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Growth and photosynthesis of Najas marina L. as affected by light intensity
Aquatic Botany, 9 (1980) 285--289
285
Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands
Short Communication GROWTH AND PHOTOSYNTHESIS OF N A J A S M A R I N A L. AS AFFECTED BY LIGHT INTENSITY
M. AGAMI*, S. BEER and Y. WAISEL.
Department of Botany, Tel Aviv University, Tel Aviv (Israel) (Accepted 19 May 1980)
ABSTRACT Agami, M., Beer, S. and Waisel, Y., 1980. Growth and photosynthesis of Najas marina L. as affected by light intensity. Aquat. Bot., 9: 285--289.
Najas marina L. is a submerged aquatic weed which infests many water reservoirs due to its fast growth rates. Its vertical distribution varies with the light penetration of the water. In the sources of the Yarkon river (Israel) it appears no deeper than 1.5 m, even during the optimal growth season. Plants which had been transplanted to greater depths deteriorated within less than three weeks. Laboratory experiments showed a light compensation point for net photosynthesis at about 5 ~E In- 2 s-'. This value correlates well with the light intensity found in the field at the maximal depths of Najas growth.
INTRODUCTION
Submerged water plants are among the most threatened groups of plants in Israel. During the last 30 years, some 20 species have become extinct (Dafni and Agami, 1976). This is mainly a result of the intensive appropriation of freshwater resources for agricultural use, as well as water pollution. Conversely, some species of water plants which previously grew in small quantities in Lake Kinneret and other water bodies of northern Israel, have become abundant in the newly constructed artificial water reservoirs of the water transport system of Israel. Such plants may have negative or even hazardous effects on the management of the reservoirs, e.g. plants bloom causing clogging of water pipes and an increased organic strain on the water. Najas marina L. is one of these species, being found at various sites on the water system since 1966, only one year after its completion. The distribution of Najas marina is very widespread throughout the world. The plant is mainly found in waters of high pH and high electrolyte concentrations (Barry and Jermy, 1952; Forsberg and Forsberg, 1961} and its germination is favoured by reducing conditions (Forsberg, 1965). Thus, the distribution of this spe*This paper was presented in partial fulfilment of the requirements for the Ph.D. degree at the Tel Aviv University.
0304-3770/80/0000--0000/$02.50 © 1980 Elsevier Scientific Publishing Company
286
cies may depend on such conditions. In the U.S.A. where Najas spp. have also become a nuisance in various water reservoirs, factors such as water level, substrate and turbidity affected its distribution and productivity patterns {Martin et al., 1969). Because of the scarcity and high value of freshwater in Israel, only biological and ecological measures may be taken to control Najas and other macrophytes which grow in water reservoirs. The aim of the present work was to investigate productivity and photosynthesis rates of Najas marina as a function of light intensity and water depth. Such information might have practical value, in determining whether alterations in the water level and, thus, in the light regime can be used to control the intensive production of this plant. MATERIALS AND METHODS
Plants of Najas marina were collected at the source of the R. Yarkon, near Tel Aviv, Israel. Uniform vegetative shoots (6 cm long) were taken from established stands and transferred to buckets which were lowered down to various depths below a floating raft. In this way the plants were kept at a constant water depth, despite severe fluctuations in the water level of the source during the course of the experiments. Four plants were planted in each bucket and the experiment was duplicated. Productivity was measured by weighing the plants (fresh weight} from each container once a week, during a three-week period. Immediately subsequent to weighing, the plants were returned to their previous containers which were lowered down to their respective depths. Four plants o f each depth were taken to the laboratory for measurements of photosynthesis. Net phosynthesis was measured using an 02 electrode in a closed system, as described b y Beer et al. (1977). The temperature was kept at 20°C, in order to prevent rapid deterioration of the plants. Light intensities were varied by placing neutral density filters between a 150 W incandescent light bulb and the plant chamber of the experimental set-up. Light intensity were varied either in different experiments or during the same experiment at steady-state photosynthetic rates. The change of slopes on the oxygraph recorder following alterations in light intensity were obtained in less than one minute. No effects of increasing O3 concentrations within the measured range ( 2 0 - - 1 8 0 pM) could be observed. Light intensities were measured using a Lambda LI 185A light meter equipped with an underwater quantum sensor measuring photosynthetically active radiation (400--700 rim). Experiments were carried o u t during August--September, 1978. The surface water temperature at the Yarkon reservoir was then 31°C, and ca. 29°C at a depth of 250 cm.
287 RESULTS
The course of biomass change of Na]as over time, and at different water depths, is presented in Fig. 1. While plants in the upper 50 cm water layers showed a rather uniform growth throughout the experiment, plants growing deeper were negatively affected. Plants at 100 cm depth attained, after 3
A=O B=25 C=50
v
D=IOO ro~ LL E = 200 F = 250
I ]
I 2
L 3
Weeks
1
2
3
of g r o w t h
Fig. 1. Growth of N ~ marina plants at different water depths: A = surface; B= 25 era; C= 50 cm; D = 100 cm; E= 200 cm; F= 250 cm. Growth measurements initiated in August 1978. The data represent means of eight replications at every depth. Standard deviations were less than 15%. 20
(a)
I.
i~ I0
d
{°
560
10
Light i n t e n s i t y
( # E r a - 2 s -1 )
(b)
5~ ?
Light intensity
( / z E m - 2 s -1)
Fig. 2. Rates o f n e t p h o t o s y n t h e s i s as a f u n c t i o n o f light intensities: (a) w i d e range; (b) narrow range. Data represent means o f three e x p e r i m e n t s . Standard deviations were less t h a n 10%.
288 weeks, weights of only some two thirds of those in the upper water layers. Nevertheless, at 100cm depth, a net increase in weight was still observed. At depths of 200 and 250 cm, the plants increased in fresh weight during the first week only, kept the same weight during the second week, and lost weight and deteriorated during the third week. Net photosynthesis as a function of fight intensity is presented in Fig. 2. Light saturation was reached at about 280 pE m -2 s-1, (Fig. 2a), while the light compensation point for net photosynthesis was about 5 ~E m -2 s-1 (Fig. 2b). Light intensities measured at different water depths in the Yarkon reservoir are presented in Fig. 3. Measurements were performed during a clear summer day. The data are presented in two scales: left--data of the upper water layers and right-data for the deeper water layers. Light intensity reached the compensation point at depth o f 170 cm. 30
1500 (a)
(b)
~,ooo
~
1o:nh
.
~ 500
-5
\.,, ~o
ISO
120
150
200
W a t e r depth (crn)
Fig. 3. Light intensity in the source of the Yarkon river as a function of water depth at mid-day of August 13th, 1978: (a) upper layer; (b) deeper layer. DISCUSSION AND CONCLUSIONS In spite of the gradual reduction in light intensity which was observed below the water surface, growth of Najas plants was n o t affected at intensities as low as 360 ~E m -2 s-1 (or a water depth of 50 cm). The course of growth over time was close to linear during the three weeks, and even the high light intensity at the water surface did not reduce this. At a depth o f 100 cm, i.e. at light intensities o f 60 pE or less, growth declined as compared to that at the upper water layers. At water depths o f 200 and 250 cm, i.e. when light intensities were below the compensation point (ca. 1.5 ~E), the plants' fresh weight increased only during the first week o f treatment. Such growth probably depended on carbohydrate reserves and not on current photosynthesis. As soon as the reserves were depleted, the plants started to lose weight. Net productivity on a daily basis would thus need light inten-
289
sities above that of 5 pE (i.e. the light intensity at the laboratory-determined compensation point) for at least a few hours every day. There is good correlation between light penetration and the depth of water in which Najas was able to grow in the field. At light intensities of, or below the laboratory-determined compensation point, plants either did not appear at all (in natural stands} or disappeared as in our field experiments. Furthermore, the fact that Najas marina grows only in the shallow waters of reservoirs supports the idea that the vertical distribution of this plant is primarily limited by light intensity. Plant responses to light may vary during the year. The measurements for this study were made in August, which is the optimum growth season of the plant (June--September). During other seasons, the light penetration --net photosynthesis relationships could differ. For example, during the month of June, when the water is clear, optimum water depth for Najas growth was found to be 100 cm. However, during August, when planktonic algae had infested the reservoir and light penetration was considerably lowered, optimum growth was at a depth of 50 cm. At 100 cm, growth had declined completely, (M. Agami, unpublished data). The appearance of Najas marina is inconsistent in its habitats. Plants appear at certain sites in great numbers during some years, but do not appear at all during others. The absence of the plants during such years may be caused indirectly by changes in water level, changes in water turbidity or by algal blooms. In all three cases the result would be a reduction in light intensity and inhibited growth. The marked reduction in growth which is associated with relatively slight increases in water depth, suggests that alterations in water level for short periods could be practised for the control of Najas in water reservoirs.
REFERENCES
Barry, D.M. and Jermy, A.C., 1952. Observation on Najas marina L. Trans. Norfolk Norwich Nat. Soc., 17: 294--297. Beer, S., Eshel, A. and Waisel, Y., 1977. Carbon metabolism in Seagrasses. I. The utilization of exogenous inorganic carbon species in photosynthesis. J. Exp. Bot., 106: 1180--1189. Dafni, A. and Agami, M., 1976. Extinct plants of Israel. Biol. Conserv., 10: 49--52. Forsberg, C., 1965. Sterile germination of oospores of Chara and seeds of Najas marina. Physiol. Plant., 18: 128--137. Forsberg, B. and Forsberg, C., 1961. The fresh water environment for Najas marina L. in Scandinavia. Sven. Bot. Tidskr., 55: 604--612. Martin, J.B., Bradford, B.N. and Kennedy, H.B., 1969. Factors Affecting the Growth of Najas in Pickwick Reservoir (Tennessee). Nat. Fert. Develop. Centre, Tennessee Valley Authority, Muscle Shoals, AL, 47 pp.
285
Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands
Short Communication GROWTH AND PHOTOSYNTHESIS OF N A J A S M A R I N A L. AS AFFECTED BY LIGHT INTENSITY
M. AGAMI*, S. BEER and Y. WAISEL.
Department of Botany, Tel Aviv University, Tel Aviv (Israel) (Accepted 19 May 1980)
ABSTRACT Agami, M., Beer, S. and Waisel, Y., 1980. Growth and photosynthesis of Najas marina L. as affected by light intensity. Aquat. Bot., 9: 285--289.
Najas marina L. is a submerged aquatic weed which infests many water reservoirs due to its fast growth rates. Its vertical distribution varies with the light penetration of the water. In the sources of the Yarkon river (Israel) it appears no deeper than 1.5 m, even during the optimal growth season. Plants which had been transplanted to greater depths deteriorated within less than three weeks. Laboratory experiments showed a light compensation point for net photosynthesis at about 5 ~E In- 2 s-'. This value correlates well with the light intensity found in the field at the maximal depths of Najas growth.
INTRODUCTION
Submerged water plants are among the most threatened groups of plants in Israel. During the last 30 years, some 20 species have become extinct (Dafni and Agami, 1976). This is mainly a result of the intensive appropriation of freshwater resources for agricultural use, as well as water pollution. Conversely, some species of water plants which previously grew in small quantities in Lake Kinneret and other water bodies of northern Israel, have become abundant in the newly constructed artificial water reservoirs of the water transport system of Israel. Such plants may have negative or even hazardous effects on the management of the reservoirs, e.g. plants bloom causing clogging of water pipes and an increased organic strain on the water. Najas marina L. is one of these species, being found at various sites on the water system since 1966, only one year after its completion. The distribution of Najas marina is very widespread throughout the world. The plant is mainly found in waters of high pH and high electrolyte concentrations (Barry and Jermy, 1952; Forsberg and Forsberg, 1961} and its germination is favoured by reducing conditions (Forsberg, 1965). Thus, the distribution of this spe*This paper was presented in partial fulfilment of the requirements for the Ph.D. degree at the Tel Aviv University.
0304-3770/80/0000--0000/$02.50 © 1980 Elsevier Scientific Publishing Company
286
cies may depend on such conditions. In the U.S.A. where Najas spp. have also become a nuisance in various water reservoirs, factors such as water level, substrate and turbidity affected its distribution and productivity patterns {Martin et al., 1969). Because of the scarcity and high value of freshwater in Israel, only biological and ecological measures may be taken to control Najas and other macrophytes which grow in water reservoirs. The aim of the present work was to investigate productivity and photosynthesis rates of Najas marina as a function of light intensity and water depth. Such information might have practical value, in determining whether alterations in the water level and, thus, in the light regime can be used to control the intensive production of this plant. MATERIALS AND METHODS
Plants of Najas marina were collected at the source of the R. Yarkon, near Tel Aviv, Israel. Uniform vegetative shoots (6 cm long) were taken from established stands and transferred to buckets which were lowered down to various depths below a floating raft. In this way the plants were kept at a constant water depth, despite severe fluctuations in the water level of the source during the course of the experiments. Four plants were planted in each bucket and the experiment was duplicated. Productivity was measured by weighing the plants (fresh weight} from each container once a week, during a three-week period. Immediately subsequent to weighing, the plants were returned to their previous containers which were lowered down to their respective depths. Four plants o f each depth were taken to the laboratory for measurements of photosynthesis. Net phosynthesis was measured using an 02 electrode in a closed system, as described b y Beer et al. (1977). The temperature was kept at 20°C, in order to prevent rapid deterioration of the plants. Light intensities were varied by placing neutral density filters between a 150 W incandescent light bulb and the plant chamber of the experimental set-up. Light intensity were varied either in different experiments or during the same experiment at steady-state photosynthetic rates. The change of slopes on the oxygraph recorder following alterations in light intensity were obtained in less than one minute. No effects of increasing O3 concentrations within the measured range ( 2 0 - - 1 8 0 pM) could be observed. Light intensities were measured using a Lambda LI 185A light meter equipped with an underwater quantum sensor measuring photosynthetically active radiation (400--700 rim). Experiments were carried o u t during August--September, 1978. The surface water temperature at the Yarkon reservoir was then 31°C, and ca. 29°C at a depth of 250 cm.
287 RESULTS
The course of biomass change of Na]as over time, and at different water depths, is presented in Fig. 1. While plants in the upper 50 cm water layers showed a rather uniform growth throughout the experiment, plants growing deeper were negatively affected. Plants at 100 cm depth attained, after 3
A=O B=25 C=50
v
D=IOO ro~ LL E = 200 F = 250
I ]
I 2
L 3
Weeks
1
2
3
of g r o w t h
Fig. 1. Growth of N ~ marina plants at different water depths: A = surface; B= 25 era; C= 50 cm; D = 100 cm; E= 200 cm; F= 250 cm. Growth measurements initiated in August 1978. The data represent means of eight replications at every depth. Standard deviations were less than 15%. 20
(a)
I.
i~ I0
d
{°
560
10
Light i n t e n s i t y
( # E r a - 2 s -1 )
(b)
5~ ?
Light intensity
( / z E m - 2 s -1)
Fig. 2. Rates o f n e t p h o t o s y n t h e s i s as a f u n c t i o n o f light intensities: (a) w i d e range; (b) narrow range. Data represent means o f three e x p e r i m e n t s . Standard deviations were less t h a n 10%.
288 weeks, weights of only some two thirds of those in the upper water layers. Nevertheless, at 100cm depth, a net increase in weight was still observed. At depths of 200 and 250 cm, the plants increased in fresh weight during the first week only, kept the same weight during the second week, and lost weight and deteriorated during the third week. Net photosynthesis as a function of fight intensity is presented in Fig. 2. Light saturation was reached at about 280 pE m -2 s-1, (Fig. 2a), while the light compensation point for net photosynthesis was about 5 ~E m -2 s-1 (Fig. 2b). Light intensities measured at different water depths in the Yarkon reservoir are presented in Fig. 3. Measurements were performed during a clear summer day. The data are presented in two scales: left--data of the upper water layers and right-data for the deeper water layers. Light intensity reached the compensation point at depth o f 170 cm. 30
1500 (a)
(b)
~,ooo
~
1o:nh
.
~ 500
-5
\.,, ~o
ISO
120
150
200
W a t e r depth (crn)
Fig. 3. Light intensity in the source of the Yarkon river as a function of water depth at mid-day of August 13th, 1978: (a) upper layer; (b) deeper layer. DISCUSSION AND CONCLUSIONS In spite of the gradual reduction in light intensity which was observed below the water surface, growth of Najas plants was n o t affected at intensities as low as 360 ~E m -2 s-1 (or a water depth of 50 cm). The course of growth over time was close to linear during the three weeks, and even the high light intensity at the water surface did not reduce this. At a depth o f 100 cm, i.e. at light intensities o f 60 pE or less, growth declined as compared to that at the upper water layers. At water depths o f 200 and 250 cm, i.e. when light intensities were below the compensation point (ca. 1.5 ~E), the plants' fresh weight increased only during the first week o f treatment. Such growth probably depended on carbohydrate reserves and not on current photosynthesis. As soon as the reserves were depleted, the plants started to lose weight. Net productivity on a daily basis would thus need light inten-
289
sities above that of 5 pE (i.e. the light intensity at the laboratory-determined compensation point) for at least a few hours every day. There is good correlation between light penetration and the depth of water in which Najas was able to grow in the field. At light intensities of, or below the laboratory-determined compensation point, plants either did not appear at all (in natural stands} or disappeared as in our field experiments. Furthermore, the fact that Najas marina grows only in the shallow waters of reservoirs supports the idea that the vertical distribution of this plant is primarily limited by light intensity. Plant responses to light may vary during the year. The measurements for this study were made in August, which is the optimum growth season of the plant (June--September). During other seasons, the light penetration --net photosynthesis relationships could differ. For example, during the month of June, when the water is clear, optimum water depth for Najas growth was found to be 100 cm. However, during August, when planktonic algae had infested the reservoir and light penetration was considerably lowered, optimum growth was at a depth of 50 cm. At 100 cm, growth had declined completely, (M. Agami, unpublished data). The appearance of Najas marina is inconsistent in its habitats. Plants appear at certain sites in great numbers during some years, but do not appear at all during others. The absence of the plants during such years may be caused indirectly by changes in water level, changes in water turbidity or by algal blooms. In all three cases the result would be a reduction in light intensity and inhibited growth. The marked reduction in growth which is associated with relatively slight increases in water depth, suggests that alterations in water level for short periods could be practised for the control of Najas in water reservoirs.
REFERENCES
Barry, D.M. and Jermy, A.C., 1952. Observation on Najas marina L. Trans. Norfolk Norwich Nat. Soc., 17: 294--297. Beer, S., Eshel, A. and Waisel, Y., 1977. Carbon metabolism in Seagrasses. I. The utilization of exogenous inorganic carbon species in photosynthesis. J. Exp. Bot., 106: 1180--1189. Dafni, A. and Agami, M., 1976. Extinct plants of Israel. Biol. Conserv., 10: 49--52. Forsberg, C., 1965. Sterile germination of oospores of Chara and seeds of Najas marina. Physiol. Plant., 18: 128--137. Forsberg, B. and Forsberg, C., 1961. The fresh water environment for Najas marina L. in Scandinavia. Sven. Bot. Tidskr., 55: 604--612. Martin, J.B., Bradford, B.N. and Kennedy, H.B., 1969. Factors Affecting the Growth of Najas in Pickwick Reservoir (Tennessee). Nat. Fert. Develop. Centre, Tennessee Valley Authority, Muscle Shoals, AL, 47 pp.