Trichechus manatus Manatí Antillano I 2 Odocoileus virginianus Venado Cola Blanca

Since the greater AGDW by barley and triticale was not due to greater ET, the difference must be in greater WUE. Water-use efficiency depends on both TE and the ratio of soil evaporation as a proportion of evapotranspiration (equation 4.2). A lower soil evaporation relative to evapotranspiration was a consistent feature of the two barleys in each of these experiments. Averaged over the three experiments for which it was calculated (C89, M89 and M90), the ratio of Eg/Ej was 0.31 for the barleys, 0.37 for the bread wheats and 0.39 for Dua triticale and Hakea oats. Siddique et al. (1990b) also observed lower Es/ET for O'Connor barley (0.34) compared to the wheats they grew (average 0.39 for wheats with similar flowering date to O'Connor). The results described in chapter 3 indicate that the lower E$/ET for barley resulted from faster leaf area development. Barley had the highest leaf area index (LAI), developed main stem leaves fastest and hence had the shortest phyllochron interval, produced the most tillers and with exception of oats had the highest specific leaf area (SLA).

These results indicate that reducing Es as a proportion of ET contributes to greater WUE by barley. It meant that T was always high for Ulandra barley, but T was still only intermediate for O'Connor, which produced the most AGDW. This indicates that variation in TE was also important, especially for O'Connor. Table 4.6 compares O’Connor with the other genotypes for TEAGDW and T as a proportion of ET averaged over the three field experiments. It shows that TE was the most important factor contributing to differences between O'Connor and Ulandra. A higher TE and also a higher T relative to ET accounted for the better performance of O'Connor compared to Rosella, Dua and Hakea. It was notable that Meteor, the only hybrid wheat in the study, was more like barley in having a high ratio of T/Et .

The higher TE of O'Connor relative to all other genotypes could arise in several ways. According to equation (4.1), high TE of leaf gas exchange may result from either a lower e^-ea and/or a lower PJPX Carbon isotope discrimination (A) is an integrated measure of Pj/Pa (Farquhar et al. 1982), so if PJP^ is the dominant factor then A should be negatively related to TE. This was not found. In fact A averaged over all experiments was positively associated with TEAGDW at each site (C89, r=0.89, P<0.01; M89, r=0.87, P<0.01; M90, r=0.60, P<0.05). The value of A was also significantly associated with final AGDW at maturity (Fig.4.8) (AGDW (%)=8.42(A)-72.5, r=0.70, PcO.Gl). This has also been found in field grown plants in wheat (Condon et al. 1987; Ehdaie et

Table 4.6 Comparison between O'Connor barley and the other genotypes for TEagdw anc* transpiration (T) relative to soil evaporation (Es). Values are the percentage by which O'Connor exceeds the other genotypes.

Genotypes _{t^ a g d w} <%) t/e t

m

Fig 4.8 Relationship between Final above-ground dry weight (AGDW) at maturity and carbon isotope discrimination (A ) for barley ( ■ ), bread wheat ( ▲ ), durum wheat ( a ), triticale ( □ ) and oats ( • ) across Five Field environments. (AGDW = 8.42( A )-72.5, r=0.70, P<0.01)

al. 1991), barley (Craufurd et al. 1991) and crested wheat grass (Read et al. 1991). In these latter studies water use, soil evaporation and root biomass were not determined, which may account for the positive associations they observed. However, water use and soil evaporation were measured here. Root biomass was also measured but accounting for root biomass had little impact on the relationship between TE and A. The slope of the relationship between TEAGDW and A at C89 (Fig. 4.9) indicates that TEAGDW increased by 43% with a change in mean A from 19.5 x 10*3 to 21.5 x 10'3. For the same change in A, there was still a 43% increase in TEB. In other words the relationship between TEB and A was still strongly positive (r=0.97, PcO.Ol), not negative as might be expected from leaf gas exchange theory.

Two factors may account for this positive relationship. The first, which will almost certainly be important, is the greater growth by barley (and hence greater T) when VPD is low. The influence of seasonal changes in VPD can be accounted for by calculating, for each genotype, an "effective transpiration" (=transpiration/average VPD) for each of the periods between soil water measurements. This procedure, based on the analysis presented by Sinclair et al. (1983), is similar to that used by Hubick and Farquhar (1989) in accounting for differences in VPD between two glasshouse experiments with different barley genotypes. Because of data limitations the calculations were only possible for C89. At this site, when summed over the whole season, the values of "effective transpiration" were 458, 403, 345, 323, 346 and 347 mm.kPa*1 for O'Connor, Ulandra, Meteor, Rosella, Dua and Hakea respectively. On average, the value for the barleys was 26% greater than for the other species. This means that transpired water was used 26% more effectively in producing dry matter in the barleys than in the other species. Fischer (1981) pointed out that growth during the winter months is cheap in terms of transpirational cost. In these experiments, by maximising its early growth, barley achieved a greater "effective transpiration" than the other species, and thereby a higher TE. When this advantage is accounted for in calculating TEB, the strong positive relationship between TEB and A is largely eliminated (Fig. 4.9). Nevertheless, the relationship between TEB and A is still not negative.

This may be due to yet another factor, the effect of boundary layer conductance on transpiration from field plots. In calculating "effective transpiration" it was assumed that the driving force for transpiration was VPD. This is not strictly the case. The true driving force is ex-ea. This will be proportional to VPD if ex (the vapour pressure inside the leaf) is constant, but this will only occur if leaf temperature is constant. If variation in A is the result of variation in stomatal conductance, then leaf temperature is unlikely to be constant. In the field, genotypes with low conductance will tend to have hotter canopies because of the greater influence of the boundary layer on leaf gas exchange. A hotter canopy means that transpiration will be driven faster per unit conductance, reducing any gain in TE from lower conductance (Cowan and Troughton, 1971; Jarvis

Fig 4.9 Relationships between transpiration efficiency at C89 and carbon isotope discrimination ( A, mean of all sites) for barley ( ■ ), bread wheat ( ▲ ), triticale ( □ ) and oats ( • ). Transpiration efficiency was calculated on the basis of above­ ground dry weight (TEAGDW = 8.15A-121.05, r=0.89, PcO.Ol), on the basis of total biomass (TEB= 11.09A-164.29, r=0.97, PcO.Ol) and with TEB normalised for seasonal variation in VPD (Normalised TEB=0.0793A+1.148, r=0.86, PcO.Ol).

and Me Naughton, 1986; Cowan, 1988). Little is known of the stomatal conductance in these species, but given the large range in A values, it is likely that conductance is lower and hence the field canopies warmer in Ulandra and Rosella than in O'Connor and Meteor (Condon et al. 1990).