Introduction
Maize (Zea mays L.) is
one of the main cereal crops, food grains and industrial in many parts of the
world for cultivated area and production. For many agricultural goods, maize is
a staple human food, a feed for livestock and a raw material. It is an
important food crop grown by many resource-poor farmers on a large scale for
subsistence. There are many agricultural goods prepared from maize including
corn sugar, corn oil, corn flour, starch, syrup, brewer's grit and alcohol
(Dutt 2005). Under a broad spectrum of soil and climatic conditions, maize is
grown, however, moderately susceptible to abiotic stresses including salinity
(Farooq et al. 2015).
Salt
stress is one of the most significant barriers to crop development in
salt-affected areas of the world. Almost 8.5 percent of the world's entire area
and about 25 percent of the agricultural land is affected by salinity (Billah et
al. 2019). One of the main farming problems in semi-arid regions is soil
salinity. Plants are vulnerable to extreme climatic conditions in Egypt, such
as high temperatures and drought. Dissolved salts can accumulate in soils due
to inadequate ion leaching. An accumulation of salt in the upper layers of the
soil can also be due to improper management of irrigation (Mohamed et al.
2007).
Proline
is an amino acid that accumulates as a result of stress in different tissues of
the plant, particularly in the leaves. In the regulation of osmosis in the
cell, the accumulation of proline is concentrated in the cytoplasm to
counterbalance the osmosis effect. Under stress conditions, proline protects
enzymes (Meister 2012) and maintains water-balance in the plant (Tarighaleslami
et al. 2012). Exogenous proline application has reduced the negative
effect of salt stress by controlling cellular osmotic equilibrium (Deivanai et
al. 2011). Proline as an osmoregulator specifically controls osmotic
pressure in the plant to absorb water and play an essential role in many of the
plant's critical processes. Proline also protects chloroplast membranes,
increases the efficiency of photosynthesis and has the potential to protect
cell walls and membranes, thus, playing an important role in scavenging free
radicals, thereby mitigating the adverse impact of stress and improving plant
development, productivity and quality (Wu et al. 2017). Thus, proline
plays an important role in promoting plant growth and seed yield under stress
conditions including maize (Abdelhamid et al. 2013). As a proteinogenic
amino acid, proline plays an important role within plant tissues for different
vital metabolic processes (Slama et al. 2014; El-Nasharty et al.
2017). Proline helps to retain the status of cell water, subcellular structures
and protect membranes and proteins from osmotic stress denaturation (Ashraf and
Fooland 2007).
Proline plays three major roles during stress, i.e., as a metal
chelator, an antioxidative protection molecule, and a signaling molecule (Hayat
et al. 2012). Furthermore, exogenously applied proline protects enzymes,
scavenges free radicals and prevents salinity stress oxidation (Wutipraditkul et
al. 2015). Wu et al. (2017) found that the toxicity of salinity can
be decreased by controlling the Na+/K+ ratio and
increasing proline accumulation. This can provide physiological insights into
the understanding of the salinity tolerance mechanisms in exogenous proline-treated
plants. Perveen and Nazir (2018) and Sary et al. (2020) found that
proline indicates differential response by regulating different physicochemical
parameters not only in different plant species but also under various
environmental conditions. Szabados and Savoure (2009) and El-Nwehy et al.
(2020) explained that multiple proline roles in plants include protein
synthesis, osmolyte protection, redox balance maintenance, and mitochondrial
function mediated metabolic signaling. Proline improved nutrient acquisition,
water uptake and nitrogen fixation are primarily motivated by these positive
effects. Exogenous proline also alleviates salt stress by enhancing the
activities of antioxidants and reducing the absorption and translocation of Na+
and Cl- while improving the assimilation of K+ by plants.
In addition, L-proline is synthesized by plants in the cytosol and accumulates
in chloroplasts. Accumulation in plants is a well-recognized physiological
response to salinity-induced osmotic stress (El-Samad et al. 2010). The
present study therefore investigated the role of foliar applied proline as osmoprotectant
in alleviation of salinity stress on growth, yield
and quality of maize grown in saline calcareous soil.
Materials and
Methods
A field experiment
was carried out at the farm of El-Nubaria
Agricultural Research Station, Behaira Governorate, Agric. Res.
Center (ARC) and Ministry of Agriculture and land
Reclamation (MALR), Egypt during the summer
seasons of 2019 and 2020 to evaluate the effect of proline foliar application
on maize (Zea mays L.) cultivar. The geographical situation features of the farm are 30ş 90´ N, 29° 96´
E, with an altitude of 25 m above sea level. The soil samples (0–30 cm depth)
were analyzed according to the method described by (Page et al. 1982).
Soil texture was sandy loam and had the following characteristics: pH 8.3,
organic matter 0.9%, CaCO3 33.6%, EC 4.9 dS/m (3136 mg/kg), K 600,
Ca 900, Na 1200, Mg 400, Fe 6.7, Mn 2.9, Zn 1.4 and Cu 2.5 mg/kg.
Experimental design and treatments
The experiment was conducted in a randomized complete
blocks design arrangement with three replications. The net plot size was of 10.5 m2. The maize crop was planted in each plot with 0.75 m row spacing and
plant to plant spacing of 0.20 m.
Treatments were as follows
including control, 100 mg/L, 200 mg/L, 300 mg/L and 400 mg/L of foliar
application of proline applied three times in a season with one month interval
using (L-proline: C5H9NO2, M.W 115.13).
Maize cultivar Giza 310 obtained from Corn Research Section, Agricultural
Research Center, Giza, Egypt was used. Maize seed was sown on the 1st of
June and harvested on the 3rd of September in both seasons. Nitrogen
fertilizer as ammonium sulfate (20.5% N), phosphorus fertilizer as superphosphate (15.5% P2O5)
and K fertilizer as potassium sulfate (48% K2O)
were added according to the recommendation of the Ministry of Agriculture and
Land Reclamation, Egypt. All other farming practices (i.e., fertilizers,
irrigation, weeds and diseases control, etc.)
were done following the recommended practices for the maize crop. Soil samples
were taken during each season in June, July and August months from different
locations in the experimental site in a randomized way to determine salinity as
shown in Table 1.
Growth, yield and yield components determination
At harvest, three plant samples were taken from each
plot to determine, plant height (m), fresh and dry weights of plant (kg), ear
weight (g), length of ear (cm), the diameter of ear (cm) per plant and the
number of row per ear as mean values for two seasons. To determine grain yield
(ton/fed), grain was removed and cleaned within 1m2 at the center of
the plot. Then grain yield is recorded on a dry weight basis. Replicated
samples of clean grain (broken grain and foreign material removed) were sampled
randomly and 100-grains were counted and weighed.
Biochemical analysis
After the third foliar
application of proline, leaves samples were taken to determine:
(1) The
chlorophyll content using
Chlorophyll meter Spad502 at 9 am according to Woods et al.
(1992) and expressed as chlorophyll index.
(2) Leaf-free proline content was determined according to Bates et
al. (1973).
Table 1: Mean soil EC
values (mg/kg) in June, July and August at the different locations in the experiment site for the two
experimental seasons
|
location |
June |
July |
August |
|
1 |
3050 |
2850 |
2175.6 |
|
2 |
3216 |
2875 |
2221.5 |
|
3 |
3285 |
2900 |
2128 |
|
4 |
3174 |
2925 |
2240.6 |
|
5 |
3173 |
2500 |
2083.2 |
|
6 |
3233 |
2750 |
2256.8 |
|
7 |
2991 |
2840 |
2486.4 |
|
8 |
3124 |
2880 |
2562 |
|
Mean .C (mg/L) |
3156 |
2815 |
2269.3 |
|
Mean EC (dS/m) |
4.93 |
4.39 |
3.54 |
(3) Carbohydrate
contents in aqueous solutions according to DuBois et al. (1956) while
nutrient content from grain were determined by method of Cottenee et al. (1982).
Nutrient content
The harvest samples from leaves were also taken for
determination of nutrients (N, K, Ca, Na, Mg, Fe, Mn, Zn and Cu) by method as described by Cottenee et al. (1982).
At harvest grains samples were
taken to determine:
Seed oil percentage
Seed oil contents was estimated according to AOAC (1990) and expressed as oil content (%) = (weight of the flask +
oil - empty flask weight/ weight of sample) x 100.
Statistical analysis
Statistically analysis was performed to compare the
means of two seasons (Combined analysis of two successive seasons) data by using the least differences (L.S.D) (Snedecor and Cochran 1990).
Results
Effects on plant growth
Foliar application of proline caused a significant
increase in fresh and dry weights of the plant, weights of ear /plant and
number of rows per ear compared with control except for foliar applied 100 mg/L.
Plant height, length and diameter of ear per
plant were not affected. Foliar applied 400 mg/L resulted in an increase of
fresh weight of plant (kg) and weight of ear per plant (g) with a relative
increase of 106 and 72%, respectively compared with control (Table 2).
Effects on biochemical
parameters
The foliar applied proline had a significant improved effect on the biochemical
parameters of maize plants except for protein contents.
Foliar applied proline with 300 and 400 mg/L gave the highest chlorophyll index
value with a relative increase of 35 and 32%, respectively when compared with
control without significant differences between the two treatments. Likely,
higher proline content was recorded for foliar applied 300 and 400 mg/L with a
relative increase of 520 and 544%, respectively compared with control without
significant differences between the two treatments. Foliar application
increased carbohydrates significantly compared with control but without
significant differences between proline treatments (Table 3).
Effects on leaves nutrient content
Foliar application of proline had a significant improved
effect on macro and micronutrients in leaves of maize plants. Regarding N% the
highest increase was recorded with foliar applied 300 mg/L proline with a
relative increase of 83% compared with control. Foliar
applied with 300 and 400 mg/L proline gave the highest values of K, K/Na, Mg,
Fe and Mn compared with control without significant
differences between the two treatments. The similar foliar application showed
the lowest Na concentration in leaves of maize plants. While Ca, Zn
and Cu had highest increase for 400 mg/L proline, as depicted in Table 4.
Effects on yield and its components
Foliar application of proline significantly
enhanced yield and its components including oil percent. Highest yield and its
components including oil percent was recorded
for foliar applied 300 and 400 mg/L proline without significant differences between the two
treatments. The relative increase in grain yield with foliar applied 300 and 400
mg/L were 125.5 and 157.2%, respectively compared with control without significant differences between the two
treatments. The relative increase in oil percent (%) with foliar applied 300 and
400 mg/L were 54 and 79%, respectively compared with control without significant differences between the two
treatments (Table 3 and Fig. 1).
Effects on grain nutrients content
Foliar application of proline has a significant effect
on some macro and micronutrients in grains of maize plants. The results for N
concentration in grains were not significant as a result of proline treatments
foliar application. K, Ca and Zn increased significantly with foliar applied 200,
300 and 400 mg/L proline without significant differences between these
treatments. K/Na, Mg and Mn increased significantly with foliar applied 300 and
400 mg/L proline without significant differences between these treatments.
While Na decreased significantly and recorded the lowest value with foliar
applied 400 mg/L proline with a relative decrease of 23% compared with control.
Fe and Cu recorded the highest value of increase with foliar applied 400 mg/L
proline foliar application, as shown in Table 5.
Table 2: Effect of proline application on plant growth of maize grown
in calcareous soil under salinity stress
|
Foliar
application of proline (m/L) |
Plant height (m) |
Fresh weight of plant (kg) |
Dry weight of plant (kg) |
Weight of ear per plant (g) |
Length of ear per plant (cm) |
Diameter
of ear per plant (cm) |
No. of rows per ear |
|
Control |
2.24 |
0.71 c |
0.17 b |
154.22
b |
16.31
a |
5.07 a |
11.88
b |
|
100 mg/L |
2.34 |
0.92
bc |
0.24 a |
167.33
b |
18.89
a |
5.13 a |
12.67
ab |
|
200 mg/L |
2.45 |
1.06 b |
0.26 a |
238.33
a |
19.89
a |
5.23 a |
12.73
a |
|
300 mg/L |
2.51 |
1.14 b |
0.26 a |
260.78
a |
19.98
a |
5.23 a |
13.10
a |
|
400 mg/L |
2.55 |
1.46 a |
0.23 a |
265.44
a |
19.99
a |
5.33 a |
13.23
a |
|
LSD 5% |
N.S |
0.2486 |
0.0471 |
52.892 |
N.S |
N.S |
0.8241 |
Combined analysis of two successive seasons
Table 3: Effect of foliar proline application on biochemical
parameters, yield and its components of Maize grown in saline calcareous soil
|
Foliar application of proline
(mg/L) |
Leaves |
Grains |
100-grains weight (g) |
Grain yield (t ha-1) |
Grain oil contents (%) |
||
|
Chlorophyll index |
Proline µg/g |
Protein % |
Carbohydrates % |
||||
|
Control |
29.67
c |
17.95
d |
5.48 |
86.02
b |
30.67
c |
3.48c |
1.74 b |
|
100 |
33.73
bc |
37.62
c |
5.63 |
87.37
a |
35.33
b |
4.03c |
1.94 b |
|
200 |
34.10
abc |
75.93
b |
5.83 |
87.30
a |
38.67
ab |
6.12b |
2.10 b |
|
300 |
40.17
a |
111.32
a |
5.83 |
87.15
a |
39.67
a |
7.85a |
2.68 a |
|
400 |
39.17
ab |
115.53
a |
5.75 |
87.73 a |
41.33
a |
8.95a |
3.11 a |
|
LSD 5% |
6.42 |
17.06 |
N.S |
0.89 |
3.51 |
1.45 |
0.52 |
Combined
analysis of two successive seasons
Table 4: Effect of foliar proline application on leaves nutrients content of Maize grown
in saline calcareous soil
|
Foliar application of proline
(mg/L) |
% |
mg/L |
||||||||
|
N |
K |
Ca |
Na |
K/Na |
Mg |
Fe |
Mn |
Zn |
Cu |
|
|
Control |
1.37 c |
1.90 c |
0.45 d |
2.83 a |
0.67 c |
0.28 c |
90.67
c |
33.0 b |
18.0 d |
11.0 d |
|
100 |
1.80 b |
2.10 b |
0.46
cd |
2.67
ab |
0.79 b |
0.28 c |
106.67
bc |
35.67
ab |
20.67
cd |
18.0
cd |
|
200 |
2.03 b |
2.17
ab |
0.49 c |
2.60 b |
0.83
ab |
0.29
bc |
108.33
bc |
36.67
ab |
23.33
bc |
26.67
bc |
|
300 |
2.50 a |
2.20
ab |
0.54 b |
2.57 b |
0.86
ab |
0.33 a |
147.33
a |
38.67
a |
27.33
b |
34.0 b |
|
400 |
2.07 b |
2.33 a |
0.70 a |
2.53 b |
0.92 a |
0.31
ab |
128.0
ab |
40.67
a |
36.33
a |
43.33
a |
|
LSD 5% |
0.3902 |
0.1758 |
0.0407 |
0.2048 |
0.1058 |
0.0247 |
21.485 |
5.478 |
4.481 |
8.9373 |
Combined
analysis of two successive seasons
Table 5: Effect of foliar proline on nutrients content in grains of Maize grown
in saline calcareous soil
|
Foliar application of proline
(mg/L) |
% |
mg/L |
||||||||
|
N |
K |
Ca |
Na |
K/Na |
Mg |
Fe |
Mn |
Zn |
Cu |
|
|
Control |
0.88 |
0.31 b |
0.16 b |
0.43 a |
0.71 c |
0.055
d |
33.50
d |
2.50 c |
36.0 c |
80.50
c |
|
100 |
0.90 |
0.32
ab |
0.16
ab |
0.41
ab |
0.77 c |
0.057
cd |
36.0 c |
3.50 c |
45.50
b |
84.67
c |
|
200 |
0.93 |
0.34 a |
0.17 a |
0.38
bc |
0.89 b |
0.061
bc |
36.75
c |
5.0 b |
60.0 a |
103.67
b |
|
300 |
0.93 |
0.33 a |
0.17 a |
0.35
cd |
0.94
ab |
0.066
a |
40.50
b |
5.50
ab |
56.0 a |
105.33
b |
|
400 |
0.92 |
0.33
ab |
0.17
ab |
0.33 d |
0.98 a |
0.063
ab |
46.0 a |
6.50 a |
53.50
ab |
153.50
a |
|
LSD 5% |
N.S |
0.02 |
0.01 |
0.04 |
0.08 |
0.01 |
1.73 |
1.15 |
9.26 |
14.88 |
Combined analysis of two
successive seasons
Discussion
%20270-276_files/image002.png)
Fig. 1: Effect of foliar proline application
on grain yield and oil percent of maize grown in saline calcareous soil
The present study showed that the role of proline in
stress tolerance as a compatible osmolyte for osmotic adjustment by affecting
the uptake and accumulation of inorganic nutrients in maize plants. The proline
counteracts the detrimental effects of salinity stress on nutrient uptake since
it encouraged K+, Ca2+ and N uptake in maize as evident
from present study. Molazem et al. (2010) found an increase in contents
of Na+ of maize leaves when grown under saline conditions. Increased
Na+ content in maize thus decreased calcium and potassium content
with increased salinity levels, leading to decreased K+/Ca2+,
ratio (Akram et al. 2010) also evident from present study. In the
rhizosphere, high Na+ due to salinity decreases plant uptake of
nitrogen, potassium and calcium, causing serious nutritional imbalances in
maize (Farooq et al. 2015). The selectivity of Na+, K+
and Ca++ in maize was markedly changed by amino acids, especially
proline treatments. Proline spraying limited Na+ uptake and improved
the K+, K+/Na+ ratio and Ca2+
selectivity uptake in maize. Zheng et al. (2015) noted that in reaction
to exogenous proline under salt stress, the increase in water potential of
leaves triggered by proline activates K+ accumulation that helps
plants change their cellular osmotic potential and therefore retain higher
water content. Cuin and Shabala (2007) showed that solutes such as proline
significantly decreased cell K+ efflux and probably retained
cytosolic K+ homeostasis through improved H+-ATPase
activity. In turn, this controls voltage-dependent outward-rectifying K+
channels and created the electrochemical gradient needed for secondary
processes of ion transport (Cuin and Shabala 2005). As well as nutrient
absorption, exogenous proline is involved in nutrient assimilation under salty
conditions. Nitrate reductase is one of the main enzymes involved in the
assimilation of nitrogen (Khan et al. 2014). Proline is a perfect way to
store and recycle nitrogen under conditions of stress (Mansour and Ali 2017).
Nitrogen deficiency has explained that proline can be used as a source of
nitrogen to develop (Hayat et al. 2012). Results of present study are
also supported by El-Samad et al. (2010) that salinity increased the Na+
content in maize shoots and roots, while Mg2+ accumulation
decreased. Proline application had a significantly increased effect on the
concentration of Mg2+ in shoots and roots under stress conditions
(Ali et al. 2008). High Na+ decreases plant absorption of Mg
and Fe due to salinity in the rhizosphere and thus induces serious nutritional
imbalances in maize (Farooq et al. 2015).
Foliar applied proline increase
in growth are consistent with Deivanai et al. (2011), where major effect
on growth traits was also reported for exogenous applied proline. Khan et al.
(2014) found improved shoots and roots, higher fresh and dry weights of shoots
and roots by exogenous applied proline under salt stress showing mitigation
effects on plant growth. Under salt-stressed condition, exogenous applied
proline significantly increased plant height (Teh et al. 2016). The
findings of present study obtained are in agreement with Perveen and Nazir
(2018) that proline regulating various physiochemical parameters under
environmental conditions in increasing development. It seems that foliar
application of proline at vegetative stage show differential response in
increasing growth by regulating different physico-chemical traits under
salinity stress (Perveen and Nazir (2018).
The
foliar applied proline had a positive significant effect on the biochemical
parameters of maize plants which are also observed by
Al-Shaheen and Soh (2016) showing higher chlorophyll content when proline was
sprayed on maize leaves. The possible reason can be regulatory function of (proline)
in detoxification of free radicals under salinity stress, causing lipid
oxidation in the cell membrane (El-Samad et al. 2010; Abuzar et al.
2011).
Al-Shaheen and Soh (2016) also
reported an increase in endogenous the concentration of leaf proline with
exogenous application of proline. Taie et al. (2013) found that stressed
plants induce a tenfold increase in the proline content of maize leaves which
gradually returned to normal level when the stress level decreased.
Lama et al. (2016)
reported that exogenous application of proline (30 mM), increased relative to the untreated plant subjected to stress.
Exogenous proline also decreased salt stress by improving antioxidant
activities and reducing the absorption and translocation of Na+
while improving plant assimilation of K+ (Bokobana et al.
2019; Moukhtari et al. 2020). Proline also plays a role in cytoplasmic
pH control or constitutes a reserve of nitrogen used by the plant under water
deficit (Kishor et al. 2005). Likely, foliar applied proline resulted in an increase of carbohydrates and
these findings are in agreement with El-Samad et al. (2010), where a
large accumulation of soluble sugar was found with foliar application of
proline. Zheng et al. (2015) also observed that exogenous proline under
salt stress showed an increase in leaf water content triggered by the
aggregation of certain organic compounds such as soluble sugars.
The
proline under stress act as a metal chelator, an antioxidative protection
molecule, and a signaling molecule (Hayat et al. 2012). Proline as an osmoregulator
specifically controls osmotic pressure in the plant to absorb water and play an
essential and efficient role in many of the plant's critical processes. It also
preserves chloroplast membranes and thus increases the efficiency of
photosynthesis and has the potential to protect cell walls and membranes and
playing an important role in scavenging free radicals, thereby mitigating the
adverse impact of stress and improving plant development, productivity, and
quality (Wu et al. 2017). Moreover, proline showed an important role in
promoting plant growth and seed yield under stress conditions, as observed in
maize (Abdelhamid et al. 2013). Proline helps to retain the status of
cell water, subcellular structures and protect membranes and proteins from
osmotic stress denaturation (Ashraf and Fooland 2007).
The maize plants, with foliar
application of proline showed an increased plant growth with a positive impact
on yield characteristics under salt stress, as reported by Alam et al.
(2016). In different plant species, proline increases salt stress tolerance.
and modulate plant growth with increases in photosynthesis and grain yield (Moukhtari
et al. 2020). Under stress, exogenous applied proline improvement in
100-grain weight and grain yield (Alam et al. 2016; Rady et al.
2019).
Conclusion
Foliar application of proline especially 400 mg/L can
help to improve salt resistance in maize as osmoprotectants or osmoregulator by
modulating growth and nutrient dynamics finally by improving grain yield and
oil percent.
Acknowledgments
This study was carried out by the
National Research Centre (NRC), the Fertilization Technology Department as part
of the Egypt-German Project "Micronutrients and Other Plant Nutrition
Problems" (Coordinator, Prof. Dr. M. M. El-Fouly) and the Institute for
Soil, Water & Climate Research, the Agriculture Research Center.
Author Contributions
All authors significantly contributed to all parts and
aspects of the paper.
Conflict of Interest
The authors declared that the present study was
performed in absence of any conflict of interest.
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