The effect of water availability and quality on photosynthesis and productivity of soilless-grown cut roses

Scientia Horticulturae 88 (2001) 257±276

Review

The effect of water availability and quality on photosynthesis and productivity of soilless-grown cut roses Michael Raviva,*, Theo J. Blomb a

Agricultural Research Organization, Department of Ornamental Horticulture, Newe Ya'ar Research Center, PO Box 1021, Ramat Yishay 30095, Israel b Department of Plant Agriculture, HRIO University of Guelph, 4890 Victoria Ave., Vineland Station, Guelph, Ont., Canada L0R 2E0 Received 24 June 1999

Abstract Low matric and to a lesser extent osmotic potential reduce signi®cantly leaf area and rose yield. Net assimilation rate and transpiration are also negatively affected although less dramatically. Low water potential causes an increase in the water use ef®ciency of greenhouse roses when tested in closed, no-discharge systems. When a stable osmotic potential is maintained in open systems, using increased leaching fraction (LF), low osmotic potential results in lower water use ef®ciency. Osmotic potential in porous media serving for greenhouse cut-rose production is usually lower than the matric potential. However, low matric potential in porous media is usually accompanied by very low unsaturated hydraulic conductivity, causing localized zones of very low matric potential adjacent to the root±medium interface. This phenomenon, that cannot be measured using tensiometers, is the main limiting factor to water uptake by plant roots. Restricted water uptake results in low leaf water potential and cessation of leaf and shoot expansive growth. Combined effects of drought and salinity on photosynthesis have been studied for a number of agronomic crops but studies on roses have been limited. In most greenhouse crops a close shoot relationship between total water potential in the root zone …Csoil † is found t † and in the shoot …Ct and there are good indications about the plant's ability to make osmotic adjustments in order to lower Cshoot and prevent excessive water losses from the leaves thus maintaining the plant's t turgidity. Future studies conducted with roses can provide a better insight into the adaptive

Abbreviations: EC, electrical conductivity; ET, evapotranspiration; NAR, net assimilation rate; NCER, net carbon exchange rate; ODR, oxygen diffusion rate; WUE, water use ef®ciency * Corresponding reviewer. Tel.: ‡972-4-9539505; fax: ‡972-4-9836936. E-mail address: [email protected] (M. Raviv). 0304-4238/01/$ ± see front matter # 2001 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 4 2 3 8 ( 0 0 ) 0 0 2 3 9 - 9

258

M. Raviv, T.J. Blom / Scientia Horticulturae 88 (2001) 257±276

processes within the plants when exposed to salt or water stresses. # 2001 Elsevier Science B.V. All rights reserved. Keywords: Leaf water potential; Matric potential; Net assimilation rate; Osmotic potential; Rosa hybrida; Water use ef®ciency

1. Introduction Globally, water is one of the main limiting factors in agricultural production. Water is scarce and its quality is decreasing in many parts of the world. Nonetheless, a large amount of water is consumed while producing plant mass. The term water use ef®ciency (WUE) is commonly used to quantify this fact (Steduto, 1996). WUE can be de®ned in four different ways: (i) photosynthetic WUE as the ratio of leaf net assimilation (mmol CO2 mÿ2 sÿ1) and leaf transpiration (mmol H2O mÿ2 sÿ1); (ii) biomass WUE as the ratio of cumulative aboveground dry weight biomass of a crop canopy (g) to cumulative evapotranspiration (ET) used by the crop (kg); (iii) yield WUE as the product of yield dry mass (g) and ET (kg); (iv) effective WUE (WUE1) as the yield dry weight biomass (g) per amount of the water input …precipitation ‡ irrigation† in kg. Data regarding WUE of roses are relatively scarce. We found no reliable data for photosynthetic WUE of roses. Cabrera (1997) reported a biomass WUE of 2.3 g kgÿ1 for `Royalty', grown in a recirculating nutrient solution, based on distilled water. For practical, horticultural purposes measuring WUE1 is more relevant than biomass WUE due to the large fraction of leached water (that is not re¯ected in the latter). Typically, ¯ow-through growing systems based on porous soilless media exhibit low WUE1 values. Under Mediterranean conditions, where 40±50% of the irrigated water is used to prevent salinity build-up, the WUE1 of roses is often near 0.7 g kgÿ1 (Raviv, unpublished). Surprisingly, similar WUE1 values were reported in the Netherlands using water of much higher quality (Bloemhard et al., 1993). Minimizing the water discharge in closed, recirculated growing systems increases the WUE1 considerably. Canadian data for `Sonia' and `Laser' roses grown in a recirculated system based on water of low salt content came to a WUE1 of 2.8 g kgÿ1 (Blom, unpublished). At higher salinity levels, WUE1 is lower as a result of the increased leaching requirement. Raviv and Korkmaz (unpublished) found WUE1 of 1.7 g kgÿ1 for `Mercedes' grown in recirculated system when the solution was based on tap water having an EC of 1.1 dS mÿ1 with a ®nal EC (including fertilizers) of 2.0 dS mÿ1. Although the role of water potential and quality in plant life is of paramount importance, we are not aware of any review that addresses its effect on both photosynthesis and productivity of soilless-grown cut roses. This is the main

M. Raviv, T.J. Blom / Scientia Horticulturae 88 (2001) 257±276

259

subject of the current review. The following discussion aims to describe the components of plant's water availability in relation to transpiration, to evaluate water effect on the photosynthetic process and relate it to plant productivity. Practical conclusions based on the available information will be discussed and gaps in knowledge will be identi®ed for future research. 2. The soil±plant±atmosphere continuum Although water constitutes most of the biomass, only a minute fraction of the water taken up by the plant is actually retained by the plant itself or is served a source of hydrogen for the reduction of CO2. The remainder is lost to the atmosphere in the process of transpiration. This apparently wasteful process is, in fact, indispensable since dry matter production is affected by the ratio of actual to potential ET (de Wit, 1958). This relation is determined by the following factors:  The transpiration process is a key factor in plant life in general and in photosynthesis in particular.  Water, being a solvent, enables translocation of ions (including essential nutrients) and organic molecules into, within and out of the living plant, driven by the transpirational stream.  Transpiration has a key role in maintaining plant temperature that enables normal plant life. Water ¯ows along the potential gradient from the soil, where high (slightly negative) total water potential …Csoil t † exists, through the plant vascular system to the leaves and to the atmosphere, where the water potential is lower (more negative) (Fig. 1). The ¯ow rate is proportional to the potential gradient and inversely proportional to the resistance imposed by the soil, soil/plant interface, plant, plant/atmosphere interface and atmosphere. 2.1. Soil (medium) water potential, Csoil t Total water potential (Ct) is divided into the following components: Ct ˆ Cg ‡ Co ‡ Cm

(1)

The gravitational potential, Cg, is negligible for roses in the substrate. The osmotic potential, Co, describes the forces between dissolved particles (mainly ions) and water molecules. For common nutrient solutions (e.g. Hoagland), Co can be estimated from electrical conductivity (EC) using the approximate relationship: Co  ÿ0:036EC (MPa) with EC in dS mÿ1. This proportionality factor is somewhat depending upon the composition of the nutrient solution. The

260

M. Raviv, T.J. Blom / Scientia Horticulturae 88 (2001) 257±276

Fig. 1. Illustration of the water potential gradient in the soil/plant/atmosphere system.

matric potential, Cm, combines adhesion forces between solid soil surfaces and water and cohesion forces among water molecules. This combined potential can be measured in situ by a tensiometer. The relationship of soil water content to the matric-pressure potential is of basic importance to the understanding of soil water status and water availability to the plants. The soil water retention curve (known also as desorption or suction curve) is a unique soil or medium property. For production in a substrate, Co varies usually between ÿ0.05 and ÿ0.15 MPa, while Cm varies between ÿ0.001 and ÿ0.01 MPa. 2.2. Total medium porosity and its components The volumetric amount of water y that saturates a given volume of the substrate is de®ned as effective pore space. The difference between total porosity and effective pore space is the volume of closed pores, not accessible to water.

M. Raviv, T.J. Blom / Scientia Horticulturae 88 (2001) 257±276

261

Container capacity is de®ned as the amount of water remaining in the container after water stops draining after saturation. The amount of available water in media is de®ned as the difference in water content between container capacity (usually de®ned as ÿ1 kPa) and unavailable water (usually de®ned as ÿ10 kPa). Under intensive cultivation with a high canopy/root ratio, maintaining high Csoil t has a bene®cial effect on rose productivity. This is probably due to its effect on photosynthate distribution between the root system and the canopy (Dasberg and Feigin, 1978). However, under these conditions, lack of oxygen may pose a severe problem in many soil types, while this risk is rare in most porous media. Physical properties of porous substrates are more adequate than those of soils for cut-rose production. Available water content is smaller on a per plant basis, but higher on a volumetric basis to that of normal ®eld soils. Much weaker matric forces in substrates, compared to soil, hold this water content. Consequently, plants grown in porous media at or near container capacity require less energy in extracting water, while experiencing a lower risk of oxygen de®ciency, than encountered by plants grown in a soil at ®eld capacity. 2.3. Water uptake by roots The ¯ux of water, Jw , from the medium to the atmosphere is regulated by the gradients in matric and osmotic potential and the sum of the inverse of the respective resistances (or inverse of re¯ection coef®cients) for the whole soil (substrate)/plant/atmosphere system: rCm rCo ÿP Jw ˆ ÿ P Rm Ro

(2)

where rCP m is the gradient in matric potential between substrate and atmosphere, Rm the total resistance to water ¯ow due to the matric potential DCm/Dx; in osmotic potential between substrate and gradient; rCo the gradient P Ro the total resistance to water ¯ow due to the osmotic atmosphere, DCo/Dx; potential gradient. The resistance of the individual components (soil, plant and atmosphere) and their interfaces (soil±plant and plant±air) depends on the re¯ection coef®cient between the water ¯ux and its driving force within soil, plant or atmosphere. So, the coef®cient relating water ¯ux with the gradient of the matric potential in a substrate is different from the coef®cient relating water ¯ux with the osmotic gradient in a substrate. For instance, Rm is small (large re¯ection coef®cient) in porous media while Ro is very large (small re¯ection coef®cient), making the contribution of the osmotic gradient to the water ¯ux negligible. On the other hand, the water ¯ux due to the osmotic gradient may be more signi®cant in a soil with a high cation exchange capacity (due to diffuse double layer and a large re¯ection coef®cient). Based on the above reasoning, for the context of this

262

M. Raviv, T.J. Blom / Scientia Horticulturae 88 (2001) 257±276

review, we will ignore the osmotic component and the water ¯ux will be described as rCm Jw ˆ ÿ P Rm

(3)

Water ¯ows from high to low potentials and is considered passive. Ceteris paribus, the steeper the gradient, the faster is the ¯ux. In addition to the potential gradient, water ¯ux is affected by the hydraulic conductance (C) of the continuum. This parameter is determined by the unsaturated soil hydraulic conductivity (USHC), root density, volume of soil/substrate occupied by roots, conductance of the xylem elements leading from the roots to the leaves and the conductance of the boundary layer between the canopy and air. It is possible to affect some components of C. Speci®cally, USHC is affected by some medium properties and is a function of its water content. Once the choice of the medium has been made, it is essential to know how y affects USHC as its decrease in porous substrates is much sharper than in soil. A minute decrease in y (e.g. of several percents) may decrease USHC by an order of magnitude and thus greatly affect water availability to the roots (Wallach et al., 1992). It was shown that in substrates, the main limiting factor to water uptake is the USHC and not the matric potential per second (da Silva et al., 1993). Unfortunately, USHC cannot be measured in situ in porous media. However, detailed analysis of USHC vs. water content and matric potential conducted ex situ enables the use of either y or Cm for control purposes, as demonstrated by Raviv et al. (1999). High root density and even root distribution within the substrate facilitate unimpaired water ¯ow in normal situations and can delay development of stress in case of transient water de®cit. These factors are strongly affected by water, air and nutrient distribution in the rhizosphere (Morvant et al., 1997). 2.4. Transpiration Water moves from the soil to the root and throughout the plant to the leaf in the liquid and in the gaseous phase from the mesophyll to the atmosphere. The latter process is called transpiration. The rate of transpiration is governed by two main components, namely the difference in water potential between stomatal air and the external atmosphere (or vapor pressure de®cit, VPD) divided by the sum of the boundary and stomatal resistances. The resistance of water vapor within the air boundary layer is relatively large within the soil/plant/atmosphere continuum (see Fig. 1). Also, the resistance in the boundary layer is about 10 times as large during the night (2400 S mÿ1) compared to the day (160±800 S mÿ1) (Hanan, 1998). Temperature of the leaf plays a very important, albeit indirect, role in the rate of transpiration through its effect on the water potential (or VPD). The leaf

M. Raviv, T.J. Blom / Scientia Horticulturae 88 (2001) 257±276

263

temperature is determined by the difference between the gains in short- and longwave radiation and the combined losses of both sensible (convection and conduction) and latent heat. This energy balance of a leaf (J mÿ2 sÿ1) can be described by Qg ˆ Rs ‡ Rl ÿ Qe ÿ Qh

(4)

Qg is the net energy gain, Rs the net heat ¯ux in short-wave radiation, Rl the net heat ¯ux in long-wave radiation, Qe the latent heat ¯ux associated with transpiration from leaf to air, Qh is the sensible heat ¯ux (convective and conductive) from the leaf. In a greenhouse, Qe is normally much higher than Qh. During summer, ET is closely related to the global irradiance intercepted by the canopy, while during the winter, it is determined by the sum of short- and long-wave radiation (esp. heating pipes). For example, Stanhill and Scholte Albers (1974) found that under Mediterranean conditions latent heat loss from `Baccara' roses was 87% of the global irradiance intercepted by the canopy. Transpirational cooling of a greenhouse-grown crop is more important than that of ®eld-grown crop. Daytime air temperature within a greenhouse is normally higher than outside ambient. Moreover, radiative and convective heat losses from plant surfaces are smaller due to the presence of cladding materials and due to reduced wind speed compared to ®eld situation. It is therefore clear that the well-being of a greenhouse-grown plant is more dependent on uninterrupted transpiration than its ®eld-grown counterpart. Stomatal conductance is high when the plant is exposed to visible light and water potential in the leaf is high. Moreover, stomatal and hydraulic conductivity within the xylem does not decrease appreciably until plants wilt (Dixon et al., 1988). Guard cell turgor pressure is the functional mechanism by which stomata open and close. Stomatal conductance is, in most cases, the rate-limiting step for transpiration (Meinzer and Grantz, 1991). This mechanism has been found to respond to root signals (Gollan et al., 1986; Meinzer et al., 1991), to light, water vapor pressure de®cit (VPD), [CO2] and to plant hormones in a complex interaction. The other factors affecting water vapor losses is the boundary layer resistance (rbl). The last factor is of great importance under greenhouse conditions where, in the absence of active air movement, air may be still resulting in a very high rbl. Air movement can reduce rbl considerably. Several models were suggested to predict potential ET as a function of these climatic factors, variables of the leaf surface physical characteristics and leaf area. Not all these models yield good predictions of actual ET. Janoudi et al. (1993) found that high leaf±air VPD induced an increase in transpiration in cucumber in spite of the expected stomatal closure, in agreement with many previous ®ndings. On the other hand, Duchein et al. (1995) found that high temperature- and VPD-induced rose stomatal closure resulted in reduced transpiration and WUE. Baille et al. (1996)

264

M. Raviv, T.J. Blom / Scientia Horticulturae 88 (2001) 257±276

demonstrated that a model based only on microclimatic factors (Jarvis, 1976) is less satisfactory, when tested with greenhouse-grown roses than a model based on physiological ¯uxes (Ball et al., 1987). One should take these ®ndings as a warning against over-simpli®cation; typical to many climate-based irrigation models, using constant values for stomatal conductance. Models that take into account the effect of VPD on stomatal conductance (e.g. Stanghellini, 1987) gave better predictions for ET as a result of changing climatic factors for tomato (Jolliet and Bailey, 1992). The above-mentioned model comparisons suggest that adding the root zone water status to climate-based irrigation models may improve irrigation control. Transpiration rate per second is not a good indicator for the onset of stress: Transpiration rate of rose leaves does not decrease appreciably until the plant wilts (Raviv et al., 2000). However, well before this point is reached, many physiological processes are severely affected. Cell expansion, for example, is dependent on full turgidity of cells, as is stomatal aperture. Most horticultural crops can produce maximal yields only when water availability is kept high throughout the whole growing period. 2.5. Effect of drought stress Plants may experience water stress whenever water loss from the canopy is greater than water uptake by the roots. The effect of the water stress depends on the respective ¯ows, duration of the de®cit period and the plant's adaptation mechanism. Water stress has both a direct and indirect in¯uence on photosynthesis and on plant growth. Plants under stress close their stomata, thereby restricting CO2 uptake directly. Water stress reduces plant's turgidity, which reduces expansive growth, ¯ow of both CO2 and carbohydrates through cell membranes and in¯uences the storage of sugars and sink/source relationships. Under extreme conditions water stress may induce cavitation in the xylem vessels, which greatly increases plants' hydraulic resistance (Dixon et al., 1988). In a horticultural context, every event of water de®cit that involves some yield loss is undesirable. Schulze and Hall (1982) determined that drought affects the stomatal conductance of C3 plants to a relatively greater extent than it affects photosynthetic metabolism. This response to drought has an adaptive signi®cance since photosynthetic WUE increases as the soil water supply decreases. Plants remain turgid when water uptake is balanced with water loss through transpiration or when the range of their osmotic adjustment capacity is not exceeded. When water loss is greater than water uptake, water stress will start to develop. This is expressed by a decrease in leaf water potential, Cleaf t . Decrease in depends on the effects of the water loss on the increased water resistance Cleaf t within the soil±plant system as well as changes in the osmotic and pressure

M. Raviv, T.J. Blom / Scientia Horticulturae 88 (2001) 257±276

265

potentials within the plant. Soil drought decreases the root and leaf water of roses usually varies between ÿ0.4 and ÿ1.0 MPa depending on potential. Cleaf t irradiance and stage of growth, while wilting of `Forever Yours' roses was shown to occur at a water potential of ÿ1.3 MPa (Aikin and Hanan, 1975). Auge et al. (1990) reported an osmotic adjustment in leaves of `Samantha' roses grown under drought conditions. Their results indicated a decrease in the foliar osmotic water 0 potential at full turgor, C100 o , and at the point of turgor loss, Co , of about ÿ0.4 MPa compared to well-watered conditions. Heavily shaded plants showed less osmotic adjustment than plants grown at full sunlight. The osmotic adjustment evoked by drought causes the accumulation of solutes in the leaves and facilitates water uptake under stress. Numerous stress phenomena are described in the scienti®c literature. The following description deals only with those directly related to NAR and growth with emphasis on roses. Cell enlargement is affected by two elements: a continuous supply of photosynthates, nutrients and plant hormones and a positive pressure (turgor) on the cell walls. Even a small decrease in turgor arrests wall stretching which leads to the cessation of expansive growth. In young, not fully expanded foliage, the transpiration rate is much higher than that of mature leaves (Jones, 1992), which makes young leaves very sensitive to transient and moderate water stress events. The resulting reduced expansive growth leads to reduced leaf area available for photosynthesis and thus to reduced yield. Greenhouse roses are harvested continuously. This practice in combination with continued production of young leaves makes the plant very sensitive to water stress. When increased stomatal resistance (rst) appears in the canopy, CO2 diffusion into the mesophyll drop, thereby lowering NAR (Blackman and Davies, 1985). More severe water de®cit may affect photosynthesis and thereby NAR via its effect on additional mechanisms, collectively de®ned as mesophyll resistance (Schulze and Kuppers, 1979); decrease in activity and concentration of enzymes along the Calvin cycle (Vu and Yelenosky, 1988) and feedback inhibition resulted from inhibited photoassimilate unloading (Ackerson, 1980; Janoudi et al., 1993). Maximum photosynthetic rates for `Samantha' rose leaves are about 20 mg CO2 mÿ2 hÿ1 for young, almost fully expanded, leaves and about 15 mg dmÿ2 hÿ1 for older and shaded leaves at an irradiance of about 500 mmol mÿ2 sÿ1 (Bozarth et al., 1982). Aikin and Hanan (1975) reported values for `Royalty' that were about one-half of the above stated values at about the same irradiance, while the maximum NCER was reached about 6 days after the red color disappeared at the adaxial side of the leaf. Aikin and Hanan (1975) showed that NCERmax declined with decreasing shoot . Moreover, the light saturation point was reached at a water potential, Cshoot t lower irradiance with decreasing shoot water potential. They suggested that ˆ ÿ0:4 MPa at an irradiance maximal photosynthesis would be reached at Cshoot t of 500 mmol mÿ2 sÿ1. Plaut et al. (1975) measured diurnal changes of NCER,

266

M. Raviv, T.J. Blom / Scientia Horticulturae 88 (2001) 257±276

Cshoot , stomatal resistance and relative water content (RWC) of `Baccara' roses t grown under two irrigation regimes (wet vs. dry). NCER increased rapidly during the morning hours and reached saturation before irradiance reached its maximum. did not appear to have a During morning hours, the irrigation regime and Cshoot t shoot decreased during morning hours and reached a strong effect on NCER. Ct minimum during late morning and early afternoon, before increasing again during was 0.2± late afternoon for both irrigation regimes. During this period, the Cshoot t 0.5 MPa lower in the dry than in the well-irrigated regime. During the middle of the day, NCER was lower in the dry treatment. It was concluded that NCER is later in the day. controlled by irradiance during early morning hours and by Cshoot t The water content (WC) of leaves can indicate whether or not leaves are turgid or ¯accid. At dawn, when plants have recovered from the water loss of the previous day, the water content is the highest (WCmax). The relative water content, RWC ˆ 100  WC=WCmax , is usually the lowest at noontime. Plaut et al. (1975) found that RWC was lower under dry than under the well-irrigated regime. 2.6. Effect of low water potential in growing media Normally, plants react to an increase in the EC of the nutrient solution by a reduced NAR, relative growth rate, leaf area, speci®c leaf area, leaf osmotic potential, predawn xylem potential, increased stomatal resistance and an increase in photosynthetic WUE. The last phenomenon is caused by greater effect of salinity on stomatal resistance than on NAR (Zozor and Marler, 1992; Brugnoli and Bjorkman, 1992; Bethke and Drew, 1992). However, mild osmoregulation, e.g. against EC of 3.8 dS mÿ1 in a well-aerated soilless medium, can be accomplished without signi®cant effect on rose leaf water relations (Urban et al., 1994). RWC of `Sonia' leaves, grown in rockwool were not affected by increasing the EC of the solution from 1.8 to 3.8 dS mÿ1. However, there was a seasonal effect as minimal RWC was lower by 6±7% during July and August compared to the rest of the year. Contrary to what has been found by Auge et al. (1990) under drought conditions, Urban et al. (1993) found no evidence for increased tissue elasticity under high EC. In a recirculating production system (Baas et al., 1997) in which sodium and chloride ions were allowed to accumulate to 12 mmol lÿ1 and EC to go up to 4.8 dS mÿ1, `Madelon' roses showed a small but signi®cant decrease (ÿ2% per dS mÿ1) in number of harvested ¯owers. Uptake of sodium was limited and no speci®c toxicity for sodium or chloride was observed. An increase of EC in the nutrient solution from 0.06 to 0.18 MPa (in absolute values) caused a from ÿ1.0 to slightly higher decrease in the osmotic potential of the leaves (Cleaf o ÿ1.20 MPa) indicating that an osmotic adjustment mechanism had taken place or that there had been a change in the resistance to water ¯ow in the root system and/ or xylem system. de Kreij and van den Berg (1989) showed similar results with

M. Raviv, T.J. Blom / Scientia Horticulturae 88 (2001) 257±276

267

Fig. 2. The effect of electrical conductivity (EC, dS mÿ1) and photosynthetic photon ¯ux density (PPFD, mmol mÿ2 sÿ1) on leaf net carbon exchange rate (NCER, mmol CO2 mÿ2 sÿ1) of `Mercedes' rose leaves.

`Sonia' (ÿ2% in production per dS mÿ1), in which the EC was varied with different nutrient concentrations, while maintaining a ®xed ratio among the major elements. In both latter studies, no photosynthesis measurements were taken. In a study conducted in Israel, `Mercedes' plants were grown in perlite, with a ¯owing, recirculated solution, which was allowed to reach a predetermined EC value before replacing the solution with a fresh one. Nutrient solutions reached an EC of up to 7.5 dS mÿ1 with no negative effect on net carbon exchange rate (NCER) of rose leaves (Fig. 2, taken from Raviv and Korkmaz, in preparation). The results presented in Fig. 2 represent NECR values typical of a gradual acclimatization process of roses to increasing EC from 2.5 to 7.5 dS mÿ1. It can be concluded that such gradual acclimatization enables practically unimpaired NECR by `Mercedes' rose leaves. Leaf area, on the other hand, was reduced by 5.3% per dS mÿ1 (Fig. 3, taken from Raviv and Korkmaz, in preparation). It therefore appears that adaptation mechanism for leaf expansive growth was less ef®cient than that of photosynthesis, under the studied conditions, suggesting a need for precise determination of EC threshold value for this important productivity-related parameter. Speci®c transpiration exhibited an even sharper response with a decrease of 7.6% per dS mÿ1 (Fig. 3). Transpiration per plant was therefore lower by 13.3% per dS mÿ1, as well as the number of ¯owers declined by 2.9% (similar to Baas et al., 1997; de Kreij and van den Berg, 1989) and stem length declined by 0.6 cm per dS mÿ1. Shoot diameter appeared insensitive to

268

M. Raviv, T.J. Blom / Scientia Horticulturae 88 (2001) 257±276

Fig. 3. The effect of average electrical conductivity (EC, dS mÿ1) on average leaf area (cm2 per plant) and speci®c transpiration rate (cm3 cmÿ2 per day) of `Mercedes' roses.

high salinity. WUE yield was increased from 1.7 to 2.6 g kgÿ1 when EC was increased from 2.5 to 7.5 dS mÿ1. Plants can develop adjustment mechanisms to severe water stress (Janoudi et al., 1993). This process consumes time (acclimatization process) and energy and results in yield loss. Osmotic adjustment is an adaptive mechanism evoked by drought or by high salinity. It involves a lower osmotic potential, Co, within the leaves (Auge et al., 1990), which facilitates water uptake by the plant and minimizes turgor pressure decrease. The osmotic adjustment is achieved through accumulation of organic (carbohydrates) and inorganic (ions) solutes and/or partition of water into apoplastic and symplastic fractions. The in¯uence of drought on osmotic and turgor adjustment was examined in `Samantha' rose fully expanded leaves under varying irradiance levels (Auge et al., 1990). They concluded that turgor pressure adjustments were high under both well-watered and drought conditions under relatively high irradiance (100 and 70% of full sunlight). Heavy shading (30% irradiance) inhibited osmotic and turgor adjustments in leaves under drought conditions. 3. Irrigation control Irrigation control involves the determination of both timing and quantity of water application. It can be approached from the point of view of potential ET

M. Raviv, T.J. Blom / Scientia Horticulturae 88 (2001) 257±276

269

according to one of the existing climatic models or using one of several sensors reporting the state of soil water or using various plant response parameters or any combination of the above (Norrie et al., 1994). The ®rst approach can be generally described as ETm ˆ Kc  ETa

(5)

where, ETm is the maximum ET under the speci®c climatic conditions, Kc the crop coef®cient, ETa the actual ET. The Penman±Montieth equation is the most widely used method for ETm prediction, based on the relevant climatic data such as net radiation absorbed by leaves, temperature, VPD and wind speed. A simpli®ed but still accurate version of the Penman±Montieth equation was developed for Ficus benjamina (Bailey et al., 1993). A simpler equation, to be used in commercial circumstances that are not equipped with sophisticated computerized control, based on pan evaporation and plant-canopy height and width as input variables has been generated (Stanley and Harbaugh, 1989) with partial success. However, the applicability of the Penman±Montieth equation and presumably other climatic models for irrigation control of roses in a greenhouse is still questionable (Munoz-Carpena et al., 1996). The fact that roses, unlike most other crops, are being constantly harvested and thereby exhibiting large ¯uctuation of the transpiring area must be taken into consideration when attempting to formulate any climatic model. Inputs typical to greenhouse intensive cultivation such as supplementary lighting, heating pipes, air humidity and CO2 concentration are not well predicted by classical, ®eldoriented models (Blom-Zandstra et al., 1995; Baille et al., 1996). If the physical characteristics of the growing medium are well understood, speci®c water content can be determined which will enable simultaneous optimal air and water contents in the root zone. Ideally, irrigation should then compensate for any amount of water that evaporated from the medium and transpired by the plants. If evaporation is minimized (e.g. by mulching) and transpiration is measured constantly (e.g. using weighing lysimeter), then water content in the medium can be maintained at the predetermined level. However, precise balance between ET and irrigation is rarely practical, mainly due to increased salinity in the root zone and lack of uniformity in the delivery of the irrigation water and in leaf area among plants. Non-essential ions such as sodium are not taken up by rose plants, causing gradual salinity build-up (Baas and van den Berg, 1999). To prevent this, excessive amounts of water must be supplied, in order to leach accumulated salts. If the root zone is leached frequently, the roots are never exposed to excessive low Csoil o . This practice is de®ned as micro-irrigation. Micro-irrigation has three clear advantages: water and oxygen are always at optimal levels in the root zone, nutrient concentration in the rhizosphere is constantly maintained at optimal level, and Co of the soil solution is maintained close to that of the irrigation water.

270

M. Raviv, T.J. Blom / Scientia Horticulturae 88 (2001) 257±276

Plant parameters, which can be used to determine irrigation timing, are: leaf water potential (with pressure chamber); stomatal resistance for water (with diffusion porometer); canopy temperature (with thermocouples or infrared thermometers); ¯ow of water in the stem (with the heat pulse method); and changes in stem diameter (with dendrometer). The use of the ®rst three parameters is not practical in commercial growing systems and these serve exclusively as research tools. The last two parameters can be used as an aid in commercial greenhouse production. They can be used to identify post factum events of water shortage when no visual signs are apparent. Analyzing such events can help the grower in the future. However, they cannot be used for irrigation control due to their relatively slow response time. In spite of the fact that stem diameter was suggested as a sensitive measure for plant water potential (Urban et al., 1996), until now, this technique did not achieve wide practical use and serves mostly for research purposes. Rosa et al. (1991) found in is better correlated with stomatal conductance and soybean that Cplant t photosynthetic rate than leaf water potential. Off-the-shelf irrigation control systems appear in the form of computer programs that are based on climatic algorithms and include real-time sensing of meteorological variables (e.g. total radiation, VPD, etc.). The other alternatives are the use of FD or TDR sensors or soil-tension actuated irrigation or the combination of both climate and soil parameters (Norrie et al., 1994). Each of these sensors has drawbacks such as cost and sensitivity to EC (TDR), lower accuracy and slow equilibration time (FD) or maintenance (tensiometers). All the three sensors have sensor to medium contact problems. It appears that only a system that combines plant, meteorological and medium parameters can give satisfactory answer for irrigation control. Oki et al. (1996) found that higher yield, reduced run-off and a higher WUE were achieved with tensiometer-based irrigation of roses compared to the oftenrecommended commercial time-based irrigation schedules. Csoil m was also used by Raviv et al. (1992, 1993) to control rose and melon irrigation as compared to conventional pulse irrigation. Higher NAR values were obtained with the tensiometer-based irrigation. Within the relevant range of water potential, maintained in scoria, a decrease of ca. 10% in photosynthetic rate was found per unit (kPa) decrease in Csoil m . Such a minute decrease in matric potential per second cannot explain this signi®cant physiological response. It appears that this response is caused by the marked decrease in USHC of the substrate, with decreasing water content as shown by Wallach et al. (1992). In some speci®c cases, the above-mentioned irrigation control principles are not suf®cient for optimal water application. Other factors, such as convective and advective heat transfer may increase the energy load on the plant above its maximal transpiration rate. Typically this may occur shortly after planting small transplants, drip-irrigated individually, encircled by dry soil surfaces (Oasis

M. Raviv, T.J. Blom / Scientia Horticulturae 88 (2001) 257±276

271

effect). Other factors that should be taken into account are the effect of irrigation timing and frequency on nutrients (especially calcium) availability and on greenhouse microclimate. 4. Irrigation water quality As explained above, when water ¯ow rate to roots of rose plants grown in porous media is balanced with the transpiration rate, little negative effects on yield quality and quantity of roses can be found at relatively high EC values, up to 4.5 dS mÿ1 (Gislerod et al., 1993, 1996). When transpiration rate is greater than water supply rate, low soil water potential may develop locally at the rhizoplane, resulting from both matricpressure forces and rapidly concentrating ions, excluded from the roots. This may result in locally increasing osmotic stress without any appreciable increase in salinity when measured in the bulk substrate or in the leachate. Since transport of ions by water mass ¯ow is about 10 000 times faster than by diffusion, more frequent irrigation is the obvious solution to this problem. Isaac and Urban (1996) found that stomatal conductance of rose leaves was not affected by bulk-EC of 4.5 dS mÿ1, as compared to control of 1.4 dS mÿ1, provided that the supply rate was high enough to prevent salt accumulation at the root zone. In addition to osmotic stress, negative effects of salinity on soil-grown roses (Fernandez Falcon et al., 1986; Yaron et al., 1969) may indirectly result from oxygen de®ciency, damaging the ATPase-Na‡ exclusion mechanism and enabling sodium entry into the cytoplasm with the transpirational stream. This situation is typical to media having low ODR such as soil but rarely prevails in porous media. 5. Practical conclusions Most rose growers who are using substrates, irrigate with a 30±50% LF in order to prevent decreased osmotic potential and build-up of both essential and nonessential ions in the rhizosphere. Although this may sound as a signi®cant waste, economically, the total cost of water and fertilizers constitutes less than 3±5% of the farm sales (Canada and Israel, respectively). Compared to other production costs and considering the potential damage of water stress, the commercial rose grower has no strong incentive to save water. His/her main task is, rather, to enable uninterrupted transpiration in order to avoid any stress event, facilitate maximal photosynthesis and consequently to increase yields. In order to do so, it is essential to prevent sharp ¯uctuations in water availability. Frequent water replenishment prevents decreased matric and osmotic potentials on the rhizoplane and enables maintaining a gradient of nutrients from the substrate solution to the

272

M. Raviv, T.J. Blom / Scientia Horticulturae 88 (2001) 257±276

root (Jaffrin et al., 1994). In practice, this can be done with micro-irrigation (Section 3), that enables almost constant water application. Simultaneously, nutrients are constantly supplied to the rhizoplane and excluded ions are leached. As normal irrigation practices, micro-irrigation also requires some sort of control of both water quantity and scheduling. Water quantity is usually determined by using climate-based control computers. Timing should be determined so as to ensure maximal water availability. Since both USHC in the rhizosphere and matric potential on the rhizoplane cannot be measured in situ, other parameters should be used, that maintain a predictable correlation with the above parameters. A positive correlation exists between USHC and the matric potential in the growing medium as well as with y. The latter parameters can both serve to optimize irrigation and can be measured with tensiometer and with TDR or FD sensors, respectively. Direct measurement of transpiration (e.g. using weighing lysimeter) is also an elegant method of irrigation control, although less convenient for the commercial grower. If water should be conserved and excess nutrients contained, it is due to environmental rather than economical considerations. The immediate solution to these requirements is ef¯uent recycling. This technique is now widely used in temperate countries and even under semi-arid conditions (Raviv et al., 1998). An inherent advantage of closed systems is the easy application of micro-irrigation, with the above-mentioned advantages. The subject of recycled water treatment (nutrient and disease management) is beyond the scope of the present review. Transpiration is also governed by the water potential gradient inside a leaf and that of the external air. Extremely low water potential in the air (high VPD) may create a water stress within the plant reducing leaf expansion and growth. On the other hand, a high water potential in the air may decrease the transpiration rate and increase the susceptibility to foliar diseases and/or physiological disorders. Commercially, the water potential is controlled through a combination of heating, cooling (ventilation), shading and sometimes misting. 6. Required future research Combined effects of drought and salinity on photosynthesis have been studied for a number of agronomic crops such as wheat (Sepaskah and Boersma, 1979), maize (Stark and Jarrell, 1980), and sorghum (Richardson and McCree, 1985), but studies on roses have been limited. Many of these studies on agronomic crops with both salinity and moisture stress used growth chambers to obtain steady state conditions and measurements on water potential of the plants and were made at a given time during the day (Richardson and McCree, 1985). Plaut et al. (1975) conducted short-term measurements (hourly), involving rose plants under nonsteady state conditions. Information from similar studies can provide more insight

M. Raviv, T.J. Blom / Scientia Horticulturae 88 (2001) 257±276

273

into the adaptive processes within rose plants when exposed to salt or water stresses. The changes at the rose rhizosphere during and between irrigation pulses are not well understood and should be studied closer. Main concern should be devoted to deciphering processes related to rate of water uptake, oxygen supply for root respiration, nutrient delivery, and pH changes at the medium±root interface and its effect on changes in nutrient status over time. Tissue elasticity (e) has often been used to characterize a plant's ability to drought adaptation. Auge et al. (1990) reported an increase in e in `Samantha' rose leaves due to drought. More quantitative work is required with additional cultivars. No information on the effect of salts on elasticity was found and similar effort should be devoted to this direction, as well. Although it appears that roses are more salt tolerant than previously believed, it is essential to determine how both expansive growth and photosynthetic rate are affected by EC. Precise quantitative description of the relations between soils' solution osmotic potential and plants' physiological responses will facilitate the much-needed ef¯uent reuse. The interaction between the temperature in the root zone and the effects of osmotic potential on the plants should also be studied, due to the marked effect of temperature on the concentration of dissolved oxygen. Another factor to be considered in this respect is the use of re¯ective mulch and its effect on the ODR in the substrate. Our knowledge regarding the ability of various rose rootstocks to cope with low osmotic and matric potential and with low concentrations of dissolved oxygen in the soil solution is very limited. This should be a subject of thorough physiological research and breeding work that may result with improved rootstocks for osmotic adjustment, water and nutrient uptake and rose productivity. Acknowledgements A workshop organized by Roses Inc., Haslett, MI stimulated us to write this review. MR is grateful to Cihangir Gurler Korkmaz of Uludag University, Turkey for the permission to use results of experiments conducted during Korkmazs' stay at his laboratory. We thank Dr. Asher Bar Tal of the Agricultural Research Organization, Dr. Pieter H. Groenvelt and Dr. Mike Dixon of the University of Guelph for a thorough review of the manuscript and their useful suggestions.

References Ackerson, R.C., 1980. Stomatal responses of cotton to water stress and abscisic acid as affected by water stress history. Plant Physiol. 65, 455±459.

274

M. Raviv, T.J. Blom / Scientia Horticulturae 88 (2001) 257±276

Aikin, W.J., Hanan, J.J., 1975. Photosynthesis in the rose: effect of light intensity, water potential and leaf age. J. Am. Soc. Hort. Sci. 100, 551±553. AugeÂ, R.M., Stodola, A.J.W., Pennell, B.D., 1990. Osmotic and turgor adjustment in Rosa foliage drought-stressed under varying irradiance. J. Am. Soc. Hort. Sci. 115, 661±667. Baas, R., van den Berg, D., 1999. Sodium accumulation and nutrient discharge in recirculation systems: a case study with roses. Acta Hort. 507, 157±164. Baas, R., van den Berg, T.J.M., Braamhorst, P., 1997. Natrium-ophoping bij rose `Madelon': effecten op productie, kwaliteit and nutrientenopname. Proefstation voor Bloemisterij en Glasgroente, Aalsmeer, The Netherlands. Report No. 87, 30 pp. ISSN 1385-3015. Bailey, B.J., Montero, J.I., Biel, C., Wilkinson, D.J., Anton, A., Jolliet, O., 1993. Transpiration of Ficus benjamina: comparison of measurements with predictions of the Penman±Monteith model and a simpli®ed version. Agric. Forest. Meteorol. 65, 229±243. Baille, M., Romero-Aranda, R., Baille, A., 1996. Gas-exchange response of rose plants to CO2 enrichment and light. J. Hort. Sci. 71, 945±956. Ball, J.T., Woodrow, I.E., Berry, J.A., 1987. A model predicting stomatal conductance and its contribution to the control of photosynthesis under different environmental conditions. In: Biggins, J. (Ed.), Progress in Photosynthesis Research, Proceedings of the Seventh International Congress. Kluwer Academic Publishers, Boston, MA. Bethke, P.C., Drew, M.C., 1992. Stomatal and nonstomatal components to inhibition of photosynthesis in leaves in Capsicum annum during progressive exposure to NaCl salinity. Plant Physiol. 99, 219±226. Blackman, P.G., Davies, W.J., 1985. Root to shoot communication. J. Exp. Bot. 36, 39±48. Bloemhard, C., van den Bos, A., van der Burg, N., de Graaf, R., Klapwijk, D., de Kreij, C., Post, W., van Schie, W., Sonneveld, C., Straver, N., Voogt, W., van der Wees, A., 1993. Plantenvoeding in de glastuinbouw. Proefstation voor Tuinbouw onder Glas, Naaldwijk, The Netherlands. Informatiereeks 87, 232 pp. Blom-Zandstra, M., Sander Pot, C., Maas, F.H., Schapendonk, H.C.M., 1995. Effect of different light treatments on the nocturnal transpiration and dynamics of stomatal closure of two rose cultivars. Sci. Hort. 61, 251±262. Bozarth, C.S., Kennedy, R.A., Schekel, K.A., 1982. The effects of leaf age on photosynthesis in rose. J. Am. Soc. Hort. Sci.107, 707±712. Brugnoli, E., Bjorkman, O., 1992. Growth of cotton under continuous salinity stress: in¯uence on allocation pattern, stomatal and nonstomatal components of photosynthesis and dissipation of excess light energy. Planta 187, 335±347. Cabrera, R.I., 1997. Water use by roses. Roses Bull. (August 1997), 38±42. Dasberg, S., Feigin, A., 1978. The effect of irrigation, fertilization and organic matter on roses grown in four soils in the greenhouse. Sci. Hort. 9, 181±188. da Silva, F.F., Wallach, R., Chen, Y., 1993. A dynamic approach to irrigation scheduling in container media. In: Proceedings of the Sixth International Conference on Irrigation, Tel Aviv, pp. 183±198. de Kreij, C., van den Berg, T.J.M., 1989. Lage EC beinvloedt kwaliteit roos positief. Vakblad voor de Bloemisterij 44 (4), 154±155. de Wit, C.T., 1958. Transpiration and crop yields. Versl. Landbouwk. Onderz. 64, 6. Inst. of Biol. and Chem. Res. on Field Crops and Herbage, Wageningen, The Netherlands. Dixon, M.A., Butt, J.A., Murr, D.P., Tsujita, M.J., 1988. Water relations of cut greenhouse roses: the relationship between stem water potential, hydraulic conductance and cavitation. Sci. Hort. 36, 109±118. Duchein, M.-C., Baille, M., Baille, A., 1995. Water use ef®ciency and nutrient consumption of a greenhouse rose crop grown in rockwool. Acta Hort. 408, 129±135.

M. Raviv, T.J. Blom / Scientia Horticulturae 88 (2001) 257±276

275

Fernandez Falcon, M., Alvarez Gonzalez, C.E., Garcia, V., Baez, J., 1986. The effect of chloride and bicarbonate levels in irrigation water on nutrient content, production and quality of cut roses `Mercedes'. Sci. Hort. 29, 373±385. Gislerod, H.R., Fjeld, T., Mortensen, L.M., 1993. The effect of supplementary light and electrical conductivity on growth and quality of cut roses. Acta Hort. 342, 51±60. Gislerod, H.R., Revhaug, V., Mikkelsen, H., 1996. Effect of electric conductivity of the growing medium on growth and yield of roses pruned in summer. Acta Hort. 424, 123±127. Gollan, T., Passiura, J.B., Munns, R., 1986. Soil water status affects the stomatal conductance of fully turgid wheat and sun¯ower leaves. Aust. J. Plant Physiol. 13, 459±464. Hanan, J., 1998. Greenhouses: Advanced Technology for Protected Horticulture. CRC Press, Boca Raton, FL. ISBN 0-8493-1698-7. Isaac, S., Urban, L., 1996. Effect of electrical conductivity and supply rate of the nutrient solution on stomatal conductance of rose plant leaves. Acta Hort. 424, 131±133. Jaffrin, A., Champeroux, A., Pertusier, N., 1994. Water and mineral demand of greenhouse soilless rose crops grown under summer conditions. Acta Hort. 361, 258±266. Janoudi, A.K., Widders, I.E., Flore, J.A., 1993. Water de®cit and environmental factors affect photosynthesis in leaves of cucumber (Cucumis sativus). J. Am. Soc. Hort. Sci. 118, 366±370. Jarvis, P.G., 1976. The interpretation of the variation in leaf water potential and stomatal conductance found in canopies in the ®eld. Phil. Trans. R. Soc. London, Ser. B 273, 593±610. Jolliet, O., Bailey, B.J., 1992. The effect of climate on tomato transpiration in greenhouses: measurements and model comparison. Agric. Forest. Meteorol. 58, 43±62. Jones, H.G., 1992. Plants and Microclimate, 2nd Edition. Cambridge University Press, Cambridge, UK. Meinzer, F.C., Grantz, D.A., 1991. Coordination of stomatal, hydraulic, and canopy boundary layer properties: do stomata balance conductances by measuring transpiration? Physiol. Plant 83, 324±329. Meinzer, F.C., Grantz, D.A., Smit, B., 1991. Root signals mediate coordination of stomatal and hydraulic conductance in growing sugarcane. Aust. J. Plant Physiol. 18, 329±338. Morvant, J.K., Dole, J.M., Allen, E., 1997. Irrigation systems alter distribution of roots, soluble salts, nitrogen, and pH in the root medium. HortTechnol. 7, 156±160. Munoz-Carpena, R., Socorro-Monzon, A.R., Manisto, P., Trujillo, J., 1996. Irrigation management based on water consumption for greenhouse roses growing on terrace soils in the Canary Islands. Acta Hort. 424, 111±114. Norrie, J., Graham, M.E.D., Gosselin, A., 1994. Potential evapotranspiration as a means of predicting irrigation timing in greenhouse tomatoes grown in peat bags. J. Am. Soc. Hort. Sci. 119, 163±168. Oki, L.R., Lieth, J.H., Tjosvold, S., 1996. Tensiometer-based irrigation of cut-¯ower roses. Report Submitted to the California Cut-¯ower Commission, 8 pp. Plaut, Z., Halevy, A.H., Diskin, Y., 1975. Diurnal pattern of plant water status and CO2 ®xation of roses as affected by irrigation regimes. J. Am. Soc. Hort. Sci. 100, 191±194. Raviv, M., Medina, Sh., Shamir, Y., Ben Ner, Z., 1992. Very low medium moisture tension Ð a feasible criterion for irrigation control of container-grown plants. Acta Hort. 342, 111±119. Raviv, M., Nerson, H., Medina, Sh., Berdugo, R., Shamir, Y., Krasnovsky, A., Ben Ner, Z., 1993. Moisture tension of the growing medium and irrigation control of container-grown plants. In: Proceedings of the Sixth International Conference on Irrigation, Tel Aviv, pp. 199±209. Raviv, M., Krasnovsky, A., Medina, Sh., Reuveni, R., 1998. Assessment of various control strategies for recirculation of greenhouse ef¯uents under semi-arid conditions. J. Hort. Sci. Biotechnol. 73, 485±491.

276

M. Raviv, T.J. Blom / Scientia Horticulturae 88 (2001) 257±276

Raviv, M., Wallach, R., Silber, A., Medina, Sh., Krasnovsky, A., 1999. The effect of hydraulic characteristics of volcanic materials on yield of roses grown in soilless culture. J. Am. Soc. Hort. Sci. 124, 205±209. Raviv, M., Lieth, J.H., Wallach, R., 2000. The effect of root-zone physical properties of coir and UC mix on performance of cut rose (cv. Kardinal). Acta Hort., in press. Richardson, S.G., McCree, K.J., 1985. Carbon balance and water relations of sorghum exposed to salt and water stress. Plant Physiol. 79, 1015±1020. Rosa, L.M., Dillenburg, L.R., Forseth, I.N., 1991. Response of soybean leaf angle, photosynthesis and stomatal conductance to leaf and soil water potential. Ann. Bot. 67, 51±58. Schulze, E.D., Hall, A.E., 1982. Stomatal responses, water loss and CO2 assimilation rates of plants in contrasting environments. In: Lange, O.L., Nobel, P.S., Osmond, C.B., Ziegler, H. (Eds.), Physiological Plant Ecology. II. Water Relation and Carbon Assimilation, Encyclopedia of Plant Physiology, New Series, Vol. 12B. Springer, Berlin, pp. 181±230. Schulze, E.D., Kuppers, M., 1979. Short term and long term effects of water de®cits on stomatal responses to humidity in Corylus avellana L. Planta 146, 319±326. Sepaskah, A.R., Boersma, L., 1979. Elongation of wheat leaves exposed to several levels of matric potential and NaCl induced osmotic potential of soil water. Agron. J. 71, 848±852. Stanghellini, C., 1987. Transpiration of greenhouse crops, an aid to climate management. Ph.D. Dissertation. Agricultural University, Wageningen, 150 pp. Stanhill, G., Scholte Albers, J., 1974. Solar radiation and water loss from glasshouse roses. J. Am. Soc. Hort. Sci. 99, 107±110. Stanley, C.D., Harbaugh, B.K., 1989. Poinsettia irrigation based on evaporative demand and plant growth characteristics. HortScience 24, 937±939. Stark, J.C., Jarrell, W.M., 1980. Salinity-induced modi®cations in the response of maize to water de®cits. Agron. J. 72, 745±748. Steduto, P., 1996. Water use ef®ciency. In: Periera, L.S., Feddes, R.A., Gilley, J.R., Lesaffre, B. (Eds.), Sustainability of Irrigated Agriculture, Proceedings of the NATO Advanced Research Workshop, Vimeiro, Portugal, pp. 193±209. Urban, L., Jaffrin, A., Chraibi, A., 1993. Analysis of pressure±volume curves of leaves of Rosa hybrida cv. `Sonia'. J. Exp. Bot. 44, 605±613. Urban, L., Brun, L., Pyrrha, P., 1994. Water relations of leaves of `Sonia' rose plants grown in soilless greenhouse conditions. HortScience 29, 627±630. Urban, L., Fabret, C., Barthelemy, L., 1996. Changes in stem diameter depend upon variations in water content in rose plants. Acta Hort. 424, 67±72. Vu, J.C.V., Yelenosky, G., 1988. Water de®cit and associated changes in some photosynthetic parameters in leaves of `Valencia' orange (Citrus sinensis L. Odnrvk). Plant Physiol. 88, 375± 378. Wallach, R., da Silva, F.F., Chen, Y., 1992. Hydraulic characteristics of tuff (scoria) used as a container medium. J. Am. Soc. Hort. Sci. 117, 415±421. Yaron, B., Zieslin, N., Halevy, A.H., 1969. Response of `Baccara' roses to saline irrigation. J. Am. Soc. Hort. Sci. 94, 481±484. Zozor, Y., Marler, T.E., 1992. Photosynthetic responses of giant Granadilla (Passi¯ora quadrangularis L.) to salinity. HortScience 27, 1231.