Regulated deficit irrigation in potted Dianthus plants: Effects of severe and moderate water stress on growth and physiological responses

Scientia Horticulturae 122 (2009) 579–585

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Regulated deficit irrigation in potted Dianthus plants: Effects of severe and moderate water stress on growth and physiological responses ˜ o´n b,c, M. Jesu´s Sa´nchez-Blanco a,b,* Sara A´lvarez a, Alejandra Navarro a, Sebastia´n Ban a

Centro de Edafologı´a y Biologı´a Aplicada del Segura (CEBAS-CSIC), P.O. Box 164, E-30100 Murcia, Spain Horticultura Sostenible en Zonas A´ridas, Unidad Asociada al CSIC (CEBAS-UPCT), Murcia-Cartagena, Spain c Departamento de Produccio´n Agraria, Universidad Polite´cnica de Cartagena Paseo Alfonso XIII, 52, 30203 Cartagena, Spain b

A R T I C L E I N F O

A B S T R A C T

Article history: Received 20 February 2009 Received in revised form 23 June 2009 Accepted 25 June 2009

The purpose of this study was to analyze the physiological and morphological response of carnation plants to different levels of irrigation and to evaluate regulated deficit irrigation as a possible technique for saving water through the application of controlled drought stress. Carnations, Dianthus caryophyllus L. cultivar, were pot-grown in an unheated greenhouse and submitted to two experiments. In the first experiment, the plants were exposed to three irrigation treatments: (control); 70% of the control (moderate deficit irrigation, MDI) and 35% of the control (severe deficit irrigation, SDI). In the second experiment, the plants were submitted to a control treatment, deficit irrigation (DI, 50% of the control) and regulated deficit irrigation (RDI). After 15 weeks, MDI plants showed a slightly reduced total dry weight, plant height and leaf area, while SDI had clearly reduced all the plant size parameters. RDI plants had similar leaf area and total dry weight to the control treatment during the blooming phase. MDI did not affect the number of flowers and no great differences in the colour parameters were observed. RDI plants had higher flower dry weight, while plant quality was affected by the SDI (lower number of shoots and flowers, lower relative chlorophyll content). Leaf osmotic potential decreased with deficit irrigation, but more markedly in SDI, which induced higher values of leaf pressure. Stomatal conductance (gs) decreased in drought conditions more than the photosynthetic rate (Pn). Osmotic adjustment of 0.3 MPa accompanied by decreases in elasticity in response to drought resulted in turgor less at lower leaf water potentials and prevented turgor loss during drought periods. ß 2009 Elsevier B.V. All rights reserved.

Keywords: Ornamental quality Potted floricultural crops Regulated deficit irrigation Stomatal conductance Water relations

1. Introduction The nursery industry produces many species and cultivars of ornamental plant that differ greatly in their cultivation and water requirements. Water use in the nursery is an increasingly important factor due to limited water supply, and there is considerable pressure on the ornamental plant industry to produce crops more efficiently and to reduce water use (Sweatt and Davies, 1984). In addition, irrigation management and the modification of seedling growth is of the utmost importance for nurserymen in order to promote ornamental quality (Morvant et al., 1998). A number of growth controlling strategies using different approaches have been studied in recent years (Cerny et al., 2003; Montgomery et al., 2004), especially involving the applica-

* Corresponding author at: Centro de Edafologı´a y Biologı´a Aplicada del Segura (CEBAS-CSIC), P.O. Box 164, E-30100 Murcia, Spain. Tel.: +34 968 396318; fax: +34 968 396213. E-mail address: [email protected] (M.J. Sa´nchez-Blanco). 0304-4238/$ – see front matter ß 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.scienta.2009.06.030

˜ o´n et al., 2002). Nevertheless, tion of plant growth regulators (Ban restricting the water supply has been also used as a technique to avoid excessive vegetative growth in many species (Cameron et al., 2006). One of the consequences of exposing plant to drier regimes in terms of plant growth is the production of smaller leaves and shorter internode sections and reductions in flower number, size and/or quality (Sa´nchez-Blanco et al., 2002; Cameron et al., 1999). Also physiological responses to drought such as stomatal closure, decreased photosynthetic rates, changes in cellular elasticity or osmotic adjustment have been described (Davies et al., 2002; Sa´nchez-Blanco et al., 2004). However, differences in sensitivity to drought between different species and/or cultivars (Zollinger et al., 2006; Clary et al., 2004; Save´ et al., 2000) and even between growth stages have been demonstrated for many plants (Sionit et al., 1987; Mingeau et al., 2001). The importance of factors such as the degree of water stress imposed, and the timing and duration of reduced irrigation have been documented. Thus, a desirable level of deficit irrigation may result in stocky stress-resistant seedlings, but, if the water restriction is too severe the effects may be very negative as seedlings die (Franco et al., 2006). For all this, increasing our

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understanding of morphological and physiological shoot and root responses of seedlings to water management is critical for optimising the production of high quality seedlings. Regulated deficit irrigation (RDI) involves restricting irrigation in order to apply a controlled drought stress that is sufficient to reduce vegetative growth, but not so much as to reduce the quality of plant. Interest in RDI has centred on saving water and/or to controlling excessive vegetative growth in fruit and nut crops (Goldhamer and Beede, 2004; Ruiz-Sa´nchez et al., 2000). However, its application to ornamental crops has so far received limited attention, because controlling water stress in containers is technically more difficult (Cameron et al., 2006). Carnations have long been grown as a cut flower in many areas, ˜ o´n et al., although its cultivation as pot plant is more recent (Ban 2002). The purpose of this study was to analyze the physiological and morphological response of these plants to different levels of irrigation and to evaluate the regulated deficit irrigation as a useful technique to save water by applying controlled drought stress while not affecting the economic value of the plant. 2. Materials and methods 2.1. Plant material Single rooted cuttings of dwarf Dianthus caryophyllus L. cultivar propagated by Barberet and Blanc S.A. (Puerto Lumbreras, Murcia, Spain) were pot-grown in an unheated greenhouse on the southeast of Spain. Rooting cuttings of 6–7 cm were potted into 12 cm  10 cm (1.1 dm3) filled with a mixture of black peat, coconut fibre and perlite (4:4:1) and amended with osmocote plus (2 g dm3 substrate) (14:13:13 N, P, K + microelements). 2.1.1. Experiment 1 – Dianthus response to severe and moderate water stress The experiment was conducted from November to March 2005–2006. The weather conditions during greenhouse cultivation were 5–12 8C minimum and 18–29 8C (maximum) and the relative humidity ranged between 25% and 70%. The average maximum photosynthetically active radiation (PAR) was 960 mmol m2 s1. Plants were into three lots (75 plants per treatment) and irrigated 3–5 times per week, depending on the evaporative demand, using a drip irrigation system with one emitter per plant, each delivering 2 l h1. The control treatment was watered so that 15% (v/v) of the applied water was leached, while deficit irrigation plants received 70% of the control (moderate deficit irrigation, MDI) or 35% of the control (severe deficit irrigation, SDI). The amount of water applied to the control varied between 140 and 630 ml per pot per week. The average of water was 58 ml/day for the control and 41 and 20 ml/day for MDI and SDI, respectively. 2.1.2. Experiment 2 – Dianthus response to regulated deficit irrigation (RDI) During 4 months (from June to September 2006), plants were into three lots and three irrigation treatments were applied, the control treatment was watered so that 15% (v/v) of the applied water was leached, deficit irrigation plants received 50% of the control during all experiment (DI) and regulated deficit irrigation. The latter treatment included a DI event during the initial development phase (phase I, 4 weeks) followed by control treatment during the flowering phase (tight bud, flower not fully open and flower fully open) (phase II, 7 weeks) and then another event of DI (50%) after flowering (flower wilting, symptoms of petal in-rolling) until the end of the season (phase III, 3 weeks) was applied. Plants were watered by computer-controlled drip irrigation system 3–7 times per week, depending on the evaporative demand. The volume of water varied between 840 and 1050 ml per pot per week for the control, while the

average was 160 ml/day for the control and 120 and 80 ml/day for RDI and RD treatments, respectively. 2.2. Growth and ornamental measurements At the end of the experimental irrigation treatments (experiment 1) and during the different phases of experiment 2, the soil was gently washed from roots, and the plants were divided into shoots (stems, leaves and flowers) and roots. These were oven dried at 70 8C until they reached a constant mass to measure the respective dry weights. Five plants per treatment were harvested and their height and width were measured. The number of leaves per plant, the number of open flowers per plant, and leaf and flower colour were also calculated. Leaf and flower colour was measured with a Minolta CR-10 colorimeter, which provided the colour coordinates hue angle, chroma and lightness (McGuire, 1992), using three leaves and three flowers for each plant and five plants per treatment. The leaf area, shoot number and relative chlorophyll content (RCC) were calculated. Leaf area of 10 randomly selected plants per treatment was measured using a Delta-T Leaf Area Meter (Device Ltd., Cambridge, UK). RCC was measured at the midpoint of three mature leaves per plant and five plants per treatment with a Minolta SPAD-502 chlorophyll meter. 2.3. Physiological measurements At the end of experiment 1, midday leaf water potential (Cl), stomatal conductance (gs), and net photosynthesis (Pn) were measured in 10 plants per treatment. In experiment 2, midday water potential was measured throughout the experimental period in 10 plants per treatment. The leaf water potential was estimated according to the method described by Scholander et al. (1965), using a pressure chamber (Soil Moisture Equipment Co., Santa Barbara, CA, USA), for which leaves were placed in the chamber within 20 s of collection and pressurised at a rate of 0.02 MPa s1 (Turner, 1988). The osmotic potential (Co) was measured using a Wescor 5520 vapour pressure osmometer according to Gucci et al. (1991) while estimates of leaf turgor potential (Ct) were based on the difference between leaf water potential and leaf osmotic potential. Stomatal conductance (gs) and the net photosynthetic rate (Pn) were determined using a gas exchange system (LI-6400, LI-COR Inc., Lincoln, NE, USA). Measurements were made on attached leaves. Estimates of the bulk modulus of elasticity (e), leaf osmotic potential at full turgor (Cos) and leaf water potential at turgor loss point (Ctlp), were obtained at the end of the differential irrigation treatments in three leaves per plant and five plants per treatment, via pressure–volume analysis of leaves, as outlined by Wilson et al. (1979). Bulk modulus of elasticity (e) at 100% RWC was calculated using the formula: ðRWC

C

Þ

os tlp e ¼ ð100RWC where e is expressed in MPa, Cos is the osmotic tlp Þ

potential at full turgor (MPa) and RWCtlp is the relative water content at turgor loss point. Leaves were excised in the dark, placed in plastic bags and allowed to reach full turgor by dipping the petioles in distilled water overnight. Pressure–volume curves were obtained from periodic measurements of leaf weight and balance pressure as leaves dried on the bench at constant temperature of 20 8C. Dryingleaves period in each curve was about 3–5 h. 2.4. Statistical analysis The data were analyzed by one-way ANOVA using Statgraphics Plus for Windows. Treatment means were separated with Duncan’s multiple range test (P  0.05).

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Table 1 Influence of irrigation treatments on growth of potted carnation plants at the end of experiment 1. Treatments

Shoot dry weight (g plant1)

Root dry weight (g plant1)

Plant (cm)

height

Control MDI SDI Significance

13.65 11.02 9.08 ***

10.73 9.90 8.25 **

14.93 12.89 10.93 ***

a b c

a b c

a b c

Foliage height/plant height ratio 0.49 0.54 0.51 *

a b a

Foliage width (cm)

Number of shoot/plant

8.85 7.78 5.04 ***

2.70 2.60 1.90 **

a a b

Total leaf area (cm2)

a a b

141.44 109.67 98.56 **

a b c

Means within a column without a common letter are significantly different by Duncan0.05 test. Each value is the mean of 10 plants per treatment.

Table 2 Influence of irrigation treatments on the flowering quality of potted carnation plants at the end of experiment 1. Treatments

Number of flowers/plant

Control MDI SDI Significance

3.02 2.80 1.30 **

Flower colour

RCC (leaf)

L a a b

46.59 48.70 49.82 *

Chroma b b a

39.87 33.10 30.43 *

Hue angle a a b

342.52 346.30 342.18 ns

a a a

39.70 38.90 34.39 *

a a b

Means within a column without a common letter are significantly different by Duncan0.05 test. Each value is the mean of three leaves and three flowers per plant and five plants per treatment.

3. Results

3.2. Experiment 2

3.1. Experiment 1

Plants exposed to deficit irrigation during the first and third growth phases and to control conditions during the second phase (blooming) (RDI) exhibited more equilibrated plant growth throughout the experimental period (Fig. 3). During the flowering phase (phase II) the aerial dry weight values for control and RDI treatments were similar. DI produced the smallest plants throughout all the experiment (Fig. 3A). At the end of the experiment root dry weight was reduced in DI compared with the control (Fig. 3B), but higher root/shoot dry weight rate during the phases I and II were observed in this treatment (Fig. 3C). DI decreased the number of flowers, but RDI, rewatering after drought treatment, presented similar values to the control at the beginning of the blooming phase followed by a decrease. During the phase III, similar values of flowers number in RDI compared with DI were observed (Fig. 4). Higher flower dry weight values during flowering were observed in RDI (Fig. 3D). As regards colour parameters measured in the flowers (Fig. 5), significant differences were observed for chroma, DI generally showing the lowest values (Fig. 5B). Not differences were found between treatments for the other colour parameters (L and hue angle). Leaf area was reduced in DI, whereas RDI maintained similar values to the control during phase II (Fig. 6A). At the end of the experiment the control plants showed the strongest growth were the tallest and had a higher number of shoots (Fig. 6B and C). Leaf water potential values (Cl) at midday reflected the different substrate water conditions and the climatic conditions (Fig. 7). Maximum values of Cl were observed during the first phase in both treatments followed by a marked decrease: values between 1.0 MPa for the control and RDI and 1.4 MPa for DI. This was followed by a gradual increase until at the end of the experiment in all treatments.

There were significant differences in the plant growth of the carnation plants with different levels of irrigation. Deficit irrigation reduced shoot and root dry weight, plant height and total leaf area proportionally to the imposed drought level (Table 1). However, the number of shoots per plant and foliage width were significantly inhibited only by the severe deficit irrigation (SDI) compared with the control and MDI treatments. Moderate deficit irrigation (MDI) produced higher values of foliage height/plant height rate than SDI and the control treatments. As regards flower parameters (Table 2), the number of flowers per plant decreased in the SDI treatment and no differences between control and MDI treatments were found. No differences in the flowers colour parameters (lightness, chroma and hue angle) were observed in MDI compared with the control. The relative chlorophyll content decreased significantly in SDI (Table 2). At the end of the experiment, leaf water potential (Cl) was reduced in both deficit irrigation treatments, showing values of 0.62, 0.84 and 0.86 MPa in control, MDI and SDI, respectively (Fig. 1A). Leaf osmotic potential was decreased by deficit irrigation, although more markedly in SDI, which induced higher values of leaf pressure potential in the latter treatment (Fig. 1B and C). Stomatal conductance (gs) and the photosynthetic rate (Pn) are shown in Fig. 2. Both parameters decreased in drought-exposed plants in relation to the control; although gs reductions were greater (Fig. 2A) than the reductions in Pn (Fig. 2B). Parameters derived from the pressure–volume curve are shown in Table 3. At the end of the experimental period, leaf osmotic potential values at full turgor (Cos) were lower in both deficit irrigation treatments, pointing to the osmotic adjustment that occurred due to drought. The difference between the values obtained in the control and deficit irrigated plants were taken as an estimate of this adjustment (0.36 and 0.46 MPa for MDI and SDI, respectively). The water potential at turgor loss point (Ctlp) was significantly affected by the lowest irrigation level (Table 3). The bulk modulus of elasticity (e) increased in both deficit irrigation treatments, the values of this parameter being statistically equal at both drought levels studied (Table 3).

4. Discussion Water limitation has an impact on plant growth (Franco et al., 2006), although the exact effect may vary depending on the intensity of the water stress imposed (Cameron et al., 1999). A moderate restriction of the water available to container-grown Dianthus slightly reduced the total dry weight, plant height and leaf area (Table 1), and improved the relationship between foliage

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S. A´lvarez et al. / Scientia Horticulturae 122 (2009) 579–585 Table 3 Pressure–volume curve. Influence of irrigation treatments on potted carnation plants at the end of experiment 1. Treatments

Cos (MPa)

Control MDI SDI Significance

1.776 2.143 2.244 *

e (MPa)

Ctlp (MPa) b a a

2.337 2.413 2.915 *

b b a

4.89 9.17 8.55 **

a b b

Means within a column without a common letter are significantly different by Duncan0.05 test. Each value is the mean of three leaves per plant and five plants per treatment.

Fig. 1. Leaf water potential (Cl, A), leaf osmotic potential (Co, B) and leaf turgor potential (Ct, C) at midday in potted carnation plants at the end of experiment 1. Each histogram represents the mean of 10 values and the vertical bars indicate standard errors.

height and plant height (0.49 for the control and 0.54 for MDI). In contrast, severe deficit irrigation clearly reduced all the plant size parameters (Table 1). Growth responses to reduce irrigation were also influenced by the timing of irrigation (Cameron et al., 1999). Plants stressed during the vegetative stage (phase I) and after blooming (phase III), which supposed a 25% of reduction of water applied compared with the control, had a similar leaf area and total dry weight to the control treatment during the blooming phase and showed a less pronounced decline at the end of experimental period than the plants stressed throughout the experiment. The root/shoot ratio of the plants stressed throughout the experiment was higher than in control and RDI plants. This redistribution of dry matter in favour of the roots at the expense of shoots (Brugnoli and Bjorkman, 1992; Montero et al., 2001) is probably due to the plants needing to maintain root surface area under drought conditions in order to absorb water from the substrate (Bradford and Hsiao, 1982). An advantage for the smaller surface area, as we can observe in our experiment, is its contribution in reducing water consumption, since canopy transpiration is a function of the net sunshine energy absorption and lower leaf area will reduce light intercep˜ o´n et al., 2002). The timing and tion (De Herralde et al., 1998; Ban degree of water stress also influenced floral development (Table 2 and Fig. 4). Moderate deficit irrigation did not affect to the number of flowers in carnation plants and no great differences in the colour space coordinate values were observed, suggesting that the colour is not modified by this level of deficit irrigation and meaning that plants can cope with water shortage without losing their ornamental value (Brawner, 2003). Plant quality was affected by the severe deficit irrigation treatment (lower number of shoots and flowers, lower RCC values). In general, the RDI treatment had higher flower dry weight than dry weight of the other treatments. Also, a deficit irrigation applied during the initial development phase produced plants with similar flowering intensity to the control, although this was maintained through the rest of the experimental period. According to Cameron et al. (1999) the highest number of flowers per plant in Rhododendron occurred followed a moderate drought, which was also observed in other

Fig. 2. Stomatal conductance (gs, A) and net photosynthetic rate (Pn, B) at midday in potted carnation plants at the end of experiment 1. Each histogram represents the mean of 10 values and the vertical bars indicate standard errors.

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Fig. 4. Number of open flowers per plant. Values are means of all plants and the vertical bars indicate standard errors.

Fig. 3. Shoot dry weight (A), root dry weight (B), root/shoot ratio (C) and flowers dry weight (D) in potted carnation plants during experiment 2. Vertical lines indicate irrigation change in RDI. Each histogram represents the mean of five values and the vertical bars indicate standard errors.

ornamental species (Carden, 1995). Water deficit may influence flowering by inhibiting vegetative growth (Cameron et al., 2006). In our conditions RDI during phase II led to greater flowering intensity without a marked decrease in foliage height and foliage width. Nevertheless, further research is required to determine the most appropriate timing, duration and of degree stress during each growth phase in order to optimise shoot and flower development,

Fig. 5. Evolution of colour parameters, lightness (A), chroma (B) and hue angle (C) in carnation flowers during experiment 2. Each histogram represents the mean of three flowers per plant and five plants per treatment and the vertical bars indicate standard errors.

because these factors have significant effect on shoot growth and flower induction (Cameron et al., 1999). A decrease in leaf water potential by deficit irrigation could be the cause of the stomatal reductions and other physiological

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Fig. 7. Evolution of leaf water potential at midday in carnation potted plants during experiment 2. Values are means of 10 plants and the vertical bars indicate standard errors.

Enhanced drought resistance through osmotic adjustment has been reported in many species (Hinckley et al., 1980; Serrano et al., 2005). This, together with increases in the tissue elastic modulus, indicates that in addition to solute accumulation, there were changes in cell wall rigidity in stressed leaves, which resulted in turgor less at lower leaf water potentials (2.33 MPa for control and MDI and 2.9 MPa for SDI). Turgor maintenance may be mediated either through the accumulation of solutes or by changes in wall elasticity (Radin, 1983). Drought has been shown to both increase and decrease wall elasticity (Serrano et al., 2005; Schulte, 1993). In our conditions, carnations plant showed osmotic adjustment and significant decreases in elasticity in response to drought, as Meinzer et al. (1990) observed in coffee and Sa´nchezBlanco et al. (2009) in geranium plants. In species which show osmotic adjustment, more rigid cell walls may be necessary to maintain cell tissue integrity upon rehydration following a period of stress (Clifford et al., 1998). Leaf water potential values below the value of Ctlp were not found for the deficit irrigated plants at any sampling time during the experiments. Thus, the turgor was maintained and was even higher at some moments for the deficit irrigation treatments. Therefore, the inhibition of growth at both deficit levels was not associated with turgor (Nabil and Coudret, 1995) but with an inhibition of photosynthesis. Fig. 6. Evolution of leaf area (A), plant height (B), plant width (C) and numbers of shoots per plant (D) in potted carnation plants during experiment 2.Values are means of 10 plants in (A), five plants in (B) and (C) and all plants in (D) and the vertical bars indicate standard errors.

adaptations such as lower leaf area development, which both responses could contribute to reduce the total water consumption (Kang et al., 2000). Deficit irrigation has been seen to reduce the diurnal stomatal conductance as a result of leaf water potential decreases (Gollan et al., 1985; Pereira and Chaves, 1993; Munne´Bosch et al., 1999). In our conditions, leaf water potential (about 0.8 MPa at midday in deficit irrigation) may have caused a substantial decrease in stomatal conductance (approximately 60%, Fig. 2A). It has been reported that the threshold level for a drop in water potential to cause a decrease in stomatal opening ranges from 0.7 to 1.2 MPa for different species (Ackerson, 1985; Hsiao, 1973). In Dianthus, gradual drought stimulated a lowering of the osmotic potential at full turgor of around 0.3 MPa, an effect that was observed in both deficit irrigation treatments (Table 3).

5. Conclusion We conclude that deficit irrigation may improve water use efficiency by reducing water consumption and can be used to control growth in potted Dianthus plants, but the degree of the water stress imposed is critical to the response of this species. SDI reduced plant size and decreased its ornamental quality (lower number of shoots and flower per plants and less intense colour of flowers). However, MDI reduced dry mass and plant height while maintaining a good overall quality in the ornamental value. The mechanisms of tolerance and avoidance assayed (stomata closure, osmotic adjustment accompanied by decreases in elasticity) shown by this species prevent turgor loss during drought periods. Periods of water stress during the vegetative phases had almost no effect on head dimensions and it increased flowering intensity, practically, during all blooming phase. However, in spite of these results, further research is required to ascertain the optimal timing, frequency, duration and severity of regulated deficit irrigation in ornamental plants.

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