Catabolism of ABA to phaseic acid cannot explain declining ABA levels during drought
Foliage phaseic acid (PA) levels displayed a peaking-type dynamic as Ψl declined during drought in the angiosperm speciesU. californica (Figure 2). PA levels were found to be significantly correlated with ABA levels in U. californica(Pearson’s r57 = 0.74, p < 0.0001) while no significant correlation between PA and ABA levels was observed in the conifer species C. rhomboidea (Pearson’sr23 = 34, p = 0.09234). In U. californica mean foliage PA levels were 0.292 ± 0.095 µg g-1 FW (± SE) at -0.7 MPa prior to the drought, then as Ψl decreased to -3.3 MPa foliage PA levels increased reaching a maximum of 0.969 ± 0.063 µg g-1 FW (±SE) (Figure 2). In U. californica as drought progressed foliage PA levels declined to a minimum of 0.545 ng g-1 FW at -6.02 MPa, at a rate of 0.167 µg g-1 FW MPa-1 (Figure 2). In the conifer species C. rhomboidea , there was very little change in PA levels as drought progressed (Figure 2). PA levels prior to drought were low at a mean of 0.107 ± 0.1 µg g-1 FW at -0.65 MPa, there was one period during drought where there was a wide variation in PA levels to a maximum of 0.504 ± 0.08 µg g-1 FW around -3.51 MPa (Figure 2). However, PA levels remained at 0.15 ± 0.013 µg g-1 FW (± SE) once Ψl had declined to -5 MPa.