- Email: [email protected]
Field studies on seed germination and seedling development in Zostera marina L.
Aquatic Botany, 16 (1983) 21--29
21
Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands
FIELD STUDIES ON SEED GERMINATION AND SEEDLING DEVELOPMENT IN ZOSTERA MARINA L.
A. COOLIDGE CHURCHILL
Department of Biology, Adelphi University, Garden City, N Y 11530 (U.S.A.) (Accepted for publication 20 January 1983)
ABSTRACT Churchill, A.C., 1983. Field studies on seed germination and seedling development in Zostera marina L. Aquat. Bot., 16: 21--29. Seeds of Zostera marina L. were collected in July 1978 and July 1979 and planted within two weeks in sediment-filled containers in shallow water at Northwest Creek, New York. The containers were retrieved at periodic intervals over the next 12 months and the seeds and seedlings examined. Between 76 and 93% of the recovered seeds germinated; primarily during October--December, 3--4 months after release. Seedling growth and development occurred during the a u t u m n and spring, but n o t in winter. A progressive loss of seedlings was observed during the winter and spring, however, and none survived past May. Three stages of seedling development were distinguished. Stage 1 represented germination including emergence and straightening of the cotyledon. Stage 2 seedlings exhibited continued growth of the cotyledon and variable, but often extensive, elongation (2--50 ram) of the axial hypocotyl. The plumule remained undeveloped (length < 3 ram), and there were no hairs on the basal hypocotyl. Stage 3 seedlings showed the same range in axial hypocotyl length, b u t they were distinguished by the onset of plumule growth and hypocotyl hair formation.
INTRODUCTION
The process of flowering, seed development and the period of seed release in Zostera marina L. have recently been described in some detail (Phillips,
1972; Churchill and Riner, 1978; de Cock, 1980, 1981; Jacobs and Pierson, 1981). Less is known about the germination capacity of the seeds, particularly under natural conditions, and the time of seed germination. Several laboratory studies have indicated that seed germination in Z. marina, although variable, is generally low and ranges between 1--20% (Tutin, 1938; Phillips, 1971; Lamounette, 1977). Results of field test plantings support these observations (Phillips, 1972; Orth et al., 1981). Conditions of low salinity (5--10°/00) appear to enhance germination in the laboratory and may yield germination values as high as 70% (Phillips, 1972; Lamounette, 1977). 0304-3770/83/$03.00
© 1983 Elsevier Science Publishers B.V.
22 The time of seed germination has been variously described. Arasaki (1950), Taylor (1957) and Phillips (1972) concluded that germination occurred in the spring, approximately 8--10 m o n t h s following seed release. Addy (1947) found that germination m a y also occur in the autumn since a number of seeds harvested in August germinated in the field by the end of October. This finding has been confirmed b y an examination of seeds and seedlings obtained from sediment cores (Orth, 1976). A later study conducted at the same field site (Orth et al., 1981), however, indicated that seeds held in sediment-free containers germinated t h r o u g h o u t most of the year, b u t with a maximum occurring in March and April. In the present study, seeds of a k n o w n age were planted in the field, and systematically retrieved at various times for examination. The time of seed germination, the germination capacity and the pattern of seedling developm e n t are described. MATERIALS
AND METHODS
The seeds used for planting were obtained from flowering shoots growing at the study site near Northwest Creek on Long Island, NY (Fig. 1). The shoots were harvested on 9 July, 1978 and 7 July, 1979, and placed with seawater in a plastic wading pool. After 4~ ~, :he shoots were removed and the water strained through screening (1.0 mm 2) to collect the released seeds. A 100% viability was indicated after staining with tetrazolium red. Seeds were planted in plastic containers, 21 cm in diameter and 4 cm high, with nylon screen (1.0 mm 2) bottoms. Each container was initially filled with 5 cm o f w e t beach sand and 100 seeds spread on the sand surface. An additional 4 cm of wet sand was then added, and the container buried with its t o p surface even with the b o t t o m sediments. The planting site was an open sandy area, proximate to the near shore boundary of a Zostera meadow. The water depth was approximately 0.3 m below mean low water and the tidal range was 0.76 m. T w e n t y containers were set o u t on 16 July, 1978 and 8 containers on 15 July, 1979. They were planted 0.6 m apart and in rows with a 1.0 m spacing. T w o containers (three in May) were h a r v e s t e d from the first planting at each collection time, yielding a total of 15 o u t of the original 20. The remainder couldn't be traced. Only four containers were recovered from the second planting. The containers were harvested by hand and transported to the laboratory in buckets with a small a m o u n t of sea water. The seeds and seedlings were recovered b y first washing the container sediments through nylon screening (1.0 mm 2) and then carefully examining the retained sediment fraction. Even though care was taken during this process, seedlings with long delicate hypocotyls tended to fragment. On several occasions, a n u m b e r of wild seedlings growing near the planting site (within 15 m) were harvested for comparison with those in the containers.
23
O N G ISLA
D SOUND
GARDNERS BAY
STUDY SITE
41
ATLANTIC OCEAN
I ~9 KILOMETERS
.
Fig. 1. Map of Long Island and enlargement showing location of study area. RESULTS
Information from the seed containers is partially summarized in Table I. The number of seeds recovered varied from none to 95 (x=70.4+22.7), b u t there was no apparent correlation between recovery success and the time period prior to harvest. Germination, defined by splitting of the seed coat and emergence of the cotyledon, occurred primarily during the autumn, for 56--76% o f the seeds had germinated by mid-November. Ungerminated seeds were found in April, however, indicating the possibility of delayed germination. The percentage increase of germinated seeds in the autumn was followed in the first planting by a decrease during the winter and spring, and by June there were no seedlings remaining. Results from the second planting also show a marked seedling loss b e t w e e n January and April, b u t the failure to recover any seeds in June suggests that the original container sediments had been scoured. The loss o f seedlings was reflected by a concomitant increase in seeds with split seed coats that were either e m p t y or contained small amounts o f decomposing plant material. These seeds could not, strictly
24 TABLE I The number of seeds recovered from each container (initial n=100) and the percentage representing germinated, ungerminated and empty seeds. Seeds recovered between September 1978 and June 1979 were collected and planted in July 1978 (first planting). Seeds recovered between November 1979 and June 1980 were collected and planted in July 1979 Collection date
Number recovered
% Germinated
% Ungerminated
% Empty
First planting 30 Sept. 1978 10 Nov. 1978 15 Dec. 1978 03 Mar. 1979 06 Apr. 1979 19 May 1979 22 June 1979 Second planting 17 Nov. 1979 17 Jan. 1980 12 Apr. 1980 02 June 1980
77 68 95 82 39 66 63 46 91 72 83 73 65 66 90
0 4 57 71 77 80 71 46 54 44 3 0 0 0 0
100 95 43 28 8 2 0 0 1 3 0 0 0 0 0
0 1 0 1 15 18 29 54 45 53 97 100 100 100 100
88 90 85 0
76 93 42
24 2 1
0 5 57
speaking, be classified as having g e r m i n a t e d o r n o t b u t the f o l l o w i n g p a t t e r n is clear: seeds g e r m i n a t e d , d e v e l o p e d into seedlings and t h e n e i t h e r d e t e r i o r a t e d o r w e r e u p r o o t e d , leaving e m p t y seed c o a t s as remains. T h e d e v e l o p m e n t o f seedlings was s e p a r a t e d into t h r e e stages. Stage 1 (Fig. 2A) r e p r e s e n t e d seed g e r m i n a t i o n . T h e p l u m u l e was n o t r e a d i l y visible, and t h e axial h y p o c o t y l and t h e c o t y l e d o n s h o w e d little e l o n g a t i o n ( < 4 and < 8 m m , respectively). T h e seedlings h a r v e s t e d in S e p t e m b e r 1 9 7 8 w e r e stage 1 (Table II) and t h e y were f o u n d in small n u m b e r s in March and April 1 9 7 9 ; suggesting again t h a t g e r m i n a t i o n m a y be d e l a y e d until the spring f o l l o w i n g seed release. Stage 2 seedlings (Fig. 2B) w e r e m a r k e d b y variable b u t o f t e n e x t e n s i v e e l o n g a t i o n o f t h e axial h y p o c o t y l and c o t y l e d o n . T h e y w e r e distinguished f r o m stage 1 b y having a c o t y l e d o n t h a t was t>8 m m in length and f r o m stage 3 seedlings b y t h e u n d e v e l o p e d n a t u r e o f t h e p l u m u l e (length < 3 m m ) and t h e lack o f hairs o n t h e basal h y p o c o t y l . T h e s e seedlings w e r e relatively m o s t a b u n d a n t in N o v e m b e r 1 9 7 8 and N o v e m b e r 1 9 7 9 (Table II), b u t t h e y d e v e l o p e d stage 3 characteristics d u r i n g t h e ensuing m o n t h s .
25 TABLE II The number of planted seedlings in each of the three developmental stages. Seedling fragments without plumules were classifiedas stage 3 or stage 2 based on the presence or absence of hypocotyl hairs, respectively
First planting (1978)
Stage I Stage 2 Stage 3
Second planting (1980)
(1979)
Sept.
Nov.
Dec.
Mar.
Apr.
May
Nov.
Jan.
Apr.
3 0 0
12 36 10
2 20 61
3 23 40
1 20 60
0 0 2
18 44 5
0 8 76
0 1 35
C
lOmrn
Fig. 2. Stages in seedling development. A: Stage 1 seedlings representing germination. B: Stage 2 seedlings characterized by the undeveloped plumule within the cotyledon sheath and by the lack of hairs on the basal hypocotyl. Marked elongation of the axial hypocotyl and cotyledon are evident. C : Stage 3 seedlings showing the characteristic production of hairs on the basal hypocotyl and various stages of plumule growth. The plumule at far right has two leaves which have emerged from the sheath. CS, cotyledon sheath ; BH, basal hypocotyl; C, cotyledon ; AH, axial hypocotyl; P, plumule. S t a g e 3 s e e d l i n g s ( F i g . 2C) w e r e d e f i n e d b y t h e o n s e t o f p l u m u l e g r o w t h and by the f o r m a t i o n of hairs on the e x p a n d e d basal h y p o c o t y l . These t w o c h a r a c t e r i s t i c s w e r e a s s o c i a t e d i n v a r i a b l y w i t h e a c h o t h e r , m a k i n g it p o s s i b l e to classify b o t h f r a g m e n t e d and d e c o m p o s i n g h y p o c o t y l p o r t i o n s based on
26 t h e p r e s e n c e o r absence o f h y p o c o t y l hairs. Stage 3 seedlings w e r e r e c o v e r e d f r o m N o v e m b e r t o May and in t h e s e c o n d planting s h o w e d a m a r k e d increase in relative a b u n d a n c e during the w i n t e r and spring. T h e y w e r e t h e o n l y seedling stage observed in t h e field. L e n g t h m e a s u r e m e n t s o f seedlings w i t h i n t a c t axial h y p o c o t y l s and p l u m u l e s are s h o w n in Table III. T h e length o f stage 2 h y p o c o t y l s had a range o f 2 - - 5 0 m m and m e a n values e x t e n d i n g f r o m 9 ( N o v e m b e r 1 9 7 9 ) t o 23 m m ( J a n u a r y 1980). Stage 3 h y p o c o t y l s had a similar range in length ( 2 - - 5 8 m m ) , b u t the m e a n values including wild seedlings were m o r e u n i f o r m and e x h i b i t e d no seasonal trend. T h e results suggest t h a t h y p o c o t y l elongat i o n ceases during the transition f r o m stage 2 t o stage 3 seedlings. TABLE III Mean hypocotyl and plumule length (mm -+ SD) of planted and wild seedlings. Only intact stage 2 and stage 3 seedlings were measured _Fir s__~_tl~lant__ip_g_............................ (1978) (1979) Nov.
Apr.
Second planting (1980)
Dec.
Mar.
May
Nov.
Jan.
Apr.
21-+1 12 20-+2 17
16±5 10±4 10 7 30-+4 28-+4 33 18 20 2 24±3 17
9±1 41 18±3 5 16±3 25
23±4 7 24±1 64 16-+2 34
32 1 15-+2 20 19±2 20
16±2 17
15±3 18
19-+9 21±2 5 64 29±4 25
June
Hypocotyl 16-+3 n 25 Stage3 (planted) 25-+10 n 3 Stage3 (wild) n Stage 2 (planted)
Plumule Stage3 (planted)
n Stage 3 (wild) n
15±10 3
26±4 20
90 2 130±10 17
85-+7 20 72±4 127±7 34 20
T h e p l u m u l e s o f stage 3 seedlings increased in length d u r i n g the a u t u m n and spring and in wild seedlings r e a c h e d a m e a n length o f 130 m m b y May 1 9 7 9 a n d J u n e 1 9 8 0 , 1 0 - - 1 1 m o n t h s a f t e r seed release. Results f r o m t h e first planting indicate t h a t p l u m u l e g r o w t h did n o t o c c u r d u r i n g t h e winter, for length values r e m a i n e d c o n s t a n t f r o m N o v e m b e r t o March. R e n e w e d growth, h o w e v e r , was e v i d e n t b y t h e first w e e k in April. DISCUSSION T w o generalizations m a y be m a d e f r o m this s t u d y ; a high p e r c e n t a g e o f seeds g e r m i n a t e , and a distinct seasonality exists in t h e t i m e o f g e r m i n a t i o n .
27 A minimum of 76 and 93% of the seeds germinated from the first and second planting, respectively. These values agree with the field observations of Orth (1976), but they contradict a general impression from laboratory experiments that seeds of Z. marina germinate poorly (Tutin, 1938; Phillips, 1971, 1972; Lamounette, 1977; Churchill et al., 1978; Keddy and Patriquin, 1978). The high germination yields are not a result exclusively of field planting, for I have obtained similar values in the laboratory after incubating seed batches in low salinity water (70/00). An important consideration may be that the seeds used in the present study were harvested only after they had been released and were presumably fully mature. The time of seed germination has been noted by a number of investigators, and a pattern of germination during the winter and spring, 5--10 months after seed release, exists in widely separated areas (SetcheU, 1929; Miki, 1933; Tutin, 1938; Taylor, 1957; Phillips, 1972; Keddy and Patriquin, 1978; Orth et al., 1981). The present results do not fit this pattern, for most seeds germinated in the autumn, 3--4 months after release. Similar observations were made by Addy (1947) and more recently by Orth (1976). It is n o t known whether the interval between seed release and germination represents a true dormancy or a quiescent period lasting until the onset of favorable conditions. Orth et al. (1981) have suggested that dormancy does n o t occur and that low water temperature (~15°C) is the primary variable initiating germination. The present study supports this view, for water temperature at the time of the 1978 and 1979 November harvests was 10 and 13°C, respectively. In contrast, laboratory studies indicate that low salinity (5--10°/00) is the overriding variable promoting germination (Amsaki, 1950; Phillips, 1972; Lamounette, 1977), and that significant germination may occur in 10%0 seawater even at 25°C (Lamounette, 1977). The relevance of this low salinity effect at Northwest Creek is questionable, however, since salinity values rarely fall below 20%o. The pattern of germination at Northwest Creek indicates that, at any one time, seeds from only one flower crop occur in the sediments (seed release starts in mid~Iune). This may not be true in other areas, for Phillips (1972) and Orth et al. (1981) have noted that seeds may remain viable for at least one year. In addition, I have successfully germinated seeds that were maintained in sediments (in seawater of 30%o) at room temperature for 22 months. When the seeds were transferred to low salinity water (7%0), 55% (n=45) developed into stage 3 seedlings within three weeks. Theoretically, viable seeds from two and perhaps three flowering seasons may occur together in the sediments. All seedlings from the plantings perished by June indicating that recruitm e n t from seeds is a function primarily of seedling survival and not seed germination. The causes for the seedling loss are unknown. The increase in the percentage of empty and decomposing seeds (Table I) reveals that approximately 50% of the seedlings disappeared during the late a u t u m n and winter and that the remainder were lost during the spring. Seedling mortality
28
occurred, therefore, during periods of both active and inactive growth and primarily after they had reached stage 3 in development. Small numbers of healthy wild seedlings were observed in June 1979 and 1980; so potential existed for extended seedling survival in the experimental area. It seems unlikely that the high density of planted seeds or the confinement in containers were deleterious to seedling vigor, as there was little difference in the growth characteristics of wild and planted stage 3 seedlings. Foraging activities and predation may be significant factors, for rasp-like wounds were frequently observed along the axial hypocotyl and plumule base and appeared to initiate a general process of seedling deterioration. The distinction between stage 2 and stage 3 seedlings has n o t been made previously. When seedlings are raised in the laboratory, they exhibit limited hypocotyl elongation and plumule growth commences with the emergence of the cotyledon (Taylor, 1957). This study has shown that conditions in the field can inhibit plumule growth while still allowing extensive hypocotyl and cotyledon elongation. Taylor (1957) collected stage 3 seedlings with axial hypocotyls of varying lengths and noted a direct relationship between length and the depth to which the basal hypocotyl was buried in the sediments. Such a relationship might explain the variation in hypocotyl length found at Northwest Creek, for although seeds were buried initially at a uniform depth, marked differences were observed when the containers were processed. It is tempting to speculate that hypocotyl elongation of stage 2 seedlings functions to push the rudimentary plumule (protected by the cotyledon sheath) to a point near the sediment surface where conditions stimulate the transition to stage 3 development.
REFERENCES A d d y , C.E., 1947. Germination o f eelgrass seed. J. Wildl. Manage., 11 : 279. Arasaki, M., 1950. Studies on the ecology of Z o s t e r a marina and Z o s t e r a nana. Bull. Jpn. Soc. Fish., 16: 70--76. Churchill, A.C. and Riner, M.I., 1978. Anthesis and seed production in Z o s t e r a marina L. from Great South Bay, New York, U.S.A. Aquat. Bot., 4 : 83--93. Churchill, A.C., Cok, A.E. and Riner, M.I., 1978. Stabilization of subtidal sediments by the seagrass Z o s t e r a marina L. New York Sea Grant Report Series. (NYSSP-RS-78-15), 65 pp. De Cock, A.W.A.M., 1980. Flowering, pollination and fruiting in Z o s t e r a marina L. Aquat. Bot., 9: 201--220. De Cock, A.W.A.M., 1981. Development o f the flowering shoot of Z o s t e r a marina L. under controlled conditions in comparison to the development in two different natural habitats in The Netherlands. Aquat. Bot., 10: 99--114. Jacobs, R.P.W.M. and Pierson, E.S., 1981. Phenology of reproductive shoots o f eelgrass, Z o s t e r a marina, at Roscoff (France). Aquat. Bot., 10: 45---60. Keddy, C.J. and Patriquin, D.G., 1978. An annual form o f eelgrass in Nova Scotia. Aquat. Bot., 5: 163--170. Lamounette, R., 1977. A study of the germination and viability of Z o s t e r a marina L. seeds. M.S. Dissertation, Adelphi Univ., Garden City, NY, 41 pp.
29 Miki, S., 1933. On the seagrasses in Japan (I) Zostera and PhyUospadix, with special reference to morphological and ecological characters. Bot. Mag., 47: 842--862. Orth, R., 1976. The demise and recovery of eelgrass, Zostera marina, in the Chesapeake Bay, Virginia. Aquat. Bot., 2: 141--159. Orth, R.J., Moore, K.A., Roberts, M.H. and Silberhorn, G.M., 1981. The biology and propagation of eelgrass, Zostera marina, in the Chesapeake Bay, Virginia. Final Report (Contract No. R 805953), U.S. Environ. Prot. Agency, Washington, DC, 227 pp. Phillips, R.C., 1971. Seed germination in Zostera marina L. Am. J. Bot., 58: 459. Phillips, R.C., 1972. Ecological life history of Zostera marina L. (eelgrass) in Puget Sound, Washington. Ph.D. Dissertation, University of Washington, Seattle, WA, 154 PP. Setchell, W.A., 1929. Morphological and phenologicai notes on Zostera marina L. Univ. Calif. Publ. Bot., 14 : 389--452. Taylor, A.R.A., 1957. Studies of the development of Zostera marina L. II. Germination and seedling development. Can. J. Bot., 35: 681--695. Tutin, T.G., 1938. The autecology of Zostera marina L. in relation to its wasting disease. New Phytol., 37 : 50--71.
21
Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands
FIELD STUDIES ON SEED GERMINATION AND SEEDLING DEVELOPMENT IN ZOSTERA MARINA L.
A. COOLIDGE CHURCHILL
Department of Biology, Adelphi University, Garden City, N Y 11530 (U.S.A.) (Accepted for publication 20 January 1983)
ABSTRACT Churchill, A.C., 1983. Field studies on seed germination and seedling development in Zostera marina L. Aquat. Bot., 16: 21--29. Seeds of Zostera marina L. were collected in July 1978 and July 1979 and planted within two weeks in sediment-filled containers in shallow water at Northwest Creek, New York. The containers were retrieved at periodic intervals over the next 12 months and the seeds and seedlings examined. Between 76 and 93% of the recovered seeds germinated; primarily during October--December, 3--4 months after release. Seedling growth and development occurred during the a u t u m n and spring, but n o t in winter. A progressive loss of seedlings was observed during the winter and spring, however, and none survived past May. Three stages of seedling development were distinguished. Stage 1 represented germination including emergence and straightening of the cotyledon. Stage 2 seedlings exhibited continued growth of the cotyledon and variable, but often extensive, elongation (2--50 ram) of the axial hypocotyl. The plumule remained undeveloped (length < 3 ram), and there were no hairs on the basal hypocotyl. Stage 3 seedlings showed the same range in axial hypocotyl length, b u t they were distinguished by the onset of plumule growth and hypocotyl hair formation.
INTRODUCTION
The process of flowering, seed development and the period of seed release in Zostera marina L. have recently been described in some detail (Phillips,
1972; Churchill and Riner, 1978; de Cock, 1980, 1981; Jacobs and Pierson, 1981). Less is known about the germination capacity of the seeds, particularly under natural conditions, and the time of seed germination. Several laboratory studies have indicated that seed germination in Z. marina, although variable, is generally low and ranges between 1--20% (Tutin, 1938; Phillips, 1971; Lamounette, 1977). Results of field test plantings support these observations (Phillips, 1972; Orth et al., 1981). Conditions of low salinity (5--10°/00) appear to enhance germination in the laboratory and may yield germination values as high as 70% (Phillips, 1972; Lamounette, 1977). 0304-3770/83/$03.00
© 1983 Elsevier Science Publishers B.V.
22 The time of seed germination has been variously described. Arasaki (1950), Taylor (1957) and Phillips (1972) concluded that germination occurred in the spring, approximately 8--10 m o n t h s following seed release. Addy (1947) found that germination m a y also occur in the autumn since a number of seeds harvested in August germinated in the field by the end of October. This finding has been confirmed b y an examination of seeds and seedlings obtained from sediment cores (Orth, 1976). A later study conducted at the same field site (Orth et al., 1981), however, indicated that seeds held in sediment-free containers germinated t h r o u g h o u t most of the year, b u t with a maximum occurring in March and April. In the present study, seeds of a k n o w n age were planted in the field, and systematically retrieved at various times for examination. The time of seed germination, the germination capacity and the pattern of seedling developm e n t are described. MATERIALS
AND METHODS
The seeds used for planting were obtained from flowering shoots growing at the study site near Northwest Creek on Long Island, NY (Fig. 1). The shoots were harvested on 9 July, 1978 and 7 July, 1979, and placed with seawater in a plastic wading pool. After 4~ ~, :he shoots were removed and the water strained through screening (1.0 mm 2) to collect the released seeds. A 100% viability was indicated after staining with tetrazolium red. Seeds were planted in plastic containers, 21 cm in diameter and 4 cm high, with nylon screen (1.0 mm 2) bottoms. Each container was initially filled with 5 cm o f w e t beach sand and 100 seeds spread on the sand surface. An additional 4 cm of wet sand was then added, and the container buried with its t o p surface even with the b o t t o m sediments. The planting site was an open sandy area, proximate to the near shore boundary of a Zostera meadow. The water depth was approximately 0.3 m below mean low water and the tidal range was 0.76 m. T w e n t y containers were set o u t on 16 July, 1978 and 8 containers on 15 July, 1979. They were planted 0.6 m apart and in rows with a 1.0 m spacing. T w o containers (three in May) were h a r v e s t e d from the first planting at each collection time, yielding a total of 15 o u t of the original 20. The remainder couldn't be traced. Only four containers were recovered from the second planting. The containers were harvested by hand and transported to the laboratory in buckets with a small a m o u n t of sea water. The seeds and seedlings were recovered b y first washing the container sediments through nylon screening (1.0 mm 2) and then carefully examining the retained sediment fraction. Even though care was taken during this process, seedlings with long delicate hypocotyls tended to fragment. On several occasions, a n u m b e r of wild seedlings growing near the planting site (within 15 m) were harvested for comparison with those in the containers.
23
O N G ISLA
D SOUND
GARDNERS BAY
STUDY SITE
41
ATLANTIC OCEAN
I ~9 KILOMETERS
.
Fig. 1. Map of Long Island and enlargement showing location of study area. RESULTS
Information from the seed containers is partially summarized in Table I. The number of seeds recovered varied from none to 95 (x=70.4+22.7), b u t there was no apparent correlation between recovery success and the time period prior to harvest. Germination, defined by splitting of the seed coat and emergence of the cotyledon, occurred primarily during the autumn, for 56--76% o f the seeds had germinated by mid-November. Ungerminated seeds were found in April, however, indicating the possibility of delayed germination. The percentage increase of germinated seeds in the autumn was followed in the first planting by a decrease during the winter and spring, and by June there were no seedlings remaining. Results from the second planting also show a marked seedling loss b e t w e e n January and April, b u t the failure to recover any seeds in June suggests that the original container sediments had been scoured. The loss o f seedlings was reflected by a concomitant increase in seeds with split seed coats that were either e m p t y or contained small amounts o f decomposing plant material. These seeds could not, strictly
24 TABLE I The number of seeds recovered from each container (initial n=100) and the percentage representing germinated, ungerminated and empty seeds. Seeds recovered between September 1978 and June 1979 were collected and planted in July 1978 (first planting). Seeds recovered between November 1979 and June 1980 were collected and planted in July 1979 Collection date
Number recovered
% Germinated
% Ungerminated
% Empty
First planting 30 Sept. 1978 10 Nov. 1978 15 Dec. 1978 03 Mar. 1979 06 Apr. 1979 19 May 1979 22 June 1979 Second planting 17 Nov. 1979 17 Jan. 1980 12 Apr. 1980 02 June 1980
77 68 95 82 39 66 63 46 91 72 83 73 65 66 90
0 4 57 71 77 80 71 46 54 44 3 0 0 0 0
100 95 43 28 8 2 0 0 1 3 0 0 0 0 0
0 1 0 1 15 18 29 54 45 53 97 100 100 100 100
88 90 85 0
76 93 42
24 2 1
0 5 57
speaking, be classified as having g e r m i n a t e d o r n o t b u t the f o l l o w i n g p a t t e r n is clear: seeds g e r m i n a t e d , d e v e l o p e d into seedlings and t h e n e i t h e r d e t e r i o r a t e d o r w e r e u p r o o t e d , leaving e m p t y seed c o a t s as remains. T h e d e v e l o p m e n t o f seedlings was s e p a r a t e d into t h r e e stages. Stage 1 (Fig. 2A) r e p r e s e n t e d seed g e r m i n a t i o n . T h e p l u m u l e was n o t r e a d i l y visible, and t h e axial h y p o c o t y l and t h e c o t y l e d o n s h o w e d little e l o n g a t i o n ( < 4 and < 8 m m , respectively). T h e seedlings h a r v e s t e d in S e p t e m b e r 1 9 7 8 w e r e stage 1 (Table II) and t h e y were f o u n d in small n u m b e r s in March and April 1 9 7 9 ; suggesting again t h a t g e r m i n a t i o n m a y be d e l a y e d until the spring f o l l o w i n g seed release. Stage 2 seedlings (Fig. 2B) w e r e m a r k e d b y variable b u t o f t e n e x t e n s i v e e l o n g a t i o n o f t h e axial h y p o c o t y l and c o t y l e d o n . T h e y w e r e distinguished f r o m stage 1 b y having a c o t y l e d o n t h a t was t>8 m m in length and f r o m stage 3 seedlings b y t h e u n d e v e l o p e d n a t u r e o f t h e p l u m u l e (length < 3 m m ) and t h e lack o f hairs o n t h e basal h y p o c o t y l . T h e s e seedlings w e r e relatively m o s t a b u n d a n t in N o v e m b e r 1 9 7 8 and N o v e m b e r 1 9 7 9 (Table II), b u t t h e y d e v e l o p e d stage 3 characteristics d u r i n g t h e ensuing m o n t h s .
25 TABLE II The number of planted seedlings in each of the three developmental stages. Seedling fragments without plumules were classifiedas stage 3 or stage 2 based on the presence or absence of hypocotyl hairs, respectively
First planting (1978)
Stage I Stage 2 Stage 3
Second planting (1980)
(1979)
Sept.
Nov.
Dec.
Mar.
Apr.
May
Nov.
Jan.
Apr.
3 0 0
12 36 10
2 20 61
3 23 40
1 20 60
0 0 2
18 44 5
0 8 76
0 1 35
C
lOmrn
Fig. 2. Stages in seedling development. A: Stage 1 seedlings representing germination. B: Stage 2 seedlings characterized by the undeveloped plumule within the cotyledon sheath and by the lack of hairs on the basal hypocotyl. Marked elongation of the axial hypocotyl and cotyledon are evident. C : Stage 3 seedlings showing the characteristic production of hairs on the basal hypocotyl and various stages of plumule growth. The plumule at far right has two leaves which have emerged from the sheath. CS, cotyledon sheath ; BH, basal hypocotyl; C, cotyledon ; AH, axial hypocotyl; P, plumule. S t a g e 3 s e e d l i n g s ( F i g . 2C) w e r e d e f i n e d b y t h e o n s e t o f p l u m u l e g r o w t h and by the f o r m a t i o n of hairs on the e x p a n d e d basal h y p o c o t y l . These t w o c h a r a c t e r i s t i c s w e r e a s s o c i a t e d i n v a r i a b l y w i t h e a c h o t h e r , m a k i n g it p o s s i b l e to classify b o t h f r a g m e n t e d and d e c o m p o s i n g h y p o c o t y l p o r t i o n s based on
26 t h e p r e s e n c e o r absence o f h y p o c o t y l hairs. Stage 3 seedlings w e r e r e c o v e r e d f r o m N o v e m b e r t o May and in t h e s e c o n d planting s h o w e d a m a r k e d increase in relative a b u n d a n c e during the w i n t e r and spring. T h e y w e r e t h e o n l y seedling stage observed in t h e field. L e n g t h m e a s u r e m e n t s o f seedlings w i t h i n t a c t axial h y p o c o t y l s and p l u m u l e s are s h o w n in Table III. T h e length o f stage 2 h y p o c o t y l s had a range o f 2 - - 5 0 m m and m e a n values e x t e n d i n g f r o m 9 ( N o v e m b e r 1 9 7 9 ) t o 23 m m ( J a n u a r y 1980). Stage 3 h y p o c o t y l s had a similar range in length ( 2 - - 5 8 m m ) , b u t the m e a n values including wild seedlings were m o r e u n i f o r m and e x h i b i t e d no seasonal trend. T h e results suggest t h a t h y p o c o t y l elongat i o n ceases during the transition f r o m stage 2 t o stage 3 seedlings. TABLE III Mean hypocotyl and plumule length (mm -+ SD) of planted and wild seedlings. Only intact stage 2 and stage 3 seedlings were measured _Fir s__~_tl~lant__ip_g_............................ (1978) (1979) Nov.
Apr.
Second planting (1980)
Dec.
Mar.
May
Nov.
Jan.
Apr.
21-+1 12 20-+2 17
16±5 10±4 10 7 30-+4 28-+4 33 18 20 2 24±3 17
9±1 41 18±3 5 16±3 25
23±4 7 24±1 64 16-+2 34
32 1 15-+2 20 19±2 20
16±2 17
15±3 18
19-+9 21±2 5 64 29±4 25
June
Hypocotyl 16-+3 n 25 Stage3 (planted) 25-+10 n 3 Stage3 (wild) n Stage 2 (planted)
Plumule Stage3 (planted)
n Stage 3 (wild) n
15±10 3
26±4 20
90 2 130±10 17
85-+7 20 72±4 127±7 34 20
T h e p l u m u l e s o f stage 3 seedlings increased in length d u r i n g the a u t u m n and spring and in wild seedlings r e a c h e d a m e a n length o f 130 m m b y May 1 9 7 9 a n d J u n e 1 9 8 0 , 1 0 - - 1 1 m o n t h s a f t e r seed release. Results f r o m t h e first planting indicate t h a t p l u m u l e g r o w t h did n o t o c c u r d u r i n g t h e winter, for length values r e m a i n e d c o n s t a n t f r o m N o v e m b e r t o March. R e n e w e d growth, h o w e v e r , was e v i d e n t b y t h e first w e e k in April. DISCUSSION T w o generalizations m a y be m a d e f r o m this s t u d y ; a high p e r c e n t a g e o f seeds g e r m i n a t e , and a distinct seasonality exists in t h e t i m e o f g e r m i n a t i o n .
27 A minimum of 76 and 93% of the seeds germinated from the first and second planting, respectively. These values agree with the field observations of Orth (1976), but they contradict a general impression from laboratory experiments that seeds of Z. marina germinate poorly (Tutin, 1938; Phillips, 1971, 1972; Lamounette, 1977; Churchill et al., 1978; Keddy and Patriquin, 1978). The high germination yields are not a result exclusively of field planting, for I have obtained similar values in the laboratory after incubating seed batches in low salinity water (70/00). An important consideration may be that the seeds used in the present study were harvested only after they had been released and were presumably fully mature. The time of seed germination has been noted by a number of investigators, and a pattern of germination during the winter and spring, 5--10 months after seed release, exists in widely separated areas (SetcheU, 1929; Miki, 1933; Tutin, 1938; Taylor, 1957; Phillips, 1972; Keddy and Patriquin, 1978; Orth et al., 1981). The present results do not fit this pattern, for most seeds germinated in the autumn, 3--4 months after release. Similar observations were made by Addy (1947) and more recently by Orth (1976). It is n o t known whether the interval between seed release and germination represents a true dormancy or a quiescent period lasting until the onset of favorable conditions. Orth et al. (1981) have suggested that dormancy does n o t occur and that low water temperature (~15°C) is the primary variable initiating germination. The present study supports this view, for water temperature at the time of the 1978 and 1979 November harvests was 10 and 13°C, respectively. In contrast, laboratory studies indicate that low salinity (5--10°/00) is the overriding variable promoting germination (Amsaki, 1950; Phillips, 1972; Lamounette, 1977), and that significant germination may occur in 10%0 seawater even at 25°C (Lamounette, 1977). The relevance of this low salinity effect at Northwest Creek is questionable, however, since salinity values rarely fall below 20%o. The pattern of germination at Northwest Creek indicates that, at any one time, seeds from only one flower crop occur in the sediments (seed release starts in mid~Iune). This may not be true in other areas, for Phillips (1972) and Orth et al. (1981) have noted that seeds may remain viable for at least one year. In addition, I have successfully germinated seeds that were maintained in sediments (in seawater of 30%o) at room temperature for 22 months. When the seeds were transferred to low salinity water (7%0), 55% (n=45) developed into stage 3 seedlings within three weeks. Theoretically, viable seeds from two and perhaps three flowering seasons may occur together in the sediments. All seedlings from the plantings perished by June indicating that recruitm e n t from seeds is a function primarily of seedling survival and not seed germination. The causes for the seedling loss are unknown. The increase in the percentage of empty and decomposing seeds (Table I) reveals that approximately 50% of the seedlings disappeared during the late a u t u m n and winter and that the remainder were lost during the spring. Seedling mortality
28
occurred, therefore, during periods of both active and inactive growth and primarily after they had reached stage 3 in development. Small numbers of healthy wild seedlings were observed in June 1979 and 1980; so potential existed for extended seedling survival in the experimental area. It seems unlikely that the high density of planted seeds or the confinement in containers were deleterious to seedling vigor, as there was little difference in the growth characteristics of wild and planted stage 3 seedlings. Foraging activities and predation may be significant factors, for rasp-like wounds were frequently observed along the axial hypocotyl and plumule base and appeared to initiate a general process of seedling deterioration. The distinction between stage 2 and stage 3 seedlings has n o t been made previously. When seedlings are raised in the laboratory, they exhibit limited hypocotyl elongation and plumule growth commences with the emergence of the cotyledon (Taylor, 1957). This study has shown that conditions in the field can inhibit plumule growth while still allowing extensive hypocotyl and cotyledon elongation. Taylor (1957) collected stage 3 seedlings with axial hypocotyls of varying lengths and noted a direct relationship between length and the depth to which the basal hypocotyl was buried in the sediments. Such a relationship might explain the variation in hypocotyl length found at Northwest Creek, for although seeds were buried initially at a uniform depth, marked differences were observed when the containers were processed. It is tempting to speculate that hypocotyl elongation of stage 2 seedlings functions to push the rudimentary plumule (protected by the cotyledon sheath) to a point near the sediment surface where conditions stimulate the transition to stage 3 development.
REFERENCES A d d y , C.E., 1947. Germination o f eelgrass seed. J. Wildl. Manage., 11 : 279. Arasaki, M., 1950. Studies on the ecology of Z o s t e r a marina and Z o s t e r a nana. Bull. Jpn. Soc. Fish., 16: 70--76. Churchill, A.C. and Riner, M.I., 1978. Anthesis and seed production in Z o s t e r a marina L. from Great South Bay, New York, U.S.A. Aquat. Bot., 4 : 83--93. Churchill, A.C., Cok, A.E. and Riner, M.I., 1978. Stabilization of subtidal sediments by the seagrass Z o s t e r a marina L. New York Sea Grant Report Series. (NYSSP-RS-78-15), 65 pp. De Cock, A.W.A.M., 1980. Flowering, pollination and fruiting in Z o s t e r a marina L. Aquat. Bot., 9: 201--220. De Cock, A.W.A.M., 1981. Development o f the flowering shoot of Z o s t e r a marina L. under controlled conditions in comparison to the development in two different natural habitats in The Netherlands. Aquat. Bot., 10: 99--114. Jacobs, R.P.W.M. and Pierson, E.S., 1981. Phenology of reproductive shoots o f eelgrass, Z o s t e r a marina, at Roscoff (France). Aquat. Bot., 10: 45---60. Keddy, C.J. and Patriquin, D.G., 1978. An annual form o f eelgrass in Nova Scotia. Aquat. Bot., 5: 163--170. Lamounette, R., 1977. A study of the germination and viability of Z o s t e r a marina L. seeds. M.S. Dissertation, Adelphi Univ., Garden City, NY, 41 pp.
29 Miki, S., 1933. On the seagrasses in Japan (I) Zostera and PhyUospadix, with special reference to morphological and ecological characters. Bot. Mag., 47: 842--862. Orth, R., 1976. The demise and recovery of eelgrass, Zostera marina, in the Chesapeake Bay, Virginia. Aquat. Bot., 2: 141--159. Orth, R.J., Moore, K.A., Roberts, M.H. and Silberhorn, G.M., 1981. The biology and propagation of eelgrass, Zostera marina, in the Chesapeake Bay, Virginia. Final Report (Contract No. R 805953), U.S. Environ. Prot. Agency, Washington, DC, 227 pp. Phillips, R.C., 1971. Seed germination in Zostera marina L. Am. J. Bot., 58: 459. Phillips, R.C., 1972. Ecological life history of Zostera marina L. (eelgrass) in Puget Sound, Washington. Ph.D. Dissertation, University of Washington, Seattle, WA, 154 PP. Setchell, W.A., 1929. Morphological and phenologicai notes on Zostera marina L. Univ. Calif. Publ. Bot., 14 : 389--452. Taylor, A.R.A., 1957. Studies of the development of Zostera marina L. II. Germination and seedling development. Can. J. Bot., 35: 681--695. Tutin, T.G., 1938. The autecology of Zostera marina L. in relation to its wasting disease. New Phytol., 37 : 50--71.