The Role of Shading on Growth, Yield and Biochemical Composition of Crisp Lettuce in Soilless Culture

Abstract

Aim of this study was to evaluate influence of various shading treatments on yield, growth and biochemical composition of lettuce (Lactuca sativa L. var. crispy) in Spring season at soilless culture. Green (35% and 55%) and white (35% and 55%) shading nets were used in the study. Plants were covered with shading materials when they reached to 9-10 true leaves. Unshaded plants were used as control treatment. The experimental design was conducted randomized split-plots with three replications. Marketable head weight, plant growth parameters, such as leaf number, head length and head diameter and biomass, dry matter, nitrate, ascorbic acid, titratable acidity, soluble solid matter (o brix) and pH were evaluated.

Full text

The Role of Shading on Growth, Yield and
Biochemical Composition of Crisp Lettuce in Soilless
Culture
Mine Aydin1*, Necdettin Saglam1, Naif Gebologlu1, Sezer Sahin2,
Emin Yilmaz1, Hakan Yucel1, Mehmet Rüştü Karaman2
1Gaziosmanpasa Univ., Agricultural Faculty, Dept. Hort., Tokat, Turkey
*E-mail : mine.aydin@gop.edu.tr
2Gaziosmanpasa Univ., Agr. Faculty, Dept. Soil Sci. Plant Nut., Tokat, Turkey
Abstract: Aim of this study was to evaluate influence of various shading
treatments on yield, growth and biochemical composition of lettuce (Lactuca sativa L.
var. crispy) in Spring season at soilless culture. Green (35% and 55%) and white (35%
and 55%) shading nets were used in the study. Plants were covered with shading
materials when they reached to 9-10 true leaves. Unshaded plants were used as control
treatment. The experimental design was conducted randomized split-plots with three
replications. Marketable head weight, plant growth parameters, such as leaf number,
head length and head diameter and biomass, dry matter, nitrate, ascorbic acid, titratable
acidity, soluble solid matter (obrix) and pH were evaluated.
Keywords: Crisp lettuce, shading, head weight, biochemical compositions
1. Introduction
Solar radiation is one of the most important factors in greenhouse during late
spring and summer season. It increases plant physiological disorders and decreases
yield and quality (Gent, 2007). High light intensity increases photochemical
damage of leaf and reduces photosynthesis during late spring and summer. Many
methods, such as fogging systems and cooling pads have been used to prevent
effects of excess solar radiation in greenhouse. In addition, use of nets for shading
is one of applications to decrease negative effects of solar radiation in greenhouse
and field conditions (Sandri et al., 2003). Different shading methods, such as
whitewashing, shading nets and curtains have been used for shading in agriculture
(Lorenzo et al., 2004; Fernandez-Rodrigez et al., 2000; Kittas et al., 2001). Lettuce
(Lactuca sativa L. var. crispa) is one of the most cultivated vegetable crops both
greenhouse and open field. It is usually consumed raw and without any restriction
for daily consuming. Lettuce contains high level of ascorbic acid (Rattler et al.,
2005) and nitrate (Burns et al., 2011). It also contains low level or moderate level
of soluble solid dry matter (obrix) and titratable acidity. The objective of this study
was to evaluate the agronomic and biochemical characteristics of lettuce grown
under different shading nets in soilless culture.
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2. Materials and Methods
Determination of Temperature, Solar Radiation and Relative Humidity:
Temperature, solar radiation level and humidity of shaded and unshaded
atmosphere were measured before 20 days until harvest in the experiment.
Temperature (oC), solar radiation (lux) and relative humidity (%) measurements
were observed two times a day (11 am and 3 pm).
Plant Material and Growing System: The experiment was conducted out in
unheated greenhouse conditions in Tokat (39º 52’- 40º 55’ North latitude and 35º
27’- 37º 39’ east longitude), Turkey. Lettuce (Lactuca sativa L.var. crispa) var.
‘Funly’ was used in the experiment. Cocopeat was used for growing medium in
Spring season (April-May), 2013. Macro and micro nutrients were added at
Hoagland solution (Arnon and Hoagland, 1940). White (35% and 55%) and green
(35% and 55%) shading nets were used in the study. Shading nets was installed on
plants as low tunnel in greenhouse and they were used when plants reached 9-10
true leaves. Unshaded plants were used as control treatment. Ten plants were
cultivated in each plot. Plants were harvested after 55 days from planting. Plant
diameter, leaf number, biomass, marketable head weight and dry weight were
measured as quantitative criteria. Dry weight was measured after leaves dried at 70
ºC for 48 hours.
Biochemical Analyses: Nitrate was measured with a Shimadzu UV mini
1240 spectrophotometer (Shimadzu, Kyoto, Japan), using the salicylic-sulfuric acid
method (Cataldo et al., 1975). Each sample (200 mg of fresh matter) was extracted
with 10 ml hot water (at 90-950C) and shaken form in, then centrifuged at 5000
rpm for 15 min. The supernatant was recovered and 200µl were added to 800µl of
5% salicylic-sulfuric acid, then 19 ml of sodium hydroxide (2N NaOH) were
slowly added. The spectrophotometer readings were taken at 410 nm. Data
compared with a standard nitrate reference. The ascorbic acid concentration was
measured with a Shimadzu UV mini 1240 spectrophotometer (Shimadzu, Tokyo,
Japan) using titrimetric method modified from Huang and Liu (2001).
Metaphosphoric acid solution (6%) was used for extraction of the ascorbic acid.
After that extraction was filtrated with Whatman No 1.Filtrated solution was
recovered and 200 µl were added to 800 µl of 6% metaphosphoric acid solution,
then 4 ml 2,6 dichlophenolindophenol sodium were added. The spectrophotometer
readings were taken at 520 nm. Data compared with a standard ascorbic acid
reference. The soluble solid dry matter was measured by digital refractometer. The
titratable acidity expressed relative to citric acid was determined by recommended
procedures. The samples were diluted with deionized water and titrated with
aqueous NaOH 0,1 N using ethanol phenolphthalein (1%) as indicator. The pH
measurements were carried out on a model Hanna HI 2211 pH meter. The
experimental design was randomized split plots with three replications. Data were
analyzed to one-way analysis of variance (ANOVA). The means were evaluated by
Duncan's multiple range test, P ≤ 0.05.
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3. Results and Discussion
Temperature, Solar Radiation and Relative Humidity: The highest daily
average temperature was determined under white 55% shading net, and it was
followed by white 35%, unshaded, green 55% and green 35%, respectively. The
highest daily average relative humidity was observed under white 35% shading net,
and it was followed by white 55%, green 35%, unshaded and green 55%,
respectively. Daily average temperature and relative humidity observations were
given in Fig1. Daily mean light intensity (solar radiation) was determined between
5110-13500 lux in unshaded treatment, 3780-8800 lux in white 35% shading net,
3135-7000 lux in green 35% shading net, 2960-6900 lux in white 55% shading net
and 1700-4250 lux in green 55% shading net. Daily average solar radiation was
given in Fig 2.
A
b
Fig 1. Daily changes of temperature (a) and relative humidity (b) under the
shaded and unshaded treatments
Interaction between shading application and quantitative characteristics:
There were significant interactions between shading treatments and quantitative
characteristics in lettuce. Biomass was determined between 654.43-976.92 g
according to shading applications. Biomass was higher under white shading net
than green. Marketable head weight was found out between 496.22-801.62 g
according to shading applications. Control was not affected significantly by 35%
white and green shading nets. The highest leaf number was determined in control
plants. Leaf number was decreased when shading rate increased. Differences
between control plants and 35% white and green shading nets were no significant.
Head diameter was found out between 33.00-38.17 cm. The highest head diameter
was measured on plants, cultivated under white 55% shading nets. There were no a
linear interaction between head diameter and shading materials. In the study, dry
weight was determined between 4.28-5.37 %. The highest dry weight was found
out in control plants. Dry weight was decreased when shading rate increased.
Those observations were given in the Table 1.
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Fig 2. Daily changes of solar radiation under the shaded and unshaded treatments
All the yield parameters were significantly affected by shading net rate and
color. Unshaded plants were stronger than shaded plants. Marroua et al., (2013)
indicated that the number of leaves and fresh weight of lettuce plants were greater
under full sunlight cultivation compared with shaded plants. Similarly, Thicoipe
(1997), found that fresh weight of lettuce grown under low solar radiation during
winter was lower than plants grown under high solar radiation during summer. In
another study, Zhou et al., (2009), found that dry weight was decreased from
5.99% (high light) to 0.97% (low light). Similar results represented by Sanchez et
al., (1989). In the another study similar results with lettuce dry matter yield of
10.14, 7.69 and 3.29 g/plant were observed in an experiment with shading at 0,
18% and 50% respectively. There was also a reduction in biomass production of
lettuce when shading rate increased from 10 to 90%, falling from 225.7 to 81.0
g/plant (Bryne et al., 2002). Our results are in agreement with the literature.
Interaction between shading application and qualitative characteristics:
There are important interactions between levels of shading and nitrate content.
Nitrate content was changed between 1714.52 mg.kg-1 (unshaded control plants)
and 2290.72 mg.kg-1 (shaded with green 55% nets). Nitrate content was increased
with increasing shading rate. Nitrate contents were no determined over level of
2500 mg NO3.kg-1 (European Union limit for lettuce) of fresh weight in all
treatments. Leafy vegetable crops, such as lettuce and spinach, are known to
accumulate high level of nitrate. Furthermore, shading causes higher nitrate
accumulation when it is compared with full light conditions. Low light intensity
encourages nitrate accumulation due to absorption rate of nitrate (Bloom-Zandstra
et al., 1988). Light intensity is the most important environmental factor for nitrate
accumulation in plants because plants usually accumulate more nitrates when they
are subjected to low light intensity (Ohse, 2000). This explains why nitrate level is
higher in our experiment. Cometti et al., (2011), showed that nitrate accumulation
is highly related to shading. Results are similar to them.
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Table 1. Effect of shading treatments on quantitative traits.
Shading
treatments
Shading
Rate,%
Biomass
g
Marketable
head w., g
Leaf nu.
leaves.plant-1
Head
diameter,cm
Dry w.
%
White
35
948.24 ab**
757.99 a**
39.30 ab*
37.97 a*
4.76 b**
55
811.62 b
558.29 b
34.33 b
38.17 a
4.28 c
Green
35
911.14 ab
727.97 a
37.83 ab
33.93 ab
4.59 bc
55
654.43 c
496.22 b
33.17 b
33.00 b
4.46 bc
Control
0
976.92 a
801.62 a
42.50 a
37.17 ab
5.37 a
(*) and (**) refers to significant at P≤0.05 and P≤0.01. Values marked in columns with the
same letter do not differ significantly for 5% level.
Table 2. Effect of shading treatments on qualitative traits.
Shading
treatments
Shading
rate (%)
Nitrate
(mg.kg-1)
Ascorbic acid
(mg.100 g-1)
(oBrix)
(%)
Titr.acidity
(%)
pH
White
35
1912.15 b**
134.76 bc**
3.53 a*
0.24 a*
5.53ns
55
2228.81 a
125.53 cd
1.40 ha
0.18 bc
5.59
Green
35
2183.57 a
145.38 ab
3.43 a
0.21 ab
5.55
55
2290.72 a
119.88 d
3.17 a
0.18 bc
5.41
Control
0
1714.52 b
151.13 a
2.43 b
0.16 c
5.41
(*) , (**) and (ns) refers to significant at P≤0.05 and P≤0.01 and insignificant, respectively.
Values marked in columns with the same letter do not differ significantly for 5% level.
When shading rate increased, ascorbic acid level was decreased. It is known
that light conditions affect ascorbic acid level of fruit and vegetables. Lower light
intensity generally decreases ascorbic acid concentration (Mozafar, 1994). The
highest ascorbic acid was obtained from higher light intensities. Similar results
were obtained by Ohashi-Kaneko et al., (2007) and Li and Kubota (2009) in
spinach and lettuce. Soluble solid dry matter (obrix) is an important quality criteria
in vegetables and fruits. Soluble solid dry matter content was affected significantly
from shading treatments. Titratable acidity was increased by shading nets, but there
was no linear relationship. It was affected significantly. There was no significant
difference between treatments for pH values..
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