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Water relations of tomato (Lycopersicon esculentum Mill. cv. Early Dwarf Red) infected with Meloidogyne javanica (Treub), Chitwood
Physiotogicut P&n! Pdwtagy (1978)
13,275-281
Water relations of tomato (Lycopersicon esculentum Mill. cv. Early Dwarf Red) infected with Meloidogyne javanica (Treub), Chitwood S. MEON,t H. R. WALLACE andJ. M. FISHER D+artment of Plant Pathology, Wad+?Agriculturat ResearchIn&u&, Glen Osmond, South Austratia 5064 (Accefitedfor publication April 1978)
Infection of tomato plants by Meloid&w jaoanica resulted in increased suction pressure in the root system probably due to abnormality of the xylem elements. A change in the suction pressure and/or transpirational behaviour appeared to account for the low water potential values in the infected plants. There was no difference in the rate of transpiration between infected and non-infected plants; however, diffusive resistance increased in infected plants as infection progressed and was higher than that in healthy plants.
INTRODUCTION
All plant physiological processes depend on water and if growth and development are to proceed normally, internal water stress must not develop within the tissues [S]. Moisture stress in leaves inhibits such processes as photosynthesis [2] and transport to the shoot system of cytokinins synthesized in root tips [6], thereby causing reductions in metabolic activity which ultimately result in reduced growth [4, 51. The influence of pathogens in altering water potential gradients is not well understood. Because the movement of water through a plant system is determined in part by the sum of the resistances encountered in the pathway, and since resistance to water movement is greatest in the intervening tissue between the root epidermis and the xylem [9], appreciable changes in water transport can result from changes in the resistance of roots to water flow. Yield reductions of a wide variety of crops result from root infestations by the root-knot nematode, Meloidogyne javanica (Treub), Chitwood. Stunting resulting from root-knot infection is often associated with root tissue damage. Infected plants have been reported to appear more susceptible to wilt than healthy plants during periods of slight water stress, although the effect of M. javanica on host water relations has never been quantitatively measured. The purpose of this investigation was to determine the effect of M. javanica on water relations of tomatoes (Lycopersicon esculentum Mill. cv. Early Dwarf Red). t Present Malaysia.
address:
0048-4059/78/1101-0275 PO
Department $02.00/O
of Plant
Protection,
University
of Agriculture,
@ 1978 Academic
Press Inc.
Serdang, (London)
Selangor, Limited
S. Meon, H. R.Wallace and J. M. Fisher
276 MATERIALS
AND
METHODS
Galls of various sizes and of varying age were collected from tomato plants infected with M. javatka and fixed in F.A.A. for histological studies. After 3 days in fixative, the galls were washed with tap water and dehydrated in the tertiary butyl alcohol series. They were embedded in paraffin and then sectioned on a sliding microtome at a thickness of 15 l.~in transverse and longitudinal planes. The sections were stained in safranin and fast green. After dehydration in an ethanol series, temporary mounts were prepared for microscopic examinations. To study the effect of infection on the flow of water in the plant, single tomato seedlings were grown in John Innes potting compost in 10 cm pots for 40 days, after which larvae of M. javanica were inoculated into the soil at the base of each plant. Inoculum densities used were 0, 500, 1000, 2000, 5000 or 10 000 larvae. The plants were then subjected to high (25%), medium (16%) and low (8%) moisture contents, 3 days after inoculation. Moisture in each system was kept constant by weighing to constant weight. The pots were randomly arranged in the growth cabinet with temperatures of 20 “C in the dark (12 h per day) and 25 “C in the light (12 h per day). Forty days after inoculation, the plants were cut off approximately 4 cm from the soil level and the suction was measured using the modified apparatus of Scholander et al. [8]. The pressure required to drive the first drops of water to the cut surface of the stem was taken to be directly proportional to the suction in the root system. For determination of diffusive resistance of stomata and water potential of leaves, tomato plants grown in 12-5 cm pots were randomly arranged in the growth cabinet with temperatures of 20 “C in the dark (12 h per day) and 25” C in the light (12 h per day). Each plant was inoculated with 6000 larvae of M. javanica and uninoculated plants were used as controls. Measurements commenced 1 week after inoculation and continued at intervals of 7 days for a period of 8 weeks. Soil was brought to field capacity at the beginning of each measurement. The diffusive resistance of the abaxial leaf surface of healthy and infected tomatoes was measured with an aspirated diffusive porometer [3]. Measurements were taken 4 h after saturation of the rooting medium with water in the light period. Data collected from diffusive resistance measurements of the third pair of true leaflets at similar physiological ages were converted to s cm-i. The same leaf was then detached and immediately placed into a pcltier-cooled thermocouple psychrometer to determine its water potential [I]. The leaves were wrapped around a wire mesh insert protecting the thermocouple and pushed into the psychrometer chamber, which was then stoppered. The chamber was placed in a water bath (25 _+O*Ol“C) and allowed to equilibrate for at least 2 h before the thermocouple output was read. The water potential was calculated by comparing the recorded deflections with the deflections obtained from a graded series of sodium chloride solutions. Transpiration rate was studied by gravimetric analysis involving growing plants in 500 ml waxed paper cups containing sand. Each plant was inoculated with 1000 larvae of M. javanica. Uninoculated plants were used as controls. A closed system surrounding the roots was provided as follows. A drain hole was made in the bottom of the cup and a watering tube inserted into the sand. The surface was then covered with polystyrene to prevent water loss from the soil surface and pots. Each cup was inserted into a second cup and the watering tube sealed with clay. At the beginning
Water relations
of infected tomato
277
of each measurement the outside cup was removed and Hoagland’s solutions added through the watering tube until the sand was saturated. The system was allowed to drain for 30 min and resealed. The difference in weight of the saturated system after 24 h was expressed as the total rate of the whole-plant transpiration (g/24 h). All the results were analysed statistically by regression analysis and are represented graphically as untransformed data. RESULTS
Histological studies showed that giant cells were formed from the parenchyma cells of the vascular system in response to infection which also inhibited cambium formation, hence no secondary xylem was formed [Plate 1(a)]. Normal conducting vessels were present away from the infection sites. The cluster of giant cells was usually surrounded by a large number of abnormal xylem-like elements [Plate 1 (b)]. The abnormal xylem was not arranged along the longitudinal axis of the root but was disposed in a diffuse manner and, furthermore, its elements were shorter and narrower than normal ones. Although vessels of large diameter were not totally suppressed, the continuity of the larger vessels was often broken by the intrusion of giant cells into the vascular system. It seems likely, therefore, that their efficiency as conducting elements was impaired. Population density and soil moisture had a significant effect (P > 0.01) on suction in the roots. Suction increased with population density of nematodes and with decreasing soil moisture content (Fig. 1).
I
2000
I
I
4000
6000
I
8000
10000
No. of nematodes
FIG. 1. The tiuence of population density of M. jamnicu on the suction in roots of tomato plants at various soil moisture contents, 40 days after inoculation. Soil moisture contents used were 8% (o), 16% (A) and 25% (A) w/w. Each point is the mean of four replicates. Vertical lines indicate L.S.D. at P
The diffusive resistance of tomato plants infected with M. javanica increased as infection progressed and was always higher than in uninfected plants (Fig. 2). Regression coefficients were significantly different (P > 0.05) between the infected
S. Meon, H. R.Wallace and J. M. Fisher
278
and non-infected plants. The resistances of leaves obtained Corn non-infected plants were lower than those from infected plants at any given time after inoculation. The infected plants did not show any significant stunting or other foliar symptoms at the end of the experimental period. Water potential of leaves from infected and healthy plants over an extended period of 8 weeks after inoculation is shown in Fig. 3. The regression coefficients were 16
I 2 Time
FIG. 2. Regression tomato
plants
with
01
(0)
after
lines showing and without
I
I
6
8
inoculation
the relationship ( 0) M. javaak.
I 2 Time
I 4
after
( weeks
between time and diffusive resistance of Each point is the mean of six replicates.
I 4
I 6
inoculotlon
(weeks)
FIG. 3. Regression lmes showing the relationship tomato plants with ( l ) and without ( 0) M. juvunicu.
1
)
I 8
between time and water potential of Each point is the mean of six replicates.
roots infected by M. javanica. PLATE 1. Tomato (a) Longitudinal section showing absence of secondary xylem in the region of giant cells. (b) A transverse section showing a cluster of giant cells surrounded by abnormal xylem. Abbreviations used in the plate: a, nematode; b, giant cells; c, abnormal xylem vessels; d, normal xylem vessels. r facing pap 2781
Water relations
279
of infected tomato
significantly different (P > O-05). Water potential in the infected plants decreased with time after inoculation at a greater rate than that of the healthy plants. Inoculation of the tomato plants with 6000 larvae of M. jam&a did not produce stunting or other foliar symptoms. Fig. 4 shows the relationship between leaf diffisive resistance and water potential for both the infected and healthy plants. The leaf diffusive resistance increased nonlinearly as water potential decreased. Even though the data were somewhat scattered,
I
I
1
2
4
6
Water
FIG. 4. Regression lines showing resistance of tomato plants with (0) of six replicates.
potential
6
(-bar)
the relationship between water potential and without (0) M. jananica. Each point
and diffiive is the mean
45s
Time
after
inoculation
( weeks)
FIG. 5. Regression lines showing the relationship between piration rate of tomato with ( l ) and without ( 0) M. jam&a. replicates.
time and the whole-plant transEach point is the mean of six
280
S. Meon, H. R.Wallace
and J. M. Fisher
the regression line drawn indicates that the diffusive resistance of leaves following inoculation was invariably higher (P > 0.05) than the diffusive resistance of healthy plants at the same water potential. Transpiration rate of whole infected tomato plants over an extended period of 8 weeks of inoculation did not differ significantly (Fig. 5) from those of the healthy plants. The plants did not show any significant visual stunting, wilting or other foliar symptoms throughout the experimental period. DISCUSSION It is currently accepted that initial density of nematodes is related to decrease in the yield of, and the amount of damage to, plants [ZO] and that this relation can be influenced markedly by changes in environmental factors. Thus, the larger the root system or the more ideal the environmental factors, the larger the number of nematodes necessary to enable measurement of damage. This relation was demonstrated in the results for the effect of M. javanica on suction in the roots, where in the presence of adequate moisture (high moisture content), no increased suction could be demonstrated but as the water content of the soil was reduced, smaller initial numbers of nematodes caused increased root suction. Histopathological studies indicated that the disruption and abnormality of xylem vessels caused by the formation of giant cells was one of the reasons for this increased root suction. In an attempt to remove the effects of increased root suction on the other parameters of water behaviour in the plant, conditions of soil moisture content, nematode density and plant size were chosen so that no measurable effect on root suction could be obtained. Under these conditions water potential in the leaves and stomata1 diffusivity decreased but a measurable effect on transpiration was not obtained. These results could be explained by assuming that root suction was increased but not sufficiently to be measured by the technique or by interference to hormone levels. As nematodes were the only variable in these experiments, the results must be related to nematode infection. These results were obtained in the absence of any visible foliar symptoms so presumably the plant is able to overcome and compensate for the effects of these levels of nematode infection even though effects on the plant such as water potential can be measured. At higher initial levels of infection, we would expect to relate the changes within the plant to noticeable foliar symptoms. Uninfected healthy plants maintain high turgor when water is limiting through closure of stomata that effectively reduces transpiration [7] and it is likely that infected plants have a similar regulatory response. However, the critical question is: Are infected plants more efficient at conserving water than uninfected plants ? The experimental results in this paper indicate that they are, because at the same water potentials, infected plants have a lower stomata1 diffusivity than uninfected plants (Fig. 4). Transpiration measurements, on the other hand, failed to indicate a statistically significant difference between infected and uninfected plants (Fig. 5). The mechanisms whereby water is conserved in the infected plant are not understood although it is likely that the stimulus for the response is hormonal. Thus, Itai & Vaadia [6’J have shown that moisture stress is associated with a decrease in transport
Water relations
of infected tomato
281
to the shoot system of cytokinins synthesized in root tips. It is possible that xylem disruption following infection by M. javanica not only reduces translocation of water to shoots but hormones also, thereby eliciting stomata1 closure. Experimental data (in preparation) did in fact suggest that amounts of cytokinins and gibberellin translocated from roots to shoots decreased and amounts of abscisic acid increased in the leaves following infection. It is also concluded that where wilting and stunting do occur in infected plants, either the growth conditions are probably sub-optimal, thereby inhibiting the plant’s ability to conserve water, or the infection level is so high that the plant’s regulatory mechanisms are insufficient to cope with the disruption associated with infection. REFERENCES 1. BARRS, H. D. (1968). Determination of water deficits in plant tissues. In Water D&its and Plant Growth, Ed. by T. T. Kozlowski, Vol. I, pp. 235-370. Academic Press, New York. Differing sensitivity of photosynthesis to low leaf water potentials in corn 2. BAYER, J. A. (1970). and soybean. Plant Physiology 46, 236-239. 3. BYRNE, G. F., Rosa, C. W. & SLATYER, R. 0. (1970). An aspirated diffusive porometer. Agric. Met-m. 7,3w. 4. DUNIWAY, J. M. (1973). Pathogen induced changes in host water relations. Phytoputhology 63,
45846. 5. GATES, C. T. (1972). 6.
Water deficits and growth of herbaceous plates. In Water D.$cits and Plant Growth, Ed. by T. T. Kozlowski, Vol. II, pp. 135-190. Academic Press, New York. ITAI, C. & VAADIA, Y. (1971). Cytokinin activity in water-stressed shoots. Plant Pathology 47,
87-90. 7. MACHARDY,
W. E., BUSCH, L. V. & HALL, R. (1976). Ve&illium wilt of chrysanthemum: quantitative relationship between increased stomata1 resistance and local vascular dysfunction preceding wilt. Canadian Journal of Botany 54, 1023-1034. 8. SCHOLANDER, P. F., HAMMEL, H. T., BROADSTXEET, E. D., & HEMMINGSEN, E. A. (1965). Sap pressure in vascular plants. S&ace 148, 339-346. 9. SLAYTER, R. 0. (1967). P&vat-water Rebiomhips. 378 pp. Academic Press, New York. 10. SEINHORST, J. W. (1965). The relation between nematode density and damage to plants. .Nematologica 11, 137-154.
13,275-281
Water relations of tomato (Lycopersicon esculentum Mill. cv. Early Dwarf Red) infected with Meloidogyne javanica (Treub), Chitwood S. MEON,t H. R. WALLACE andJ. M. FISHER D+artment of Plant Pathology, Wad+?Agriculturat ResearchIn&u&, Glen Osmond, South Austratia 5064 (Accefitedfor publication April 1978)
Infection of tomato plants by Meloid&w jaoanica resulted in increased suction pressure in the root system probably due to abnormality of the xylem elements. A change in the suction pressure and/or transpirational behaviour appeared to account for the low water potential values in the infected plants. There was no difference in the rate of transpiration between infected and non-infected plants; however, diffusive resistance increased in infected plants as infection progressed and was higher than that in healthy plants.
INTRODUCTION
All plant physiological processes depend on water and if growth and development are to proceed normally, internal water stress must not develop within the tissues [S]. Moisture stress in leaves inhibits such processes as photosynthesis [2] and transport to the shoot system of cytokinins synthesized in root tips [6], thereby causing reductions in metabolic activity which ultimately result in reduced growth [4, 51. The influence of pathogens in altering water potential gradients is not well understood. Because the movement of water through a plant system is determined in part by the sum of the resistances encountered in the pathway, and since resistance to water movement is greatest in the intervening tissue between the root epidermis and the xylem [9], appreciable changes in water transport can result from changes in the resistance of roots to water flow. Yield reductions of a wide variety of crops result from root infestations by the root-knot nematode, Meloidogyne javanica (Treub), Chitwood. Stunting resulting from root-knot infection is often associated with root tissue damage. Infected plants have been reported to appear more susceptible to wilt than healthy plants during periods of slight water stress, although the effect of M. javanica on host water relations has never been quantitatively measured. The purpose of this investigation was to determine the effect of M. javanica on water relations of tomatoes (Lycopersicon esculentum Mill. cv. Early Dwarf Red). t Present Malaysia.
address:
0048-4059/78/1101-0275 PO
Department $02.00/O
of Plant
Protection,
University
of Agriculture,
@ 1978 Academic
Press Inc.
Serdang, (London)
Selangor, Limited
S. Meon, H. R.Wallace and J. M. Fisher
276 MATERIALS
AND
METHODS
Galls of various sizes and of varying age were collected from tomato plants infected with M. javatka and fixed in F.A.A. for histological studies. After 3 days in fixative, the galls were washed with tap water and dehydrated in the tertiary butyl alcohol series. They were embedded in paraffin and then sectioned on a sliding microtome at a thickness of 15 l.~in transverse and longitudinal planes. The sections were stained in safranin and fast green. After dehydration in an ethanol series, temporary mounts were prepared for microscopic examinations. To study the effect of infection on the flow of water in the plant, single tomato seedlings were grown in John Innes potting compost in 10 cm pots for 40 days, after which larvae of M. javanica were inoculated into the soil at the base of each plant. Inoculum densities used were 0, 500, 1000, 2000, 5000 or 10 000 larvae. The plants were then subjected to high (25%), medium (16%) and low (8%) moisture contents, 3 days after inoculation. Moisture in each system was kept constant by weighing to constant weight. The pots were randomly arranged in the growth cabinet with temperatures of 20 “C in the dark (12 h per day) and 25 “C in the light (12 h per day). Forty days after inoculation, the plants were cut off approximately 4 cm from the soil level and the suction was measured using the modified apparatus of Scholander et al. [8]. The pressure required to drive the first drops of water to the cut surface of the stem was taken to be directly proportional to the suction in the root system. For determination of diffusive resistance of stomata and water potential of leaves, tomato plants grown in 12-5 cm pots were randomly arranged in the growth cabinet with temperatures of 20 “C in the dark (12 h per day) and 25” C in the light (12 h per day). Each plant was inoculated with 6000 larvae of M. javanica and uninoculated plants were used as controls. Measurements commenced 1 week after inoculation and continued at intervals of 7 days for a period of 8 weeks. Soil was brought to field capacity at the beginning of each measurement. The diffusive resistance of the abaxial leaf surface of healthy and infected tomatoes was measured with an aspirated diffusive porometer [3]. Measurements were taken 4 h after saturation of the rooting medium with water in the light period. Data collected from diffusive resistance measurements of the third pair of true leaflets at similar physiological ages were converted to s cm-i. The same leaf was then detached and immediately placed into a pcltier-cooled thermocouple psychrometer to determine its water potential [I]. The leaves were wrapped around a wire mesh insert protecting the thermocouple and pushed into the psychrometer chamber, which was then stoppered. The chamber was placed in a water bath (25 _+O*Ol“C) and allowed to equilibrate for at least 2 h before the thermocouple output was read. The water potential was calculated by comparing the recorded deflections with the deflections obtained from a graded series of sodium chloride solutions. Transpiration rate was studied by gravimetric analysis involving growing plants in 500 ml waxed paper cups containing sand. Each plant was inoculated with 1000 larvae of M. javanica. Uninoculated plants were used as controls. A closed system surrounding the roots was provided as follows. A drain hole was made in the bottom of the cup and a watering tube inserted into the sand. The surface was then covered with polystyrene to prevent water loss from the soil surface and pots. Each cup was inserted into a second cup and the watering tube sealed with clay. At the beginning
Water relations
of infected tomato
277
of each measurement the outside cup was removed and Hoagland’s solutions added through the watering tube until the sand was saturated. The system was allowed to drain for 30 min and resealed. The difference in weight of the saturated system after 24 h was expressed as the total rate of the whole-plant transpiration (g/24 h). All the results were analysed statistically by regression analysis and are represented graphically as untransformed data. RESULTS
Histological studies showed that giant cells were formed from the parenchyma cells of the vascular system in response to infection which also inhibited cambium formation, hence no secondary xylem was formed [Plate 1(a)]. Normal conducting vessels were present away from the infection sites. The cluster of giant cells was usually surrounded by a large number of abnormal xylem-like elements [Plate 1 (b)]. The abnormal xylem was not arranged along the longitudinal axis of the root but was disposed in a diffuse manner and, furthermore, its elements were shorter and narrower than normal ones. Although vessels of large diameter were not totally suppressed, the continuity of the larger vessels was often broken by the intrusion of giant cells into the vascular system. It seems likely, therefore, that their efficiency as conducting elements was impaired. Population density and soil moisture had a significant effect (P > 0.01) on suction in the roots. Suction increased with population density of nematodes and with decreasing soil moisture content (Fig. 1).
I
2000
I
I
4000
6000
I
8000
10000
No. of nematodes
FIG. 1. The tiuence of population density of M. jamnicu on the suction in roots of tomato plants at various soil moisture contents, 40 days after inoculation. Soil moisture contents used were 8% (o), 16% (A) and 25% (A) w/w. Each point is the mean of four replicates. Vertical lines indicate L.S.D. at P
The diffusive resistance of tomato plants infected with M. javanica increased as infection progressed and was always higher than in uninfected plants (Fig. 2). Regression coefficients were significantly different (P > 0.05) between the infected
S. Meon, H. R.Wallace and J. M. Fisher
278
and non-infected plants. The resistances of leaves obtained Corn non-infected plants were lower than those from infected plants at any given time after inoculation. The infected plants did not show any significant stunting or other foliar symptoms at the end of the experimental period. Water potential of leaves from infected and healthy plants over an extended period of 8 weeks after inoculation is shown in Fig. 3. The regression coefficients were 16
I 2 Time
FIG. 2. Regression tomato
plants
with
01
(0)
after
lines showing and without
I
I
6
8
inoculation
the relationship ( 0) M. javaak.
I 2 Time
I 4
after
( weeks
between time and diffusive resistance of Each point is the mean of six replicates.
I 4
I 6
inoculotlon
(weeks)
FIG. 3. Regression lmes showing the relationship tomato plants with ( l ) and without ( 0) M. juvunicu.
1
)
I 8
between time and water potential of Each point is the mean of six replicates.
roots infected by M. javanica. PLATE 1. Tomato (a) Longitudinal section showing absence of secondary xylem in the region of giant cells. (b) A transverse section showing a cluster of giant cells surrounded by abnormal xylem. Abbreviations used in the plate: a, nematode; b, giant cells; c, abnormal xylem vessels; d, normal xylem vessels. r facing pap 2781
Water relations
279
of infected tomato
significantly different (P > O-05). Water potential in the infected plants decreased with time after inoculation at a greater rate than that of the healthy plants. Inoculation of the tomato plants with 6000 larvae of M. jam&a did not produce stunting or other foliar symptoms. Fig. 4 shows the relationship between leaf diffisive resistance and water potential for both the infected and healthy plants. The leaf diffusive resistance increased nonlinearly as water potential decreased. Even though the data were somewhat scattered,
I
I
1
2
4
6
Water
FIG. 4. Regression lines showing resistance of tomato plants with (0) of six replicates.
potential
6
(-bar)
the relationship between water potential and without (0) M. jananica. Each point
and diffiive is the mean
45s
Time
after
inoculation
( weeks)
FIG. 5. Regression lines showing the relationship between piration rate of tomato with ( l ) and without ( 0) M. jam&a. replicates.
time and the whole-plant transEach point is the mean of six
280
S. Meon, H. R.Wallace
and J. M. Fisher
the regression line drawn indicates that the diffusive resistance of leaves following inoculation was invariably higher (P > 0.05) than the diffusive resistance of healthy plants at the same water potential. Transpiration rate of whole infected tomato plants over an extended period of 8 weeks of inoculation did not differ significantly (Fig. 5) from those of the healthy plants. The plants did not show any significant visual stunting, wilting or other foliar symptoms throughout the experimental period. DISCUSSION It is currently accepted that initial density of nematodes is related to decrease in the yield of, and the amount of damage to, plants [ZO] and that this relation can be influenced markedly by changes in environmental factors. Thus, the larger the root system or the more ideal the environmental factors, the larger the number of nematodes necessary to enable measurement of damage. This relation was demonstrated in the results for the effect of M. javanica on suction in the roots, where in the presence of adequate moisture (high moisture content), no increased suction could be demonstrated but as the water content of the soil was reduced, smaller initial numbers of nematodes caused increased root suction. Histopathological studies indicated that the disruption and abnormality of xylem vessels caused by the formation of giant cells was one of the reasons for this increased root suction. In an attempt to remove the effects of increased root suction on the other parameters of water behaviour in the plant, conditions of soil moisture content, nematode density and plant size were chosen so that no measurable effect on root suction could be obtained. Under these conditions water potential in the leaves and stomata1 diffusivity decreased but a measurable effect on transpiration was not obtained. These results could be explained by assuming that root suction was increased but not sufficiently to be measured by the technique or by interference to hormone levels. As nematodes were the only variable in these experiments, the results must be related to nematode infection. These results were obtained in the absence of any visible foliar symptoms so presumably the plant is able to overcome and compensate for the effects of these levels of nematode infection even though effects on the plant such as water potential can be measured. At higher initial levels of infection, we would expect to relate the changes within the plant to noticeable foliar symptoms. Uninfected healthy plants maintain high turgor when water is limiting through closure of stomata that effectively reduces transpiration [7] and it is likely that infected plants have a similar regulatory response. However, the critical question is: Are infected plants more efficient at conserving water than uninfected plants ? The experimental results in this paper indicate that they are, because at the same water potentials, infected plants have a lower stomata1 diffusivity than uninfected plants (Fig. 4). Transpiration measurements, on the other hand, failed to indicate a statistically significant difference between infected and uninfected plants (Fig. 5). The mechanisms whereby water is conserved in the infected plant are not understood although it is likely that the stimulus for the response is hormonal. Thus, Itai & Vaadia [6’J have shown that moisture stress is associated with a decrease in transport
Water relations
of infected tomato
281
to the shoot system of cytokinins synthesized in root tips. It is possible that xylem disruption following infection by M. javanica not only reduces translocation of water to shoots but hormones also, thereby eliciting stomata1 closure. Experimental data (in preparation) did in fact suggest that amounts of cytokinins and gibberellin translocated from roots to shoots decreased and amounts of abscisic acid increased in the leaves following infection. It is also concluded that where wilting and stunting do occur in infected plants, either the growth conditions are probably sub-optimal, thereby inhibiting the plant’s ability to conserve water, or the infection level is so high that the plant’s regulatory mechanisms are insufficient to cope with the disruption associated with infection. REFERENCES 1. BARRS, H. D. (1968). Determination of water deficits in plant tissues. In Water D&its and Plant Growth, Ed. by T. T. Kozlowski, Vol. I, pp. 235-370. Academic Press, New York. Differing sensitivity of photosynthesis to low leaf water potentials in corn 2. BAYER, J. A. (1970). and soybean. Plant Physiology 46, 236-239. 3. BYRNE, G. F., Rosa, C. W. & SLATYER, R. 0. (1970). An aspirated diffusive porometer. Agric. Met-m. 7,3w. 4. DUNIWAY, J. M. (1973). Pathogen induced changes in host water relations. Phytoputhology 63,
45846. 5. GATES, C. T. (1972). 6.
Water deficits and growth of herbaceous plates. In Water D.$cits and Plant Growth, Ed. by T. T. Kozlowski, Vol. II, pp. 135-190. Academic Press, New York. ITAI, C. & VAADIA, Y. (1971). Cytokinin activity in water-stressed shoots. Plant Pathology 47,
87-90. 7. MACHARDY,
W. E., BUSCH, L. V. & HALL, R. (1976). Ve&illium wilt of chrysanthemum: quantitative relationship between increased stomata1 resistance and local vascular dysfunction preceding wilt. Canadian Journal of Botany 54, 1023-1034. 8. SCHOLANDER, P. F., HAMMEL, H. T., BROADSTXEET, E. D., & HEMMINGSEN, E. A. (1965). Sap pressure in vascular plants. S&ace 148, 339-346. 9. SLAYTER, R. 0. (1967). P&vat-water Rebiomhips. 378 pp. Academic Press, New York. 10. SEINHORST, J. W. (1965). The relation between nematode density and damage to plants. .Nematologica 11, 137-154.