Influence of Light Quality and Quantity on Heterophylly
in the Aquatic Plant Nymphaea odorata subsp. tuberosa (Nymphaeaceae)
Shelley A. Etnier, Philip J. Villani, and Travis J. Ryan
Northeastern Naturalist, Volume 24, Issue 2 (2017): 152–164
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22001177 NORTHEASTERN NATURALIST 2V4(o2l). :2145,2 N–1o6. 42
Influence of Light Quality and Quantity on Heter ophylly
in the Aquatic Plant Nymphaea odorata subsp. tuberosa
(Nymphaeaceae)
Shelley A. Etnier1,*, Philip J. Villani1, and Travis J. Ryan1
Abstract - Heterophylly, the production of different leaf forms on the same plant, is a widespread
phenomenon in terrestrial and aquatic plants and provides an opportunity to study
how sessile organisms sense and respond to changes in environmental factors. Nymphaea
odorata subsp. tuberosa (American White Water Lily) produces 2 distinct leaf forms: a
floating surface leaf and an aerial form in which the lamina is held above the water. Previous
research suggests that changes in the light environment may be a critical determinant of heterophylly
in Nymphaea. In this study, we tested the hypothesis that changes in light quantity
and light quality stimulate the production of aerial leaf forms in water lilies. Specifically,
shade cloth was used to reduce light intensity (quantity), and varying plant densities were
used to increase leaf cover (affecting light quality) in artificial ponds. Aerial leaf production
was not stimulated by reduction in light quantity alone but was when leaf cover exceeded
30–40%. We suggest that as the surface of a pond becomes covered with a canopy of leaves,
American White Water Lily responds with the production of aerial leaves that rise above
the surface of the water to gain access to light. Interestingly, water lilies exhibit an atypical
shade response in that aerial leaves have short, thick petioles that allow them to rise above
the surface of the water, rather than displaying the elongated phenotype associated with
etiolation, which is the typical shade response of other floweri ng plants.
Introduction
Heterophylly, or the production of multiple leaf types on a single plant, is a
widespread phenomenon in both terrestrial and aquatic plants. In aquatic environments,
heterophylly typically occurs when a plant grows in 2 physically dissimilar
environments (e.g., terrestrial and aquatic) that present dramatically different physical
and physiological challenges to different parts of the plant. Numerous studies
have investigated the causal factors leading to heterophylly in aquatic plants. These
factors include red/far-red ratios, photoperiod, light intensity, water depth, levels
of dO2/dCO2, osmotic stress, temperature, and desiccation (Anderson 1982, Bristow
1969, Deschamp and Cooke 1984, Goliber and Feldman 1989, Johnson 1967,
Kuwabara et al. 2003; for reviews, see Franklin 2008, Wells and Pigliucci 2000).
In some cases, heterophylly results from a combination of environmental factors,
rather than any single cue (Franklin 2008, Vandenbussche et al. 2005).
Nymphaea odorata Ait. subsp. tuberosa (Paine) Wiersema & Hellquist (American
White Water Lily) exhibits marked heterophylly. Plants can produce 2 distinct
mature leaf forms (Sculthorpe 1967) that differ in their position relative to the
1Department of Biological Sciences, Butler University, Indianapolis, IN 46208. *Corresponding
author - setnier@butler.edu.
Manuscript Editor: Thomas Philbrick
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surface of the water: the most common leaf type floats on the surface of the water,
whereas the lamina of the alternative leaf type is held above the surface of the water.
These 2 forms can exist simultaneously on a single plant, but aerial forms are
only produced under certain conditions (Villani and Etnier 2008). In the situation
where aerial leaves are produced, surface leaves predominate at the beginning (May
and June in central Indiana) and end (September) of the growing season, and aerial
leaves predominate during the middle (Villani and Etnier 2008). Interestingly, these
different leaf forms do not reflect the growth continuum of a single leaf. Rather, an
individual mature leaf has either a surface lamina or an aerial lamina (Villani and
Etnier 2008). There are minor differences in these 2 leaf forms (Sculthorpe 1967).
The surface area of aerial leaves tends to be larger (Villani and Etnier 2008), and
our personal observations suggest that the aerial lamina is more structurally robust.
Anatomically, stomata are only located on the upper surface of the lamina regardless
of its position relative to the water (Sculthorpe 1967).
While the mature laminae are relatively similar, the petioles of aerial and surface
leaves can be differentiated morphologically early in development. Aerial leaves
have a significantly larger petiole diameter as they emerge from the sediment at the
bottom of the pond (Villani and Etnier 2008). This early morphological differentiation
suggests that the site of stimulus perception is underwater at the rhizome tip
and/or in developing leaves, but it is unclear which factor(s) are responsible for
triggering different developmental pathways during leaf formation.
The seasonal changes in the proportion of surface and aerial leaves suggest that
the factor(s) influencing their production may also vary seasonally. A previous field
study in central Indiana indicates that temperature, dO2, and dCO2, while highly
variable from day to day, oscillate above and below a central mean across the growing
season, suggesting that they are not associated with the observed heterophyllic
variation; additionally, ammonia concentration and water pH remained nearly constant
across the growing season, also eliminating them as possible signals (Etnier
and Villani 2008). While the specific factors influencing the production of surface
and aerial leaves in Nymphaea are unknown, crowding (and thus competition for
light) may play an important role (Sculthorpe 1967, Villani and Etnier 2008). As
noted above, leaf production, and hence the competition for light, is strongly seasonal
in central Indiana ponds. A dense canopy of leaves, whether in a terrestrial
or aquatic system, will affect both light quantity and quality (red/far-red ratio) as
specific wavelengths are absorbed (Salles et al. 1995, Smith and Whitelam 1997,
Leyser and Day 2003). Both of these factors have been shown to influence heterophylly
in diverse aquatic species, including Proserpinaca (marsh mermaid weed),
Porphyra (a red algae), and Marsilea (an aquatic fern) (Goliber and Feldman 1989,
Leyser and Day 2003, Lin and Yang 1999, Monroe and Poore 2005, Salles et al.
1995, Schmidt and Millington 1968, Smith and Whitelam 1997). In a prior field experiment,
we observed that controlled maintenance of open water at the surface of
a pond completely repressed the appearance of aerial leaf forms in Nymphaea (Villani
and Etnier 2008). In contrast, aerial leaves appeared if the surface leaves were
allowed to form a dense canopy on the water’s surface (Villani and Etnier 2008),
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thus limiting light penetration to the rhizome and developing leaves. We hypothesized
that the open water permits more natural light irradiance to penetrate to the
underlying rhizomes, which signals for the production of surface leaves, whereas
a canopy of surface leaves alters the underwater light environment and stimulates
the production of aerial leaves.
In this study, we tested the hypothesis that changes in the underwater light environment
over the growing season stimulate the production of aerial leaf forms in
water lilies. Specifically, we were interested in the impact of the competition for
light and the resultant changes in underwater light intensity and light quality that
might impact individual leaf development. Results from 2 field seasons provided
information on the relative impact of light intensity and light quality on heterophyllic
growth patterns American White Water Lily.
Materials and Methods
Plant material
Plants were collected in June 2007 from a population of American White Water
Lily growing in a 2000-m2 pond at Eagle Creek Park (39º52'3"N, 86º18'1"W) in
Indianapolis, IN. Tuberous rhizomes were collected and established in two 1000-L
water-filled polyethylene tanks in an unshaded area on Butler University’s campus
during the summer of 2007. In May 2008, pieces of rhizome ~10 cm in length were
removed from established plants, transferred to 3-gallon plastic pots, and covered
with ~15 cm of potting soil. Pea-sized gravel was placed over the soil to help maintain
the position of the rhizome in the pot. Pots were submerged and overwintered
in the same large tanks. These plants were used during both field seasons, as described
below.
Field season 1 (summer 2008)
We used a randomized block design to test whether light intensity and/or light
quality serves as the signal for the appearance of aerial leaves. We transferred potted
plants to 375-L Rubbermaid® stock tanks that served as artificial ponds. The
tanks had a surface area of ~0.8 m2 (oval tanks approximately 1.2 m x 0.8 m), with
a water depth of ~0.5 m. Plant density was the main experimental variable, with 1,
2, or 4 plants (1 per pot) per tank. Increased plant density should increase biotic
shading, thus impacting both the quality (red/far-red ratio) and quantity of light
available to the plants. The second variable was the presence or absence of abiotic
shading, which should alter only the intensity, but not the quality, of light. We used
50% neutral shade cloth (as rated by the manufacturer) mounted 24 cm above the
surface of the water on structures built from 2.5-cm PVC pipe to shade the tanks.
The combination of crowding and shading resulted in 6 treatment combinations
that were replicated 4 times each for a total of 24 experimental units. We positioned
tanks in a grid with 4 rows and 6 columns in an open field with ~1 m between each
tank. Each row consisted of a complete set of treatment combinations, with the
exact position of each treatment determined randomly. This approach was used
to insure that all tanks received similar light exposure over the course of a day.
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We treated each tank with algaecide (Green Clean Granular Algaecide, BioSafe
Systems, LLC, East Hartford, CT) on May 16 and May 22 to limit the competition
for light from other biotic sources.
Field season 2 (summer 2009)
During this field season, we more closely examined the influence of light quality
on water lily heterophylly, again using a randomized block design. In field season 1,
the use of shade cloth was associated with increased leaf cover without the production
of aerial leaves (see Results section for statistical analysis). Thus, during field
season 2, all stock tanks were covered with 50% shade cloth to enhance leaf cover.
We did not control for the presence or absence of algae in field season 2, as our goal
was to increase biotic shading. For this growing season, plant density was the main
variable; we randomly assigned stock tanks either 2 or 5 plants. The second variable
was the presence or absence of other competing plants. For the competing plants
treatment, we seeded tanks with ~1 L of mixed aquatic plants that were added after
surface leaves appeared in early June. These plants were collected from the same
natural pond from which we obtained the original lily plants. This 2 x 2 experimental
design yielded 4 treatment combinations that were replicated 4 times for a total
of 16 experimental units. As in field season 1, we arranged the tanks in a grid, with
each row containing all treatment combinations in a randomized distribution.
Leaf counts and cover analysis
Data collection began as soon as water lily plants started producing leaves in
early to mid-June. We counted the number of aerial and surface leaves twice a week
throughout the growing season, from about the start of June through the end of September.
Surface leaves were counted only if they were >5 cm in diameter (smaller
leaves were present, but were relatively rare and thus not counted). Every 2 weeks,
we estimated how much of the water’s surface was covered by surface leaves
(proportion leaf cover). This measure included all water lily leaves, regardless of
size, but did not factor in leaves from other aquatic plants. To estimate leaf cover,
we constructed a rectangular frame made of PVC pipe measuring 120 cm x 80 cm.
This frame was subdivided into an 8 x 12 grid of square sections, delineated with
string, with each square measuring 10 cm x 10 cm. We placed the grid over each
tank and coded each square on a scale of 0–4 depending on the density of leaf cover
(numbers corresponding to 0%, 25%, 50%, 75%, or 100% cover, respectively).
Note that this measure included all surface leaves regardless of size. Because the
tanks were oval and our grid was rectangular, 32 of 96 squares overlapped the edges
of the tanks and could have a maximum of only 50% cover (and thus a maximum
score of 2). After coding all of the squares, we added these numbers together and
divided by the total possible number (352 when corrected for the partial squares) to
estimate the proportion of the water surface covered with leaves.
Light measurements
We measured light intensity (photosynthetically active radiation [PAR]) and
quality of light (red/far-red ratio) weekly during the growing season (early June
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through mid-September) between 12:00 and 13:00 hours. Light intensity (μmol
m-2 sec-1) was measured with a field photometer (Quantum Scalar Laboratory PAR
Irradiance Sensor, Biospherical Instruments, Inc., San Diego, CA), while the red/
far-red ratio was measured with a red/far-red light meter (Fieldscout, Spectrum
Technologies, Inc., Elysburg, PA). PAR was first measured in the open air to determine
overall light intensity, which may vary both seasonally and daily. We then
measured both light intensity and light quality in the tanks themselves at the depth
of a developing rhizome, ~40 cm underwater. We collected PAR measurements
in open, sunlit water, to determine the effects on light parameters of water alone,
as well as under the shade cloth, and collected red/far-red measurements in open
water, in water under the shade cloth, and also in the shade under a single leaf to
determine how the ratio was affected by light passing through water, shade cloth,
and biotic materials. Single PAR measurements were recorded, whereas red/far-red
measurements were taken 3 times and averaged to provide a value for daily light
quality. For consistency, recorded values reflect measurements taken in tanks with
only 2 plants, although the actual number of plants should not have an impact on
the measurements.
Data analysis
In field season 1, no aerial leaves were produced regardless of the treatment.
For the data from field season 1, we used 2-way analysis of variance (ANOVA)
with proportion of the water surface covered with leaves (hereafter referred to as
“leaf cover”) as a response variable to test the effects of lily density and the presence
of shade cloth. For field season 2, we used 2-way ANOVA with leaf cover as
a response variable to test the effects of lily density and the presence of competing
surface plants. Maximum leaf cover was used in all data analyses because it
was under this condition that the difference in light availability to the developing
leaves was at its most extreme. In field season 1, that date was approximately 15
July for all treatments, while it was slightly more variable during field season 2.
We then used a 1-way ANOVA to determine whether leaf cover differed in tanks
that produced one or more aerial leaves from those that did not. Finally, we used
simple linear regression to determine whether the number of mature surface leaves
affected the surface area covered. In all analyses, we arcsine-transformed leaf cover
and inverse-transformed the number of mature surface leaves to meet the assumptions
of parametric tests.
Results
Experimental variables
As expected, treatment of American White Water Lily plants with 50% shade
cloth reduced overall light intensity (Table 1), although daily light-intensity data
differed considerably among dates due to variable cloud cover (Fig. 1). For example,
on 6 June 2008 in an unshaded tank, the mean underwater PAR was 581.25
μmol m-2 sec-1, while the value was 277 μmol m-2 sec-1 in a shaded tank, representing
a 48% reduction in light intensity due to the shade cloth. Similar reductions in
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light intensity were obtained on an overcast day (6 July), with a 52% reduction
in light intensity under the shade cloth. This same general trend is seen when looking
at the mean values for field season 1, where the presence of shade cloth reduced
the light intensity by 46% (Table 1). Changes in underwater light intensity were also
evident across the growing season (Fig. 1). In general, underwater light intensity
decreased in all treatments from the start of the monitoring period until late July.
Underwater light intensity then remained relatively constant for ~1 month (August)
and subsequently began to rise again until the end of the monitoring period
Table 1. A comparison of light parameters between field seasons 1 and 2. Light intensity was measured
weekly from June to September (season 1 n =11; season 2 n =14) in 4 replicate tanks containing 2
plants per tank. Light intensity, for both shade and no shade, was measured underwater at the level of
the rhizome; shade was generated using a 50%-reduction shade cloth. Light intensity was measured
in PAR (μmol m-2 sec-1). Standard deviation is in parentheses.
Season Statistic Open air Water and shade Water and no shade
1 Mean (SD) 2014 (545) 190 (80) 415 (161)
Max. 2569 280 679
Min. 890 162 200
2 Mean (SD) 2098 (595) 216 (118)
Max. 3468 494
Min. 772 77
Figure 1. Measures of seasonal light intensity under a lily pad leaf canopy during field season
1. Treatments included 4, 2, and 1 plants per tank. Treatments were either in full sun or
under 50% shade cloth (+ shade).
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(September) (Fig. 1). Decreasing light intensities (Fig. 1) roughly coincided with
higher leaf cover (Fig. 2). In general, these same general trends were seen in field
season 2 (in which all tanks were covered in shade cloth to promote leaf production).
Average seasonal light intensity was similar in both seasons, as was the
average light intensity under the shade cloth (Table 1).
The red/far-red ratios measured underwater in the open sunlight water and under
the shade cloth were similar to typical values measured in the air (Fig. 3). In contrast,
the ratio was reduced under the canopy of surface leaves and was comparable
to shade values recorded under a leaf canopy in both terrestrial (Franklin 2008) and
aquatic systems (Salles et al. 1995, Vandenbussche et al. 2005). Recorded values
were also similar to those measured in a natural field setting. For example, on 24
June 2008 in a pond at Eagle Creek Park, Indianapolis, IN (the original source of
experimental plants), the red/far-red ratio below surface leaves was 0.29 ± 0.08
(n = 3), while the red/far-red ratio in the open water was 1.30 ± 0.005 (n = 3).
Plant growth
During both field seasons, the timing and patterns of leaf production were generally
comparable to previous observations from a natural pond (Villani and Etnier
Figure 2. Mean percent cover of surface water with lily pad leaves among treatments with
4, 2, or 1 plants per tank during field season 1. Treatments were either in full sun or under
50% shade cloth (+ shade). Standard deviations were omitted for visual clarity.
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2008). The plants appeared healthy, with all treatments producing flowers from
approximately early June until the end of July. The one observed difference was that
overall leaf production and leaf cover in the tanks did not reach the same levels seen
in a natural setting, where leaf cover approaches 100% late in the growing season
(Villani and Etnier 2008). In all treatments, the number of surface leaves increased
rapidly early in the growing season, remained fairly stable during the middle of
the growing season, and gradually (compared to earlier in the growing season) decreased
toward the end of the growing season. In field season 1, we found that both
the density of plants and the presence of shade cloth had a significant effect on the
maximum percentage of area covered by large surface leaves (2-way ANOVA with
percentages arcsine-transformed: density F2,18 = 80.59, P < 0.0001; shade F1,18 =
5.96, P = 0.0252; interaction F2,18 = 1.46, P = 0.2579). In field season 2, maximum
leaf cover occurred in tanks with 5 plants, with coverage exceeding 45% for brief
periods (Fig. 4).
Figure 3. Underwater measures of red/far-red ratios in experimental tanks. All measurements
were taken ~40 cm below the surface of the water. Values under the leaf were taken
at approximately the same depth, at the level of the rhizome, and thus are a measure of the
light after passing through the leaf. Shaded bar represents treatments with 50% shade cloth.
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Aerial leaf production
No aerial leaves were observed in any of the tanks during field season 1, regardless
of plant density or the presence of shade cloth. In field season 2, aerial leaves
were produced in 7 of the 16 tanks. Aerial leaf production was not random; aerial
leaves were more often found in high-density (5 plants) tanks (6 of 8) than in lowdensity
(2 plants) tanks (1 of 8) (goodness-of-fit test: χ2 = 6.34, df = 1, P = 0.01174).
Regardless of treatment combination, aerial leaves were produced in tanks where
the mean maximum leaf cover (%) was significantly higher (with aerial leaves =
42.7%, S.D. = 2.3; no aerial leaves 21.9%, S.D. = 2.4; t-test assuming equal variances:
t = 12.6572, df = 14, P < 0.0001). Accordingly, surface-area coverage was
strongly influenced by the density of plants in the tanks (F1,12 = 11.1097, P = 0.006),
with high-density tanks having 60% more coverage than low-density tanks (highdensity
mean = 40.2% [SD = 10.61], low-density mean = 24.7% [SD = 8.58]). The
presence of other plants had no significant effect on aerial leaf production (F1, 12 =
0.3557, P = 0.5620), nor was the interaction between density and the presence of
other plants significant (F1,12 = 2.5786, P = 0.1343).
There was a clear and not surprising relationship between the number of surface
leaves and the percentage of leaf cover (Pearson’s r = 0.8811, P < 0.0001; Fig. 5),
and there appears to be a threshold value of leaf cover required to induce aerial leaf
Figure 4. Mean percent cover of surface water with lily pad leaves among treatments with
5 or 2 plants per tank during field season 2. Treatments were either with (+ competition) or
without competing algae. Standard deviations were omitted for visual clarity.
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production. Aerial leaves were produced only in tanks with >40% surface coverage
(Figs. 5, 6). As we noted above, no aerial leaves were produced during field season
1, and only 1 of the 24 tanks had a surface area coverage of >40% for more than 1
Figure 5. Relationship between the mean number of large surface leaves and the maximum
percent leaf cover during field season 2. Filled symbols represent treatments where no aerial
leaves were observed; open symbols represent treatments were aerial leaves were recorded.
Figure 6. Maximum number of aerial leaves produced in a given treatment relative to the
maximum percent leaf cover during field season 2.
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observation period. In the second field season, 6 of the 7 tanks that produced aerial
leaves showed >40% leaf coverage for more than 1 month.
Discussion
In this study, we tested the hypothesis that changes in the underwater light environment
stimulate the production of aerial leaves in American White Water Lily.
The results from field season 1 strongly suggest that changes in light intensity alone
are not a sufficient cue, as a 50% reduction in light intensity (by way of shade cloth)
was not enough to induce the appearance of aerial leaves. Somewhat surprisingly,
the presence of the shade cloth served to modestly increase leaf production and
subsequent leaf-coverage in tanks. In contrast, the results from field season 2 suggest
that changes in light quality (red/far-red ratio) are a more reliable signal for the
heterophyllic response in water lilies.
We suggest that water lilies use changes in the red/far-red ratio to monitor the
competition for light, such that aerial leaves are produced when leaf cover is high,
but surface leaves are produced when leaf cover is low, which occurs at both the
beginning and the end of the growing season (Villani and Etnier 2008). While there
may be other density-dependent environmental cues involved, the changes in leaf
cover and subsequent changes in the red/far-red ratio correspond well with the patterns
we observed in both natural and artificial settings. As light passes through a
leaf, the red/far-red ratio decreases due to differential absorption of red and far-red
light. When leaf cover is low, unfiltered light will penetrate the water to the developing
rhizome during part or all of the day, as the sun moves across the sky. In
contrast, as leaf cover increases, the developing rhizome will be exposed to longer
periods of time in which the red/far-red ratio is decreased. As the surface cover
of shading plants increases, more and more of the underwater environment will
become red-light-depleted as those wavelengths are absorbed by the leaf canopy.
Thus, as the surface area of a pond becomes covered with a canopy of leaves, water
lilies produce aerial leaves that rise above the surface of the water. In field season
2, aerial leaves were produced in tanks in which there was more biotic shading. We
suggest that ~40% cover by the water lily surface leaves (Fig. 4) may represent the
minimal signal necessary to produce aerial leaves. The absence of aerial leaves during
field season 1, despite briefly reaching higher surface leaf cover, suggests that
there may be a temporal threshold which would be worthy of further investigation.
The aerial leaves, which rise above the existing surface leaf canopy, may allow
water lilies to increase their photosynthetic capabilities when there is a canopy of
leaves on the surface of the water. Similarly, the water lily could respond in the
same manner if there was a heavy layer of algae or other competing plants. As
the surface leaves naturally decay over the course of the growing season, the red/
far-red ratios will be less affected, and then aerial leaves will no longer be produced.
Based on this hypothesis, aerial leaves will be produced only when there
is sufficient density of surface leaf cover, suggesting that they may be unnecessarily
costly to build or too expensive to maintain when light quality is not limiting.
This strategy uses a light signal that is sensitive to any type of biotic surface cover,
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assuming it impacts the light quality in a significant manner. The exact impact is
going to depend on the properties of the shading leaf in question, as a water lily leaf
will potentially have greater impact than filamentous algae. Thus, this mechanism
could initiate a response by lily plants to both intraspecific and interspecific competition,
as well as self-shading.
The binary response of heterophylly in water lilies is determined early in leaf
development in response to signals that occur well before an actual leaf is visible
as a discrete structure on the plant (Villani and Etnier 2008). Thus, there is a time
lag associated with the signal being received and the ultimate response of the plant,
as seen in the delayed appearance of aerial leaves as leaf cover increases during the
growing season (P.J. Villani and S.A. Etnier, pers. observ.). As there are potential
costs to both types of leaves (e.g., changes in herbivory patterns [Kouki 1993], desiccation,
etc.), we predict that this strategy allows lily plants to adjust leaf-growth
patterns to maximize photosynthetic capabilities, while also potentially minimizing
unwanted costs due to growing a poorly adapted leaf form.
Shade-induced decreases in the red/far-red ratio are commonly used by plants
as an indicator of shading and the competition for light (Goliber and Feldman 1989,
Leyser and Day 2003, Lin and Yang 1999, Schmidt and Milington 1968). Typically,
plants grow long and spindly (etiolation) with increased lamina area when light is
limited (Holbrook et al. 1991, Leyser and Day 2003, Smith 1982). In contrast, water
lilies produce a robust petiole that allows the leaf to rise above the water’s surface
due to increased cross-sectional area (Etnier and Villani 2007), but petiole length
and lamina area do not increase (Villani and Etnier 2008). In some types of plants, increases
in far-red light also inhibit leaf expansion (Taylor and Davies 1988). As aerial
leaves grow up through the water column, the leaf lamina of water lilies stays tightly
wrapped in a cone-shaped structure, expanding only when the aerial leaf pushes
above the water’s surface (S.A. Etnier and P.J. Villani, unpubl. data). This coneshaped
lamina may make it easier for the growing leaf to actually penetrate through
the surface canopy and reach above the level of the water. As typified by shade-avoidance
mechanisms in most flowering plants (Franklin 2008), this suite of responses
in water lilies is likely phytochrome-mediated, allowing them to respond to the
competition for light. Importantly, the overall response in water lily plants is quite
different from the typical shade response that has been well-studied in both aquatic
and terrestrial systems (Franklin 2008, Vandenbussche et al. 2005) and thus presents
interesting questions about heterophylly and phytochrome signaling in water lilies.
Acknowledgments
The authors thank C. Bowman for his invaluable assistance in data and chigger collection,
Dr. E. Gerecke for critical review of the manuscript, and The Butler Institute for
Research and Scholarship at Butler University for funding of this project.
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