Peaking-type ABA dynamics in Umbellularia and Callitris
During a soil drought, the evergreen angiosperm speciesUmbellularia californica , like the conifer speciesCallitris rhomboidea , displayed a peaking-type dynamic in foliage ABA levels (Figure 1). Prior to drought at a water potential of -0.7 MPa, mean foliage ABA levels in U. californica were 0.714 ± 0.61 µg g-1 FW (± SE, Figure 1A). As leaf water potential (Ψl) declines, ABA levels in U. californica rose to a mean peak of 0.4924 ± 0.4 µg g-1 FW (± SE) once Ψl had reached to -3.1 MPa (Figure 1A). As U. californica experienced more negative Ψl ABA levels gradually declined, decreasing back to a mean of 0.109 ± 0.6 µg g at a Ψl of -6.02 MPa (Figure 1A). ABA levels in U. californica decreased at an average rate of 0.15 µg g-1 FW MPa-1 during this time. InC. rhomboidea mean initial ABA level prior to drought was 0.236 ± 0.11 µg g-1 FW (±SE), this increased to a mean peak ABA level of 0.845 ± 0.8 µg g-1 FW (±SE) at -2.7 MPa (Figure 1B). After ABA levels peaked they declined by an average of 0.26 µg g-1 FW MPa-1 to 0.275 ± 0.13 µg g-1 FW (±SE) at -5 MPa (Figure 1B). Evidence of leaf death and a lack of recovery in maximum photosynthetic rate following rewatering for both species occurred once Ψl declined less than -7 MPa (data not shown). In both species, canopy conductance (gc) declined exponentially as Ψldeclined, with an 83% to 99% reduction in gconce Ψl had declined to the Ψl at which peak levels of foliage ABA occurred in U. californica andC. rhomboidea , respectively (Figure 1C and D). In U. californica mean (± SE) maximum gc prior to drought was 40.9 ± 0.2 mmol m-2 s-1, this exponentially declined to a minimum of 0.42 ± 0.2 mmol m-2 s-1 by -6.02 MPa (Figure 5). InC. rhomboidea, mean (± SE) maximum gcprior to drought was 17 ± 0.1 mmol m-2s-1, this declined to a minimum of 0.026 ± 0.13 mmol m-2 s-1 by -6.02 MPa (Figure 6). Stomata remained closed for the duration of the drought in both species.