EFFECTS OF SUPPLEMENTARY NITROGEN ON BIOMASS AND
NITROGEN ACCUMULATION IN THE SUNDEW
DROSERA CAPENSIS L.
by
Bri an M. Bruzzese

THESIS SUBMITTED IN PARTIAL FULFILMENT OF
THE REQUIREMENTS FOR A DEGREE OF
BACHELOR OF SCIENCE
In

PLANT SCIENCE

© Brian M. Bruzzese
THE UNIVERSITY OF NORTHERN BRITISH COLUMBIA
December, 2002

All rights reserved. This work may not be reproduced
in whole or in part, by photocopy or
by other means, without the permission of the author.

QK

495

.076

878

2002
c.2

UNIVERSITY OF NORTHERN
BRITISH COLUMBIA
UBRARY
Prfnce George, BC

TABLE OF CONTENTS
1. List of tables ........................................ .............................. iv
2. List of figures ...................................................................... v
3. Acknowledgements .............................................................. vi
2. Abstract ......... .. ........ . ..... . ... .. .. ........................................... 1
3. Introducticn ........................................................................ 2
3.1 Insectivorous plant perspective ............................. 2
3.2 Rationale . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .... 4
3.3 Objectives...... .............................................. 4
4. Materials and Methods ........................................................... 5
4.1 Plant material, Media, and Lighting ........................ 5
4.2 Nutrients ........................................................ 6
4.3 Sample Design ................................................ 7
4.4 Statistical Analysis ............................................ 9
5. Results ................. . ... ......................................................... 10
5.1 Cultivation and Morphology ........................ ......... 10
5.2 Biomass ...... ................................................... 13
5.3 Nitrogen and Carbon Accumulation ........................ 14
6. Discussion ......................................................................... 15
7. References .......................................................................... 23

iii

LIST OF TABLES
Table 1. NJ4Cl fertilizer concentrations .................................................. 7
Table 2. Average initial and final leaf totals and total flower stalks ................... 10
Table 3. Average leaf, root, and total biomass in shoot and root (dry weight) ....... .13
Table 4. Average total nitrogen and carbon concentrations in shoots and roots ..... .14

iv

LIST OF FIGURES
Figure 1. Floral light shelf and random plant setup ...................................... 6
Figures 2-3. Spray collar methodology .................................................... 8
Figures 4-15. Initial and final individual plant morphologies ..................... ..... 11
Figures 16-17. Initial and final plant morphologies and randomization patterns ..... 12

v

1. Acknowledgements
I would like thank Dr. Hugues Massicotte for the supervision of my undergraduate thesis
and all the experience gained over the last 8 months. Many thanks also to Steve Storch
and John Orlowski (EFL technicians) for the invaluable help and initiating me to the
growth of insectivorous plants and all the technical support during the experiment, and
Alan Tyee for nutrient analysis. I am very grateful to Michael van Adrichem for the van
Adrichem Undergraduate Summer Research Bursary that financially supported this
project. Finally, I am very grateful to Dr. Arthur Fredeen for accepting to be the second
reviewer of my undergraduate thesis.

VI

4. ABSTRACT
The effect of nutrient supplementation on biomass and nitrogen accumulation in
the insectivorous plant Drosera capensis L. was studied under laboratory conditions
using supplementary feeding of ammonium chloride (Nr4CI) to plant leaf surfaces. The
nitrogen solution used was based on a standard concentration of 157.6 mM N}4CI.

Drosera capensis did not differ significantly in biomass or nitrogen accumulation from
concentrations of 50%, 75 %, 100%, 150%, or 200% of nutrient fertilizer when applied to
the foliage. Reproductive structures were observed in higher numbers in those plants
treated with higher concentrations of Nr4Cl but no relationship was found between
reproductive structures and biomass or nitrogen concentrations. The results of this
experiment potentially indicate that D. capensis can grow and initiate reproductive
structures without additional nutrients above which already exist in the soil mixture. The
results also suggest that D. capensis does not readily absorb NH4CI through its leaves.

Keywords: Drosera capensis, nutrient supplementation, biomass, nitrogen accumulation.

5. INTRODUCTION
5.1 Insectivorous plant perspective
The kingdom Plalltae consists of fi ve phyla. Anthophyta represents the largest
and most diverse phylum , containing approximately 235,000 species (Raven et al. 1999).
A small group of plants within this phylum, consisting of approximately 538 species
occurring in 18 genera and 8 families worldwide, are known as insectivorous plants
(Givnish 1989). Insectivorous plants are also referred as carnivorous plants, however, for
the purpose of this thesis, the term insectivorous will be used and the term carnivorous
will only be referred to in the context of cited literature. Insectivorous plants have been
known to grow in nutrient poor environments where readily available nutrients are
limiting. It has often been proposed that these plants fulfill their basic nutrient
requirements by insect prey capture. This benefit is generally believed to be an effect of
mineral nutrient uptake from prey, mainly nitrogen and phosphorus (Hanslin and
Karlsson 1995).
However, some researchers question the reliability these plants have on the
capture of prey for fulfilling such nutrient deficits (Chapin and Pastor 1999).
Insectivorous plants have adapted to their unfavorable conditions by growing slowly.
These plants function as a normal plant would but do not require a high supply rate of
mineral nutrients from the soil due to the ability to store and re-utilize nutrients
efficiently (Adamec 2002). Roots of most insectivorous plants are weakly developed but
have the ability to regenerate quickly and withstand anoxic conditions (Adamec 1997).
The capacity for nutrient uptake by roots is limited, and compensated by nutrient uptake
from prey capture in modified leaves or traps (Adamec 1997). Nutrient uptake by

2

insectivorous plant leaves are either signaled by prey capture or nutritive stimulation and
generally are aided by digestive enzymes then reabsorbed through specialized stalked
and/or sessile glands (Slack 2000).
By definition, an insectivorous plant must meet two requirements . First, it must
have the ability to absorb nutrients from captured prey and gain some benefit of fitness in
terms of growth or reproduction. Second, the plant must have some obvious adaptation
or resource allocation whose primary result is the active attraction, capture, and digestion
of prey (Givnish 1989). Consequently, this definition implies that these plants will
benefit nutritionally from insectivory.
Although there have been studies conducted on the effects of supplementary
feeding in insectivorous plants, both positive and negative results have been documented.
Adamec (2002) showed an increase in plant growth and nutrient accumulation in Drosera

capilleris Poir., D. aliciae Hamet., and D. spathulata Labill. through supplemental foliar
feeding using a nutrient solution consisting of Nf4N0 3 , KH 2P04. MgS0 4, CaC1 2 and
FeCi). Similarly, in a study of supplementary feeding on three Pinguicula species in the
subarctic, Thoren and Karlsson (1998) found the plants exhibited an increase in growth
when fed extra nutrients. On the contrary, Stewart and Nilsen (1992) found that nutrient
supplementation decreased vegetative growth in D. rotundifolia L., suggesting this
species had optimally adapted to low nutrient environments and any nutrient
enhancement actually produced a toxic effect. To complicate the issue further, Chapin
and Pastor (1995) found that Sarracenia purpurea L. did not increase in biomass from
nutrient supplementation but had significant increases in nutrient concentrations within
plant tissues.

......
3

5.2 Rationale
We selected Drosera capensis as a model plant for this thesis. Drosera capensis
is a native insectivorous plant of the Cape peninsula in South Afiica, and is one of the
easiest Drosera species to cultivate under greenhouse conditions (D' Amato 1998). This
particular genus of insectivorous plants has shown positive, negative, and neutral effects
to nutiient supplementation on biomass and nitrogen accumulation in previous
expeiiments (Adamec 2002; Hanslin and Karlsson 1995; Karlsson and Pate 1992;
Stewart and Nilsen 1991). The diversity of effects from nutiient supplementation found
in vaiious Drosera species provides an opportunity to explore these differences within
the genus Drosera and determine how the effects of nutiient supplementation on another

Drosera species compares to the other species studied in this genus.

5.3 Objective
Our objectives of this study were 1) to successfully grow D. capensis in media
under laboratory conditions and 2) determine if nitrogen supplementation treatments to
the foliage under different concentrations has any significant effect on plant biomass and
total nitrogen accumulation within the tissues of D. capensis.

4

6. MATERIALS AND METHODS
6.1 Plant Material, Media. and Lighting

Drosera capensis is a herbaceous perennial insectivore from the plant family
Droseraceae. The leaves fotm around a center stalk with total length of petiole and leaf
extending between 3 and 6 inches. The leaf develops two types of glands found on the
adaxial side of the leaf. The first gland is a stalked gland used in the capture of prey,
release of digestive enzymes, and most of the digestible material. The second gland is a
sessile gland found on the leaf surface, and together with microscopic hairs, is used to a
small degree for aiding in the re-absorption of digestible materials (Slack 2000; Givnish

1989).
Forty- five D. capensis specimens from an identical seed source were grown in 4"
square containers using a soil mixture consisting of 60% peat (Premier Sphagnum peat
moss), 25% #4 washed sand (Kodies sand pit, Prince George, BC), 10% 2-3mm Forestry
sand (Target products, Abbotsford, BC) and 5% perlite (Supreme perlite, Portland,
Oregon). Ambient nitrogen in the peat was 2.5 ppm or 0.00025 % (analysis provided by
Premier Horticulture dept.).
From 45 plantlets, 36 of the most homogeneous appearing plantlets were selected
for the experiment. Plantlets for all treatments were of relative equal size averaging six
leaves and a length of 4cm per leaf. At that stage, 3 plantlets were dried and measured for
initial biomass; the average dry weight of a plantlet was 0.024g. Plant containers were
placed on germination trays covered with clear raised dome plastic lids. Lighting was
provided in a floral light shelf system with a photoperiod of 16 h light and 8 h dark (fig.
2

1

1). Light intensity averaged 41.32 J.tmol m- s- throughout the lighting system.

5

Temperature ranged between 20-25°C for the duration of the experiment. Plants were
watered with reverse osmosis (RO) water to eliminate additional nutrient effects from tap
water.

Figure 1. Floral light shelf with random arrangement of treatments in plants.

6.2 Nutrients
We used Chapin's nitrogen solution, based on an analysis of collected houseflies
that showed nitrogen to be approximately 9% by mass. A solution of 157 .6mM of NI4Cl
was created in an attempt to simulate this percentage into a working form. This solution
was used in a previous supplement feeding experiment on Sarracenia purpurea (Chapin
and Pastor 1995).

6

Table 1 illustrates treatment number and application concentration for fertilization
of D. capensis leaves based on Chapin ' s standard concentration of Nf4CI (157.6mM =
8.4 mg/L).
Table 1. Fertilization concentrations (Nf4CI) for nutrient applications to D. capensis.
1
2
3
4
5
6

Treatment
Control
50% of standard
75% of standard
100% standard
150% of standard
200% of standard

Concentration
Omg/L
4.2 mg/L
6.2 mg/L
8.4 mg/L
12.6 mg/L
16.8 mg/L

6.3 Sample Design and Fertilization Regime
Thirty-six plants were divided into 6 treatment groups, each consisting of 6 plants
and numbered accordingly. Plants were then set up in a random block design and moved
on a biweekly rotation (mid rows to outer rows) to eliminate edge effects.
Calibrated spray bottles were used to apply nutrient fertilizer to the foliage of the
plants. Each bottle dispensed approximately 0.82 ml of liquid per spray. Through
preliminary trials, it was found that approximately 50% of the spray would stick to the
leaves, and 2 sprays per plant (a total of 0.82 ml hitting the leaf surface) during each
application period would sufficiently cover plant leaf surfaces. Plants were tilted at a 45°
angle and sprayed from a distance of about 10 em. Filter paper stem collars were placed
at the base of each plant, exposing only the leaves during fertilization, to eliminate
fertilizer from entering the media (figs. 2-3).

Plant trays were covered with a plastic

dome lid to prevent flying insects from being captured. Some fungus gnats were

7

observed in the media randomly among trays, any mature gnats that did become trapped
on leaves were quickly removed.

Figures 2-3. Spray collars around plants to prevent soil fertilization.
Nutrient supplementation extended over a 4 month period (May 1/02 to Sept.
1/02) with treatments occurring every two weeks for a total of 8 applications per plant.
Over the 4 month period, observations of growth within each treatment were recorded.
At the end of the treatment period, plants were carefully harvested and separated into
leaves and roots. All plant parts were dried at 70oc for 48 h. Dry weights were recorded
from each plant to determine biomass of each treatment. Leaves and roots were ground
separately. Leaves were ground using a Wiley mill and roots were ground using a mortar
and pestle. Each sample was stored separately in glass sampling containers at room
temperature until nutrient analysis. Total nitrogen and carbon concentration of leaf and
root tissue was determined using Micro-Dumas combustion (NA1500 C/H/N Analyzer,
Carlo Erba Strumentazione, Milan). Thirty six leaf and 36 root samples were processed

8

separately in 5 mg samples. The frequency of reproductive structures (flowering scapes)
was recorded.

6.4 Statistical Analyses
Single factor ANOY A (excel version 5.1) were performed to assess significance between
fertilization treatments in terms of biomass and nitrogen accumulation.

9

7. RESULTS
7.1 Cultivation and Morphology
All 36 plants selected grew successfully in the media provided over the 4 months
of the experiment. There were no treatment effects on the visual aspect of the plants
except in week eight, 2 plants from treatment 5 appeared to lose turgor and show signs of
yellow discolouration. In terms of number of leaves, plants exhibited an increase over
the period of the experiment, ranging from 6 to 33 (control) and 6 to 23 (treatment 6).
The presence of flower stalks in all treatments was noted (Table 2). Figures (4-17) show
the initial and the final condition of plants for each treatment.

Table 2. Average initial (1-leaf) and final (F-Ieaf) leaf numbers, total number of flower
stalks (TFS) grown, total number of flower stalks aborted (TFA), and total number of
flower stalks which flowered (FSF).
Treatment No.

1: Control
2: [50%]
3: [75%]
4: [ 100%] standard
5: [150%]
6: [200%]

1-leaf
(n=6)
6 ±0.3
5 ±0.5
6 ±0.3
7 ±0.2
7 ±0.0
6 ±0.2

F-leaf
(n=6)
33 ± 1.0
28 ± 0.7
24 ± 0.5
30 ± 0.9
26 ± 0.7
23 ± 0.9

TFS

TFA

FSF

2
3

1
2
0
0
1
1

1

1

4
4
4

Note: Flower stalks aborted= stalks which senesced before producing flowers
Flower stalks tlowered =stalks which flowered (alive or dead at end of trials)

10

1

1
4
3
3

Figures 4-15. Initial and final plant morphologies (representatives of each treatment).
Treatment 1 (figs. 4-5); treatment 2 (figs. 6-7); treatment 3 (figs. 8-9); treatment 4 (figs.
10-11); treatment 5 (figs. 12-13); treatment 6 (figs. 14-15).

11

Figures 16 and 17. Initial plantlet size and randomization (Fig. 16) prior to experimental
treatments. Final plant size and randomization (Fig. 17).

12

7.2 Biomass (dry weight)
At time of harvest, plants averaged an increase of 1.49 gin dry weight over four
months of growth. Combined total leaf and root biomass ranged between 1.24 g
(treatment 5) to 1.72 g (treatment 4). The average shoot, root, and total biomass of each
treatment are shown in Table 3. Supplementary foliar feeding did not result in any
significant differences at the 0.05 levei in leaf biomass (p =0.347) or root biomass (p =
0.489).

Table 3. Average leaf, root, and total plant biomass (dry weight± SE) from each
treatment, n = 6 per treatment.
Treatment No.
1: Control
2: [50%]
3: [75%]
4: [100%] standard
5: [150%]
6: [200%]

Leaf biomass (g)
0.14 ± 0.02
0.20 ± 0.04
0.15 ± 0.02
0.20 ± 0.03
0.14 ± 0.02
0.16 ± 0.02

13

Root biomass (g)

Total biomass (g)

0.11 ± 0.01
0.08 ± 0.02
0.10 ± 0.02
0.09 ± 0.02
0.07 ± 0.02
0.08 ± 0.01

0.25 ± 0.02
0.28 ± 0.05
0.25 ± 0.03
0.29 ± 0.05
0.21 ± 0.04
0.24 ± 0.03

7.3 Nitrogen and Carbon Accumulation
Total nitrogen and carbon accumulation in plant tissues is shown in Table 4.
Nitrogen concentration s were not significantly different in leaf tissues (p = 0.744) or root
tissues (p = 0.51) at the 0.05 level between treatments .

Table 4. Average leaf and root nitrogen and carbon concentrations(% of dry weight) in
leaves and roots (n=6).
Treatment No.

Leaf %N

Leaf %C

Root %N

Root %C

1: Control
2: [50%]
3: [75 %]
4: [100%] standard
5: [150%]
6: [200%]

0.89 ±0.08
0.81 ±0.04
0.89 ±0.09
0.85 ±0.03
0.93 ±0.07
0.84 ±0.03

39.34±0.17
36.46 ±1.17
39.62 ±0.48
40.13 ±0.29
39.82 ±0.82
39.54 ±0.77

0.98 ±0.13
0.83 ±0.07
0.86 ±0.13
1.03±0.13
1.08 ±0.11
1.11 ±0.14

40.11 ±0.74
40.48 ±0.95
41 .69 ±0.35
42.44 ±0.25
41.83 ±0.31
41.30 ±0.92

14

Leaf
N/C %
2.25 ±0.17
2.21 ±0.07
2.24 ±0.21
2.11 ±0.08
2.35 ±0.16
2.11 ±0.04

Root
N/C%
2.43 ±0.31
2.06 ±0.14
2.07 ±0.29
2.43 ±0.30
2.58 ±0.26
2.68 ±0.33

8. DISCUSSION
The objectives of this experiment were to successfully cultivate D. capensis, and
determine if a supplementary feeding treatment to the foliage using N&Cl provided any
significant contribution in terms of biomass and nitrogen accumulation within the plant.

The cultivation of D. capensis was successful under the protocol used for this
experiment. Plants grew to an average of 1.49g per plant across all of treatments.
Compared to treatments with added fertilizer, the fact that plants grew as much in our
experiment without using nutrient fertilizer (i .e. control plants), indicates that D. capensis
has the ability to establish and grow at very low levels of nitrogen. This suggests that
sources of nitrogen (and other nutrients) were obtained in sufficient amounts from the
media to sustain growth, an observation also noted in the literature (Chapin and Pastor
1995; Adamec 2002). Owen and Lennon (1999) also showed that the tropical pitcher
plant Nepenthes alata Blanco. could be grown in greenhouse conditions without the
addition of fertilizer. Our result indicates that, under laboratory conditions and with a
proper media, D. capensis can solely grow on its own photosynthetic carbon and root
nutrient acquisition capabilities.

For the duration of the experiment, flower stalk production was observed in
several plants for all treatments. A greater number of flower stalks were observed in
plants under higher concentrations compared to that of lower concentration or the
absence of fertilizer. These observations may suggest that flower production is linked to
nutrient concentrations. However, the similarity in plant tissue nitrogen concentrations

15

and biomass in all treatments makes this suggestion very unlikely. Furthermore, D.
capensis is known to repeatedly produce flower stalks throughout a growing season
(D' Amato 1998). In addition, flower stalk abortion prior to flower production occurred
in both high and low treatment concentrations. It has previously been documented that
flowering frequency increases in some insectivorous plants with higher nutrition
availability (Thoren and Karlsson 1998). However, with the addition of nutrients to D.
rotundifolia, flower production decreased (Stewart and Nilsen 1991). This suggests is
that these plants have specifically adapted to their habitat and vary in limitations and need
for additional nutrient input. Seed production or seed viability was not tested in this
experiment.

As noted in Table 2, there were differences in the average biomass between all
treatments, however, none were statistically significant. This indicates that D. capensis
did not benefit in biomass from supplementary nitrogen as control plants grew apparently
as well as those receiving supplementary nitrogen. Chapin and Pastor (1995) also
showed that S. purpurea did not differ in terms of biomass as compared to the control
when supplemented with an N~CI fertilizer into the pitchers. These results suggest
three possibilities: 1) the concentration of N~CI was not high enough to show effective
results; 2) the fertilizer N~CI is not effective in terms of biomass production and may
require the presence and/or action of other nutrients which could lead to increased
biomass or 3) the fertilizer N~CI is not readily taken up by the absorbing mechanisms
found on the leaves of D. capensis.

16

The last scenario has not been tested in other Drosera plants using an NI-4CI
fertilizer. However, Karlsson and Pate (1992) documented that two pygmy species of

Drosera (D. closterostigma March . & Low. and D. glanduligera Lehm .) were
unresponsive to a nutrient supplementation (Hoaglands) when applied to the roots.
Trapping mechanisms of prey capture are known (Givnish 1989; D' Amato 1998; Slack
2000) , however, the exact mechanism of enzymatic release and digestion involving
chemical and physical signals is poorly understood. If chemical signals are involved,
then we need to know what particular chemical(s) are involved and at what
concentrations or levels they are required.
Zamora et al. (1997) found that Pinguicula vallisneriifolia did not increase in
biomass under a nutrient solution fertilizer applied to the leaves but rather showed
remarkable growth when fed live insect prey. These findings suggest that some species
of insectivorous plants could require the capture of live prey rather than a supplementary
nutrient spray application to stimulate enzymatic digestion.
The results found in our experiment contradict those of Adamec (2002), where he
found the biomass of D. capillaris, D. aliciae, and D. spathulata increased 81- 120%
greater than that of the control plants when leaf fed with a nutrient supplementation.
Similarly, Thoren and Karlsson (1998) documented that supplementary feeding by use of
insect prey increased plant growth and biomass in three sub-arctic Pinguicula species (P.

alpina L., P. villosa L., and P. vulgaris L.). This again may suggest that such
insectivorous plants respond to natural prey far more efficiently than that of supplemental
ferti Iizati on.

17

Nitrogen concentrations within the plants (Table 3) did not significantly differ
between treatments. The results indicate that nitrogen levels within the plant were not
affected by the supplementary NH4Cl fertilizer applied to the leaves. Interestingly, the
nitrogen and carbon percentages found in the shoot and root of D. capensis are similar to
the concentrations found in most vascular plants (Raven et al. 1999). Adamec (2002)
found that the final root or shoot nitrogen content in three Drosera species did not
significantly differ from that of the controls even though their biomass did increase. This
observation was also noted in three Pinguicula species studied by Thoren and Karlsson
(1998), however, the only difference was these plants were supplementary fed with fruit
flies (Drosophila melanogaster).
On the contrary, some nutritional experiments on insectivorous plants do suggest
that nutrient concentrations do increase with supplementation. The study by Chapin and
Pastor (1995) on S. purpurea showed a significantly higher amount of nitrogen content
within plant tissue of fertilized plants (NfuCI fertilizer, similar to ours) than that of the
controls and other treatments. The non-absorption of NfuCl through the leaves of D.

capensis during our experiment may directly be correlated to trap and absorption
mechanisms. As well, in other studies, the supplementary nitrogen source had been
added in different forms and, in many cases, with other nutrients, suggesting a possible
link between combinations of nutrients being more effective (Hanslin and Karlsson 1996;
Adamec 2002; Adamec 1997; Zamora et al. 1997).
The results of our experiment indicated that foliar NfuCl fertilization had no
significant effect on the growth of D. capensis and this raises additional points related to
the physiology of this particular species. First, how can D. capensis successfully grow in

18

nutrient poor media without any additional nutrients? Second, what mechanisms are
involved with nutrient uptake in the leaves of D. capensis?
It has been documented that insectivorous plants have evolved to live in naturally
nutrient poor habitats where available nutrients such as nitrogen, phosphorus, potassium
and many micronutrients are lacking or are not available in usable forms due to
environmental conditions such as low pH, water logged soils and low decomposition
rates (Adamec 1997; Ellison and Gotelli 2002). In our experiment, although nutrient
levels in the media were minute, there were enough nutrients present to provide an
adequate supply for significant gain in biomass. Presumably these conditions (including
the light regime) were shown to be sufficient for the growth of D. capensis. Clearly, the
ability to capture and utilize insects is not the primary source of nutrient acquisition in D.

capensis (or perhaps other species of insectivorous plants), but is merely a mechanism to
enhance nutrient capture or acquisition when available. Ellison and Gotelli (2002)
showed that when the ambient nitrogen availability in the soil was high, S. purpurea did
not produce or rarely produced its trapping structures compared to the phyllodia (keel
leaves) which are more effective at photosynthesis than the pitchers. Similarly, Knight
and Frost (1991) found the trap production in Utricularia macrorhiza LeConte was
directly related to the nutrient status of the water. This suggests that the production of
traps in some carnivorous plant species is directly related to nutrient availability. In the
case of D. capensis, the trapping mechanism is found on the photosynthetic leaf as one
unit. Since the plant grew sufficiently in the experimental conditions of our study, the
possibility of not producing digestive enzymes may be a prevalent trait under sufficient
or optimal growing conditions. In addition if photosynthesis is optimal, testing the

19

quality of light or photosynthetic rates against the rate of insectivory in these unique
plants should be further investigated. If these plants were to solely rely on insectivory for
survival, they might have become extinct long ago . On a yearly basis, the insect capture
rate of most insectivorous plants is usually fairly low compared to the amount of insects
available. In many experiments attempting to quantify the benefits of insect nutrition,
insectivorous plants have been over-supplemented with insects (Thoren and Karlsson
1998), which can mislead us to believe what the actual response to insect nutrition is in a
natural habitat. We need to quantify the amount of prey capture first in order to fully
understand the affects of insectivory. In addition, trapping becomes unlikely on larger
types of insect prey resulting in a decrease in capture success (Gibson 1991).
Mechanisms involved in nutrient uptake in leaves of D. capensis are quite
complex. Although poorly understood, it has been suggested that the process of digestion
is initiated by a stimulant acting upon the stalked glands of the leaf. Such stimulation can
occur naturally through insect contact or artificially by the addition of solid or liquid
nutritive substance (Slack, 2000). However, the stimulant must still in some form trigger
the movement action of the stalked glands to signal the process to start. To protect
against unnecessary activation of digestion from rain water or dust, these glands have a
mucilage envelope which surrounds the actual gland. Therefore, the stimulant must be
able to diffuse through this mucilage and contact the actual gland (Pietropaolo and
Pietropaolo 2002). This process of stimulation may explain why D. capensis in our
experiment did not absorb the fertilizer from the treatments. Perhaps the nutrient is
unable to penetrate the mucilage barrier to activate uptake by the sessile glands or the
quantity of actual nutrient on the glands was insufficient. In addition, it is unknown

20

whether the mucilage of the glands has an affinity or repulsion towards some chemical
compounds. It has been shown that high levels of ammonium may produce toxic effects
in plants (de Graaf eta/. 1998). Again, based on assumption, the D. capensis plants
grown in this experiment may have had sufficient amounts of nitrogen in the tissue from
other sources which possibly may deter the additional uptake to prevent toxicity.

In this experiment, it was shown that the insectivorous plant D. capensis did not
significantly benefit from Nt4Cl fertilizer treatments. However, it was able to
successfully grow and sometimes produce flower stalks without any additional nutrients.
This experiment suggests that D. capensis can grow without the addition of nutrients
(rom which that is present in the soil in its natural habitat or in our case, in the
greenhouse trial. Through nutritional experiments using insectivorous plants, a variety of
surprising results have indicated the diversity which surrounds the effects of nutrition on
such plants. Difference in effects can only suggest a species specific regime.
Further experimentation is needed both in laboratory and field condition to
provide a better understanding of insectivory in general and to quantify on a per species
basis how nutritional supplementation may benefit the plant. Other tests should include
several nutrients (N, P, K, and micronutrients) with and without stimulation of leaves and
insect prey to justify the relationship between stimulation and nutrient uptake in both
leaves and roots. In addition, studies directed towards the involvement of insectivory on
the enhancement of photosynthesis should be explored. It should be recognized that the
adaptation of plants to trap and utilize insect nutrients was a step forward in the evolution
of plants. Many other non-insectivorous plants such as Geranium viscosissimum F. & M.

21

and Potentilla arguta Pursh . exhibit protocamivory traits, which are morphological
features such as sticky hairs similar to that of true carnivorous plants (Spomer 1999).
These traits may indicate that such a mechanism of increasing mineral nutrition is more
common among plants than first realized. I would conclude that insectivorous plants,
given the nature of their nutrient poor habitats, the ability to survive without additional
nutrients as shown in this experiment with D. capensis, could be considered opportunists
taking advantage of an additional nutrient resource available in abundance in their
habitat.

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