Effects of air humidity and nutrient solution concentration on growth, water potential and stomatal conductance of cucumber seedlings

Scientia Horticulturae, 50 (1992) 173-186

173

Elsevier Science Publishers B.V., Amsterdam

Effects of air humidity and nutrient solution concentration on growth, water potential and stomatal conductance of cucumber seedlings Pieter A.C.M. van de Sanden and Bob W. Veen Centre for Agrobioiogical Research, P.O. Box 14, 6 700 AA Wageningen, Netherlands (Accepled 6 November 1991 )

ABSTRACT Van de Sanden, P.A.C.M. and Veen, B.W., 1992. Effects of air humidity and nutrient solution concentration on growth, water potential and stomatal conductance of cucumber seedlings. Scientia Hottic., 50: 173-186. The influence of air humidity and the concentration of the nutrient solution on the growth of cucumber seedlings in water culture was studied in a split-plot design. Three air humidities (55%, 75% and 95%) and three solution concentrations (electrical conductivity i mS cm-~, 4 mS cm-~ and 8 mS cm- ~) were examined. Classical growth analysis was used to derive parameters, which were then related to single epidermal cell area, stomatal conductance and leaf water status. Increased air h,lmidity enhanced relative growth rate (RGR). At relatively low humidity (55-75%) this was attributed to an increase of net assimilation rate (NAR), caused by an increased stomatal conductance. At high humidity (75-95%) the prime cause seemed to be a higher specific leafarea. Individual leaf area was increased at high humidity, but the enhancement of leaf elongation was not always associated with high leaf turgor. Increased electrical conductivity of the nutrient solution reduced RGR, although NAR was pro~noted. Shoot growth and leafelongation were negatively affected, but root growth was not. Leaf water potential changed in parallel with the water potential of the root environment. The inhibition of leaf elongation by electrical conductivity coincided with reduced leafturgor. At high electrical conductivity some osmotic adjustment occurred. Keywords: cucumber; Cucumis sativus; dry matter distribution; electrical conductivity; growth analysis; humidity; leafelongation; net assimilation rate; specific leaf area; stomatal conductance; water potential. al ! * ,* ~,OUreVliitlOiiS. t- It . ~ t . t . = t t a y s he, o.. *,-~,~,~lantino. l : : ¢ " ~ e i a c t r i e ~ i c n n d u c t i v i ~ y , --

.l

~l

IIAR =leaf

area ratin;

LWK = leaf weight ratio; NAR = net assimilation rate: RGR = relative growth rate; RH = relative humidity; RWR=root weight ratio; SLA=specific leaf area; SWR= stem weight ratio; u.n.s. = universal nutrient solution; VPD = vapour pressure deficit. Correspondence to: P.A.C.M. van de Sa,den, Centre for Agrobiological Research, P.O. Box 14, 6700 AA Wageningen, Netherlands.

© 1992 Elsevier Science Publishers B.V. All rights reserved 0304-4238/92/$05.00

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P.A.C.M. VAN DE SANDEN AND B.W. VEEN

INTRODUCTION

The effect of aerial factors (e.g. light intensity, air temperature and carbon dioxide concentration) on photosynthesis, growth and yield of (greenhouse) crops have been well studied and integrated into mechanistic models (Chailla and Schapendonk, 1986; Gijzen and Goudriaan, 1989), which can be used in the automated control of greenhouse culture. Modern greenhouse culture has not only benefited from the automation of climate control, but also from improved control of the root environment through the nutrient fil~ technique and substrate culture. Manipulation of plant water status to improve growth and production is now feasible. Marcelis (1989) included climatic factors such as leaf-to-air vapour pressure difference and root environment water potential in his explanatory model of the water relations of greenhouse crops, but the effects of these on plant growth have not yet been quantified. Leaf toair vapour pressure difference affects the plant's water loss, and the water potential of the root environment influences the availability of water. Thus both affect the plant's water status. In turn, plant water status influences stomatal conductance and leaf gas exchange (Schulze, 1986), tissue extension (Tyree and Jarvis, 1982; Cosgrove, 1986; Hsiao and Jing, 1987), morphology, and the distribution of dry matter and fresh weight (Mclntyre, 1987). Respective direct effects on stomatal conductance (Schulze, 1986) and on tissue extension (Passioura, 1988 ) have also been reported. Our study aimed to quantify the effect of leaf-to-air vapour pressure difference and root environment water potential on the growth of cucumber seedlings, which are sensitive to salinity and humidity (Bakker et al., 1987), and to study the consequences for plant water status, stomatal conductance and the size of epidermal leaf cells. Seedlings were exposed to different leaf-to-air vapour pressure differences and to different water potentials in the root environment by using different concentrations of the nutrient solution. The latter is common practice in greenhouse cultivation on artificial substrates. MATERIALS AND METHODS

Cucumber (Cucumis s a t i v u s L. cultivar 'Corona') seeds were sown at 30°C in moist Perlite. At emergence (4 days after sowing), they were transferred to a growth chamber, the next day transplanted in water culture units, containing aerated nutrient solution, and grown at 25 °C air temperature, 400 /~1 I-! CO2 concentration and 28 W m -2 photosynthetically active radiation (fluorescent tubes TLD Super 80) with an 8 h day length (simulating standard winter conditions in greenhouses in the Netherlands). Experiments lasted for 23 days after transplanting (d.a.t.). In each water culture unit 30 seedlings were available for measurements and growth analysis. The growth of these 'standard winter plants' has been described by Challa (1976). Growth. -

EFFECT OF HUMIDITY AND NUTRIENTS ON GROWTH OF CUCUMBERS

| 75

Experimental design. - Nine consecutive experiments were performed. Usi~lg a split-plot design, the effect of three levels of relative air humidity (RH) 55+5%, 75+2% and 95+2%, corresponding to 1.43, 0.79 and 0.16 kPa vapour pressure deficit (VPD), was studied in triplicate and within each humidity treatment three levels of nutrient solution concentration were used. Leaf-to-air temperature differences (measured occasionally using a porometer) were less than 1 °C and therefore VPD was an acceptable approximation of leaf-to-air vapour pressure difference. In the chamber seedlings were grown in three water culture units, each with nutrient solution of differing concentrations and electrical conductivities (EC). The 'universal nutrient solution' (u.n.s.) (Steiner, 1968) was used and concentrations equalled 0.5 u.n.s., 2 u.n.s, and 4 u.n.s. Their EC were 1 mS era-~, 4 mS crn-l, and 8 mS era-t, corresponding to osmotic potentials of - 36 kPa, - 144 kPa and - 288 kPa, respectively. The seedlings were spaced to avoid mutual shading. In one of three replications water status, stomatal conductance and number of epidermal cells and stomates were measured. Growth analysis. - Classical growth analysis (Evans, 1972; Hunt, 1982 ) was used, relative growth rates (RGR) were calculated and analysed with respect to net assimilation rate (NAR), leaf morphology and dry matter distribution. From each water culture unit five seedlings were harves~ted at random, just before the start of the light period, at 9, 12, 14, 16, 19 and 23 d.a.t. Leaf area and fresh and dry weight of roots, stem plus petioles, and individual leaves were determined. RGR were calculated using polynomial fits of natural logarithm transformed data of dry weight over time. To determine whether RGR was constant different order polynomials were tested; in all treatments the first-order polynomial gave a very good fit (average percentage variance accounted for 98.7%). Mean leaf area ratio (LAR) was calculated from actual instantaneous values at day of harvest instead of u~ing the functional approach. Also mean specific leaf area (SLA) and mean weight ratios were calculated from actual instantaneous values. Mean NAR was calculated using NAR × LAR = RGR. Thus parameters were derived as stationary averages to be used for comparative purposes. They were subjected to analysis of variance to test for least significant differences or (non-)linear contra~ts. Number o f epidermal cells and stomates per unit leaf area. - A non-destructive replica technique (Pieters, 1974) was usedo Replicas were taken from a central interveinal area of the abaxial and adaxial sides of the third leaf above the cotyledons by applying a thin-flowing, silicone based, precision impression material (BayerDental Xantropren light body ) and a polymerising agent (BayerDental Elastomer Activator ). To obtain a transparent copy, a thin layer of polystyrene, dissolved in toluol, was applied to the replica, allowed to dry, removed and studied under the microscope. Single epidermal cell area was

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P.A.C.M. VAN DE SANDEN AND B.W. VEEN

calculated by dividing the area of measurement by the amount of epidermal cells counted. Stomatal index was calculated as the ratio of the number of stomata to number of non-stomatal epidermal cells per unit leaf area. Total number of epidermal and stomatal guard cells was calculated by multiplying number of cells counted per unit leaf area by leaf area. Six seedlings from each treatment were selected for measurement:is 26 d.a.t., just before the start of the light period. Measurements were done at 55% and 75% RH and at all concentrations of the nutrient solution. Water status and stomatal conductance. - A pressure chamber (Ritchie and Hinckley, 1975 ) was used to measure the pressure potential of the xylem of the third leaf above the cotyledons. Desiccation was prevented by inserting the leaf in a plastic bag prior to cutting. Xylem sap was collected from the petiole cut end at a pressure slightly above the balancing pressure and osmotic potential was measured with a vapour pressure osmometer (Wescor 5000). Assu,ning equilibrium, leaf water potential equc, ls the sum of pressure and osmotic potential of the xylem. The leaf osmotic potential was measured after freezing and thawing the leaf material and expressing the cell sap. Turgor was calculated as the difference between leaf water potential and leaf osmotic potential. The measurements were taken at the end of the dark and light periods 23 d.a.t. In each treatment three seedlings were harvested for water potential measurements. Twenty-one and 22 d.a.t, stomatal conductance of the abaxial side of the third leaf above the cotyledons was measured with a steady-state porometer (Licor LI- 1600 ) on five seedlings per treatment. The data presented are the averages of several measurements taken during the light period. As water status, stomatal conductance and epidermal cell area were determined in the third replication only, a randomised block design was assumed for statistical analysis. Since the design was not randomised with respect to RH the significance of its effect on these measurements should be interpreted with some caution. RESULTS

Water status and stomatal conductance Water potential, osmotic potential and turgor were generally the same at 55% and 75% RH. At 95% RH o~rnotic Dotential was higher, but the water potential was higher only at low EC; as a result there was high turgor at low EC and low turgor at high EC (Table 1 ). The leaf water potential changed almost in parallel with the osmotic potential of the nutrient solution (linear regression coefficient, 1.06). At I and 4 mS cm-~ leaf osmotic potential was approximately the same and as a conse-

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EFFECT OF HUMIDITY AND NUTRIENTS ON GROWTH OF CUCUMBERS

TABLE 1 The effect of RH and EC on leaf water potential, leaf osmotic potential and turgor (MPa) of the third leaf 23 d.a.t. EC (mS c m - ~)

End of dark period

End of light period

~.H (%)

RH (%)

55

75

95

55

75

95

-0.34 -0°39 -0.58

-0.26 -0.42 -0.56

-0.45 -0.57 -0.71

-0.46 -0.52 -0.70

-0.30 -0.49 -0.64

-0.69 -0.71 -0.79

-0.69 -0.70 -0.76

-0.60 -0.64 -0.68

-0.70 -0.74 -0.86

-0.70 -0.73 -0.83

-0.63 -0.63 -0.73

0.37 0.25 0.25

0.35 0.31 0.18

0.34 0.22 0.12

0.25 0.17 0.15

0.24 0.21 0.13

0.33 0.14 0.09

Leafwaterpotentml 1 4 8

-0.32 -0.46 -0.54

Leafosmoticootential 1 4 8

Leafturgor 1 4 8 Main effects

RH (%) 55

EC (mS c m - J)

lsd I

75

95

1

4

8

-0.44 -0.56

-0.41 -0.48

-0.31 -0.40

-0.42 -0.53

-0.56 -0.68

-0.73 -0.77

-0.72 -0.75

-0.64 -0.67

-0.66 -0.68

-0.68 -0.70

-0.74 -0.81

0.29 0.19

0.28 0.19

0.23 0.19

0.35 0.27

0.26 0.17

0.18 0.13

Leaf watep potential dark light

0.08

-0.44 -0.58

Leaf osmotic potential dark light

0.03

Leaf turgor dark light

0.09

qsd is least significant difference of means of RH or EC ( P = 0.05 ); see also Materials and Methods.

quence turgor was decreased when EC was increased. At 8 mS cm-~ leaf osmotic potential was lower than at the lower levels of EC. This reduction was sufficient to compensate for the negative effect of EC on leaf water potential only at 55% RH, resulting in a limited further loss of turgor (Table 1 ). In the dark water potential was approximately 0.1 MPa higher than in the light. Diurnal differences in leaf osmotic potential were negligible, at 8 mS crn-l seemingly somewhat lower in the light than in the dark. There was a positive effect of RH on stomatal conductance with mean values of 0.34 cm s - i , 0.82 cm s -1 and 1.25 cm s -~ at 55%, 75% and 95% RH.

178

P.A.C.M. VAN DE SANDEN AND B.W VEEN

TABLE 2

Growth analysis parameters RGR (mg g- ~d a y - i ), NAR (/tg cm -2 d a y - ~), LAR and SLA (din 2 g- ~) and dry weight ratios (%) as affected by RH and EC Treatments

55% RH

75%RH

95%RH

EC ( mS cm - ~)

EC ( mS cm - ~)

EC ( mS c m - ~)

I RGR NAR LAR SLA LWR SWR RWR

4

1

4

8

1

4

lsd 95

163 175 181 298 322 320 5.48 5.43 5.63 7.72 7 64 7.87 71.1 70.6 71.6 18.3 19.1 19.0 10.6 10.3 9.5

EC (mS c m - t ) 1

15 21 0.38 0.44 1.2 1.3 i.2

4

lsd (EC) a

12 19 0.34 0.41 1.1 1.3 1.3

3 11 0.20 0.27 0.8 1.0 1.1

8

171 165 153 179 176 169 185 183 173 291 296 307 308 323 336 317 315 328 5.87 5.57 5.00 5.80 5.47 5.03 5.83 5.80 5.27 7.99 7.84 7.32 7.94 7.69 7.29 8.00 8.09 7.53 73.6 71.4 68.4 72.6 70.6 68.6 73.0 71.5 70.0 17.9 18.6 18.4 18.8 19.2 I9.3 19.2 19.4 18.4 8.6 10.0 13.2 8.7 10.2 12.1 7.7 9.1 11.6

Main RH (%) effects w 75 55 RGR NAR LAR SLA LWR SWR RWR

8

lsd I

lsd' 8

178 175 165 305 311 323 5.83 5.61 5.10 7.98 7.88 7.38 73.1 71.2 69.0 18.6 19.1 18.7 8.3 9.8 12.3

2 6 0.12 0.15 0.5 0.6 0.6

tlsd is least significant difference at P=0.05. Zlsd (EC) is Isd when comparing means of EC within the same level of RH.

EC had a negative effect with meat values of 0.96 cm s-!, 0.85 cm s-i and 0.60 cm s- t at 1 mS era- t, 4 mS era-t and 8 mS em- t EC, respectively. Within the scope of our observations (similar to the range of variation in the greenhouse culture) stomata seem to be more sensitive to 'atmospheric drought' (RH) than to 'soil drought' (EC).

Growth analysis The effects of RH and EC of the nutrient solution on the parameters derived from growth analysis are shown in Table 2 and their relative effects summarised in Fig. 1.

Effects of relative humidity. - The RGR showed a significant ( P < 0.05 ) linear relation with RH. However~ the mechanism underlying this positive response depended on whether the relative humidity was high (75% ( - ) 95%) or low (55% ( - ) 75%). At the lower humidity there was a significant positive effect

EFFECT OF HUMIDITYAND NUTRIENTSON GROWTH OF CUCUMBERS

| 79

% difference from reference (75% RH)

......

I

-6 -9

RGR

NAR

LAR

SLA

LWR

SLA

LWR

D 55% D 95% % difference from reference (,4 mS/cm)

3½ 0

I

-3 -6

....

-9

RGR

8 .I NAR

LAR

[_~ 1 mS/cm [~ 8 mS/crn Fig. 1. Summaryof main effects of RH (%) ( A ) and EC (mS cm-') of lhe n ulrient sol u lion (B) on growth parameters. Effects of RH are percentage differences from 75% RH as reference. Effects of EC are percentage differences from 4 mS c m - ~as reference. Values are means of nine data. An (s) inside a bar indicates a significant difference from reference at P < 0.05.

on NAR and no effect on LAR. In the higher humidity, NAR was not affected and the effect on RGR may be ascribed to the increase (albeit not statistically significant) in LAR caused by effects on SLA and leaf weight ratio (LWR). At 95% RH other morphological changes occurred too; root weight ratio (RWR) was lower compared to the value at 55% and 75% RH; the final ratio of leaf area to root fresh weight (a measure of the relative abilities to lose and to take up water) was 0.25 cm 2 rag- i, whereas at 55% RH and 75% RH it was 0.19 cm 2 rag-~ and 0.17 cm 2 mg -~, respectively. There were no significant effects on stem weight ratio (SWR, mean value 18.8%) and plant dry matter content (mean value 5.8%); fresh and dry weight ratios were affected to the same extent, individual leaf area was about the same at 55 and 75% R H a n d higher at 95% RH (Fig. 2).

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P.A.C.M, VAN DE SANDEN AND B.W. VEEN

1 mS/cm

8 mS/cm

4 mS/cm

95 % RH

75 % RH ¢¢S

2

._=-

55 % RH

12

16

20

12

16

20

S 12

16

20

timeafterplanting(days)

Fig. 2. Tile effect of RH (%) and EC (mS cm-i ) on the growth of individual leafarca (din ~) of leaf No. 2 (-W-), leaf No. 3 (-[a-) and leaf No. 4 (-El-).

Effects ofe/ectrical conductivity. - EC had a negative effect on RGR, mainly because ~! ncreased EC, relatively less dry matter was invested in leaf growth in favour o~ root growth, thus reducing LWR and increasing RWR; root fresh and dry weight per se were not influenced by EC. The final ratio of leaf area versus root fresh weight was smaller at higher EC with mean values of 0.24 cm 2 rag- ~, 0.20 cm 2 rag- ~and 0.17 cm 2 nag- ~at 1 mS cm- ~, 4 mS c m - ~and 8 mS cm-~ EC, respectively. At high EC, leaf expansion was inhibited (Fig. 2 ), reducing SLA. The relation between EC and SLA was significantly nonlinear (P<0.03) and, as a consequence, the relation between EC and RGR was also significantly non-linear (P < 0.02 ). NAR was promoted by increased EC. However, this did not compensate for the negative effect on LWR and SLA. Stem dry weight ratio was the only parameter insensitive to EC. Plant dry matter content was significantly higher (P<0.001 ) at higher EC with

EFFECT OF HUMIDITYAND NUTRIENTSON GROWTH OF CUCUMBERS

181

TABLE 3

The effect of RH and EC on single epidermal cell area (/~m 2), ratio of number of stomata to number of non-stomatal epidermal cells (index) and the total number of epidermal and stomatal guard cells per leaf of the third leaf above cotyledons 26 d.a.t. EC (mScm -t)

Abaxial leaf side RH (%)

Adaxia~ leaf side RH (%)

Significance ~

55

75

55

75

RH

EC

NS

**

1286 972 891

1270 1115 957

973 823 679

963 875 755 NS

NS

NS

NS

Area 1 4 8 Index 1 4 8

0.32 0.31 0.33

0.31 0.32 0.33

0.12 0.12 0.13

0.14 0.14 0.13

1 4 8

3.0 3.3 2.4

3.1 3.2 3.0

2.9 3.0 2.4

3.3 3.1 2.8

Number ( × 10 7)

~NS, not significant; **P< 0.01; see also Materials and Methods.

mean values of 5.6%, 5.8% and 6.0% at 1 mS cm-1, 4 mS cm-~ and 8 mS cm-t EC, respectively. Effects on distribution of fresh weight showed the same trend as that of dry weight.

Interactive effects of R H and EC. - There was a significant interactive effect of RH and EC on RGR ( P < 0.01 ). The effect of EC was more pronounced at low RH than at high RH. At I mS cm-~ the effect of RH on RGR can be fully attributed to effects on the NAR, at higher EC to both NAR and LAR, as explained above. Single epidermal cell area and stomatal index The total (epidermal plus stomatal) number of cells per leaf was the same on the abaxial and adaxial sides of the leaf and was hardly affected by treatment, although at high EC/low RH a lower total number of cells seemed to occur (Table 3). Also the stomatal index was not influenced by the treatments, but was smaller on the adaxial than on the abaxial leaf side. Differences in leaf area were thus largely due to differences in single cell area. EC had a clear effect; individual epidermal cell area was inversely related to EC. The area of individual epidermal cells at 55% and 75% RH were approximately the same.

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P.A.C.M. VAN DE SANDEN AND B.W. VEEN

DISCUSSION

The effects of humidity on growth and development are diverse (Hoffman, 1979; Grange and Hand, 1987). Variable responses might have been caused by effects of, or interactions with, other environmental factors, like CO2 (Gisler~d and Nelson, 1989), or by physiological disorders (related to calcium deficiency) at high humidity (Bakker and Sonneveld, 1988 ). Information on the effect of RH on growth of cucumber is scarce. Mortensen (1986) found a positive effect of increased RH on fresh weight and plant height. Pareek et al. (1969) found that cucumber produced distinctly short-jointed plants when humidity was high. Bakker et al. (1987) found that high humidity enhanced growth and yield. Since control of EC of the root environment is relatively new in greenhouse culture, there are few publications on this subject (except those on NaCI salinity). Sonneveld and Welles (1988) and Adams and Ho (1989) found a lower fruit yield of tomato at high EC, although fruit quality was improved. In fruiting tomato Charbonneau et al. ( 1988 ) found that dry weight of the aerial part decreased and dry weight of the roots increased, but Ehret and Ho (1986) found no effect of EC on dry matter partitioning to fruits, vegetative shoots and roots. As Jones (1990) stated, there is an increasing number of reports mentioning physiological responses to drought without detectable changes in leaf water status. Hoffman et al. ( 1971 ) found cotton leaf water potential responded to salinity, but not to RH. Our experiments sought to influence the water status of the plant by imposing 'soil' drought in terms of EC and 'atmospheric' drought in terms of RH. Although plant water status was measured in the last week of each experiment, we assumed it to be consistent for the whole period of growth and tissue extension, because external factors governing it were constant throughout. In our experiments at 55% and 75% RH plant water status was the same. Probably at those levels whole plant transpiration rates were comparable, because at higher RH the higher stomatal conductance and larger total leaf area counterbalanced the reduced drying power of the air. Photosynthesis is but one of the determinants of NAR, but the lower NAR, and consequently lower RGR, at 55%, compared to these rates at higher humidity levels, may have been the result of a lower rate of photosynthesis, inhibited because of decreased stomatal conductance. The effect of RH on total leaf area was mediated by that on RGR, leading to a somewhat higher rate of leaf emergence, whereas effects on SLA, LWR and single epidermal cell area were not significant. At 95% RH, the reduced drying power of the air was not balanced by the increased stomatal conductance and leaf area and consequently daytime plant transpiration rate was reduced and plant water potential was increased. In this situation the increase of stomatal conductance did not influence NAR.

EFFECT OF HUMIDITY AND NUTRIENTS ON GROWTH OF CUCUMBERS

| 83

SLA tended to be higher at 95% RH, when EC was 4 and 8 mS cm- ~, thus promoting RGR. As SLA and NAR are inversely related (Evans, 1972) this might have neutralised any positive effect on NAR. The combination of higher SLA and increased individual leaf area points to increased cell elongation. As this coincided with lower leafturgor and higher osmotic potential than at 55% and 75% RH, it indicates, that this leaf water status was the result rather than the cause of enhanced leaf elongation. In his review Hoffman (1979) mentions beneficial effects of high humidity on leaf extension of several s!~ecies. This is commonly attributed to high turgor resulting from a reduced transpiration rate. But hydraulic determinants primarily control short-term extension growth. Long-term extension growth is governed by cell wall related metabolism (Tyree and Jarvis, 1982). The adaptational response to high humidity is likely to be a modification of the mechanical properties of the cell wall, such as wall plastic extensibility and wall yield threshold. Not only morphology of the leaf but also that of the whole plant changed at 95% RH. The seedlings developed a high ratio of leaf dry weight to root dry weight and leaf area to root fresh weight. Thus whole plant growth rate was maximised at the expense of root growth. According to Givnish (1986) total biomass gain of plants growing in a constant environment is max/raised by adaptation of the ratio of allocation of energy to CO2-absorbing leaves versus water-absorbing roots, thus maintaining an optimal ratio of carbon fixation to water use. As humidity increases, he stated, optimal stomatal conductance and optimal leaf allocation increases. Our observations seem to confirm this model. Leaf water potential decreased in parallel with a decrease in water potential of the root environment (increase o ~~C). This occurred in spite of a decrease in stomatal conductance and leaf are~, reducing plant transpiration rate) and an increase in water-absorbing root surface relative to water-losing leaf area. A relative increase of root growfll in response to drought or salinity is a common feature. The elongation of the root tips of maize, for example, is insensitive to a water potential of the root environment that completely inhibits shoot growth (Sharp et al., 1988 ). Roots have a strong ability to adjust under water stress. This adjustment might either be a change in wall yielding properties (Hsiao and Jing, 1987 ) or an increase of osmotic potential in root cells (Greacen, 1972 ). Leaf elongation and single cell area were reduced by higher EC and this coincided with the decrease of water potential and turgot, suggesting a causal relation. This would confirm the conclusion reached by Neumann et al. (1988), with bean seedlings subjected to mild salt stress, that decrease of turgor rather than wall extensibility reduces leaf expansion. On the other hand, Matthews et al. (1984) found a decreased wall extensibility in sunflower leaves acclimatised to moderate soil water deficit. The non-linear effect of EC on $LA (Table 2) might be attributed to the reduced number of leaf cells at high EC (Table 3 ), restricting maximum leaf

184

P.A.C.M. VAN DE SANDEN AND B.W. VEEN

area; the lower leaf osmotic potential at high EC might point to restricted cell elongation. NAR was promoted by higher EC, despite lower stomatal conductance, and probably associated with responses like the inverse relation between SLA and NAR and a slightly higher leaf-shading within the plant at low EC. CONCLUSIONS

Response to air humidity. - At 55% and 75% the effect of RH on plant tran-

spiration rate was probably limited, because effects on stomatal conductance and leaf area tended to neutralise the effect of VPD on transpiration rate. As a consequence, neither leaf water status responded, nor significant morphological adaptations were observed. Relative growth rate was higher at 55% than at 75% RH, because net assimilation rate was higher as a result of the positive stomatal response. At high humidity (95%) plant transpiration rate ~as assumed to be reduced, despite increased stomatal conductance and leaf area. The increase of stomatal conductance did not affect NAR. The morphology of the plant changed when RH increased, resulting in a higher LAR. Shoot growth was enhanced at the expense of root growth. Individual leaf area was increased. Promotion of leaf elongation was not always accompanied by high turgot, suggesting that cell wall characteristics were influenced. O

Response to concentration o f the nutrient sol:aion. - The water potential of the

root environment was transmitted to the leaf without apparent restriction, determining its turgor. RGR was negatively affected by increased EC. Root growth rate was maintained, shoot growth and leaf elongation were inhibited. The negative effect on leaf elongation and single cell area on the one hand and turgor on the other suggests a causal relation. ACKNOWLEDGEMENTS

The authors are very grateful to Ineke Ammerlaan and Pier Brouwer for their skilful execution of the experiments and would like to thank Guus Broekhuijsen, Cees Jansen and CABO Technical Service for their valuable assistance. They are indebted to Prof. dr. it. H. ChaUa for critically reading the manuscript. This work has been supported financially by the Dutch Greenhouse Energy Saving Fund.

REFERENCES Adams, r. and Ho, L.C., 1989. Effects of constant and fluctuating salinity on the yield, quality and calcium status of tomatoes. J. Hortic. Sci., 64: 725-732.

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