Dehydration triggered ABA biosynthesis is temporarily deactivated in plants sustaining low Ψl
In the angiosperm species U. californica, we found that rapid bench dehydration of never-before stressed branches triggered a fast and considerable accumulation of foliage ABA levels (Figure 6). In U. californica , foliage ABA levels increased by 0.345 µg g-1 FW h-1 as branches were dehydrated on the bench (Figure 6). In branches collected from plants under long-term drought at least 10 d after peak foliage ABA levels, Ψl decreased rapidly when bench dried while ABA levels remained low and unchanged (less than 0.767 ± 0.092 µg g-1 FW, mean ± SE, Figure 6). Ψl in never-before stressed branches of U. californica which accumulated considerable levels of ABA on rapid dehydration decreased at a slower rate than for branches that were collected from plants that had experienced long-term drought, with WP at 3.25 h after the initiation of dehydration being -0.50 MPa in never-before stressed branches and -2.47 MPa in branches collected from plants experiencing long-term drought that were not accumulating foliage ABA levels (Figure 6). Similarly, in the conifer species C. rhomboidea foliage ABA levels increased in unstressed branches from 0.153 ± 0.1 µg g-1 FW to 1.743 ± 0.3 µg g-1 FW at 6 h while in branches collected from plants under long-term drought at least 10 d after peak foliage ABA levels were measured displayed relative low levels of foliage ABA increasing from 0.090 ± 0.02 µg g-1 FW to just 0.225 ± 0.024 µg g-1 FW at 6 h (Figure 6). Ψl in never-before stressed branches of the coniferC. rhomboidea , like those of U. californica , decreased at a slower rate than that of branches in which ABA levels did not accumulate on rapid bench dehydration. Ψl decreased to -1.83 MPa in never-before stressed branches and -3.96 MPa in branches from plants under long-term drought at 6 h (Figure 6). The ability to synthesize ABA on rapid bench dehydration recovers in U. californica plants after rewatering (Figure 7). In plants that were drought stressed for 10 d after peak ABA levels were measured then rewatered to saturating soil water capacity, branches harvested 3 d after rewatering and then rapidly dehydrated on the bench were able to synthesize ABA over 10 h (Figure 7). Mean foliage ABA levels in these branches increased from 0.26 to 1.97 µg g-1 FW in 10 h (Figure 7).