Báo cáo lâm nghiệp: "Effects of exogenous ABA on photosynthesis and stomatal conductance of cut twigs from oak"

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Effects of exogenous ABA on photosynthesis and stomatal conductance of cut twigs from oak seedlings I. Scuiller E. Dreyer Laboratoire de Bioclimatologie et d’Ecophysiologie Forestiere, INRA Nancy, F-54280 Champenoux, Seichamps, France Introduction ed the reactions of cut twig photosynthesis to exogenous ABA. The effects of shoot removal gas exchange were assessed on Abscisic acid (ABA) plays a major role in of this technique with ABA. prior to use plant-water relations. It has been shown to promote stomatal closure in many spe- cies including trees (Johnson, 1987), and there is growing evidence that it could be Materials and Methods a root-produced effector for water stress reactions (Zhang et al., 1987). These Plant material assertions are based on studies with cut soil 3 yr old twigs supplied with exogenous ABA and seedlings, sand-peat on a grown (50/50, v/v) in 8I pots, were transferred into a on measured increases of ABA concentra- climate chamber (February) to accelerate bud tions in xylem sap. break prior to measurements conducted during However, many questions remain open March and April "1988. Species: Quercus robur Fig. 1.L., Q. petraea L. (seeds collected near to discussion: are the concentrations of Nancy), and Q. pubescens L. (Avignon). exogenous ABA necessary to promote a sensible reaction of the same magnitude as those of free ABA measured in the Gas exchange measurements xylem during water stress? Is the rapid These made in open flow chamber. were an stomatal closure promoted by exogenous Twig transpiration was estimated using a by- pass flow (300 1-ti-I), and net C0 assimilation 2 ABA the direct cause of the observed calculated from C0 reduction in the main 2 was decline in net photosynthesis (Downton (60 1-h- Chamber volume was 9 1; time flow ). 1 etal., 1988) or is there some direct ef- lags between apparent assimilation and transpi- fect of ABA on mesophyll photosynthesis ration appeared during rapid rate changes. Steady state calculations were therefore only (Raschke and Hedrich, 1985)? Do forest conducted after stabilization to avoid artifacts. trees display the same responses to ABA as other species? We have therefore, as a preliminary to a Climate in the chamber detailed survey of the role of ABA in reac- Photosynthetic photon flux density: about tions of oak species to water deficits, test- ; 1 -s- 2 ymol-m- temperature: 24°C; 600 ± 20
immediately plunged into a nutrient solution. molar fraction of C0 in the chamber (c 2 ): a 350 ± 5 pmol-mol- leaf to air difference in Gas exchange parameters and leaf water ; 1 water vapor molar fraction (dw): about 12-15 potential were monitored for at least 4 h after mmol-mol- depending upon leaf temperature , 1 cutting. and stomatal conductance. Leaf water potential ( was monitored in the chamber with a Wes- j/w) l cor in situ leaf micropsychrometer. ABA application (+I-)2-cis-4-trans-Abscisic acid (Aldrich Che- mie) was dissolved in the nutrient solution at 3 Gas exchange parameters concentrations: 10- 10- and 1! M. The 45 , Net C0 assimilation (A), transpiration (E), sto- 2 nutrient solution supplied to shoots was re- matal conductance for C0 and mesophyll 2(g) placed by an ABA-supplemented one and gas C0 molar fraction (c were calculated ac- 2 ) i exchange followed for at least 4 more hours. cording to von Caemmerer and Farquhar (1981).Results are presented either as time evolution of A, g and I or as A vs c graphs. ; w’ / j Effects of C0 enrichment 2 A, E and g were measured successively on Q. Twig removal pubescens under ambient (350) and enriched Twigs bearing 3-4 leaves were enclosed in the (1000 jlmol C0 mole fractions, both ) 1 mol- ’2 chamber and gas exchange parameters deter- before and after ABA supply. Each mea- mined after at least 2 h of equilibration. There- surement was made after at least 1 h of equili- after, twigs were detached and their cut end bration.
Results (0. pubescens, Fig. 3). De- relations creasing reactions with concentrations below 10! M were observed. Increasing Effects of cutting c caused additional stomatal closure a in the presence of ABA but did not even Cutting caused an immediate and steep promote the expected increase in A. Fur- decrease in stomatal conductance (g) and thermore, the application of ABA did not net C0 assimilation (A) (Fig. 1and a 2 change the relationship between A and g rapid increase of water potential (!yw), the for each c under constant humidity, this : a latter being a direct consequence of both a suggests that ,ABA affects both stomatal reduction in transpiration (E) and the conductance and mesophyll assimilation. removal of all the resistances to water flux from root to shoots. These effects were immediate (appearing after less than 1 min) and only transient, vanishing in about Discussion 1 h. A new steady state was reached thereafter, with significantly lower A and Cutting promoted quite immediate reac- g, and was maintained for at least 3-4 h. tions by leafy shoots. These kinds of In as much as it displays a new steady effects had been attributed to a hydropas- state gas exchange rate, a cut twig is a sive stomatal closure; but, like Myers et valuable tool for studying effects of exo- aL (1987) on Eucalyptus sp., we noticed genous ABA in the absence of any water that stomatal closure was accompanied by stress. quasi constant c values, which reveals a i reduction in me!sophyll photosynthetic ac- Effects of ABA application tivity. These effects were reversible and the appearance of a new steady state At 10- M, the effects were very similar to 4 enabled the use of cut twigs as an experi- those described above with two main dif- mental tool for ABA studies. ferences: 1) there was a significant time At high concentrations of about 10- M, 4 lag before leaf reaction, which may be ABA had an important effect on stomata attributed to ABA diffusion into leaves; and photosynthesis on all tested oak spe- from the original records, we may estimate cies, although lower concentrations (1-6 Q the delay to be 10.8 ± 1.9 min for A and M) had no effect. 9.1 ± 1.2 min for g (Fig. 2a); 2) no recov- Direct effects, on mesophyll photosyn- ery appeared during the 1 st hours after thesis may be inferred from A vs c curves i application, even if A and g increased which show A reductions at constant c i slightly after the first breakdown. Plotting values, and from the constant A/g ratios at these results on A vs c curves (Fig. 2b) i high c These results are in agreement . a reveals a strong reduction of mesophyll with those of Raschke and Hedrich (1985). photosynthesis. The c gradients across hypostomatous i leaves (Parkhurst et al., 1985) are not ABA reactions under increasing external large enough to modify these conclusions. C0 molar fractions (c 2 ) a Existence of ’patchy behavior’ of stomata in response to ABA (Downton et al., 1988) could contradict these conclusions, but ca temporarily increased to 1000 was there is still not enough evidence to 1 mol- ’ .lmol J just before and 1 h after ABA demonstrate the reality of this behavior. application. Results shown A as vs are g
Wong S.C., Farquhar G.D. & Parkhurst D.F., References (1988) Gradients of intercellular Cowan I.R. across the leaf mesophyll. Plant C0 levels 2 PhysioL 86, 1032-1037 Loveys B.R. & Grant W.J.R. Downton W.J.S., (1988) Stomatal closure fully accounts for the Raschke K. & Hedrich R. (1985) Simultaneous inhibition of photosynthesis by abscisic acid. and independent effects of abscisic acid on sto- New PhytoL 188, 263-266 mata and the photosynthetic apparatus in whole leaves. Planta 163, 105-118 8 Johnson J.D. (1987) Stress physiology of forest trees: the role of plant growth regulators. Plant Caemmerer S. & Farquhar G.D. (1981) von Growth Regul. 6, 193-215 5 Some relationships between the biochemistry of photosynthesis and the gas exchange of M. & Neales F.T. (1987) Myers B.A., Kuppers leaves. Planta 153, 376-387 Effect of stem excision under water on bulk leaf Zhang J., Schuir U. & Davies W.J. (1987) water potential, leaf conductance C0 assimila- 2 Control of stomatal behaviour by abscisic acid tion and stemwood water storage in Eucaly,!r which apparently originates in the roots. J. tus behriana F. Muell. Aust. J. Plant Physiol. 14, Exp. Bot 38, 1174-1181 135-145