares linear regressions are shown for statistically-significant relationships (P <0.05). Open symbols indicate genotypes with low photosynthetic capacity (adapted from Condon et al., 1990, 1993 with permission from CSIRO Publishing.).
Fig. 5. Summary of results of wheat crop growth simulations using the ‘SIMTAG’ crop growth model and long-term weather data (30–50 years) showing the average change in yield, in three environments in eastern Australia, of breeding for either higher A/T, greater early vigour, or a combination of higher A/T and greater early vigour (Condon and Stapper, 1995; AG Condon et al., unpublished results). Simulations were done for autumn-sown crops grown in (i) a northern, sub-tropical environment where crops have a strong reliance on sub-soil moisture stored from summer rainfall, (ii) a southern environment with a winter-dominant rainfall pattern where the crop relies heavily on within-season rainfall, and (iii) an environment between these two, where the average rainfall distribution is even but rainfall is highly variable in amount and seasonal distribution.
Fig. 6. (A) Average grain yield advantage of BC2 bread wheat breeding lines selected for low-13C compared with BC2 breeding lines selected for high-13C, grown in nine environments in eastern Australia (closed symbols) and five environments in Western Australia (open symbols) from 1995 to 1998. Grain yield advantage of low-13C breeding lines was calculated as [(average yield of low-13C selections)–(average yield of high-13C selections)]/[average yield of high- 13C selections] (adapted from Condon et al., 2002 with permission from The Crop Science Society of America.). (B) Grain yield at 12 locations in SE Australia in 2003 of the low-13C cultivar Drysdale relative to the grain yield of a widely-grown variety currently recommended in that region (check variety). (Data in Fig. 6B courtesy of Agritech Research Services) Solid lines indicate statistically significant relationships (P <0.05).