Annals of Botany 78 : 599–604, 1996
Ammonium Nutrition Enhances Chlorophyll and Glaucousness in Kohlrabi*
M I C H A E L M . B L A N KE†, W O L F G A N G B A C H ER†, R I C H A R D J. P R I N G‡
and E D W A R D A. B A K E R‡
† Institut fuX r Obstbau und GemuX sebau, UniŠersitaX t Bonn, Auf dem HuX gel 6, D-53121 Bonn, Germany
and ‡ IACR–Long Ashton Research Station, Department of Agricultural Sciences, UniŠersity of Bristol,
Long Ashton, Bristol BS18 9AF, UK
Received : 10 October 1995
Accepted : 11 May 1996
Kohlrabi (Brassica oleracea var. gongylodes) plants were grown in the greenhouse under autumn conditions and
fertilized either with pellets containing nitrogen as 40 % ammonium sulphate and 60 % urea or with nutrient solution
containing nitrogen predominantly as nitrate. Plants given nitrogen as ammonium ions developed glaucous leaves
compared to those supplied with nitrate which formed glossy leaves. Ammonium-induced glaucousness was the result
of a two-fold increase in the amount of epicuticular wax and a markedly altered fine structure. Leaves from
ammonium fertilized kohlrabi plants also showed a 21 % increase in chlorophyll content together with a reduction
in the chlorophyll a : b ratio and decreased ground state fluorescence compared to plants supplied with nitrate.
Photosynthesis and stomatal transpiration were unaffected by the form of supplied nitrogen.
# 1996 Annals of Botany Company
Key words : Brassica oleracea, chlorophyll, chlorophyll fluorescence, epicuticular wax, glaucousness, photosynthesis,
transpiration.
INTRODUCTION
Kohlrabi (Brassica oleracea var. gongylodes) is a popular
autumn and spring crop which grows slowly at a time when
nitrogen assimilation is limited by cool temperatures, short
days and low radiation (Hucklesby and Blanke, 1992). In an
attempt to combat slow growth, excessive nitrate fertilizer
has been used to produce a marketable crop. This may
result not only in unassimilated nitrate accumulation in
leaves, but also soil leaching and run-off of nitrate which
may contaminate drinking water (Hucklesby and Blanke,
1993). In an attempt to decrease excessive nitrate fertilizer
usage, ammonium-based CULTAN2 pellets, containing
ammonium sulphate (40 % nitrogen), urea (60 % nitrogen)
and calcium sulphate as substrate, were used as an
alternative source of nutrients (Sommer, 1993). Use of these
ammonium pellets, compared with nitrate, decreased the
nitrate content of leaves and of the edible part, the
orthotrophic swollen stem.
This study was commissioned to determine whether the
form of supplied nitrogen (nitrate Šs. ammonium) affects
plant physiology in terms of photosynthesis, photosynthetic
efficiency, transpiration, chlorophyll content, chlorophyll
fluorescence, and plant structure. This is relevant in view of
the tendency to substitute conventional nitrate fertilizers by
ammonium-based products with the aim of reducing nitrate
accumulation in plants, soil and drinking water and of
nitrate leaching from the soil.
This paper reports the results of investigations on the fine
* Dedicated to Prof. F. Lenz on his 65th birthday.
0305-7364}96}110599­06 $25±00}0
structure and quantification of epicuticular wax, chlorophyll
content, chlorophyll a : b ratio, chlorophyll fluorescence,
transpiration and photosynthesis of leaves from plants
grown with nitrate or ammonium based nutrition.
MATERIAL AND METHODS
Plants and their cultiŠation
Twenty-four kohlrabi Brassica oleracea var. gongylodes cv.
‘ Express Forcer ’ plants were grown in pots in a greenhouse
at Marhof experimental station near Bonn, Germany from
14 Sep. 1993. The greenhouse conditions were : relative
humidity 30–50 % (day) and 60–70 % (night) and 15–
21}10–13 °C day}night temperature with 12±5 (mid Sep.),
10±5 (mid Oct.) to 9 (mid Nov.) hours of natural light. The
5 l pots contained a peat mixture which was deficient in
nitrogen (less than 0±017 g total N per plant) and received
one spray against white fly in the first week after potting.
Plants developed nine to ten leaves and were harvested on 1
Dec. 1993. The same experiment was repeated on the same
dates in autumn 1994. Care was taken to avoid mechanical
leaf damage and shading or wetting of leaves with nutrient
solution and spraying was limited to the pesticide application.
Nitrogen supplementation
All plants were supplied with 0±6 g nitrogen per plant.
One set of plants was supplied with ammonium pellets,
containing 40 % nitrogen as ammonium sulphate and 60 %
# 1996 Annals of Botany Company
600
Blanke et al.—Ammonium Effects in Kohlrabi
T     1. Leaf parameters of ammonium and nitrate supplied kohlrabi plants which deŠeloped glaucous and glossy leaŠes,
respectiŠely
Leaf features and units
Ammoniumbased pellets
Nitrate based
nutrient solution
l.s.d. (5 %)
915
38±5
50±8
984
39±4
57±9
68
1±35
7
Leaf area (cm# per plant)
Specific leaf weight (mg cm−#)
Leaf area (cm# per leaf)
(w}w) nitrogen as urea in calcium sulphate as substrate. The
widest dimensions of the elliptic pellets were 2±5¬3 cm. One
pellet was placed 5 cm deep under one plant in the soil and
resulted in a temporary small pH increase which levelled out
at harvest. The other set of plants received complete nitratebased Hoagland nutrient solution (Hoagland and Arnon,
1952) containing 94 % nitrogen as nitrate and 6 % as
ammonium ions. Plants supplied with ammonium pellets
were watered with Hoagland nutrient solution without
nitrogen : potassium nitrate was substituted by potassium
sulphate at the same concentration. The volume of nutrient
solution supplied was increased from 20 ml twice a week to
80 ml as the plants developed.
Chlorophyll a fluorescence, photosynthesis, leaf area,
chlorophyll and epicuticular wax
Six kohlrabi plants of each set, selected for uniformity
and with fully expanded leaves, were analysed by methods
(a)–(c) listed below before harvest and methods (d) and (e)
after harvest on 1 Dec.
(a) Chlorophyll a fluorescence was measured with a
portable fluorometer type PAM 2000 (Walz, Effeltrich,
Germany) following the method of Schreiber (1986).
Measurements were made on a dull morning of approx.
80 µmol m−# s−" of photosynthetically active radiation
(PAR) in situ at Marhof near Bonn. Kohlrabi leaves were
illuminated with 400 µmol m−# s−" PAR for detection of
ground fluorescence (Fo« or Ft). This was followed by a pulse
of 0±6 s saturating light of 2500 µmol m−# s−" PAR, measured
at the leaf surface, to allow the determination of maximum
fluorescence (Fm«) when PS II reaction centres are closed
(primary quencher QA reduced). (b) Photosynthesis and
transpiration were measured at a PAR of 560–650 µmol m−#
s−", a temperature of 21–24 °C and relative humidity of
30–34 % (¯ VPD of 7±3–10±2 kPa) in situ on fully expanded
leaves between 0900 and 1100 h using a portable, steadystate porometer type CIRAS and a Parkinson broad leaf
chamber (PPSystems, Hitchin, Herts., UK). The Parkinson
leaf chamber was maintained at less than 3 °C above the
greenhouse temperature. (c) Leaf areas were determined in
ŠiŠo using a digital portable leaf areameter type CI 201 (CID
Inc., Moscow, Idaho, USA). (d) Chlorophyll content was
determined by extraction of leaf sections by immersion in
2±5 % (w}v) dimethylformamide (DMF). The absorbance of
the solution was read at 647 nm and 664±5 nm after 5 h, the
optimum time for pigment extraction from Brassica leaves
(Blanke, 1990). (e) Epicuticular wax was removed from
three fully expanded leaves by two washes (2–3 s) in
chloroform which were combined, dried over anhydrous
Na SO , filtered and the mass of the dried residue determined
# %
following evaporation of the solvent (Baker, 1972). Areas of
the leaves were determined from paper replicas taken before
a brief period of darkness to ensure stomatal closure prior
to solvent extraction.
Cryotechnique and scanning electron microscopy
Kohlrabi plants were transported by air courier from
Marhof near Bonn to IACR–Long Ashton Research
Station, University of Bristol, UK and examined by
cryotechnique using a Philips SEM 505 microscope equipped
with a Hexland cryo-stage (Blanke, Ho$ fer and Pring, 1994).
Intercostal leaf sections (halfway between the midrib and
leaf edge) of fully expanded leaves were mounted on stubs
with a mixture of ‘ Tissue Tek ’ (Agar Scientific, Stansted,
Essex, UK) and collodial graphite, and frozen by inserting
into the pre-chamber stage of the Hexland cryo unit at
®150 °C. Cryo-preserved samples were then transferred to
the SEM cold stage and examined for superficial ice
contamination. If present, this was removed by warming the
specimen to ®70 °C. When the surface was clean, the
sample was returned to the pre-chamber and sputter-coated
with approx. 20 nm of gold and re-inserted in the SEM cold
stage. Coated specimens were studied at 7±5 kV and a spot
size of 50 nm. Micrographs were taken with a Leitz Leica 2
camera using 21 DIN Kodak TMX 100 film.
Experimental design
The experimental design was a series of randomized
complete blocks with 11 replicates. Data were analysed
using a commercial statistical package (STATGRAPHICS).
RESULTS
Leaf area and glaucousness
During 10 weeks of growth from mid-Sep. to the beginning
of December, the leaf area per plant, specific leaf area and
average leaf areas per leaf were 8, 2±3 or 14 % larger in
kohlrabi plants supplied with nitrate-based Hoagland
solution (mean 57±9 cm#) compared with ammoniumsupplied plants (50±8 cm#) (Table 1). Fully-expanded leaves
from kohlrabi plants with ammonium nutrition became
dark green to blue, overlaid with a white-silver glaucous
bloom, whereas leaves from plants supplied with nitratebased nutrient solution were darker green and glossy (Fig.
1A).
Blanke et al.—Ammonium Effects in Kohlrabi
601
T     2. Chlorophyll content per unit leaf area and fresh weight (f.wt) of leaŠes of kohlrabi plants supplied with ammonium
and nitrate fertilizer
Ammonium-based
pellets
Chlorophyll
Chlorophyll
Chlorophyll
Chlorophyll
Chlorophyll
content (µg cm−#)
content (µg g f.wt−")
a (µg g−" f.wt)
b (µg g−" f.wt)
a : b ratio
65±4
1658
1206
453
2±67
Nitrate-based
nutrient solution
52±5
1365
1012
353
2±88 :1
l.s.d. 5 %
3±8*
96*
73*
25*
n.a.
* Significant at the 5 % level, n.a. not applicable.
A
B
C
F. 1. Photomount showing the glaucousness of kohlrabi leaves : A, Pieces of kohlrabi leaves showing the distinct and conspicuous glaucousness
of ammonium (top right) relative to nitrate-fed leaves (bottom left) (vertical view). B, Micrograph of the epicuticular wax on the adaxial surface
of a glaucous, ammonium-fed kohlrabi leaf showing a dense network of dendrites, 0±8–1±8 µm wide and 2±5–3 µm long, superimposed on small
tubes and plates, embedded within an underlying layer of amorphous wax. Height of the micrograph represents 15±6 µm. Magnification ¬5000.
C, Micrograph of the epicuticular wax on a shiny, non-glaucous, nitrate-fed adaxial leaf showing erect, separate, crystalline wax tubes and plates.
Height of the micrograph represents 15±6 µm. Magnification ¬5000.
602
Blanke et al.—Ammonium Effects in Kohlrabi
T     3. Photosynthesis and transpiration of fully-expanded leaŠes of ammonium and nitrate supplied kohlrabi plants
Ammonium-based
pellets
Nitrate-based
nutrient solution
20±0
153±9
2±57
259
19±0
168±8
2±36
249
Net photosynthesis (µmol CO m−# s−")
#
Intercellular CO (µl l−")
#
Transpiration (mmol H O m−# s−")
#
Stomatal conductance (mmol m−# s−")
l.s.d. 5 %
4±4 n.s.
39±7
0±29
62
Values are means of measurements on 12 leaves of six plants for each treatment. Conditions of measurements were a PAR of 560–650 µmol m−#
s−", temperature of 21–24 °C and relative humidity of 29–34 % (¯ VPD of 7±3–10±2 kPa) with 373–377 ppm reference CO .
#
T     4. Amount of epicuticular wax per fresh weight on leaŠes of ammonium-fed and nitrate-fed kohlrabi plants
Wax (µg cm−#)
Wax (µg g−" f.wt)
Ammoniumbased pellets
Nitrate-based
nutrient solution
l.s.d. 5 %
54±8
1424±5
27±8
704±9
2±1*
53±1*
* Significant at the 5 % level.
Chlorophyll and chlorophyll fluorescence
Photosynthesis and transpiration
Photosynthesis was measured to assess the effects of the
two forms of nitrogen nutrition. Kohlrabi leaves had photosynthetic rates of about 20 µmol CO m−# s−" under the
#
autumn conditions and a transpiration rate of 2±4 mmol
H O m−# s−", associated with a stomatal conductance gs of
#
249–259 mmol m−# s−", measured in the Parkinson broad
leaf chamber (Table 3).
Quantity and structure of epicuticular wax
Stomata were present on the abaxial and also on the adaxial surface of kohlrabi leaves with 93–133 stomata mm−#.
Their guard cells were covered by crystalline epicuticular
wax. Leaves from plants grown with ammonium nutrition
produced approximately double the epicuticular wax
expressed either on a unit area (55 µg cm−#) or fresh mass
(1424 µg g−") basis compared with leaves from plants supplied with nitrate (28 µg cm−# and 705 µg g−", respectively)
(Table 4). Dipping the leaves in chloroform removed the
bloom completely and most, but not all, of the blue appearance. Glaucousness, which first became apparent when
leaves reached half their final size (approx. 35 cm#), was
greatest at full leaf expansion. Epicuticular wax on young
kohlrabi leaves consisted of elongated rodlets of approx.
0±2 µm diameter branching into filaments superimposed
on wax platelets ; the filaments disintegrated during leaf ex-
0.45
Ground fluorescence, Fo'
Leaves from ammonium-supplied kohlrabi plants contained 21 % more total chlorophyll (Table 2), with a
reduced chlorophyll a : b ratio (Table 2), compared with
leaves of plants supplied with nitrate-based nutrient
solution. The larger chlorophyll content in leaves from
ammonium-fed kohlrabi was associated with decreased
ground state fluorescence of chlorophyll (Fo«) (Fig. 2).
0.44
0.43
0.42
0.41
0.40
1
2
3
F. 2. Ground fluorescence (Fo«) of glaucous (2) Šs. non-glaucous
leaves (1,3) measured on a dull morning with approx. 80 µmol PAR
m−# s−". Kohlrabi leaves were illuminated with a PAR of 400 µmol m−"
s−" for detection of ground fluorescence (Fo«), followed by a 0±6 s pulse
of saturating light of 2500 µmol m−# s−" PAR at the leaf surface.
pansion and their density declined markedly (Fig. 1 B, C).
Kohlrabi leaves from plants fertilized with nitrate exhibited
erect, separate, crystalline wax tubes and plates, 3–4 µm
long and 0±3–0±5 µm wide (Fig. 1B). By contrast, leaves from
plants fertilized with ammonium developed a dense network
of dendrites, 0±8–1±8 µm wide and 2±5–3 µm long, superimposed on small tubes and plates, partially embedded
within an underlying layer of amorphous wax (Fig. 1 B).
DISCUSSION
The present work was based on the environmental issue of
substituting nitrate by ammonium nutrition in order to
reduce nitrate accumulation in vegetables, soil and drinking
water. The objective was to determine whether ammonium
Blanke et al.—Ammonium Effects in Kohlrabi
fertilized Brassica plants differed from those supplied with
conventional nitrate fertilizers in terms of their fine structure
and physiology. The results showed that plants supplied
with ammonium developed glaucous leaves compared to
plants supplied with nitrate which developed glossy leaves.
Leaves from ammonium supplied plants also had a markedly
different fine structure, a two-fold increase in the amounts
of epicuticular wax, a 21 % increase in chlorophyll content,
a reduction in chlorophyll a : b ratio and decreased ground
fluorescence compared to leaves from plants supplied with
nitrate, whereas photosynthesis and stomatal transpiration
were unaffected by the form of supplied nitrogen.
Structure and amounts of epicuticular wax and light
reflection
The two-fold increase in the amount of wax on glaucous
leaves compared with that on the glossy leaves (55 µg cm−#
and 28 µg cm−#, respectively) (Table 4), is consistent with
the findings of Denna (1970) that wax per surface area was
increased 2±5-fold in glaucous compared to glossy cabbage
leaves. Like Denna (1970), we found no effect of glaucousness on stomatal transpiration in the light, while cuticular
transpiration in the dark was decreased by glaucousness.
Similarly, the water balance of cabbage was only affected
under extremes of heat and drought stress (Welker and
Furuya, 1995). The amounts of epicuticular wax recovered
from kohlrabi leaves in our experiment (Table 4) were
similar to those reported for leaves (40–53 µg cm−#) of the
same cultivar grown under comparable greenhouse conditions (Knoche, Noga and Lenz 1992 ; Schwab, Noga and
Barthlott, 1993), 6±5–60 µg cm−# for other Brassica
oleracea species (Baker, 1972, 1974) and 9–38 µg cm−# for
Brassica oleracea var. capitata (Welker and Furuya, 1995).
The bloom on leaves or fruits is a result of light-scattering
from epicuticular wax crystallites (Hall et al., 1965 ; Baker,
1972). Baker (1972) showed that approx. 30 % of incident
radiation was reflected from glaucous Brassica oleracea
leaves carrying 35–50 µg wax cm−#. This was due to the
greater reflection of radiant energy from the mesh-like
arrangements of dendrites, similar to those on leaves from
kohlrabi plants supplied with ammonium (Fig. 1 B) in
contrast to the open straight tubes similar to those on leaves
from nitrate-supplied plants (Fig. 1 C). Only 10 % of
incident light was reflected from glossy leaves of the single
gene mutants, GL1 (broccoli) and GL3 (marrow) (Macey
and Barber, 1970), which produced less than 10 µg wax cm−#
(Baker, 1972).
Baker (1974) has also shown that plants respond to stress
induced by increases in ambient temperature and incident
radiation or from decrease in relative humidity and soil
moisture content through increases in rates of wax production and marked changes in wax configuration. In the
present study, however, the nitrate and ammonium supplied
plants developed under identical growth conditions : clearly
the difference noted with the kohlrabi leaves could not be
ascribed to enviromental variation. Moreover, since the
decrease in the mean leaf area found between nitrate and
ammonium supplied plants was relatively small, the increase
in the wax deposits on the latter can not be attributed to
603
modifications to the growth habit of the plants resulting
from an alteration in the balance from mesophytic towards
xerophytic status.
Chiu et al. (1992) also found that glaucousness in
Douglas fir, was associated with altered epicuticular wax
fine structure. Douglas fir seedlings supplied with one
combination of nitrogen and potassium fertilizer showed
increased glaucousness compared to an unfertilized control.
Glaucousness of the needles was associated with more
ornate, tubular crystalline wax in the non-stomatal region
and similarly ornate, tubular wax in the stomatal region
(Chiu et al., 1992). Glaucousness in this conifer was not
associated with the amount of epicuticular wax and could
not be attributed to one nutritional element (Chiu et al.,
1992), since only one combination of nitrogen and potassium
fertilizer was used.
The stimulatory effects of the ammonium-based pellets
appear to be achieved through a direct action of ammonium
on wax metabolism. This presumably results through effects
on the synthesis and transport of short chain fatty acid
precursors prior to their elongation to long chain hydrocarbons, primary alcohols and esters. Based on the
assumption that waxes are carried to the leaf surface in
volatile solvents such as those identified around growing
plants (Baker, 1974, 1982 ; Anton et al., 1994), the differences
between the open tubular wax structures and the closed
arrangements of dendrites can be attributed to differences in
solute concentrations or rates of solvent evaporation
controlled by ambient conditions. In practical terms the
open wax structures and reduced deposits found on nitratefed kohlrabi leaves may increase surface wetting and the
uptake of applied pesticides whilst facilitating pathogen
infection compared to ammonium-fed plants.
Chlorophyll, photosynthesis, chlorophyll fluorescence and
transpiration
Chlorophyll concentrations of 0±52 g m−# in leaves of
nitrate supplied kohlrabi plants (Table 2) corresponded to
the 0±5 g chlorophyll m−# commonly found in leaves (Lawlor,
1993), while the ammonium supplied kohlrabi leaves
synthesized 0±62 g chlorophyll m−#. However, this 21 %
increase in chlorophyll content per leaf area or weight of the
glaucous leaves (Table 2) did not yield a larger net
photosynthesis (Table 3). This may be a result of saturating
chlorophyll concentration exceeding that required for
maximum photosynthesis (Lawlor, 1993), given the nearmaximum values for kohlrabi of 20 µmol CO m−# s−"
#
photosynthesis under conditions of 600 µmol PAR and
24 °C, values twice the 9–10 µmol CO m−# s−" observed
#
with the same cv. (Sritharan and Lenz, 1992) in growth
chambers (25 °C, 50 % RH and 330 µl l−" CO ). However,
#
transpiration of 2±4 mmol H O m−# s−", and stomatal
#
conductance of 249 mmol m−# s−" were only half of the
4±0–4±3 mmol H O m−# s−" or stomatal conductance of
#
403–885 mmol m−# s−" found by Sritharan and Lenz (1992)
in the growth chamber. A decreased ground state
fluorescence (Fo«) (Fig. 1 B) is commonly associated with
increased chlorophyll (Table 2), as found for the leaves from
ammonium-fed plants, and may be explained by light being
604
Blanke et al.—Ammonium Effects in Kohlrabi
trapped and re-absorbed by chlorophyll (Krause and Weis,
1984). Neither the chlorophyll content, chlorophyll a : b
ratio, chlorophyll a fluorescence, photosynthesis nor transpiration have been investigated in any previous study on
glaucousness.
The following conclusions may be drawn from this work :
(1) glaucousness in kohlrabi is under environmental as well
as genetic control ; (2) glaucousness is enhanced by
ammonium fertilization under short day conditions ; (3)
glaucousness is the result of altered fine structure of
epicuticular crystalline wax resulting in a denser, flatter
surface wax coverage, a two-fold increase in the amount of
total (crystalline plus amorphous) epicuticular wax and
increased chlorophyll content with a smaller chlorophyll
a : b ratio ; (4) the altered fine structure (tubes and plates)
and smaller amount of wax on leaves of kohlrabi plants
supplied with nitrate may provide easier wetting than leaves
of plants given ammonium fertilizer and also easier uptake
of applied pesticides, but may facilitate easier pathogen
infection ; (5) glaucousness did not affect photosynthesis or
transpiration in the light of well-watered kohlrabi, but a
slow transpiration rate may contribute to the induction of
glaucousness.
ACKNOWLEDGEMENT
We are grateful to Drs Brian A. Notton and Peter Holloway,
Long Ashton for reviewing the manuscript, Guido Schnabl,
Bonn, for the fluorescence measurements, Diana Wo$ lfel for
transporting the kohlrabi plants from Marhof to Long
Ashton and Mrs A. Krapf for the illustration.
LITERATURE CITED
Anton LH, Ewers FW, Hammerschmidt R, Klomparens KL. 1994.
Mechanisms of deposition of epicuticular wax in leaves of broccoli,
Brassica oleracea L. var. capitata L. New Phytologist 126 : 505–510.
Baker EA. 1972. The effect of enŠironmental factors on the deŠelopment
of the leaf wax of Brassica oleracea Šar. gemmifera. MSc thesis,
Long Ashton, University of Bristol.
Baker EA. 1974. The influence of environment on leaf wax development
in Brassica oleracea var. gemmifera. New Phytologist 73 : 955–966.
Baker EA. 1982. Chemistry and morphology of plant epicuticular
waxes. In : Cutler DF, Alvin KL, Price CE, eds. The plant cuticle.
London : Academic Press, 139–165.
Blanke MM. 1990. Chlorophyll determination using DMF. WeinWissenschaft 45 : 76–78.
Blanke MM, Ho$ fer M, Pring R. 1994. Stomata and structure of
tetraploid apple leaves cultured in Šitro. Annals of Botany 73 :
651–654.
Chiu ST, Anton LH, Ewers FW, Hammerschmidt R, Pregitzer KS. 1992.
Effects of fertilization on epicuticular wax morphology of needle
leaves of douglas fir, Pseudotsuga menziesii (Pinaceae). American
Journal of Botany 79 : 149–154.
Denna DW. 1970. Transpiration and the waxy bloom in Brassica
oleracea L. Australian Journal of Biological Science, 23 : 27–31.
Hall DM, Matus AI, Lamberton JA, Barber HN. 1965. Intraspecific
variations in wax on leaf surfaces. Australian Journal of Biological
Science 18 : 323–332
Hoagland DR, Arnon DJ. 1952. The water culture method for growing
plants without soil. Circular of the Californian Agricultural
Experiment Station 347.
Hucklesby DP, Blanke MM. 1992. Effect of defruiting on nitrate
assimilation, transpiration, and photosynthesis of tomato leaf.
Gartenbauwissenschaft 57 : 53–56.
Hucklesby DP, Blanke MM. 1993. In : Zakosek H, Lenz F, eds. Nitrat
in Pflanze und Boden, English Abstracts, Stuttgart : Ulmer
Publisher, 137–154.
Knoche M, Noga G, Lenz F. 1992. Surfactant-induced phytotoxicity :
evidence for interaction with epicuticular wax fine structure. Crop
Protection 11 : 51–56.
Krause GH, Weis E. 1984. Chlorophyll fluorescence as a tool in plant
physiology. II. Interpretation of fluorescence signals. Photosynthesis Research 5 : 139–157.
Lawlor D. 1993. Photosynthesis. UK : Longman, Harlow.
Macey MJK, Barber HN. 1970. Chemical genetics of wax formation in
leaves of Brassica oleracea. Phytochemistry 9 : 13–23.
Schreiber W. 1986. Continuous recording of photochemical and nonphotochemical chlorophyll fluorescence quenching with a new
type of modulation fluorometer. Photosynthesis Research 10 :
51–62.
Schwab M, Noga G, Barthlott W. 1993. Einfluß eines Na$ hrstoff- und
Wassermangels auf die epicuticula$ ren Wachse von Kohlrabi.
Angewandte Botanik 67 : 186–191.
Sommer K. 1993. Cultan Cropping System. In : Zakosek H, Lenz F,
eds. Nitrat in Boden und Pflanze. Stuttgart : Ulmer Publisher,
161–173.
Sritharan R, Lenz F. 1992. Effects of carbon dioxide enrichment and
nitrogen supply on kohlrabi (Brassica oleracea var. gongylodes L.).
1. Water use, gas exchange, and carbohydrate partitioning.
Gartenbauwissenschaft 57 : 138–145.
Welker OA, Furuya S. 1995. Influence of heat stress on growth and leaf
epicuticular structure of cabbages. Journal of Agronomy 174 :
53–62.