The role of endogenous ABA and exogenously applied ABA and ABA analogs on the gas exchange of conifer seedlings
Date
1998
Authors
Fuchs, Edgar Edmund
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Abstract
Two main sets of experiments were conducted. The first set of experiments was conducted to determine the relative influence of root and leaf water status on stomatal conductance (gc) in one-year-old Douglas fir (Pseudotsuga menziesii (Mirb.) Franco) and 2-3 month old alder (Alnus rubra (Bong)) grown in drying soil. These results have been published and are presented in the appendix. The second set of experiments was conducted to: a) determine structure-function relationships of ABA analogs in white spruce (Picea glauca (Moench) Voss) and b) evaluate which analogs might assist conifer seedlings planted on reforestation sites maintain a favorable water balance and thereby improve seedling establishment.
The relative influence of root vs leaf water status on gc was investigated by manipulating shoot water status independently of the roots by using a pressure chamber that enclosed the root system. Pressurizing the chamber increases the turgor of cells in the shoot but not in the roots. Seedling shoots were enclosed in a whole plant cuvette and transpiration and carbon assimilation rates measured continuously. In both species stomatal closure in response to soil drying was progressively reversed with increasing pressurization. Responses occurred within minutes of pressurization and almost immediately returned to pre-pressurization levels when the pressure was released. Even in wet soils, there was a significant increase in gc with pressurization. In Douglas-fir, the stomatal response to pressurization was the same for seedlings grown in dry soils for up to 120 days as for those subjected to drought stress over 40 to 60 days. The stomatal conductance of both Douglas-fir and alder seedlings was less sensitive to root chamber pressure at higher vapor pressure deficits (D) and stomatal closure in response to increasing D from 1.04 to 2.06 kPa was only partially reversed by pressurization. My results are in contrast to other studies on herbaceous species, even though I followed the same experimental approach. They suggest that it is not always appropriate to invoke a "feedforward" model of short-term stomatal response to soil drying, whereby chemical messengers from the roots bring about stomata! closure.
ABA analog structure-activity relationships were detem1ined by testing an array of 19 different ABA analogs on 1-year -old clonal white spruce (Picea glauca (Moench) Voss) raised from somatic embryos. The contribution of specific structural features to analog activity was determined from the relative effect of aeroponically applied 10-3 M analog solutions on seedling gas exchange. Seedling transpiration and carbon assimilation rates were measured continuously during treatment by means of a whole plant cuvette system which enclosed the shoot. The analogs were racemic about the C-1, chiral center and were derived from changes imposed on 6 regions of the ABA molecule. Changes were made in; the C-1 oxidation level (the acid functionality was changed to the ester, aldehyde and alcohol), the side chain bond order at C-4, C-5 ( changed from a trans double bond to an acetylenic linkage), ring saturation (varied by replacing the C-2 ', C-3 ' ring double bond with a single bond or replacing the C-5' , C-6' single bond with a double bond), the C-7' methyl (fluorinated to form CHF2), and at C-2 of the side chain (C-1 and C-2 were deleted and replaced with a ketone terminus. The activity of optically pure (+)-S-ABA and (-)-R-ABA were also determined. Activity was reduced by any change in the C-1 functionality and ring structure natural to ABA. The ring C-2 ', C-3 ' double bond was important but not essential to activity. The activity lost through changes in ring structure and C-1 functionally was, in many cases, almost fully restored by replacing the C-4, C-5 double bond with an acetylene bond. Therefore, acetylenic analogs were more active than their equivalents with a dienoic side chain overall. Fluorination of the C-7' methyl caused a relatively moderate reduction in analog activity. Truncation of C-1 and C-2 from the side chain reduced activity to near zero. Racemic ABA was as active as optically pure ( + )-ABA but required more time to achieve its full effect. The unnatural , (-) ABA enantiomer was inactive. At least 5 of the analogs caused large reductions in spruce seedling transpiration rate and also improved seedling water use efficiency. This suggests that these analogs, if applied to seedlings prior to planting on reforestation sites, may reduce seedling water stress and increase seedling survival rate.
The reduction in transpiration rate in Picea in relation to the concentration of (+)ABA applied to the roots was determined through a separate set of experiments. At least 1-3 M (±) ABA had to be applied to bring about significant stomatal closure. This contrasts with experiments in wheat (S. Kaul, University of Victoria, personal communication) in which application of 10-5 M (± ) ABA closed stomata. [3H]-ABA incorporation experiments performed on both wheat and spruce seedlings provided evidence that spruce is less responsive to exogenously applied ABA than wheat because ABA is absorbed into its shoot less efficiently and because spruce is inherently less sensitive to ABA.