Stomatal conductance, leaf chlorophyll content, growth, and yield of sweet pepper in response to plant growth regulators

Abstract

The demand for sweet pepper (Capsicum annuum L.) is increasing because of nutrition and quality factors. Research to improve pepper yield using plant growth regulators (PGRs) has occurred, but results are not consistent. This study evaluated the effects of the PGRs naphthalene acetic acid (NAA), gibberellic acid (GA3), 4-chlorophenoxyacetic acid, Kelpak® (seaweed extract), and their combinations, on stomatal conductance, leaf chlorophyll content, and growth and yield of hydroponically grown sweet pepper. Seedling root plugs were soaked in Kelpak® at 10 mL L−1, 3 h before transplanting. Thereafter, solutions were applied to foliage with NAA at 20 mg L−1, GA3 at 15 mL L−1, and Kelpak® at 5 mL L−1 and their combinations at 15, 30, and 45 days after transplanting. PGR treatments benefitted stomatal conductance. Leaf chlorophyll content of untreated plants decreased with time compared to treated plants. Foliar application of GA3, alone, and in combination with Kelpak®, increased plant height, fresh and dry biomass, and yield compared to other treatments. Further investigation with application rates, and time of foliar application, on sweet pepper will provide a better understanding of how plants respond to PGRs.

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Stomatal conductance, leaf chlorophyll content,
growth, and yield of sweet pepper in response to
plant growth regulators
Noxolo P. Mbandlwa, Helene Fotouo-M, Martin M. Maboko & Dharini
Sivakumar
To cite this article: Noxolo P. Mbandlwa, Helene Fotouo-M, Martin M. Maboko & Dharini
Sivakumar (2020) Stomatal conductance, leaf chlorophyll content, growth, and yield of sweet
pepper in response to plant growth regulators, International Journal of Vegetable Science, 26:2,
116-126, DOI: 10.1080/19315260.2019.1610925
To link to this article: https://doi.org/10.1080/19315260.2019.1610925
Published online: 14 May 2019.
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Stomatal conductance, leaf chlorophyll content, growth,
and yield of sweet pepper in response to plant growth
regulators
Noxolo P. Mbandlwa
a
, Helene Fotouo-M
a
, Martin M. Maboko
b,c
,
and Dharini Sivakumar
a
a
Phytochemical Food Network to Improve Nutritional Quality for Consumers Group, Department of
Crop Sciences, Tshwane University of Technology, Pretoria, South Africa;
b
Agricultural Research Council
- Vegetable and Ornamental Plants, Division of Crop Science, Pretoria, South Africa;
c
Department of
Agriculture and Animal Health, University of South Africa, College of Agriculture and Environmental
Science, Roodepoort, South Africa
ABSTRACT
The demand for sweet pepper (Capsicum annuum L.) is increas-
ing because of nutrition and quality factors. Research to
improve pepper yield using plant growth regulators (PGRs)
has occurred, but results are not consistent. This study evalu-
ated the effects of the PGRs naphthalene acetic acid (NAA),
gibberellic acid (GA3), 4-chlorophenoxyacetic acid, Kelpak®
(seaweed extract), and their combinations, on stomatal con-
ductance, leaf chlorophyll content, and growth and yield of
hydroponically grown sweet pepper. Seedling root plugs were
soaked in Kelpak® at 10 mL L
−1
, 3 h before transplanting.
Thereafter, solutions were applied to foliage with NAA at
20 mg L
−1
, GA3 at 15 mL L
−1
, and Kelpak® at 5 mL L
−1
and
their combinations at 15, 30, and 45 days after transplanting.
PGR treatments benefitted stomatal conductance. Leaf chlor-
ophyll content of untreated plants decreased with time com-
pared to treated plants. Foliar application of GA3, alone, and in
combination with Kelpak®, increased plant height, fresh and
dry biomass, and yield compared to other treatments. Further
investigation with application rates, and time of foliar applica-
tion, on sweet pepper will provide a better understanding of
how plants respond to PGRs.
KEYWORDS
Capsicum annuum;
gibberellic acid;
4-chlorophenoxyacetic acid;
plant biomass; plant height
Demand for sweet pepper (Capsicum annuum L.) is increasing because of
carotenoid, flavonoid, and ascorbic acid contents and their flavor and color.
High temperature and nutrient availability adversely affect productivity of
many plant species including pepper (Hatfield and Prueger, 2015).
Cultivation management systems affect production costs through the reduc-
tion of inputs. Agronomic practices, including the use of plant growth
regulators (PGRs) under open field and controlled environment, have been
CONTACT Martin M. Maboko mmaboko@yahoo.com Agricultural Research Council –Vegetable and
Ornamental Plants, Private Bag X293, Pretoria, 0001, South Africa.
Color versions of one or more of the figures in the article can be found online at www.tandfonline.com/wijv.
INTERNATIONAL JOURNAL OF VEGETABLE SCIENCE
2020, VOL. 26, NO. 2, 116–126
https://doi.org/10.1080/19315260.2019.1610925
© 2019 Taylor & Francis Group, LLC

studied to improve sweet pepper yield and quality (Farooq et al., 2015;
Nkansah et al., 2017; Sahu et al., 2017). Results from these studies on the
effects of PGRs on pepper growth and yield are not consistent. Some
reported increased yield after application of PGRs (Das et al., 2015; Sahu
et al., 2017), others reported no difference or negative effects of PGR on
sweet pepper yield (Maboko and Du Plooy, 2015; Pérez-Jiménez et al., 2015).
Manipulation of physiological processes in plants remains a challenge
because of the specific and complex self-regulated systems. Manipulating
processes in plant remain a challenge including increasing hormone endo-
genous level with PGRs and biostimulants application, to enhance specific
plant traits (Souza et al., 2016). Although there have been few studies on the
effect of PGRs on leaf stomatal conductance and photosynthesis, some
speculated on their positive influence on stomatal opening and chlorophyll
content (Bauly et al., 2000; Hao et al., 2011). An increase in plant height,
number of fruit, and fruit mass occurred after foliar application of gibberellic
acid (GA3) (Baliyan et al., 2013; Kumar et al., 2014) and application of
naphthalene acetic acid (NAA) and 4-chlorophenoxyacetic acid (4-CPA)
(Baliyan et al., 2013) in tomato (Solanum lycopersicum L.), another member
of the Solanaceae family. Similar results occurred after treating eggplant
(Solanum melongena L.), also in the Solanaceae, with liquid seaweed fertilizer
(Ramya et al., 2015). Treating bell pepper with GA3 (100 mg L
−1
) and 4-CPA
(1000 mg L
−1
) at flowering resulted in increased bell pepper fruit size,
biomass, and overall yield (Das et al., 2015). An increase in plant height
and plant biomass occurred in sweet pepper after the application of GA3 (10
and 15 mL L
−1
), and NAA (30 mL L
−1
) at flowering, but yield did not
improve compared to the control (Maboko and Du Plooy, 2015). Although
PGRs can influence mechanisms regulating plant growth, their application
demands insightful research to determine optimal concentrations and stage
of application. This study was undertaken to determine whether application
of the PGRs, alone, and in combination, affected sweet pepper’s growth and
yield.
Materials and methods
The experiment was carried out from November 2016 to March 2017 (sum-
mer/autumn season) in a temperature-controlled plastic tunnel at the
Agricultural Research Council –Vegetable and Ornamental Plants,
Roodeplaat, Pretoria, South Africa (lat. 25.59°S, long. 28.35°E, at 1200 m
above sea level). Growing conditions in the plastic tunnel were maintained at
a mean of 30°C day/15°C night, during the growing season. Seeds of sweet
pepper, cv. Citrine (Sakata Seed Southern Africa Pty. Limited, Lanseria,
South Africa), were sown in seedling trays filled with Hygromix as growing
medium (Hygrotech Pty. Ltd., Pretoria, South Africa) in November 2016 and
INTERNATIONAL JOURNAL OF VEGETABLE SCIENCE 117

maintained as described by Maboko and Du Plooy (2015). Seven-week-old
seedlings were transplanted into 10 L plastic bags filled with sawdust as
a growing medium using the open bag hydroponic system. Plants were
established at a density of 2.5 plants m
−2
. Pruning, trellising, and fertigation
of plants were described by Maboko and Du Plooy (2015).
A causal loop diagram was used to illustrate the role of different hormones
in the plant system (Figure 1). PGRs applied as treatments were NAA, GA3,
4-CPA, Kelpak® solution, and their combinations (NAA + Kelpak®,
4-CPA + Kelpak®, 4-CPA + GA3, Kelpak®+ GA3, 4-CPA + NAA, and
NAA + GA3). Plants receiving Kelpak® were first treated by drenching
seedling root plugs in Kelpak® at 10 mL L
−1
(distilled water) for 3 min, 3 h
before transplanting, thereafter, Kelpak® was applied to foliage at 15, 30, and
45 days after transplanting (DAT) at a concentration of 5 mL L
−1
. Similarly,
NAA, GA3, and 4-CPA solutions were applied to foliage at 15, 30, and
45 DAT at concentrations of 20, 15, and 30 mL L
−1
(distilled water),
respectively. Control plants were treated with distilled water only. Solutions
were applied to all aerial parts until runoff (Maboko and Du Plooy, 2015).
Treatments were arranged in a randomized complete block design with four
Flowers Fruit yield
fruit development
and maturation
fruit se t
+
fruit dr op
-
Chlrophyll
stomata
co nduct anc e
+
-
vege tative grow th
+
-
Mine ral n utr ien t
-
+
+
root system
auxins
+
+
giberrelic acid
+
+
ab scic ic Ac id -
-
-
-
cytokinin
+B4
B2
B3
B1
+
Figure 1. Simplified causal loop diagram of functions of plant hormones. B1: Balancing loop
between chlorophyll content and fruit development; B2: balancing loop between mineral
nutrients and fruit development; B3: balancing loop between chlorophyll and vegetative growth;
B4: balancing loop between mineral nutrients and vegetative growth. (+) indicates the change in
the variable at the beginning of the arrow produces a change in the same direction in the
variable at end on the arrow head, and (–) links two variables with an inverse relationship.
118 N. P. MBANDLWA ET AL.

replications. Each treatment per replicate consisted of ten data plants, with
border plants on all sides of the tunnel.
Leaf chlorophyll content was measured in actively growing leaves using
a chlorophyll meter-The Soil-Plant Analyses Development (SPAD) (model
502 plus, Konica Minolta, Tokyo, Japan). The first plant height, from the root
collar to the apical meristem, and leaf chlorophyll content measurements
were at 30 DAT and measurements were repeated on the marked plants at
2-week intervals until 110 DAT. Leaf stomatal conductance was measured
during sunny days on the abaxial surface of the youngest fully expanded leaf
with a leaf porometer (model SC-1, Decagon Devices Inc., Pullman, WA).
Measurements of leaf stomatal conductance were at 42, 56, and 91 DAT.
Fruits were harvested at full yellow maturity from 89 to 164 DAT. Fruits
exhibiting physiological disorders and of small size (<100 g) were unmarke-
table; marketable fruits were categorized as extra large (>250 g), large (250–-
200 g), medium (199–150 g), and small (149–100 g). At termination of the
experiment, plants were cut at the root collar and dried in a mechanical
convection oven (Blue M Lab oven, Thermal Products Solutions, New
Columbia, PA.) at 75°C for 48 h for dry plant mass determination.
Data were subjected to analysis of variance (ANOVA) using Statistica (ver.
6.0, Statsoft Inc, Tulsa, OK). One-way ANOVA was used to determine
treatment effects on variables. If an interaction was significant, it was used
to explain results. If interactions were not significant, means were separated
using Fisher’s least significant difference.
Results and discussion
Treatment and day of measurement affected leaf chlorophyll content and
plant height but not leaf stomatal conductance; the interaction only affected
leaf chlorophyll content (Table 1). Fresh and dry plant mass were affected by
treatment (Table 2). The GA3 and GA3 + Kelpak® treatments produced fresh
and dry plant mass greater than most other treatments, while the
Table 1. Analysis of variance for effects of treatments which were plant growth regulators
(4-chlorophenoxyacetic acid, gibberellic acid, naphthalene acetic acid, Kelpak®, and their combi-
nation), at days after transplanting, and their interaction, on stomatal conductance, leaf chlor-
ophyll content, and plant height of sweet pepper.
Stomatal conductance Leaf chlorophyll content Plant height
Source of variation df
a
MS df MS df MS
Treatment (T) 10 13507ns 10 28724*** 10 123487**
Day (D) 2 11211ns 5 26664*** 5 7748697***
T×D20 10580 ns 50 5838*** 50 6193ns
Error 66 6600 165 1703 165 5656
Total 131 263 263
ns, **, ***: not significant or significant at 1% or 0.01% probability.
a
df: Degrees of freedom; MS: means squares.
INTERNATIONAL JOURNAL OF VEGETABLE SCIENCE 119

GA3 + 4CPA and GA3 + NAA treatments were intermediate between high
and low values (Table 3).
Leaf stomatal conductance approximates rates of transpiration and gas
exchange through leaf stomata (Vico et al., 2013). Reduction in leaf stomatal
conductance is a strategy used by plants to avoid or to tolerate drought
(Lawlor, 2013). Kelpak® contains the hormones: cytokinins, auxins, abscisic
acid, and brassinosteroids (Stirk et al., 2014). Abscisic acid and cytokinin
regulate stomatal closure when there is CO
2
or water deficit (Askari-
Khorsgani et al., 2018), which was not the case in this experiment. The effect
of GA3 on stomatal conductance is not well known; however, stimulation of
stomata opening by GA3 has been related to increases in the accumulation of
carbohydrates and potassium in guard cells that may influence speed and
degree of stomata opening (Göring et al., 1990). In disagreement with our
findings, GA3 enhanced stomatal opening in tomato (S. lycopersicum L.),
Table 2. Analysis of variance for effects of treatment, plant growth regulators (4-chlorophenox-
yacetic acid, gibberellic acid, naphthalene acetic acid, Kelpak®, a seaweed extract, and their
combination) on fresh mass, dry mass, and total, marketable, and unmarketable yield of sweet
pepper.
Fresh mass Dry mass Total yield Marketable yield Unmarketable yield
Source of variation df
a
MS MS MS MS MS
Treatment 10 17963* 698.7* 27335ns 225518ns 54458ns
Error 30 7046 270.2 294960 225518 46165
Total 43
ns, *: not significant, or significant at 5% probability.
a
df: Degrees of freedom; MS: means squares.
Table 3. Sweet pepper plant fresh and dry biomass and fruit yield as affected by
application of plant growth regulators.
Treatment Fresh plant mass (g/plant) Dry plant mass (g/plant)
Control 6092b 1079b
4-CPA
a
6001b 1090b
Kelpak® 6206b 1144b
GA3 7575a 1404a
NAA 6040b 1152b
GA3 + 4CPA 6759ab 1246ab
GA3 + Kelpak® 7436a 1392a
GA3 + NAA 6753ab 1266ab
4-CPA + NAA 5577b 1032b
Kelpak®+ NAA 5703b 1095b
4-CPA + Kelpak® 5947b 1044b
SD margin 1160 2314
LSD 005 1212 2374
Means within a column followed by the same letter are not significantly different, P≤005,
Fisher’s protected t-test; ns: not significant; LSD: least significant difference.
a
4-CPA: 4-chlorophenoxyacetic acid; GA3: gibberellic acid; NAA: naphthalene acetic acid;
Kelpak®: a seaweed extract.
120 N. P. MBANDLWA ET AL.

another member of the Solanaceae (Sivakumar et al., 2018), indicating that
the response is likely not universal within related crops.
Leaf chlorophyll content of sweet pepper plants was influenced by the PGR
treatment by DAT interaction (Table 1). No particular trend emerged
between treatments, with the exception that GA3 inclusive treatments had
lower chlorophyll content from 30 to 69 DAT (Figure 2). It may be that
increases in leaf area caused by GA3 may lead to chlorophyll dilution rather
than an increase in chlorophyll content (Wheeler and Humphies, 1963).
A decrease in chlorophyll content was reported after the application of
GA3 at 100 and 200 mg L
−1
on eggplant (S. melongena); while an increase
at a concentration of 400 mg L
−1
in eggplant that was 12 and 18 days old in
the same study (Magdah, 2016). An increase in total chlorophyll in green
chile (C. annuum L.) was occurred after the application of GA3 at 10, 25, or
50 mg L
−1
after 30, 60, or 90 days of transplanting (Mahindre et al., 2018).
The increase or decrease of chlorophyll content may depend on GA3 timing
of application. Kelpak® is reported to improve chlorophyll content because it
contains glycine betaine that delays oxidation of chlorophyll (Blunden et al.,
1997). There was an increase in chlorophyll content after liquid seaweed
application (Auskelp, similar nutrient composition as Kelpak®) on chile
(Tejashvi, 2016). The increase in chlorophyll has been attributed to the link
between auxin and calcium transport and its root promoting activity (Arthur
30
35
40
45
50
55
60
65
30 42 56 69 110 139
Chlorophyll content (SPAD)
Days after transplanting
Control 4CPA Kelpak NAA
GA3 GA3+4-CPA GA3+Kelpak GA3+NAA
NAA+4-CPA NAA+Kel
p
ak 4-CPA+Kel
p
ak
4-CPA
Figure 2. Leaf chlorophyll content of sweet pepper as affected by plant growth regulators over
days after transplanting. Results are means ± standard error. 4-CPA: 4-chlorophenoxyacetic acid;
GA3: gibberellic acid; NAA: naphthalene acetic acid; Kelpak®: a seaweed extract.
INTERNATIONAL JOURNAL OF VEGETABLE SCIENCE 121

et al., 2013). This could explain the higher chlorophyll content recorded in
auxin (NAA, 4-CPA) and Kelpak®-treated plants (Figure 2B).
A correlation between stomatal conductance and photosynthetic capacity
of the plant is usually evaluated by measuring leaf chlorophyll content (Wang
et al., 2016). In the current study, a correlation between stomatal conduc-
tance and chlorophyll content was not evident so that none of the treatments
could be identified as having any positive influence on the two processes
simultaneously. There was no correlation between photosynthetic capacity
and stomatal conductance in transgenic tobacco (Nicotiana tabacum L.),
another member of the Solanaceae (von Caemmerer et al., 2004). The latter
authors suggested that the mechanism coordinating the two processes might
not be directly linked as previously reported (Hetherington and Woodward,
2003). Others reported that environmental factors affect stomatal opening
and the contribution of stomatal regulation to photosynthesis may depend
on plant species (Willmer and Fricker, 1996 in Kusumi et al., 2012). The
influence of treatments in this study may be of small magnitude (most
particularly on stomatal regulation) compared to other abiotic factors. At
139 DAT, the positive influence of PGRs was evident as control plants had
the lowest chlorophyll content compared to treated plants, demonstrating
that PGRs can delay plant senescence (Han et al., 2018).
Plant height varied due to number of days after transplantation (Figure
3A) and application of PGRs (Figure 3B). Generally, GA3 inclusive treat-
ments positively, and significantly, produced taller plants, except for
GA3 + NAA-treated plants. The NAA + Kelpak®-treated plants were shortest
(Figure 3). Increased plant height after foliar application of GA3 occurs in
sweet pepper [(Kannan et al., 2009) (25 and 50 mg L
−1
); (Maboko and Du
Plooy, 2015) (10 mg L
−1
)]. GA3 stimulates plant growth through cell divi-
sion, and by enhancing cell and shoot elongation (Han et al., 2018). Taller
40
60
80
100
120
140
160
180
30 42 56 69 110 139
)mc(thgiehtnalpegarevA
Da
y
s after trans
p
lantin
g
aa
ab
bc bc bc bc bc bc c
c
70
75
80
85
90
95
100
105
110
115
Plant height
Plant growth regulator
ab
Figure 3. Effects of treatment on plant height: (A) change in sweet pepper over time; results are
means ± standard error, (B) change due to treatment with plant growth regulators; bars with the
same letter are not statistically different, at 5% level of significance,
4-CPA: 4-chlorophenoxyacetic acid, GA3: gibberellic acid. NAA: naphthalene acetic acid;
Kelpak: a seaweed extract.
122 N. P. MBANDLWA ET AL.

plants due to treatment with Kelpak®+ GA3 is probably due to synergistic
effects of GA3, an active component of Kelpak®. Plants that were treated with
Kelpak®, 4-CPA + Kelpak®, 4-CPA + NAA, and NAA + Kelpak®, and the
control had the shortest plants. Auxin inhibits the growth when applied in
high concentration (Thimann, 2008), and it is a constituent in Kelpak® with
high concentration; the shortest plants due to NAA + Kelpak® treatment
maybe associated with high auxin concentration (Figure 3B). The findings of
this study agree with Kannan et al. (2009) who reported that NAA applied
alone had no effect on the height of sweet pepper plants, implying that auxin
may not have an influence on sweet pepper vegetative growth.
Although height of 4-CPA-treated plants was slightly greater than
untreated plants, they showed symptoms of the growth abnormalities leaf
curling and shrinking, and deformation of flower structures and fruit. Similar
symptoms, but less severe, occurred on plants treated with combinations
including 4-CPA. The 4-CPA is a synthetic auxin analog used to control
flower and fruit drop, and as a pesticide (Grossman, 2003). The action of
auxin herbicides can be summarized in three steps (Grossman, 2003); first
a stimulation characterized by the activation of metabolic processes, followed
by symptoms of abnormal growth or deregulation of plant growth (leaf
epinasty, initiation of stem curling). The second step is characterized by
inhibition of roots and decreased internode elongation and leaf area, fol-
lowed by stomatal closure, and finally plant senescence. Observations in this
study were similar to the first phase described by Grossman (2003). High
chlorophyll content, until 69 DAT, could represent stimulation followed by
a decrease in chlorophyll content from 110 DAT, and with plant growth
abnormalities. Deregulation of plant growth caused by high concentration of
4-CPA could explain low fruit yield for 4-CPA inclusive treatments.
Fresh and dry plant biomass of all treatments was positively correlated
with plant height (Table 2). Plant growth hormones are effective in partition-
ing and translocating accumulates from sources to sinks (Toungos, 2018).
The biomass of each treatment may reflect the ability of particular hormones
to enhance photosynthetic activity and photosynthates translocation
efficiency.
The treatment with GA3 alone and GA3 + Kelpak increased plant fresh
and dry mass more than most other treatments; when GA3 was combined
with 4CPA and NAA, the results were intermediate between high and low
values. The higher biomass in GA3-treated plants maybe correlated to high
stomatal conductance that increases inflow of carbon dioxide in mesophyll
tissue resulting in increased photosynthate biosynthesis (Jie et al., 2012). The
total, marketable, and unmarketable yields overall mean were 3047 g/plant,
2258.7 g/plant, and 788 g/plant, respectively, and there was no difference
among treatments.
INTERNATIONAL JOURNAL OF VEGETABLE SCIENCE 123

Treatments applied in this study did not always influence plant physio-
logical responses, but leaf chlorophyll, plant height, and plant biomass
were enhanced due to treatment with PGRs. Chlorophyll content may be
influenced by leaf area; the initial number of flower and the ability of the
plants to limit fruits drop can influence the number of fruits (Figure 1).
Information on leaf area, the initial number of flower, and number of fruit
on plant just after fruit set could bring more clarity on plant responses to
PGRs.
Acknowledgments
We acknowledge the South African System Analysis Centre for funding the High-level
Capacity Strengthening Programme, and for funding provided by the Agricultural Research
Council, the National Research Foundation, and the Department of Science and Technology
South Africa.
Funding
This work was supported by the South African Research Chairs Initiative (SARChI) program
“Phytochemical Food Network to Improve the Nutrition for the Consumers”: [Grant Number
98352].
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