The Effect of VA Mycorrhizal Infection and ...

Journal of Experimental Botany, Vol. 36, No. 168, pp. 1087-1098, July 1985

The Effect of VA Mycorrhizal Infection and Phosphorus Status on Sunflower Hydraulic and Stomatal Properties ROGER KOIDE1 Department of Botany, University of California, Berkeley, California 94720, U.S.A. Received 2 January 1985

ABSTRACT Koide, R. 1985. The effect of VA mycorrhizal infection and phosphorus status on sunflower hydraulic and stomatal properties.—J. exp. Bot. 36: 1087-1098. Mycorrhizal (M) and non-mycorrhizal (NM) sunflower plants were grown in a soil of low phosphorus availability (with and without phosphorus amendment) and in a soil of moderate phosphorus availability (without phosphorus amendment). Using the Ohm's law analogy and measured leaf water potentials, stem water potentials, and transpiration rates, hydraulic resistances were calculated for the whole plant, leaf, and below leaf components. Mycorrhizal infection (as high as 89%) was shown to have no effect on the intrinsic hydraulic properties of the soil/plant system over a wide range of transpiration rates in either soil when M and NM plants of equivalent root length were compared. When grown in the soil of moderate phosphorus availability, calculated hydraulic resistances under given environmental conditions were the same for M and NM plants, as were stomatal resistances and transpiration rates. When grown in the soil of low phosphorus availability, calculated values of hydraulic resistance were lower for M plants than for NM plants under given sets of environmental conditions. These differences in calculated hydraulic resistance were not due to a difference in the intrinsic hydraulic properties of M and NM plants. The differences were evident because stomatal resistances were lower and transpiration rates higher for M plants and because hydraulic resistance varied inversely with transpiration rate. When plants of significantly greater root length were compared to plants of lesser root length, the calculated hydraulic resistances under given environmental conditions were much lower for the plants of greater root length. This difference was largely due to a difference in the intrinsic hydraulic properties between large and small plants, and not because of differences in transpiration rate. The elevated transpiration rates exhibited by M plants were attributed to an enhanced phosphorus status. Short term phosphorus amendments made to phosphorus-deficient NM plants improved transpiration; transpiration rates were similar for M and NM plants before NM plants became phosphorus-deficient, and phosphorus-amended M and NM plants had similar transpiration rates. The data are discussed in relation to other reports of mycorrhizal influence on hydraulic and stomatal resistances. Possible mechanisms for the influence of infection on stomatal resistance are also briefly discussed. Key words—Hydraulic resistance, stomatal resistance, mycorrhizas. Correspondence to: Department of Biological Sciences, Stanford University, Stanford, CA 94305, U.S.A.

INTRODUCTION Vesicular-arbuscular mycorrhizal (VAM) fungi have been shown to enhance the phosphorus status of their hosts, at least partly because they provide additional surface area for 1

Address for correspondence—see abstract.

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Koide—Mycorrhizal Plant Water Relations

absorption above that provided by the root itself (Smith, 1980). Thus, there has been much speculation that these fungi might also directly alter the resistance to water flow in the soil/root system in an analogous way; that is, by absorbing and transporting to the root a significant amount of water above that absorbed by the root itself (Reid, 1978; Hayman, 1983). Hardie and Ley ton (1981) and Allen (1982) have suggested that outgrowing hyphae may be responsible for observed decreases in root hydraulic resistance in mycorrhizal plants. Indeed, Read and Malibari (1978) have demonstrated that ectomycorrhizal hyphae can transport water, although whether the quantity transported could significantly affect water supply to the host was not assessed. Mycorrhizal infection might also alter the resistance to water movement in an indirect, nutrient-mediated fashion. Safir, Boyer, and Gerdemann (1972) showed that in a nutrient deficient soil, mycorrhizal (M) plants had much lower root hydraulic resistances than non-mycorrhizal (NM) plants, and that this difference was eliminated if both M and NM plants were nutrient-amended. They also showed that application of a fungitoxicant had no significant effect, suggesting that movement of water through living hyphae was not significant. Phosphorus supply and mycorrhizal infection have been shown to affect the quantity of vascular tissue (Schnappinger, Bandel, and Kresge, 1969; Daft and Okusanya, 1973) and root and shoot growth (Atkinson, 1973; Smith, 1980), which could affect the hydraulic resistance of plants independently from infection status. Sands, Fiscus, and Reid (1982) showed that hydraulic resistance was independent of the amount of ectomycorrhizal infection in pines, and Levy and Krikun (1980), using citrus plants of similar shoot size, showed that infection had no effect on hydraulic resistance. Mycorrhizal infection has been shown to reduce stomatal resistance and thus increase transpiration rate (Levy and Krikun, 1980; Allen, Smith, Moore, and Christensen, 1981; Allen, 1982; Nelsen and Safir, 1982; Allen and Boosalis, 1983; Stahl and Smith, 1984). Enhanced plant phosphorus status, independent of mycorrhizal infection, has also been shown to reduce stomatal resistance (Atkinson and Davison, 1972; Terry and Ulrich, 1973; Clarkson and Scattergood, 1982). Since plant hydraulic resistance has been shown to vary with transpirational flux (Tinklin and Weatherley, 1966; Boyer, 1974), any mycorrhizal effect on stomatal resistance would also be expected to affect the calculated values of hydraulic resistance. The objective of this present study, then, was to distinguish between direct and indirect (nutrient-mediated) effects of mycorrhizal infection on plant hydraulic resistance. M A T E R I A L S AND

METHODS

Growth conditions Sunflower plants (Helianthus annuus h., cv. S304, SeedTec International, Woodland, CA) were grown in the glasshouse in six inch plastic pots filled with a soil of either low (5-0 fig g"') or moderate (15 fig g~~') available phosphorus. All plants were kept in the glasshouse for the duration of the experiments. Glasshouse temperatures were thermostatically controlled and fluctuated between 13 °C at night and 26 °C during the day. Additional photosynthetically active photon flux density (PPFD) of approximately 350-400 fimol m~2 s~' was provided by two 1000 W General Electric metal halide lamps operating 14 h each day. Maximal PPFD was 1900 /rniol m " 2 s~'. Inoculation procedures and methods for the measurement of growth, infection status, and tissue phosphorus levels were as described in Koide (1985). Root lengths were measured according to Newmann (1966) as modified by Torsell, Begg, Rose, and Byrne (1968). Stem diameter was measured with a micrometer. In Experiment 1, mycorrhizal and non-mycorrhizal (designated M —P and NM —P) plants were grown without nutrient amendment in the soil of moderate phosphorus availability. In Experiment 2, the same soil was used, but M and NM plants (also designated M — P and NM — P) were watered with a nutrient solution lacking phosphorus (Koide, 1985). In Experiment 3, M — P and NM — P plants were grown in the soil of low phosphorus availability and watered with the nutrient solution lacking


Koide—Mycorrhizal

Plant Water Relations

1089

phosphorus. Additionally, some NM plants (designated NM + P) were watered with a complete nutrient solution. Experiment 4 was carried out in order to study more carefully the effect of mycorrhizal infection and phosphorus status on stomatal properties. In this experiment, the soil of low phosphorus availability was used and M — P, NM — P, M + P, and NM + P plants were grown. All plants were watered with the nutrient solution either with ( + P) or without (— P) phosphorus. In addition, some of the originally phosphorus-unamended NM plants (designated NM — P + P) were amended with phosphorus from 8 until 11 weeks. Experimental conditions are summarized in Table 1. In all experiments, pots were watered once or twice daily, as required, and free drainage occurred. T A B L E 1. Experimental conditions for Experiments 1-4 Experiment

SoilP

Added P

Dates grown

Plant type0

No. of plants

(wig"1)

1" 1

M-P NM-P

20 20

15 15

8 Jun. 198329 Jul. 1983

2 2

M-P NM-P

20 20

15 15

16 Aug. 19834 Oct. 1983

3 3 3

M-P NM-P NM + P

18 18 18

5 5 5

5 Oct. 19835 Dec. 1983

4 4 4C 4 4

M-P NM-P NM-P+P M+P NM + P

15 10 5 15 10

5 5 5 5 5

20 Dec. 19836 Mar. 1984 —1-

+ +

" M designates mycorrhizal and NM non-mycorrhizal. * In Experiment 1, plants were watered daily from the tap. In all other experiments, a nutrient solution was applied daily. Phosphorus either was not added ( —P) or was added ( + P) as KH 2 PO 4 in the nutrient solution. c In Experiment 4, some originally unamended NM plants were amended with P between 8 and 11 weeks ( N M - P + P). Hydraulic resistance measurement For hydraulic resistance measurements, M and NM plants were removed from the glasshouse and placed in a growth chamber (Western Environmental model H-14-HL) equipped with precise (±1%) temperature and humidity control and a lamp bank capable of producing high photon flux densities. Hydraulic resistances for M and NM plants in Experiments 1, 2, and 3 were calculated over a wide range of transpiration rates using the Ohm's law analogy (Tinklin and Weatherley, 1966); i.e., R =dij/^-T, where Aij/ is the difference in water potential between two points in the catenary path, T is the transpirational flux from the first point to the second, and R is the hydraulic resistance between the two points. Whole plant hydraulic resistance was calculated using bulk soil water potential and the water potential of the most recently fully expanded transpiring leaf (leaf 3, 4, 5 or 6). All soils were well-watered prior to the measurement of hydraulic and stomatal resistances and were assumed to be at 0 bar for both M and NM plants. Component (leaf or below leaf) hydraulic resistances were calculated using bulk soil, stem, and uncovered leaf water potentials. Stem water potentials were estimated in a manner similar to that used by Begg and Turner (1970). A leaf below the leaf used for the measurement of uncovered (transpiring) leaf water potential was covered with an aluminium foil envelope. A vapour-tight seal was made using putty and plastic tape. Covered (non-transpiring) leaf water potentials were allowed to come to equilibrium for 2-5 h before measurement. Leaf water potentials were measured either with a PMS Inst. Co. model 1000 pressure chamber, or a modified PMS pressure chamber equipped with a six inch 0-30 bar full scale gauge. Precautions as recommended by Turner (1981) were followed.


1090


The rate of transpiration was varied by changing the amount of photosynthetically active photon flux density (PPFD) reaching the plants. This was done by varying the number of layers of grey shade cloth suspended below the lamps. PPFD varied from 850 /imol m~ 2 s" 1 at the most recently fully expanded leaf (no shade cloth) to 100 /imol m~ 2 s " ' (three layers). The growth chamber was held at a constant 25 °C and 50% relative humidity. 2-5 h were allowed after changes in light flux for plants to come to water potential and transpiration equilibrium before measurements were made. Independent tests showed 2-2-5 h were necessary for equilibrium to be established. Transpiration was measured by enclosing pots in plastic bags and aluminium foil, measuring water loss during one hour by weighing, and dividing this quantity by leaf area previously measured (Koide, 1985). Stomatal resistance measurements For stomatal resistance measurements, M and NM plants were removed from the glasshouse and placed in the growth chamber under the conditions previously described for hydraulic resistance measurements. Stomatal resistances for the abaxial surface of the most recently fully-expanded leaf were measured with a LI-COR Inst. Corp. model LI-1600 null balance porometer. Data from a separate study (Koide, unpublished) and from Turner and Singh (1984) demonstrated that abaxial and adaxial stomata of sunflower did not differ in their responses to PPFD, and that the abaxial stomatal resistance was significantly lower than the adaxial. Statistical tests The Student-I test was employed to determine if significant differences occurred between treatments. Ninety-five percent confidence intervals were used to compare straight line plots for various treatments (Walpole, 1974).

RESULTS Experiment 1 The plot of hydraulic resistance versus transpiration for M and NM plants is shown in Fig. 1A. It can be seen that despite high infection levels (Table 2), the plots for the various components (whole plant, leaf, and below leaf) were the same for M and NM plants. A linear transformation of the whole plant data from Fig. 1A for either M orNM plants yielded aline with a 95% confidence interval containing all the data points from the other treatment. This indicated that the intrinsic hydraulic properties of the plants had not been affected by infection. As shown in Fig. 1A, the whole plant and below leaf hydraulic resistances for M and NM varied with transpiration rate. However, since the stomatal resistances (Table 3) and hence transpiration rates (Table 4) were the same for M and NM plants under given conditions, the calculated hydraulic resistances under given conditions of PPFD were generally not significantly different between M and NM plants (Table 5). Root lengths, leaf areas, shoot dry weights and stem diameters were not significantly different between M and NM plants (Table 2). Although M plants had higher tissue phosphorus concentrations (Table 2), growth of M and NM plants was probably limited by a nutrient other than phosphorus because when plants were fertilized without phosphorus in Experiment 2, leaf area was much greater (Table 2). Experiment 2 Results similar to those from Experiment 1 were found in Experiment 2. M and NM plants had equivalent intrinsic hydraulic properties (data not shown). M and NM plants also had equivalent calculated hydraulic resistances at any given PPFD (Table 5), and stomatal resistances (Table 3) and transpiration rates (Table 4) were similar for M and NM plants. Root lengths and phosphorus concentrations were similar for M and NM plants at 4 weeks, but there were significant differences in leaf area, shoot weight, and stem diameter (Table 3).



M

1091

NM

«

O Whole

Plant

A •

A Leaf D Below

Leaf

M

NM

a a

V

•

a

O Whole A

Leaf

a

Below

Plant Leaf

3 a •a >.

I

0.4

1.2 2.0 2.8 Transpiration (g

3.6

4.4

FIG. 1. Hydraulic resistances versus transpiration. On any given line, each data point represents a separate plant, (A) Whole plant and component hydraulic resistances for 5-week-old mycorrhizal (M) and non-mycorrhizal (NM) plants in Experiment 1. (B) Whole plant and component hydraulic resistances for 8-5-week-old M and NM plants in Experiment 3. (c) Whole plant hydraulic resistance for mycorrhizal (M —P) and non-mycorrhizal (NM — P) plants grown without phosphorus amendment, and for amended non-mycorrhizal plants (NM + P) in Experiment 3 at 8 5 weeks.

Experiment 3 As in Experiments 1 and 2, in Experiment 3, the plots of hydraulic resistance versus transpiration for the whole plant, leaf, and below leaf components were the same for M - P and NM — P plants (Fig. 1B) despite high infection levels (Table 2). The linear transformation of the whole plant data from Fig. 1B for either M or N M plants again yielded a line with a 95%


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T A B L E 2. Means of various growth parameters Experiment

Age (weeks)

Root length (m)

Leaf area (cm2)

Shoot dry weight (g)

Stem diameter (mm)

Tissue %P

/o

Infection

M-P NM-P

5 5

135-6 128-8

204-4 2090

2-50 2-67

6-26 6-40

0-278a" 0-202b

65-8a 00b

2M-P 2 NM-P

4 4

1981 168-7

459-6a 543-8b

2-32a 2-78b

602a 658b

0-390 0-371

53-4a 00b

3 M-P 3 NM-P 3 NM + P

8-5 8-5 8-5

107-la 112-5a 309-7b

329-2a 305-2a 102O-7b

2-52a 315b 1315c

610a 6-98b 12-39c

0192a 0110b 0-33 lc

89-2a 00b 00b

4 4

M-P NM-P

8 8

63-7 62-3

155-7 142-4

0-99a l-43b

0-219a 0119b

85-3a 00b

4 M+ P 4 NM + P

506-2 533-8

3-55a 5-90b

0-310a 0-239b

58-4a 00b

418a 2-85b 3-70a

0108a 0097a 0-246b

93-6a 00b 00b

O-236a 0-248a

85-6a 00b

00 OO

1 1

1800 185-9

4 M-P 4 NM-P 4NM-P+P

11 11 11

117-4 930 108-9

4021a 2101b 4091a

4 M+ P 4 NM + P

11 11

423-2 441-7

846-4 923-2

13-40 1506

" Dissimilar letters indicate a significant difference between treatments (M —P, NM —P, M + P, NM + P) at the 005 level. The absence of letters indicates no significant difference between treatments.

confidence interval containing all the data points from the other treatment, indicating that infection had no effect on the intrinsic hydraulic properties of the plants. In contrast with Experiments 1 and 2, however, the calculated whole plant and below leaf component hydraulic resistances were generally significantly lower in M — P plants compared to NM — P plants at any given PPFD (Table 5). This was because stomatal resistances were lower (Table 3) and transpiration rates were higher (Table 4) in M plants, and because hydraulic resistance again varied inversely with transpiration rate. It should be stressed, however, that the differences in calculated hydraulic resistance between M and NM plants at a given PPFD were not due to differences in the intrinsic hydraulic properties of the plants. M — P plants had equivalent leaf areas and root lengths but lower shoot dry weights, thinner stems, and higher tissue phosphorus concentrations compared to NM — P plants (Table 2). The growth of NM — P plants was limited by phosphorus as evidenced by the fact that N M + P plants were much larger (Table 2). When larger (NM + P) plants were compared to smaller (NM — P and M — P) plants, hydraulic resistances were always lower in the larger plants at any given PPFD (Table 5). The reason for this was not that larger plants had higher transpiration rates (Table 4). Rather, the major reason was that they had lower hydraulic resistances at given transpiration rates. In other words, the intrinsic hydraulic properties were different between large and small plants


Koide—Mycorrhizal Plant Water Relations 1

T A B L E 3. Means of stomatal resistances (s cm' ) Experiment

Age (weeks)

PPFD (p.mo\ m

2

s ') during measurement

850

500

200

100

1 M-P 1 NM-P

5 5

0-73 0-86

111 1-33

2-42 2-67

5-32 5-20

2M-P 2 NM-P

4 4

0-62 0-58

0-79 0-77

0-92 0-91

1-30 1-53

3 M-P 3 NM-P 3 NM + P

8-5 8-5 8-5

0-74a° l-59b 0-76a

l-27a 2-47b 0-88c

306a 611b l-81c

312a 10-89b 4-60a

4M-P 4 NM-P

6 6

0-86 0-80

1-43 1-33

209 214

3-87 4-82

4 M+P 4 NM+P

6 6

0-57a 0-64b

0-68 0-72

110 104

214 1-90

4M-P 4 NM-P

8 8

0-80a 3-21b

114a 2-84b

—

2-86a 8-91b

4 M+P 4 NM + P

8 8

0-75 0-87

0-94 101

0-8 la 0-88a 0-67b

l-60a l-82a 107b

4M-P 4 NM-P 4NM-P+P

11 11 11

—

1-91 2-24 9-26a ll-78a 4-57b

° Dissimilar letters indicate a significant difference between treatments (M - P, NM - P, M + P, NM + P) at the 005 level. The absence of letters indicates no significant difference between treatments.

T A B L E 4. Means of transpiration rates n = 4 or 5. Units are g dm"2 h" 1 . Experiment

Age (weeks)

PPFD (fimo\ m

2

s ') during measurement

850

500

200

100

1 M-P 1 NM-P

5 5

4-39a" 400b

2-87 2-79

1-74 1 63

0-79 0-78

2M-P 2 NM-P

4 4

3-75 3-93

3-25 3-34

1-27 1-25

113 107

3 M-P 3 NM-P 3 NM + P

8-5 8-5 8-5

4-31a 3-51b 4-32a

2-83a 2-38a 2-9 lb

214a l-46b l-60b

0-98a 0-58b 0-92a

" Dissimilar letters indicate a significant difference between treatments (M - P, NM - P, NM + P) at the 005 level. The absence of letters indicates no significant difference between treatments.


1093

1094


TABLE 5. Means of whole plant and component hydraulic resistances for Experiments 1-3 at

four different PPFD. Units are bar dm2 h g"' Experiment

PPFD (/itnol m 850

500

2

s ') during measurement 200

100

1 M-P 1 NM-P

Whole plant 0-96 1-40 104 1 33

1-91 1-82

411 405

1 M-P 1 NM-P

Leaf 031 0-30

0-28 0-27

0-65 0-74

1 M-P 1 NM-P

Below leaf 0-94 0-65a 080b 0-99

1-63 1-61

3-46 3-45

2 M-P 2 NM-P

Whole plant 112 103a 0-92b 109

2-17 2-33

2-59 2-88

2 M-P 2 NM-P

Leaf 015 017

0-28 019

009 0-21

015 017

2M-P 2 NM-P

Below leaf 0-84 0-9 la 0-90 0-76b

209 213

2-44 2-80

3 M-P 3 NM-P 3NM + P

Whole plant l-63a 1-27 217b 1-46 l-27c

l-95a 2-85b l-34c

3-51a 6-47b 2-68c

O-32a 0-42a 011b

005 0-76 009

l-64a 2-44b l-23c

3-54a 5-7 lb 2-59c

3 M-P 3 NM-P 3NM + P 3 M-P 3 NM-P 3NM + P

Leaf 027 0-23

O-46a* O-34b

0-25 0-28 016

Below leaf 101 l-41a 1-24 l-89a 111b

° Dissimilar letters indicate a significant difference between treatments ( M - P , N M - P , NM + P) at the 005 level. The absence of letters indicates no significant difference between treatments.

when transpiration was normalized on a leaf area basis (Fig. lc). When the data from the larger and smaller plants in Fig. lc were linearly transformed, approximately 90% of the data points from the larger plants fell outside of the 95% confidence interval of the smaller plants. The NM + P plants had greater roots lengths, leaf areas, shoot dry weights, stem diameters and tissue phosphorus concentrations compared to N M - P and M - P plants (Table 2).



1095

When transpiration was normalized on a root length basis, the hydraulic resistances of the larger plants at any given transpiration rate were still significantly lower than those of the smaller plants. That there was no difference between calculations based on root length and those based on leaf area was due to the fact that the ratio of leaf area to root length was approximately the same for large and small plants. Stomatal resistance was shown to be inversely correlated with tissue phosphorus concentration when plants were grown without phosphorus amendment in the soil of low phosphorus availability regardless of infection status (Experiment 3, Tables 2 and 3). However, this was not true when plants were grown in the soil of moderate phosphorus availability (Experiment 1). In Experiment 1, statistically significant differences in tissue phosphorus concentration occurred between M and NM plants (Table 2) but did not occur for stomatal resistance (Table 3). This suggested that phosphorus deficiency in NM plants was responsible for the increased stomatal resistance. To investigate more carefully the relationship between plant phosphorus status and stomatal resistance, Experiment 4 was conducted. Experiment 4 In this experiment, prior to the decline in leaf expansion for NM — P plants relative to M — P plants (at 6 weeks, see Koide, 1985), there were no significant differences in stomatal resistance between M — P and NM — P plants (Table 3). At 8 weeks, however, NM — P plant leaf expansion had declined significantly relative to M — P plants (Koide, 1985), as had phosphorus concentration (Table 2). This coincided with an elevated stomatal resistance in NM — P plants (Table 3). By 11 weeks, however, M — P and NM — P plants had similar tissue phosphorus levels (Table 2) and similar stomatal resistances (Table 3). NM — P + P plants amended with phosphorus between 8 and 11 weeks, had increased leaf expansion (Koide, 1985) and had elevated tissue phosphorus levels (Table 2) relative to M —P and NM —P plants, as well as lower stomatal resistances (Table 3). Thus, whenever phosphorus seemed to limit the growth of plants, tissue phosphorus status was inversely related to stomatal resistance. When phosphorus seemed not to limit the growth of plants (Experiment 1), there was no relation between tissue phosphorus status and stomatal resistance. DISCUSSION Data presented in this study show that when M and NM plants of equivalent root length were compared under well-watered conditions, mycorrhizal infection did not affect intrinsic plant hydraulic properties either at the whole plant, leaf, or below leaf level. Mycorrhizal infection, however, did affect calculated values of hydraulic resistance under given environmental conditions, even when M and NM plants had equivalent intrinsic hydraulic properties. This was because hydraulic resistance varied with transpirational flux and because infection could influence transpirational flux by affecting stomatal resistance. The major effect of mycorrhizal infection on water movement under the well-watered condition, then, was at the level of the stomata and not on intrinsic hydraulic properties. Nelsen and Safir (1982) showed for onion that mycorrhizal infection reduced the calculated whole plant hydraulic resistance if plants were unamended with phosphorus. Under the one environmental condition used, mycorrhizal infection reduced stomatal resistance and increased transpiration rate while increasing leaf water potential. These investigators proposed that the difference in stomatal resistance between M and NM plants was caused by the difference in calculated hydraulic resistance. However, since hydraulic resistance might have varied with transpirational flux, it is not possible to determine if altered hydraulic


1096


properties affected stomatal resistance, or if an altered stomatal resistance affected the transpirational flux and thus the calculated hydraulic resistance. Allen (1982) concluded that infection of Bouteloua improved water uptake into host roots because a greater flux of water moved through M plants without a concomitant increase in water potential depression. It was suggested that this reduction in resistance to water flux was caused by the increased surface area provided by the hyphae. But again, differences in the calculated hydraulic resistance between M and NM plants could have been due to the way calculated resistances vary with transpirational flux. Allen et al. (1981), also using Bouteloua, showed that mycorrhizal infection, enhanced phosphorus status, and decreased stomatal resistance correlated well as in the current study. In the current study, root length seemed to be more important than shoot weight or stem diameter in its effect on intrinsic hydraulic properties because despite differences in shoot weight or stem diameter, M and NM plants had equivalent intrinsic hydraulic properties if root lengths were equivalent. This is consistent with the fact that the root is generally believed to have a much greater hydraulic resistance than the shoot (Kramer, 1969). In this study, the site of the majority of whole plant hydraulic resistance was below the leaf (Figs 1A, B). When larger plants were compared to smaller plants (Experiment 3), larger plants were shown to have intrinsically lower hydraulic resistances than smaller plants, even on a root length basis. Larger plants were shown to have hydraulic resistances that were lower than would be predicted on the basis of increased root length alone. Thus if mycorrhizal infection increased root length (it did not in the current study), hydraulic resistance might be reduced to a greater degree than expected and the difference between the expected increase and the perceived might erroneously be attributed to hyphal transport. Hardie and Leyton (1981) showed that M plants had lower root system hydraulic resistances and greater root lengths than NM plants. Even on a root length basis, resistances were lower in M plants. This difference was suggestive of a direct involvement of hyphae in water transport. However, since hydraulic resistance is not necessarily linearly related to root length (as shown by the comparison between large and small plants in Experiment 3), the observed difference could still have been due to a difference in root size. The current study is not hampered by this complication because M — P. and NM — P plants had equivalent root lengths when hydraulic properties were measured. Safir et al. (1972) indicated that while infection lowered root hydraulic resistance, application of a fungitoxicant did not influence hydraulic resistance. This suggested that any water movement through living hyphae was probably not significant. Additionally, Cooper and Tinker (1981) have shown that the amount of water used to convey typical amounts of phosphorus to hosts via mycorrhizal hyphae is probably insignificant to the water economy of well-watered plants. The current study is in complete agreement with this suggestion. The mycorrhizal effect on stomatal resistance was probably mediated by an altered plant phosphorus status since short term phosphorus amendments to phosphorus-deficient plants also reduced stonjatal resistance. In addition, mycorrhizal infection only reduced stomatal resistance relative to NM plants under phosphorus-limiting conditions. In Experiment 1 where soil phosphorus was available in moderate quantities and some nutrient other than phosphorus appeared to limit growth, or in Experiment 4 when phosphorus was supplied daily to M and NM plants, significant differences between M and NM plants in tissue phosphorus concentrations were not matched by significant differences in stomatal resistances. Mycorrhizal infection may affect stomata in a number of ways which include effects on plant growth .substances and on photosynthesis. Abscisic acid (ABA) promotes stomatal closure (Raschke, 1979) and has been reported to occur in lower quantities in M plants (Allen,



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Moore, and Christensen, 1982). Mizrahi and Richmond (1972) have shown that ABA levels increase in nutrient-deprived plants and thus mycorrhizal effects on ABA concentration may be mediated by plant nutrient status. Mycorrhizal infection (Allen et al., 1981) and enhanced phosphorus status (Terry and Ulrich, 1973) have been shown to increase the rate of photosynthesis by reducing the mesophyll resistance. A reduction in the rate of photosynthesis in NM or phosphorus-deficient plants would be expected to be associated with an increase in stomatal resistance if stomatal apertures varied to minimize the amount of evaporation for a particular rate of assimilation (Cowan, 1977). Although mycorrhizal infection did not directly affect plant hydraulic properties under well-watered conditions in this study, hyphae could conceivably transport a significant amount of water if the hydraulic conductivity of the soil decreased due to drying. Additionally, hyphae may act to bind soil to roots, thereby reducing the gaps created by soil shrinkage during drying or by root contraction during high transpiration rates, although no differences in the intrinsic hydraulic properties between M and NM plants were exhibited even at transpiration rates as high as 50 g dm" 2 h" 1 . Infection has been suggested to be an advantage under conditions of water stress (Hardie and Ley ton, 1981). Indeed, if M plants have greater rooting densities than NM plants, water extraction might be more efficient in infected plants. However, while infection can increase root length, the overall effect is often to reduce root/shoot ratios while reducing stomatal resistance. If mycorrhizal hyphae do not transport a significant amount of water as suggested by this study, infection might create a greater potential for transpirational water loss and at the same time, a proportionately less efficient means of acquiring soil water. M plants have indeed been shown to wilt faster than NM plants (Hardie and Ley ton, 1981) and to develop more severe water stress symptoms during drought (Levy, Syvertsen, and Nemec, 1983).

ACKNOWLEDGEMENTS Thanks are due to Professor Robert H. Robichaux for his invaluable moral and intellectual support throughout the course of this research. Acknowledgement is also made of the financial support from NSF Grant DEB-8206411 given to Robert H. Robichaux, and from a grant given to Roger T. Koide from the University of California Appropriate Technology Program. LITERATURE CITED ALLEN, M. F., 1982. Influence of vesicular-arbuscular mycorrhizae on water movement through Bouteloua gracilis (H.B.K.) Lag ex steud. New Phytologist, 91, 191-6. and BOOSALIS, M. G., 1983. Effects of two species of VA mycorrhizal fungi on drought tolerance of winter wheat. Ibid. 93, 67-76. MOORE, W. K. JR., and CHRISTENSEN, M., 1982. Phytohormone changes in Bouteloua gracilis infected by vesicular-arbuscular mycorrhizae. II. Altered levels of gibberellin-like substances and abscisic acid in the host plant. Canadian Journal of Botany, 60, 468-71. SMITH, W. K., MOORE, T. S. JR., and CHRISTENSEN, M., 1981. Comparative water relations and

photosynthesis of mycorrhizal and non-mycorrhizal Bouteloua gracilis (H.B.K.) Lag ex steud. New Phytologist, 88, 683-93. ATKINSON, D., 1973. Some general effects of phosphorus deficiency on growth and development. Ibid. 72, 101-11. and DAVISON, A. W., 1972. The influence of phosphorus deficiency on the transpiration of Arctium minus Bernh. Ibid. 71, 317-26. BEGG, J. E., and TURNER, N. C , 1970. Water potential gradients in field tobacco. Plant Physiology, 46, 343-6. BOYER, J. S., 1974. Water transport in plants: mechanism of apparent changes in resistance during absorption. Planta, 117, 187-207.


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potassium on alfalfa root anatomy. Agronomy Journal, 61, 805-8. SMITH, S. S. E., 1980. Mycorrhizas of autotrophic higher plants. Biological Reviews, 55, 475-510. STAHL, P. D., and SMITH, W. K., 1984. Effects of different geographic isolates ofGlomus on the water relations of Agropyron smithii. Mycologia, 76, 261-7. TERRY, N., and ULRICH, A., 1973. Effects of phosphorus deficiency on the photosynthesis and respiration of leaves of sugar beat. Plant Physiology, 51, 43-7. TINKLIN, R., and WEATHERLEY, P. E., 1966. On the relationship between transpiration rate and leaf water potential. New Phytologist, 65, 509-17. TORSELL, B. W. R., BEGG, J. E., ROSE, C. W., and BYRNE, B. R., 1968. Stand morphology of townsville

lucerne {Stylosanthes humilis). Seasonal growth and root development. Australian Journal of Experimental Agriculture and Animal Husbandry, 8, 533-43. TURNER, N. C , 1981. Techniques and experimental approaches for the measurement of plant water status. Plant and Soil, 58, 339-66. and SINGH, D. P., 1984. Response of adaxial and abaxial stomata to light and water deficits in sunflower and sorghum. New Phytologist, 96, 187-95. WALPOLE, R. E., 1974. Introduction to statistics, second edition. Macmillan Publishing Co., New York.
