Research ArticleArticles

Identification of Drought-Tolerant Woody Perennials Using Chlorophyll Fluorescence

Glynn C. Percival and Colin N. Sheriffs
Arboriculture & Urban Forestry (AUF) September 2002, 28 (5) 215-223; DOI: https://doi.org/10.48044/jauf.2002.032

Abstract

The purpose of this research was to determine whether chlorophyll fluorescence values obtained from excised leaves of woody plants subjected to dehydration in vitro provided a measurable indicator of whole-plant performance following drought in situ and to gain a greater understanding of alterations in leaf photosynthetic properties between species. Based on reductions in photochemical efficiency, as measured by chlorophyll fluorescence, of detached leaves of 30 woody plants in vitro following 24 hours of dehydration, plants were ranked in order of tolerance. Five species identified as drought tolerant, intermediate, and sensitive were subjected to 70 days of drought under glasshouse conditions. Based on mortality rates at day 70, drought tolerance followed the same order as that obtained in vitro. In addition, reductions in chlorophyll fluorescence parameters and photosynthetic rates of whole plants mirrored tolerance ranking in vitro (i.e., rates declined most rapidly in species identified as dehydration sensitive and least in species identified as drought tolerant). Alteration to leaf chlorophyll fluorescence parameters in the test species highlighted a number of previously unreported effects on the leaf photosynthetic apparatus in response to drought. Results strongly indicate that screening of detached leaf material in vitro using chlorophyll fluorescence can provide a means of gauging the drought tolerance of plants with limited whole-plant experimentation.

Key Words

It is now widely recognized that urban landscapes create an environment that is hostile to tree growth. Trees have to contend with problems such as soil compaction, waterlogging, drought and de-icing salts. The consequences can be short-lived planting schemes, as hundreds of thousands of plants degenerate and die annually in the United Kingdom (Gilbertson and Bradshaw 1985; Craul 1994; Gibbs and Palmer 1994). In areas where newly planted trees are not irrigated, initial establishment relies heavily on precipitation. If the transplant does not receive water during the period of new root regeneration, its internal water deficits increase considerably due to excessive water transpiration and nonabsorption of water from soil (Kozlowski et al. 1991). Indeed, water deficits are the major causes of failure of newly planted trees (Kaushal and Aussenac 1989). Because trees planted in urban landscapes are selected primarily on their aesthetic qualities, little information exists concerning their drought tolerance. Determining the drought tolerance of woody plants, however, can be expensive, requiring time-consuming and labor-intensive glasshouse, street trials, and/or field trials. A rapid and accurate method to identify drought-tolerant woody plants for future urban plantings is now required.

A promising approach is the use of chlorophyll fluorescence, a technique that can provide large amounts of data with a minimum of expertise and time and without injury to the plants. Chlorophyll fluorescence works on the principle that photosynthesis is one of the core functions in the physiology of plants. The functional state of photosynthesis has been considered an ideal physiological activity to monitor the health and vitality of plants (Clark et al. 2000). Assessing the health or integrity of the internal apparatus driving the photosynthetic process within a leaf (i.e., the thylakoid membrane) using chlorophyll fluorescence provides a rapid and accurate technique of detecting and quantifying plant tolerance to stress. Furthermore, previous research indicates that chlorophyll fluorescence values from detached leaves strongly correlate with wholeplant mortality in response to environmental stress (freezing, heat, chilling, salinity), permitting quantification of species and cultivar tolerance with limited whole-plant experimentation (Smillie and Hetherington 1983; Greaves and Wilson 1987; Brennan and Jefferies 1990; Yamada et al. 1996; Hakam et al. 2000).

Objectives of this investigation were to (1) determine whether chlorophyll fluorescence values obtained from excised leaves of urban woody perennials subject to dehydration in vitro provide a measurable indicator of whole-plant performance under drought conditions in situ and (2) gain a greater understanding of alterations in leaf photosynthetic properties and other key physiological traits between species in response to drought.

MATERIALS AND METHODS

Plant Material

The experiment used 4-year-old stock of a range of woody perennials commonly used in United Kingdom planting schemes obtained from a range of commercial suppliers. At each sampling, shoots consisting of current year’s growth were collected from each species and placed in paper bags for transport to the laboratory. Five fully expanded, nonsenescing leaves per shoot were excised from the base of the petiole using a razor blade; all material was prepared within 2 hours of collection. Samples were collected in late June—well beyond the initial 21 days required after leaf flush (circa early May)—to allow the photosynthetic apparatus to fully develop (Kitao et al. 1998).

Treatments

In Vitro

Immediately following detachment, leaves were dehydrated in vitro by placing abaxial surface down, in a uncovered Petri dish on Watman filter paper and placed in a Merck environmental growth chamber at 21°C (70°F) for 24 hours at a 35% relative humidity and low light intensity of 15 μmol m−2 s−1 photosynthetically active radiation (PAR) to induce stomatal opening and facilitate maximal water loss from leaf tissue (Jensen et al. 1999). The influence of leaf detachment was determined by placing detached leaves, abaxial surface down, in a Petri dish on moist Watman filter paper sealed with a thin polythene film permeable to air but not water.

Whole plants

Four-year-old stock of five woody perennials identified in vitro as dehydration tolerant (Aucuba japonica ‘Variegata’, Prunus laurocerasus ‘Zabeliana’), intermediate (Pieris ‘Forest Flame’), and sensitive (Euonymous fortunei ‘Emerald ‘n’ Gold’ and Spiraea japonica ‘Shirobana’) were placed under glasshouse conditions at a temperature of 22°C (72°F) ± 2°C (36°F) supplemented with 400-W, high-pressure sodium lamps (SON/T) providing a photoperiod of 16 hours light and 8 hours dark and 250 μmol m−2 s−1 PAR at the tree crown and allowed to grow for 1 month. After 1 month, plants were watered until the compost (loamy texture, with 23% clay, 46% silt, 31% sand, 3.1% organic carbon, pH 6.6) was fully saturated (i.e., water was observed emerging from drainage holes). Immediately thereafter, drought treatments were imposed by the cessation of watering. The experimental design used was a completely randomized block that was continually randomized on a weekly basis. Experiments were undertaken in May 1999 using eight plants per treatment and repeated in May 2001 using six plants per treatment.

Chlorophyll fluorescence

Leaves were adapted to darkness for 30 minutes by attaching light-exclusion clips to the leaf surface in situ. Chlorophyll fluorescence was measured using a portable fluorescence spectrometer (Hansatech Instruments Ltd., Kings Lynn, U.K.). Measurements were recorded up to 1 second with a data acquisition rate of 10 ms for the first 2 ms and of 1 ms thereafter. The fluorescence responses were induced by a red (peak at 650 nm) light of 600 W m−2 intensity provided by an array of six light-emitting diodes. Fluorescence values recorded include Fo: Minimal fluorescence. Increased or decreased values indicate damage or impairment of the photosynthetic apparatus.Fv/Fo: Estimates the maximum primary yield of photochemistry of photoystem II and provides an estimation of photosynthetic capacity.Fv/Fm: Represents the maximum quantum yield of photoystem II, which in turn is highly correlated with the quantum yield of net photosynthesis.

Dehydration tolerance in vitro was assessed by reductions in Fv/Fm values only because Fv/Fm values are the most popular index used as a measure of plant vitality and early diagnostic of stress in situ (Meinander et al. 1996).

Photosynthetic CO2 Fixation

The light-induced CO2 fixation (Pn) was measured in pre-darkened leaves (20 minutes) using a CO2/H2O porometer (Walz D-91090 Effeltrich). The irradiance on the leaves was 700 μmol m−2 s−1 PAR saturating with respect to Pn; the velocity of the airflow was 0.5 L min−1. The calculation of the photosynthetic rates was carried out according to von Caemmerer and Farquhar (1981).

Plant Dry Weights

At the conclusion of the experiment, surviving trees were destructively harvested and leaf, shoot, and root dry weight recorded after oven drying at 85°C (185°F) for 48 hours.

Statistical Analysis

Multivariate analysis was used to determine the significance of species and dehydration on chlorophyll fluorescence and photosynthetic rates using the Genstat V version 3.1 program. Because a significant effect of dehydration was recorded, the program was restricted to analyze for treatment effects within each species. Differences between treatment means were separated by the least significance difference (LSD) at P > 0.05. Photosynthetic CO2 fixation was measured in the May 1999 experiment only. In no instances did treatment effects on chlorophyll fluorescence values of each species differ between years. Consequently, values represent pooled data for both years.

RESULTS

In Vitro

Values of detached leaves protected from the dehydration stress by sealing in Petri dishes on moist filter paper did not significantly differ from values recorded on whole plants (Table 1), demonstrating no significant effect of leaf detachment on photochemical efficiency (Fv/Fm). Likewise, no significant intra-specific differences in Fv/Fm values between the 30 species selected for experimental purposes were recorded. Fv/Fm values of all species ranged between 0.804 and 0.818. For reasons of clarity, only results for ten species are shown (Table 1). A significant effect of species in response to dehydration was recorded. This finding is reflected by marked differences in the magnitude of the stress response recorded between species (Table 2). In all cases, dehydration reduced Fv/Fm values. Consequently, species were ranked in order of drought tolerance based on reductions in Fv/Fm values after 24 hours of dehydration. Based on Fv/Fm ranking in vitro, five species were identified as drought tolerant (Aucuba japonica ‘Variegata’, Prunus laurocerasus ‘Zabeliana’), intermediate (Pieris ‘Forest Flame’), sensitive (Euonymus fortunei ‘Emerald ‘n’ Gold’), and very sensitive (Spiraea japonica ‘Shirobana’) and used for whole-plant experimentation.

View this table:
Table 1

The effect of leaf detachment on photochemical efficiency (Fv/Fm) of woody plants tested.

View this table:
Table 2

The influence of 24-hour dehydration on photochemical efficiency (Fv/Fm) of detached leaves of woody plants.

Whole Plants

Effects on photochemical efficiency and photosynthetic capacity (Fv/Fm and Fv/Fo values, respectively) followed similar trends for all species in that reductions with time following cessation of watering were recorded (Figure 1). Effects on Pieris ‘Forest Flame’, Euonymus fortunei ‘Emerald ‘n’ Gold’, and Spiraea japonica ‘Shirobana’ were recorded by week 1 but not until weeks 3 and 4 in the case of Aucuba japonica ‘Variegata’ and Prunus laurocerasus ‘Zabeliana’. Effects on Fo as a measure of damage to the photosynthetic apparatus differed between species. In the cases of Pieris ‘Forest Flame’, Euonymus fortunei ‘Emerald ‘n’ Gold’, and Spiraea japonica ‘Shirobana’, Fo values steadily declined from day 0 until the termination of the experiment (day 70). In the cases of Aucuba japonica ‘Variegata’ and Prunus laurocerasus ‘Zabeliana’, Fo values increased until week 1 and then steadily declined until day 70. For all species, photosynthetic rates declined following the initiation of drought stress, although the rate of decline varied markedly between species.

Figure 1.
Figure 1.

The influence of drought on chlorophyll fluorescence and photosynthetic rate of five woody perennials with time.

  • ⚬ = Aucuba japonica ‘Variegata’ (control)

  • + = Spiraea japonica ‘Shirobana’

  • • = Euonymus fortunei ‘Emerald ‘n’ Gold’

  • ▲ = Pieris ‘Forest Flame’

  • ◆ = Prunus laurocerasus ‘Zabeliana’

  • ▪ = Aucuba japonica ‘Variegata’

  • All values mean of 14 plants; 5 leaves per plant except Pn (n = 8).

  • Error bars represent the least significant difference (P = 0.05) between treatments.

Growth and Mortality

Based on mortality rates after the 10-week drought treatment, species tolerance was in the order Aucuba japonica ‘Variegata’ > Prunus laurocerasus ‘Zabeliana’ > Pieris ‘Forest Flame’ > Euonymus fortunei ‘Emerald ‘n’ Gold’ > Spiraea japonica ‘Shirobana’ (35%, 40%, 75%, 95%, and 100% mortality, respectively). Time to wilting varied markedly between species. The earliest symptoms were observed visually in Spiraea japonica ‘Shirobana’, identified as the most drought-sensitive species in vitro at day 9. Wilting was not observed until day 48 in Aucuba japonica ‘Variegata’, identified as the most drought-tolerant species in vitro. Leaf drop was recorded after 21, 36, and 45 days for Spiraea japonica ‘Shirobana’, Euonymus fortunei ‘Emerald ‘n’ Gold’, and Pieris ‘Forest Flame’, respectively. No leaf drop was recorded in the species Aucuba japonica ‘Variegata’ and Prunus laurocerasus ‘Zabeliana’. Irrespective of species, drought stress significantly reduced (P > 0.05) leaf, shoot, and root dry weight of all species tested by week 10 (Table 3), with one exception: shoot dry weight of Prunus laurocerasus ‘Zabeliana’, which did not significantly differ from controls.

View this table:
Table 3

The influence of 70 days’ drought on growth and mortality of five woody plant species.

DISCUSSION

According to Ludlow (1989), perhaps the single most important determinant of drought resistance within a plant is the dehydration tolerance (i.e., the measure of tissue capacity for withstanding desiccation). Based on reductions in photochemical efficiency as measured by Fv/Fm ratios of detached leaves after 24 hours of dehydration, the dehydration tolerance of five of the 30 plants tested was in the order Aucuba japonica ‘Variegata’ > Prunus laurocerasus ‘Zabeliana’ > Pieris ‘Forest Flame’ > Euonymus fortunei ‘Emerald ‘n’ Gold’ > Spiraea japonica ‘Shirobana’. Based on whole-plant mortality after 70 days of drought, tolerance followed the same order as that obtained in vitro. Likewise, reductions in chlorophyll fluorescence parameters and photosynthetic rates of whole plants mirrored tolerance ranking in vitro (i.e., rates declined most rapidly in Spiraea japonica ‘Shirobana’, identified as drought sensitive, and least in Aucuba japonica ‘Variegata’, identified as the most drought-tolerant species). Techniques that help toward the rapid, objective testing of woody plant species have been advocated as necessary to facilitate the selection of robust trees and shrubs that will survive into maturity, reducing replacement costs and giving permanence to the landscape (Patch 1994; Cregg 1995). Results of this investigation indicate screening of detached leaf material in vitro using chlorophyll fluorescence can provide a rapid means of gauging the drought tolerance of plants with limited whole-plant experimentation. Previous work elsewhere supports this hypothesis. Greaves and Wilson (1987) were among the first to demonstrate that chlorophyll fluorescence was able to distinguish between degrees of freezing tolerance among wild and cultivated potato and rice genotypes and in turn quantify their freezing sensitivity when leaf tissue was subjected in vitro to a −5°C (23°F) freezing regime. Evaluation of chlorophyll fluorescence as a means of screening different genotypes of black currant of known hardiness to spring frosts using detached leaf material provided a rapid screening technique for assessing frost hardiness (Brennan and Jefferies 1990). Further evidence is supplied by Smillie and Hetherington (1983) using leaf material of bean, cucumber, cabbage, mandarin, grapefruit, and lime subject to freezing and chilling damage, while research in Japan by Yamada et al. (1996) successfully used chlorophyll fluorescence to quantify the heat tolerance of a range of fruit crops based on detached leaf experiments. Finally, work by Hakam et al. (2000) concluded that measurement of chlorophyll fluorescence might provide a rapid method to pre-screen rose genotypes for chilling susceptibility.

Water stress after planting is generally recognized as the major factor in the United Kingdom resulting in death of newly transplanted trees (Gilbertson and Bradshaw 1985). Owing to the limited root system moved with a tree following transplanting, inability of a plant to establish appropriate mechanisms for water harvesting and/or conservation plus, in many instances, the soil’s inability to store adequate water, physiological drought can be experienced even when climatic drought is not severe (Kendle 1995). Although the significance of drought stress on woody plant performance is broadly appreciated by professionals involved in tree use and management, when it becomes necessary to make decisions on plant selection for a given site, it is soon apparent that “hard” data on stress tolerance is poorly developed or nonexistent (Flint 1985; Hitchmough 1995; Percival and Hitchmough 1995). In such circumstances, many woody plants are selected for use on the basis of their aesthetic characteristics. Consequently, if, for example, Spiraea japonica ‘Shirobana’ was selected for wide-scale planting into a sandy soil prone to drought throughout the spring and summer, then results of this experiment indicate that by the following growing season, mortality rates could be as high as 100%. This net effect is undesirable when it occurs on a metropolitan or regional scale. Alternatively, selection of Aucuba japonica ‘Variegata’ or Prunus laurocerasus ‘Zabeliana’ could result in mortality rates between 35% and 40%, which would reduce labor and replacement costs. Such a result demonstrates how appropriate species and site selection can mean the difference between low and substantial plant mortality and emphasizes the need for further research to select woody plants with superior stress tolerance linked to desirable aesthetic characteristics. Survivability following planting will become of greater importance because future resource allocations to urban tree management are likely to decline, increasing pressure to deliver superior services at less costs.

Measurements of chlorophyll fluorescence in this experiment highlighted a number of previously unreported effects on the leaf photosynthetic apparatus between the species tested in response to drought. Functionally chlorophyll molecules within the leaf act in groups. Contained within that group are special pairs of chlorophyll a molecules, which promote electron transfer, known as reaction centers. The reaction center within photosystem II responsible for the water splitting and oxygen-evolving reaction of photosynthesis is known as P680 (Hall and Rao 1999). An increase in Fo, as demonstrated by Aucuba japonica ‘Variegata’ or Prunus laurocerasus ‘Zabeliana’, is associated with dissociation of the light-harvesting chlorophyll a/b complexes from the P680 complex of photosystem II (Yamane et al. 2000). A decrease in Fo, as in the case of Pieris ‘Forest Flame’, Euonymus fortunei ‘Emerald ‘n’ Gold’, and Spiraea japonica ‘Shirobana’, has been suggested to reflect damage to regulatory processes external to P680, such as impairment of the photo-protective process that facilitates the dissipation of excess energy within the leaf (i.e., the xanthophyll cycle (Angelopoulous et al. 1996; Hong and Xu 1999).

In response to drought, often one of the most sensitive indicators of a plant’s overall physiological state is stomatal behaviour (Croker et al. 1998). Stomatal closure permits the plant to balance water loss with carbon update, in turn improving long-term water-use efficiency and survival. Although not recorded in this investigation, reduced carbon uptake caused by stomatal closure would account for reductions in the rate of photosynthesis recorded in all species tested by day 7, while reductions in Fv/Fm (photochemical efficiency) and Fv/Fo (photosynthetic capacity) were not detected until week 3. Alterations in Fo, although differing among species, were detected by week 1. Consequently, results of this investigation indicate direct measurements of photosynthetic rate or alterations to Fo values provided are more effective indicators of drought stress than are Fv/Fm and Fv/Fo.

Stress detection in urban trees relies heavily on visual observations of, for example, leaf yellowing, crown dieback, etc. Such indicators can be subjective because they are based on human knowledge and interpretation, which can differ between individuals. Consequently, health evaluations can differ markedly between assessors. The first visible signs of drought stress in this investigation were manifested by leaf wilting. Time taken for wilting ranged from 9 to 48 days, depending on species. In all cases, effects on chlorophyll fluorescence were detected prior to visible signs of wilting. The implications of chlorophyll fluorescence as a stress detection system for use in urban forestry programs have been discussed elsewhere (Percival and Dixon 1997; Percival et al. 1998; Percival and Fraser 2001).

Results of this investigation support previous work in that chlorophyll fluorescence permits early detection of stress before physical signs of deterioration become evident, which allows remedial action prior to the tree becoming a public hazard.

Acknowledgments

The author is grateful for funding from the International Society of Arboriculture and the Stanley Smith (UK) Horticultural Trust.

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Identification of Drought-Tolerant Woody Perennials Using Chlorophyll Fluorescence
Glynn C. Percival, Colin N. Sheriffs
Arboriculture & Urban Forestry (AUF) Sep 2002, 28 (5) 215-223; DOI: 10.48044/jauf.2002.032
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