Introduction
One of
the main factors that can influence plant development is water deficit. Water
stress is considered the most important environmental factor, capable of
interfering with plant metabolism. The capacity of plants to tolerate drought
exists as a function of several anatomical, morphological and physiological
characteristics, which interact allowing the maintenance of growth and
development processes (Taiz and Zeiger 2013).
The herbaceous cotton (Gossypium hirsutum L. var. latifolium
Hutch.) is considered a relatively drought-tolerant plant, especially when
compared to crops such as soybean, rice, corn etc. However, the lack of water
during critical growth periods compromises plant growth, development and
productivity (Luo et al. 2014). As it is a plant with an indeterminate
growth habit, the cotton plant may present excessive vegetative growth and
consequent imbalance in the source/drain ratio, resulting in lower yields. In
this sense, it is essential to apply growth regulators due to the development
of this culture (Ferrari et al. 2014).
Mepiquat chloride (MC) stands out among the most used plant regulators
in modern Brazilian cotton farming. It inhibits the natural production of
gibberellins, reducing excess vegetative growth without affecting productivity
(Lamas et al. 2013). Despite the benefits arising from the application
of growth regulators, the management of these products is still a challenge due
to high temperatures and water deficit (Echer and Rosolem 2017). According to
Paixão et al. (2017) there is evidence that the regulator may interfere
with exchanges gases, modifying the photosynthetic rate of cotton plants.
Currently, numerous studies 150 report the beneficial effects of silicon (Si)
in some economically important crops. According to Filho (2010), Si plays an
important role in plant-environment relations, as it can provide crops with
better conditions to withstand climatic, edaphic and biological adversities
with the final result of increased production and better quality of crops.
Among the benefits provided by Si, we highlighted increase in cell wall
resistance and regulation of evapotranspiration, improved photosynthetic rate,
in addition to improving leaf architecture (Basagli et al. 2003). The
foliar use of Si can be a viable alternative to minimize the harmful effect of
biotic and abiotic stresses on crops (Lima et al. 2011; Sheng and Chen
2020). This is because the element can optimize some desirable
morphophysiological and biochemical processes by significantly increasing the
yield of some cultivated species, notably by the accumulation and
polymerization of silicates in epidermal cells forming a silicon-cuticle double
layer that substantially reduces transpiration, converging to lower water
consumption (Peixoto et al. 2011). Thus, it is believed that Si
represents an interesting alternative, with great potential to be used in the
improvement of Brazilian agricultural production. This study aimed to analyze
the doses of calcium silicate (CS) with the use of MC in different water
conditions, through leaf anatomical and gas exchange, to verify the possible
changes that occur in leaf providing information about silicate fertilization
for cotton.
Materials and Methods
Experiment location
The experiment
was carried out in a Pad and Fan greenhouse, with a maximum temperature of
30ºC, at Universidade Estadual Paulista, Ilha Solteira – SP, located at
20°43'09"S and 51°33'79"W, with an altitude of about 335 m.
Experimental design
The experimental
design was completely randomized in a 4 × 2 × 2 factorial scheme, with 4
replications. The factors studied were four doses of CS i.e., 0, 100, 200 and 400 g ha-1, with (10, 20 and 30%
of the 120 mL dose) and without the use of MC and two levels of soil water
[full field capacity (FC) and 1/3 field capacity (1/3 FC)].
Installation
and conduct of experiment
Pots
polyethylene of 15 liters were used, and all pots were first filled with 1 kg
of gravel nº2 (for drainage) and 14 kg of soil on top. The soil used in the
installation of the experiment was the typical Dystrophic RED LATOSOL, from the
0 to 0.20 m layer, it was corrected and fertilized based on chemical analysis:
pH (CaCl2) = 5.1; Presin = 2 mg dm3; M.O.= 17 g dm3;
K+ = 0.7 mmol/dm3; Ca+2 = 9 mmol/dm3;
Mg+2= 8 mmolc/dm3; H+Al = 16 mmol/dm3; Al = 0
mmol/dm3; CTC = 33.7 mmol/dm3; V=53%.
Four cotton seeds of the TMG 81WS genotype were sown per pot on December
29, 2018. The seedlings emerged six days after planting, two thinning were
performed, the first at 15 days after emergence (DAE) and the second at 25 DAE,
leaving one plant per pot until the end of the experiment. CS was sprayed on
the plant leaves with the aid of a sprayer, and divided equally into three
times, at 30, 45 and 60 DAE. The growth regulator applied was MC at three
concentrations (10, 20 and 30% of the 120 mL dose), after the application of
CS. To determine the FC, the pots were initially saturated with water and then
covered with polyethylene bags to prevent evaporation from the soil in the pots
and left to rest for a period of no less than 20 h, until stabilization was
closer to the FC. The soil moisture was read daily using a portable moisture
meter, which shows the water retained in the soil, thus determining the amount
of water to be placed in each pot.
Variables measured
Measurements of
gas exchanges, Infra-Red Gas Analyzer, were performed at 80 DAE, using the
LC-Pro equipment (ADC – Bioscientific Ltd., Hoddesdon, UK) and the conditions
for measurements were made under 1000 μmol m-2 s-2
photosynthetically active radiation (PAR), provided by LED lamps; 380 ppm CO2
and chamber temperature of 28°C. Measurements were performed on a sunny day,
between 8:00 and 12:00 h, taking the 2nd or 3rd adult
leaf, completely expanded from the apex of the branch. The data
were recorded for internal concentration of CO2 (Ci),
transpiration rate (E), stomatal conductance (gs) and photosynthetic rate (A).
For the evaluation of anatomical characters of the leaf blade, leaves
were collected from the middle third of the plant. The material was fixed with
FAA 70 (formaldehyde + glacial acetic acid + 70% alcohol) for 48 h and
subsequently preserved in 70% alcohol. The leaf samples were sectioned freehand
with a steel blade, in the middle region of the mesophyll. The samples were
dehydrated in an ascending ethylic series, dried by the drying process at the
critical point of CO2. The samples were mounted on aluminum “stubs”,
metallized with a 30–40 nm gold layer and observed under a scanning electron
microscope (model LEO 435VP, Germany). Images of the transverse view of the
epidermis were digitally generated. The images were processed with the
Digimizer 5.3.5 Software and the thickness of the epidermis, adaxial and
abaxial and of the palisade and spongy parenchyma was determined. The total
thickness of the leaf blade was treated from the sum of the epidermis with the
parenchyma.
Statistical Analysis
The data were
subjected to analysis of variance (ANOVA) and F values were derived. The means
were compared using the Tukey test at 5% probability. For finding the
significant regressions, regression models that best fitted the effects
obtained from doses of CS statistical software SISVAR 5.6 were applied.
Results
The water
conditions significantly altered (P < 0.01) the leaf anatomy of the cotton
plant, where the plants submitted to 1/3 CC presented greater total thickness
of the leaf, due to the greater thickness of the abaxial and adaxial epidermis,
and of palisade and spongy parenchyma, the differences reaching 15.30% for the
epidermis and 23.45% for the spongy parenchyma, when comparing the plants that
were not subjected to water deficit (Fig. 1A). A significant (P < 0.05)
effect of MC was observed only in the spongy parenchyma and the total thickness
of the leaf blade, where it is possible to verify differences of 15.34 and
9.57%, respectively when comparing the plants without the application of the
regulator (Fig. 1B).
%20221-226_files/image002.png)
Fig. 1: Leaf-blade tissue thickness (µm), the
adaxial epidermis (Ad), palisade parenchyma (PP), spongy parenchyma (SP), the
abaxial epidermis (Ab) and a total thickness of herbaceous cotton, under FC and
1/3 FC (A) as well as with and without application of MC (B)
%20221-226_files/image004.png)
Fig. 2: Thickness of abaxial epidermis in
herbaceous cotton leaves as a function of CS doses
The CS applied to cotton significantly (P < 0.01) increased the
thickness of the abaxial epidermis; the increase occurring linearly with CS
doses (Fig. 2). Using scanning electron microscopy (Fig. 3), it was possible to
verify the presence of a thicker cuticle on the lower surface of the leaves of
plants treated with CS. According to the ANOVA (Table 1), different water
conditions had significant (P<0.05) effect on gs and net photosynthesis (A)
of herbaceous cotton. MC significantly (P < 0.01) influenced E, gs and A. CS
doses did not (P > 0.05) affect any variable nor the interaction between the
studied factors.
The gs was significantly (P < 0.01) affected by water conditions,
where there was an increase in gs in treatments submitted to water stress (1/3
CF). Comparing the gs of the plants introduced to the lowest and highest water
content in the soil, there was a decrease of 34.78%. Lower soil water levels
were also responsible for the increase in net photosynthesis (Table 1), which
was 61.70% higher when compared to treatments that were not subjected to low
water availability. The application of MC significantly (P < 0.01) increased
E, gs and A. Compared with the use of the product, there was a difference of 25%
in E, 43.18% for gs and A, the greatest difference (87.51%) was observed, which
was for plants that did not receive the foliar sprays (Table 1).
Discussion
In environments
where water deficit occurs frequently, morphophysiological mechanisms are severely
affected, causing plants to quickly adapt to the new condition (Santos and Carlesso 1998;
Tombesi et al. 2018). When the plants are subjected to water deficit
from the beginning of the cycle, plant adaptation
occurs more easily, as was the case in the present study,
where the water deficit started at 30 DAE and remained throughout the development of
the cotton plant, causing the plant to adapt to the conditions of the water. The E was not
significantly affected with different water stress conditions, which may be a result of a
greater transpiration Table 1: Summary of analysis of variance for the internal
concentration of CO2 (Ci), transpiration rate (E), stomatal
conductance (gs), net photosynthesis (A), of
herbaceous cotton plants, under different hydric conditions (HC), mepiquat
chloride (MC), calcium silicate (CS) levels
|
FV |
p>F |
|||
|
Ci |
E |
Gs |
A |
|
|
Hydric conditions (HC) |
0.422ns |
0.099ns |
0.006** |
0.001** |
|
Mepiquat chloride (MC) |
0.425ns |
0.001** |
0.001** |
0.001** |
|
Calcium silicate (CS). |
0.552ns |
0.694ns |
0.851ns |
0.941ns |
|
Linear regression |
0.530ns |
0.977ns |
0.773ns |
0.658ns |
|
Quadratic regression |
0.928ns |
0.512ns |
0.428ns |
0.804ns |
|
HC × MC |
0.917ns |
0.274ns |
0.221ns |
0.264ns |
|
HC × CS |
0.703ns |
0.462ns |
0.431ns |
0.712ns |
|
MC × CS |
0.984ns |
0.993ns |
0.820ns |
0.698ns |
|
HC × MC x CS |
0.352ns |
0.968ns |
0.775ns |
0.432ns |
|
C.V (%) |
16.55 |
32.09 |
41.44 |
52.53 |
|
Hydric conditions |
||||
|
FC |
302.87 a |
2.31 a |
0.46 b |
10.21 b |
|
1/3 FC |
292.96 a |
2.64 a |
0.62 a |
16.51 a |
|
Mepiquat chloride |
||||
|
WITH |
292.93 a |
2.75 a |
0.63 a |
17.42 a |
|
WITHOUT |
302.90 a |
2.20 b |
0.44 b |
9.29 b |
ns,** and
* – non-significant and significant at 1 and 5% by the F Test, respectively
Means followed by the same letter, in the column,
do not differ from each other by Tukey's test, at a 5% probability level
%20221-226_files/image006.png)
Fig. 3: Cross sections in scanning electron microscopy of the cotton leaf
showing the adaxial epidermis (ed), palisade parenchyma (pp), sponge parenchyma
and abaxial epidermis (eb) as a function of doses of CS (A) 0, (B) 100, (C)
200, (D) 400 g ha-1)
capacity under water stress conditions.
According to Tombesi et al. (2018), this strategy can be evaluated as
positive, as it reduces the evapotranspiration and can
indirectly reduce the long-term damage associated with
the energy loss via evapotranspiration.
Leaf anatomy of the herbaceous
cotton plant showed plasticity for water deficit,
changing the thickness of the palisade and spongy parenchyma (Fig. 1A). These
resulting from low availability of water are involved in the protection of the plants
to avoid excessive loss of water by transpiration and, therefore, an important mechanism of
tolerance to water deficit (Castro et al. 2009).
The spongy parenchyma is
specialized for the temporary storage of water (Beltrão et al. 2011),
while the palisade parenchyma is well-developed and specialized for the photosynthetic
process (Castro et al. 2009), making the plant show good efficiency in
the use of water, thus being able to survive better
in an environment of water stress. The thickness of
the palisade parenchyma is one of the adaptations responsible for greater photosynthesis
in plants subjected to 1/3 CF (Batista et al. 2010). Adaptations to
water stress can be physiological, anatomical, and morphological, but these
responses vary according to the species and cultivar (Devi and
Reddy 2018).
In the plants where MC was
applied, it is possible to observe the greater thickness of the spongy
parenchyma and the total thickness of the plants (Fig. 1B). The application of
MC is responsible for making plants more compact and with smaller leaves (Stewart et
al. 2001) changing the morphology and physiological characteristics of the leaves (Leal et
al. 2020). These changes occur as a way for plants to compensate because the smallest size
of the leaf increases the thickness of the leaf and even the plants being more compact
manage to be photosynthetically efficient.
The increase in abaxial
thickness with CS doses (Fig. 2) is linked to the fact that Si is an element that
accumulates in the aerial part, next to the cuticle, mainly
in the endoplasmic reticulum, in intercellular spaces and cell walls (Taiz and Zeiger
2013). The deposition of silica in the cell wall makes the plant more resistant to the action
of fungi and insects. It is also possible to verify that, as the deposition of
Si in the leaf increased, they became more upright,
which can contribute to an improvement in the
architecture of the plants, thus allowing better penetration of light into the
canopy of the plant and improving photosynthesis (Junior et al. 2021).
However, this was not observed in the study, where CS did not
influence leaf photosynthesis (Table 1).
Understanding how each plant
will respond to water deficit is extremely important, due to climate change,
where periods of low water rainfall are becoming increasingly common. Knowing
these answers facilitates the search for techniques that can help us, such as
the use of nutritional elements such as Si, which has been shown to be a tool
for different cultures as a protector against stress, both biotic and abiotic
(Lima et al. 2011; Peixoto et al. 2011; Junior et al. 2021),
as revealed from this study.
Conclusion
Low water availability increased A, gs, and the thickness of the adaxial
epidermis, palisade parenchyma, spongy parenchyma, and abaxial epidermis. The
use of MC increased the total thickness of cotton leaves. Moreover, CS increased the thickness of the abaxial
epidermis of the leaves, resultantly increasing the protective barrier against
water stress.
Acknowledgements
This study was financed
in part by the Coordenação de Aperfeiçoamento de Pessoal de Nível Superior-Brazil
(CAPES) – Finance Code 001. CAPES contributed to the publication of this article.
Author Contributions
DBS, ARM and EFJ designed the experiments, DBS collected and analyzed the
data, DBS, APPD, NCSV interpreted the results and revised the manuscript, all
authors did the final revision of the manuscript.
Conflict of Interest
All authors declare no conflict of interest.
Data Availability
Data presented in this study will be available on a fair request to the
corresponding author.
Ethics Approval
Not applicable to this paper
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