Environmental and physiological control of stomates in Douglas-fir and other species Public Deposited

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  • This research dealt with the direct environmental influence of atmospheric vapor pressure on stomates of Pseudotsuga menziesii, Picea engelniannii, and Fragaria X ananassa, and the indirect physiological control of stomates by abscisic acid in Pseudotsuga menziesii. Two ventilated porometers (diffusion and steady-state) were compared on four broadleaf and five coniferous species. The diffusion porometer gave consistently lower conductance values for both types of species, reflecting a direct stomatal response to low chamber humidity. At high conductance values, the porometers produced nearly equal response, but the diffusion porometer was less sensitive at low conductance values. This was due to lower wind velocity (20% of the velocity in the steady-state porometer) and water vapor sorption by its acrylic plastic chamber. The rapid response of stomates to changes in atmospheric vapor pressure and its interaction with plant water stress was characterized in the three species above. All of the species showed similar curvilinear response of enhanced conductance at low vapor pressure deficit and depressed conductance at high vapor pressure deficit. The response time ranged from 30 seconds at high VPD to 2 minutes at low VPD. Engelmann spruce was more sensitive than either Douglas-fir or strawberry, which were equally sensitive. Plant water status significantly altered stomatal response to humidity. The relationship of conductance to xylem water potential was linear under ambient conditions (15 mb), but became curvilinear when conductance was measured at a vapor pressure above (20 mb) and below (7 mb) ambient. Between -0.5 MPa and -2.0 MPa, the stomates were sensitive to vapor pressure deficit, but below -2.0 MPa, this sensitivity lessened. This desensitization was attributed to the increase in abscisic acid overriding the influence of atmospheric humidity. The photoisomerization kinetics of the geometric isomers of abscisic acid were studied in solution and in seedlings of Douglas-fir. In vitro, 250-nm light caused photolysis of both isomers. The 350-nm light and sunlight were comparable in isomerizing ABA and 2-trans-ABA with a half-time of about 3 minutes, whereas isomerization under fluorescent light took longer (half-time = 7 minutes). Methylation of the isomers caused the photoisomerization half-time to increase in all of the light sources. The equilibrium concentration of 55% ABA and 45% 2-trans-ABA was consistent for the free acid and the methyl ester under all radiation treatments. The absorption spectra of the isomers provided evidence for the higher concentration of the cis-isomer at photo-equilibrium. In vivo, no significant conversions in ABA, 2-trans-ABA or their saponifiable conjugates were observed in either 350-nm or fluorescent light. However, the presence of 2-trans-ABA and its conjugate in the needle extracts suggests that isomerization occurs, in vivo. The changes in ABA and its metabolites in Douglas-fir needles were followed through two drought cycles and its effect on stomatal conductance was determined. Leaf conductance showed the typical water potential threshold, decreasing abruptly at -2.0 MP. This corresponded to the simultaneous increase in ABA level, from 500 to 850 ng g-1. No adjustment to stress was observed in any of the relationships examined, but stress progressed at a slower rate during the second cycle. Metabolism of ABA was found to differ from previous studies. A linear relationship between ABA arid its conjugate strongly implicated the importance of the interconversion of the two compounds for storage and supply of the free acid. This may have been due to tissue age and water stress preconditioning. Phaseic acid and epi-dihydrophaseic acid were the primary metabolites. Changes in trans-dihydrophaseic acid paralleled ABA providing evidence for concentration-dependent photoisomerization, either of ABA or one of its metabolites.
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