Directory UMM :Data Elmu:jurnal:T:Tree Physiology:Vol15.1995:

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Summary We investigated the effects of foliar absorption of dew by eastern white pine (Pinus strobus L.) seedlings on midday shoot water potential, as well as on other water rela-tions variables and growth. Two-year-old container-grown eastern white pine seedlings were subjected to contrasting watering regimes (normal and deficient) and three frequencies of artificial dew (none, once and three times per week) for 10 weeks in a greenhouse. Midday shoot water potential was measured on four occasions during the study. Other water relations variables (relative water content, stomatal conduc-tance, pressure-volume curves) and growth (hypocotyl diame-ter, aboveground dry mass, root dry mass) were also measured. Artificial dew significantly increased shoot water potential, stomatal conductance and seedling root growth, with greater responses observed for seedlings subjected to a deficient wa-tering regime than for well-watered seedlings. Because dew can be a frequent microclimatic event in some areas, this finding has practical implications for field studies of water relations of eastern white pine and possibly of other coniferous species.

Keywords: drought stress, eastern white pine, microclimate, pressure-volume curves, stomatal conductance.

Introduction

Foliar uptake of dew by conifers was a focus of research in the 1950s and 1960s (e.g., Breazeale 1950, Breazeale and McGeorge 1953, Stone and Fowells 1955, Stone et al. 1956, Stone 1957, Leyton and Armitage 1968). However, because it was considered of little physiological importance (Monteith 1963), the phenomenon received scant attention during the next 20 years (Chaney 1981, Rundel 1982). Recently, there has been a renewed interest in foliar uptake of water by conifers as a result of studies of air pollution effects and forest decline (e.g., Schulze et al. 1989, Schönherr and Riederer 1989). Although this new field of research has not focused on dew absorption by foliage, it has defined the parameters governing cuticle permeability and penetration (Riederer 1989, Schön-herr and Riederer 1989).

The early work of Stone et al. (1950) concluded that Pinus

coulteri G. Don was able to utilize atmospheric water vapor when growing in a soil at wilting point. Further work (Stone et al. 1956, Stone 1957) demonstrated that Pinus ponderosa Dougl. growing in a soil at ultimate wilting point survived up to 30 days longer when artificial dew was applied. Waisel (1958) observed increased water content of Pinus halepensis Mill. following overnight dew exposure and later demon-strated foliar water absorption (tritiated water), though absorp-tion was very slow (Vaadia and Waisel 1963). During studies of cuticle structure, Leyton and Armitage (1968) found that Pinus radiata D. Don was particularly well adapted to its native environment along the Californian coastal ‘‘fog belt’’ because of its ability to absorb atmospheric precipitation.

During a recent ecophysiological field study with eastern white pine (Pinus strobus L.) in Ontario, Canada (Boucher, unpublished data), we found that predawn measurements of shoot water potential were higher on nights with heavy dew than on dry nights. Moreover, in a greenhouse experiment, seedlings increased their weight up to 4.9% after an artificial dew application. Dew events occur frequently in the field and could influence the timing or interpretation of plant water relations measurements. Therefore, we tested the hypothesis that foliar absorption of artificial dew by eastern white pine increases shoot water potentials, and influences other water relations variables and growth.

Materials and methods

Biological material and pre-experimental period

Eastern white pine seedlings were obtained from the Beauce region of Québec, Canada (46°12′ N, 70°38′ W, 175 m alti-tude). Seedlings were grown in containers (110 cm3) at the St-Modeste nursery (Québec, Canada) for about 30 weeks, and subsequently transported to Université Laval and transplanted to 1000 cm3 pots containing a 3/1/1 (v/v) mixture of peat, vermiculite and perlite. During the next 55 days, the trans-planted seedlings received fertilizer weekly (20,20,20 N,P,K at 1.0 g l−1) and were gradually acclimated to a 16-h photoperiod and day/night ambient temperatures of 25/13 ± 3 °C. Relative humidity was not controlled and was around 5 to 25% during

Foliar absorption of dew influences shoot water potential and root

growth in Pinus strobus seedlings

J.-F. BOUCHER,

1

A. D. MUNSON

1

and P. Y. BERNIER

2

1

Centre de Recherche en Biologie Forestière, Université Laval, Faculté de Foresterie et de Géomatique, Laval, Québec G1K 7P4, Canada

2

Forestry Canada, Quebec Region, C.P. 3800, Sainte-Foy, Québec G1V 4C7, Canada

Received April 26, 1995

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the day and 30 to 40% during the night. Six high pressure sodium lamps were used to extend the photoperiod at a PPFD of about 150 µmol m−2 s−1 at seedling height. Apical growth of all seedlings was completed by the end of the 55-day pretreatment period.

Experimental factors and conditions

The experiment was set up as a 2 × 3 × 3 factorial design, with two watering regimes (normal (well-watered treatment) and deficient (water-stress treatment)), three artificial dew treat-ments (none, once and three times per week), and three sam-pling periods (Weeks 4, 8 and 10). The watering regimes consisted in saturating the soil every 3 to 4 days in the well-watered treatment and every 9 to 12 days in the water-stress treatment. Average gravimetric soil water content before rewa-tering was 49 ± 9% (n = 10) in the well-watered treatment and 19 ± 3% in the water-stress treatment. At 9--12-day intervals, all seedlings received fertilizer (20,20,20 N,P,K at 1.2 g l−1).

Artificial dew was applied with a mist spray either once a week (occasional dew treatment) or three times a week (fre-quent dew treatment). The control seedlings received no dew application. Each application was made at night fall and lasted about 3 s, enough to completely saturate the foliage. All seed-lings designated for dew application were removed from benches and the pots carefully covered to ensure that no water reached the soil during the dew application. Because the foli-age dried within 3 to 4 h after dew application, there was no indirect effect of dew (through reduced transpiration in the early morning, e.g., Slatyer 1960). The foliage was completely dry well before the measurement period, and no natural con-densation occurred on the foliage.

Sampling and measurements

Sampling was carried out near midday at Weeks 1, 4, 8 and 10 of the 10-week experimental period. Midday xylem water potential (Ψx) was measured on a current-year shoot with a

pressure chamber (PMS Instruments, Corvallis, OR). Relative water content (RWC) was measured on the same shoot, with the fresh shoot mass determined immediately after shoot exci-sion, the shoot mass at full turgor measured after 12 h of rehydration in distilled water, and the dry mass after 48 h at 65 °C. Stomatal conductance for water vapor (gsW) was

meas-ured with an LI-1600 steady state porometer (Li-Cor Inc., Lincoln, NE) on one or two fascicles per seedling (Edwards 1989). The detached needles were then frozen for later deter-mination of transpiring foliar area by the volume displacement method, using a Nettleton and Hattel (1981) apparatus and Johnson’s (1984) calculations for white pine needles. The pressure-volume (P-V) curves were prepared by Richards’ method as described in Ritchie (1984) with the recommenda-tions of Zine El Abidine et al. (1993). The morphological variables measured were aboveground dry mass (DMAG), root

dry mass (DMR) and shoot diameter at the hypocotyl (D).

Except for the P-V curves, all variables were measured on one randomly chosen seedling per treatment (two watering regimes × three artificial dew applications) per block (four blocks) per date (four dates). The P-V curves were measured

at Week 10 only on nine randomly selected seedlings from each of the following three treatments: well-watered + no dew, water-stress + no dew and water-stress + frequent application of dew. From each curve, six parameters were derived sepa-rately by means of the PVC program developed by Schulte and Hinckley (1985): RWC at turgor loss point (RWCTLP),

sym-plastic water fraction (ΘSYMP), maximum modulus of elasticity

(εMAX ) measured near full turgor, osmotic potential at full

turgor (ΨπFT), osmotic potential at turgor loss point (ΨπTLP),

and osmotic amplitude for turgor maintenance (∆Ψπ = ΨπFT−

ΨπTLP).

Experimental design and statistical analyses

The experiment was designed as a split-split plot (within each of the four blocks) with watering regime as the main plot and dew application as the subplot. Dates of measurement were randomly assigned to four of the seven seedlings within the subplots. Excess seedlings were included in the design to prevent large gaps forming within the physical layout and to provide extra seedlings in the event of mortality. The effect of experimental factors on the variance of the measured variables was determined by analysis of variance using the GLM proce-dure of the SAS software (SAS Institute Inc., Cary, NC). Analyses of variance were performed on the last three sam-pling dates, because the first samsam-pling date served as a test for initial homogeneity among seedlings. Specific within-treat-ment comparisons were performed by orthogonal contrasts (Steel and Torrie 1980).

The experimental design for the P-V curve parameters was a triple Latin square design. The three columns were repre-sented by three pressure chambers and the nine rows by nine rehydration periods. The Latin square design permits one to use two or more pressure chambers simultaneously, to have different rehydration periods, and to control statistically the pressure chamber and rehydration period effects (Zine El Abidine et al. 1993). Because convergence for some of the curves could not be obtained with the PVC program, only 20 of the 27 curves provided estimates of the various parameters.

Results

Water relations variables

The water-stress treatment significantly reduced Ψx, gsW and

RWC (Table 1). All three variables were also influenced by the week of measurement (Table 1). Dew application significantly affected Ψx of water-stressed seedlings only (Table 1), with a

28% increase following frequent dew application compared with the no-dew control (Figure 1A). Artificial dew strongly affected gsW under both watering regimes (Table 1), with a 68%

increase following frequent dew application compared with the no-dew control (Figure 1B). We observed a small effect of dew application on RWC in the water-stressed seedlings (Ta-ble 1), which translated into a 3% increase in RWC in response to frequent dew application compared with the no-dew control (Figure 1C).

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Table 1. Summary of ANOVA (P-values for F-test and mean square of errors) for water relations and growth variables. Abbreviations: df = degrees of freedom, Ψx = shoot xylem water potential, RWC = relative water content, gsW = stomatal conductance, D = stem basal diameter, DMAG =

aboveground dry mass, and DMR = root dry mass. Contrasts were performed on the linear effect only. Significant (P < 0.05) dew or interaction

(watering × dew) effects are in bold type.

Source of variation df Ψx gsW RWC D DMAG DMR

Block 3 0.204 0.464 0.569 0.136 0.050 0.627

Watering 1 ≤0.001 ≤0.001 0.045 ≤0.001 ≤0.001 0.006

Mean square error 3 0.01583 0.00003 14.7853 0.09166 0.33144 0.45380

Dew 2 0.003 ≤≤0.001 0.743 0.251 0.313 0.096

Watering × dew 2 0.010 0.104 0.117 0.477 0.267 0.017

Contrasts:

Dew (well-watered) 1 0.447 ≤≤0.001 0.251 0.735 0.852 0.712

Dew (water-stress) 1 ≤≤0.001 0.006 0.087 0.061 0.094 0.003

Week 2 ≤0.001 ≤0.001 0.004 0.010 ≤0.001 ≤0.001

Watering × week 2 ≤0.001 0.009 0.058 0.061 0.034 0.005

Dew × week 4 0.089 0.052 0.674 0.158 0.442 0.626

Watering × dew × week 4 0.488 0.049 0.232 0.266 0.661 0.248

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ANOVA revealed no significant effect of either watering re-gime or dew application on any of the six parameters (Table 2). Seedling growth variables

At Week 1, no treatment effects were noted for any of the growth variables, indicating initial homogeneity among seed-lings. By Week 10, all growth variables were significantly reduced by the water-stress treatment (Table 1). Of the growth variables measured, only root dry mass (DMR) of

water-stressed seedlings was significantly affected by artificial dew, resulting in a 45% increase following frequent dew application compared with the no-dew control (Figure 1F). There was a small effect of artificial dew on diameter and aboveground dry mass in the water-stressed seedlings (Table 1), which trans-lated into a 7% diameter increase and 18% aboveground dry mass increase following frequent dew application compared with the no-dew control (Figures 1D and 1E).

Discussion

Artificial dew significantly increased midday shoot water po-tentials of water-stressed seedlings, indicating that dew uptake by foliage directly influences shoot water potential. Increased water potential constitutes one of the four criteria required to prove water uptake by plant foliage (Rundel 1982).

Relative water content (RWC) of seedlings did not increase significantly after artificial dew application, although a posi-tive effect was observed in water-stressed seedlings subjected to frequent dew application. This small response of RWC to artificial dew might be associated with the small amount of water deposited on foliage or to the application of a relatively mild water stress treatment----there was only a small difference in RWC between well-watered and water-stressed seedlings (P = 0.045). Waisel (1958) measured a 13.3% increment in RWC content in well-watered P. halepensis (a Mediterranean spe-cies) saplings following a longer overnight dew exposure un-der natural conditions.

Neither dew application nor watering had a significant im-pact on any of the water relations parameters derived from the P-V curves, although both of these experimental factors had a

significant effect on shoot water potential. The lack of treat-ment effects on Ψ_πFT and ΨπTLP suggests that seedlings did not

use osmoregulation as a mechanism of stress tolerance (Mor-gan 1984). Consequently, it seems likely that the significant increase in shoot water potential in response to the frequent dew application treatment was a result of the small increase in shoot water content.

Artificial dew resulted in increased stomatal conductance. Jones (1957) found that, in Salvia spp., stomata closed at a higher water content in wet foliage than in dry foliage. In our study, the link between RWC and gsW was less clear, because

RWC was only slightly affected by artificial dew and only in the water-stressed seedlings; however, increased gsW did not

cause reductions in either RWC or Ψx. If, as suggested by

Meinzer (1993), transpirational losses are controlled by both stomatal- and canopy-level resistances to vapor transfer, then increased stomatal conductance does not necessarily translate into increased water loss by the plant.

Of the growth variables examined, only root growth re-sponded significantly to artificial dew application, and the response was greater for water-stressed seedlings than for well-watered seedlings. Breazeale and McGeorge (1953) showed that water absorption from the atmosphere by tomato plants stimulated root growth. Stone and collaborators (Stone and Fowells 1955, Stone et al. 1956, Stone 1957) have shown that water stress imposed on roots of P. ponderosa was dimin-ished by water absorbed through the foliage. Enhanced root growth in response to artificial dew is probably linked to the increases in stomatal opening and shoot water potentials fol-lowing the frequent dew applications.

Based on the finding that application of artificial dew to the foliage of eastern white pine seedlings significantly enhanced midday shoot water potential, stomatal conductance and root growth, we conclude that the higher water potential values measured in field-grown eastern white pine after heavy dew nights than after dry nights are a direct result of the foliar absorption of dew. The significant interaction between water-ing treatment and dew application and the fact that dew consti-tutes a frequent microclimatic event indicate that the effects of dew need to be considered when interpreting field results of

Table 2. Summary of ANOVA (P-values for F-test) and treatment means (with standard error) of parameters derived from pressure-volume curves based on a triple Latin square design. Abbreviations: df = degrees of freedom, ΘSYMP = symplastic water fraction, RWCTLP = relative water content

at turgor loss point, Ψ_πFT = osmotic potential at full turgor, ΨπTLP = osmotic potential at turgor loss point, ∆Ψπ = osmotic amplitude for turgor

maintenance, ε_MAX = maximum modulus of elasticity, W0D0 = well-watered + no artificial dew, W1D0 = water stress + no artificial dew, and W1D2 = water stress + frequent artificial dew.

Source of variation df ΘSYMP RWCTLP ΨπFT (MPa) ΨπTLP (MPa) ∆Ψπ (MPa) εMAX (MPa)

Rehydration time 7 0.078 0.787 0.837 0.685 0.953 0.935

Pressure chamber 2 0.626 0.417 0.877 0.030 0.512 0.732

Treatment 2 0.728 0.624 0.619 0.816 0.669 0.739

Treatment means

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irrigation and drought studies.

Acknowledgments

We sincerely thank Claude Fortin for technical assistance. Thanks also to Pierre-André Dubé for comments on an earlier version of the manuscript and to Sylvain Boisclair for advice concerning statistical analyses. Sylvie Lépine and Corine Rioux, Quebec Ministry of Natu-ral Resources, provided the seedlings for the experiment.

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