Sciences in Cold and Arid Regions  2017, 9 (2): 120-126 PDF

#### Article Information

Photosynthetic pigments accumulation and some growth indices of cowpea, maize and tomato in response to interspecific and intraspecific competition stress
Sciences in Cold and Arid Regions, 9(2): 120-126
http://dx.doi.org/10.3724/SP.J.1226.2017.00120

### Article History

Accepted: January 10, 2017
Photosynthetic pigments accumulation and some growth indices of cowpea, maize and tomato in response to interspecific and intraspecific competition stress
Department of Botany, Faculty of Science, Obafemi Awolowo University, Ile-Ife, Osun state, Nigeria
Abstract: This study aimed at investigating the photosynthetic pigment accumulation and some growth indices of cowpea, maize and tomato in response to interspecific and intraspecific competition stress. The study was carried out under a screen-house to minimize extraneous factors such as pests and rodents using a randomized complete block design (RCBD). Seeds of cowpea, maize and tomato were collected from the Department of Crop Production and Protection, Faculty of Agriculture, Obafemi Awolowo University, Ile Ife, Osun state, Nigeria. These seeds were planted at a depth of about 3 mm below the soil. The seeds were sown at the rate of six seeds per pot in the monoculture, while in the pots designed for the mixed culture of maize and cowpea, maize and tomato, cowpea and tomato, three seeds of each plant were sown. Two seeds of each plant were sown in the pots with the three crops. The treatments were then supplied with 500 mL of tap water in the morning and in the evening respectively until the seedlings become fully established. The photosynthetic pigments were determined spectrophotometrically with three replicates. Plant growth indices were determined according to Hunts (1978) using leaf area and dry matter data collected at four and six weeks after planting. Statistical analysis was performed using statistical analytical software SAS version 9.2. The results indicated that photosynthetic pigments accumulation (Chlorophyll a, b and carotenoid) in maize (15.98, 23.92 and 44.72 μM), Chlorophyll b and carotenoid in tomato plants (12.48 and 1,178.7 μM) in the sole stands were more than the mixed culture of maize with cowpea and tomato (7.195, 14.74 and 0.00 μM). Also, total Chlorophyll in maize (1,127.8 μM), Chlorophyll a and total Chlorophyll in tomato (3.95 and 1,317.5 μM) in the mixed culture were more than in the sole culture of maize (1,030.9 μM) and tomato plants (−9.40 and 546.3 μM). The different photosynthetic pigments accumulated in cowpea were greatly enhanced in the mixed culture than in the sole culture. All the photosynthetic pigments of maize, cowpea and tomato in all the treatments analysed in this study were significantly different atP>0.05. Plant growth indices such as net assimilation rate, relative growth rate, crop growth rate and tissue water contents of these plants had higher value in the intercropped than the check crop (sole). These plant growth indices were significantly different to one another atP>0.05. This study concluded that competition for shared resources in the mixed culture of tomato, maize and cowpea enhanced growth and accumulation of photosynthetic pigments.
Key words: pigments     competition     growth     monoculture     intercropping
1 Introduction

Competition is one of the biotic stress factors that influence growth, yield, reproductive capacity and survival of plants. Competition exists between plants where independent demands for environmental factors exceed supply and this can be intraspecific or interspecific competition (Weiner, 1990). Intraspecific competition is the competition among plants of the same species while interspecific competition is the competition between different plants or, with plants of different species. Plants of the same or different species do not compete together as long as water-content, nutrient material, light and heat are in excess of the needs of both. When the immediate supply of a single factor falls below the combined demands, the plants are stressed and competition begins.

Competition for light is influenced by plant species, if the species is native to shaded or sunny environments (Floss, 2008). When there is competition for light between a plant (large) and its neighbor (small), height growth will help individual plants (large) avoid the shading effects of its neighbor (small) on itself and enable it to acquire more of the light resource (Schmitt and Wulff, 1993; Schmitt et al., 1995; Dorn et al., 2000). However, as the competition for light increases further, an individual plant's (small) growth rate will become very low because the shading effects it experiences are very strong.

The intensity of light competition endured by a small plant is much greater than that endured by a larger plant owing to size-asymmetric light competition between them (Weiner, 1990). The height growth of small plants is more rapid than that of large plants (Geber, 1989). However, when the light competition intensifies further, the height growth of a small plant will weaken earlier than that of a large plant because the shading effects that the smaller plant experiences are considerably greater than those experienced by the large plant. Light interception and light use efficiency are powerful concepts for characterizing the resource capture and use efficiency of cropping systems. Most annual crop mixtures such as those involving cereals and legumes are grown almost at the same period, and develop root systems that explore soil zone for resources (Jensen et al., 2003). Under such conditions, below-ground competition for resources such as water and nutrients is mostly likely to occur.

Light interception is increased as a result of mixing two species and growing them together than when in pure stands (Keating and Carberry, 1993). This means that plants in the intercrops intercept more light energy than monocultures. Hence, it is imperative to study the effect of competition stress on pigments accumulation and growth of crops to expound a better understanding of the physiological responses of the crops under study to competition stress. The specific objective of this study was to determine the photosynthetic pigments accumulation and some growth indices of cowpea, maize and tomato in response to interspecific and intraspecific competition stress.

2 Materials and methods

The seeds of cowpea (IT07K-38-33), maize (2008 DTMA-YSTR) and tomato (ROMA VF) were utilized in this experiment. These seeds were collected from the Department of Crop Production and Protection, Faculty of Agriculture, Obafemi Awolowo University, Ile Ife, Osun state, Nigeria. A screen-house was constructed to minimize extraneous factors such as pests and rodents, supply of water other than the amount specifically applied. The mean daily temperature under the screen-house was taken with the aid of a thermometer and the average temperature was 35 °C. The intensity of light was also determined using a digital luxmeter LX 1000 and the average light intensity was 8400 LUX.

Forty two bowls were obtained (38 cm in diameter, 5.5 cm in height). Holes of about 3 mm each were bored at the bottom of the bowls, allowing for proper drainage and prevent water logging during the course of the experiment. The bowls were filled near brim with 10 kg of the analyzed soil. The seeds of cowpea, maize and tomato were then planted at a depth of about 3 mm below the soil. The seeds were sown at the rate of six seeds per pot in the monoculture, while in the pots designed for the mixed culture of maize and cowpea, maize and tomato, cowpea and tomato, three seeds of each plant were sown. Two seeds of each plant were sown in the pots with the three crops. The bowls were then supplied with 500 mL of tap water in the morning and 500 mL of tap water in the evening until the seedlings become fully established at two weeks after planting.

The seedlings were divided into seven regimes which include the following: T1 = sole maize (SM); T2 = Maize intercrop with cowpea (MC); T3 = Maize intercrop with tomato (MT); T4 = Sole tomato (ST); T5 = Tomato intercrop with cowpea (TC); T6 = Sole cowpea (SC); T7 = Maize intercrop with cowpea and tomato (MCT). All the groups of plants were made to receive 500 mL of water every morning and evening throughout the experimental period. An experiment laid out in a Randomized Complete Design (RCD) was used with six replicates.

The photosynthetic pigments were determined spectrophotometrically. Fresh green tomato leaves were plucked from the differential treatments. 5.0 g of these leaves were grinded in 20 mL of 80% acetone and a pinch of Na 2CO 3 was added to prevent the degradation of Chlorophyll b to phaeophytin. The brew obtained was filtered using Whatman No.1 filter paper. Each coloured filtrate obtained from the seven treatments was fed into separate cuvettes. The optical absorbance of the coloured solution from each treatment in the cuvettes was determined at two specified wavelengths, 647 nm and 664 nm respectively (Combs et al., 1985). A Beer-Lambert equation was used to determine the concentrations of Chlorophyll b a and Chlorophyll b b and the carotenoid in the leaf extract as follows:

 ${\rm{Chlorophyll b a }}\left(\text{μ} {{\rm{g/mL}}} \right) = 12.21{\rm{A}}663 \text{−} 2.81{\rm{A}}646$
 ${{\rm{Chlorophyll b b}}\left( \text{μ}{{\rm{ g/mL}}} \right) = 20.13{\rm{A}}646 \text{−} 5.03{\rm{A}}663}$
 \begin{aligned}\!\!\!\! & {\rm{Carotenoids}}\left( \text{μ}{ {\rm{g/mL}}} \right) = \\& \left( {1000{\rm{A}}470 \text{−} 3.27\left[ {{\rm{Chl \,\,\, a}}} \right] \text{−} 104\left[ {{\rm{Chl \,\,\, b}}} \right]} \right)/227 \qquad \end{aligned}
 ${\rm{Total \,\,\, Chlorophyll b}}\left(\text{μ}{{\rm{M}}} \right) = 7.93{\rm{A}}663 + 19.53{\rm{A}}646$

In the carotenoid equation, "[Chl a]" and "[Chl b]" refer to the calculated concentration of Chl a and Chl b from the previous equations. A663 represent the absorbance at wavelength 663 nm while A646 represents the absorbance at wavelength 646 nm.

Crop growth rate, CGR; relative growth rate, RGR; net assimilation rate, NAR; leaf area ratio, LAR; and tissue water contents, TWC were determined according to Hunts (1982) using leaf area and dry matter data collected at 4 (t1) and 6 (t2) WAP as follows:

 $\begin{array}{l}{\rm{CGR}} = \left( {W2 \text{−} W1} \right)/\left( {t2 \text{−} t1} \right);\\{\rm{RGR}} = {\rm{NAR}} \times {\rm{LAR}};\\{\rm{NAR}} = \left( {W2 \text{−} W1} \right)\left( {\ln A2 \text{−} \ln A1} \right)/\left( {A2 \text{−} A1} \right)\left( {t2 \text{−} t1} \right);\\{\rm{LAR}} = \left( {A2 \text{−} A1} \right)\left( {\ln W2 \text{−} \ln W1} \right)/\\\quad \quad \quad \, \left( {W2 \text{−} W1} \right)\left( {\ln A2 \text{−} \ln A1} \right);\\{\rm{TWC}} = \left( {{\rm{Fresh}}\;{\rm{weight}} - {\rm{dry}}\;{\rm{weight}}} \right)/{\rm{fresh}}\;{\rm{weight}} \times \\\quad \quad \quad \, 100\left( {{\rm{Black}}\;{\rm{and}}\;{\rm{Pritchard}},2002} \right)\end{array}$

W1 and W2 is weight at t1 and t2 respectively, while A1 and A2 are the respective leaf area. Leaf area was calculated from leaf length and width as follows:

 $\begin{array}{l}{\rm{LA}} = {\rm{L}} \times {\rm{W}} \times {\rm{CF}}.\;{\rm{Where}\,\, {CF}\,\, {is} \,\,{the}\,\,{correction}\,\,{ factor}}\\{\rm{CF}} = 0.64\;{\rm{for}}\;{\rm{cowpea}},0.73\;{\rm{for}}\;{\rm{maize}}\;{\rm{and}}\\\quad \quad \,\, 0.78\;{\rm{for}}\;{\rm{tomato}}\left( {{\rm{Awal}}\;et\;al.,2004} \right)\end{array}$

Statistical analysis was performed using statistical analytical software SAS version 9.0. A one way analysis of variance (ANOVA) was carried out to investigate the effect of competition stress on the growth and photosynthetic pigments accumulation of maize, tomato and cowpea. Post hoc setting testing was carried out using Duncan Multiple Range Test to separate the significance means at 0.05 confidence limit (alpha level) for the mean.

3 Results and discussion 3.1 Photosynthetic pigments accumulation of cowpea in response to interspecific and intraspecific competition stress

To capture as much light as possible, shade-grown plants typically have more light-harvesting complexes per unit area (Okunlola and Adelusi, 2014). Therefore, this was the major reason why Chlorophyll b a and b content was higher in T7-plants and lowest in T5-plants. However, the high increase in the level of carotenoid and total Chlorophyll b of T2-plants compared to other treatments was because T2-plants have a better advantage of trapping more solar radiation than other treatments. This allowed T2-plants to accumulate more of carotenoid and total Chlorophyll b compared to other treatments. The decrease in the level of carotenoids and total Chlorophyll b in T7- and T5-plants could be an indication of Chlorophyll b destruction by excess irradiance. These results corroborate many studies made with sun or high light and shade or low-light leaves. The result of ANOVA shows that Chlorophyll b a, Chlorophyll b b, carotenoid and total Chlorophyll b accumulation of cowpea in all the treatments were significantly different from one another (P>0.05).

Table 1 Photosynthetic pigments accumulation of cowpea in response to interspecific and intraspecific competition stress
 Treatments Chlorophyll a Chlorophyll b Totalchlorophyll Carotenoid T2 9.24a 9.24a 1,145.6a 22.44a T5 −3.13c −3.13c 768.6b −9.15c T6 3.13b 3.14b 1,114.2a 8.39b T7 10.95a 10.95a −490.4c −2.44d SE 2.75 2.85 30.53 4.035 CV% 184.00 126.82 121.22 184.00 Means with the same letter along the same column are not significantly different at P>0.05, T2: Maize intercrop with cowpea (MC); T5: Tomato intercrop with cowpea (TC); T6: Sole cowpea (SC); T7: Maize intercrop with cowpea and tomato (MCT); SE: Standard error; CV: Coefficient of variance
3.2 Photosynthetic pigments accumulation of maize in response to interspecific and intraspecific competition stress

As a result of competition among plants of different species, plant species present high light use efficiency when evaluated alone (Floss, 2008). Results of the present study conformed to this finding in which accumulation of Chlorophyll b a, Chlorophyll b b and total Chlorophyll b was highest in T1-plants compare to other treatments. This result is in agreement with Santos et al. (2003) who noted that the light use efficiency of bean and soybean and of weed species (Euphorbia heterophylla) accumulated more photosynthetic pigments and dry mass per unit of light interception in their sole cultures. This also contradicts the accumulation of carotenoid which was highest in T3-plants compare to other treatments in the present study. The higher accumulation of carotenoid in these plants was brought about by high light intensities. Carotenoids are very important in protecting plants against photo-damage (Ort, 2001). The decreased content of both Chlorophyll bs and carotenoid T1-, T2- and T7-plants could probably be the result of mineral deficiency. The result of ANOVA shows that Chlorophyll b a of T1-, T2-, T3- and T7-plants were not significantly different from one another (P>0.05), meanwhile Chlorophyll b b, carotenoid and total Chlorophyll b accumulation of T1-, T2-, T3- and T7-plants were significantly different from one another (P>0.05).

Table 2 Photosynthetic pigments accumulation of maize in response to interspecific and intraspecific competition stress
 Treatments Chlorophyll a Chlorophyll b Total chlorophyll Carotenoid T1 15.98a 23.92a 1,030.9b 44.72a T2 12.82a 12.03c 974.9c 27.83b T3 9.93a 15.56b 1,127.8a 28.56b T7 7.195a 14.74bc 903.7d 25.62c SE 1.11 1.26 2.97 1.56 CV% 32.91 30.95 9.37 27.72 Means with the same letter along the same column are not significantly different at P>0.05, T1: Sole maize (SM); T2: Maize intercrop with cowpea (MC); T3: Maize intercrop with tomato (MT); T7: Maize intercrop with cowpea and tomato (MCT); SE: Standard error; CV: Coefficient of variance
3.3 Photosynthetic pigments accumulation of tomato in response to interspecific and intraspecific competition stress

From the result of the present study, Chlorophyll b b accumulation and total Chlorophyll b accumulation was highest in T4-plants and was lowest in T7-plants. This is also in line with Santos et al. (2003) on bean and soybean and of weed species (Euphorbia heterophylla) in their sole cultures. This also contradicts Chlorophyll b a accumulation which was higher in T5-plants and lowest in T3-plants and carotenoid accumulation which was found to be higher in T7-plants and lower in T3-plants. High light intensity has an effect on Chlorophyll b a and carotenoid biosynthesis. This explains why T5-plants and T7-plants recorded higher Chlorophyll b a and carotenoid than T3-plants which are shaded. The level of Chlorophyll b a accumulation and proliferation of carotenoid which was lower in T3-plants compare to other treatments was as a result of shading effects of its competitor (maize). It is notable that both Chlorophyll bs observed to increase under low light conditions (Wijanarko et al., 2007). The result of ANOVA shows that Chlorophyll b a, Chlorophyll b b, carotenoid and total Chlorophyll b accumulation of tomato in all the treatments were significantly different from one another (P>0.05).

Table 3 Photosynthetic pigments accumulation of tomato in response to interspecific and intraspecific competition stress
 Treatments Chlorophyll a Chlorophyll b Total chlorophyll Carotenoid T3 9.24a 9.24a 1,145.6a 22.44a T4 −3.13c −3.13c 768.6b −9.15c T5 3.13b 3.14b 1,114.2a 8.39b T7 10.95a 10.95 −490.4c −2.44d SE 2.75 2.85 30.53 4.035 CV% 184.00 126.82 121.22 184.00 Means with the same letter along the same column are not significantly different at P>0.05, T3: Maize intercrop with tomato (MT); T4: Sole tomato (ST); T5: Tomato intercrop with cowpea (TC); T7: Maize intercrop with cowpea and tomato (MCT); SE: Standard error; CV: Coefficient of variance
3.4 Interspecific and intraspecific competition stress effects on the plant growth indices of cowpea

The significant difference at P>0.05 in leaf area ratio, crop growth rate, relative growth rate and tissue water contents of all the treatments in sole cultures and in the intercrops is a result of maximal use of resources such as space, light and nutrients (Ndakidemi, 2006). Meanwhile, plant growth indices which was highest in T2-plants compared to other treatments is probably because of growth habit and competitiveness of T2-plants intercropped with maize plants. The lower relative growth rate observed in T6-plants and to the intercrop is attributed to competition for the same growth resources such as solar energy, soil nutrients and water in the sole culture. The lower tissue water content which is observed in T7-plants to other treatments is a result of competition for shared use of resource (water) in the mixed culture.

Table 4 Interspecific and intraspecific competition stress effects on the plant growth indices of cowpea
 Treatments Net assimilation rate Leaf area ratio Crop growth rate Relative growth rate Tissue water contents (%) T2 0.01a 2.87a 0.69a 0.022a 94.37a T5 0.0065a 1.52ab 0.41ab 0.0097b 74.21b T6 0.0068a 0.96b 0.35ab 0.0055b 92.18a T7 0.0058a 1.21b 0.091b 0.0068b 59.43b SE 0.026 0.66 1.11 0.073 1.84 CV% 30.53 51.94 63.87 69.51 20.55 Means with the same letter along the same column are not significantly different at P>0.05, T2: Maize intercrop with cowpea (MC); T5: Tomato intercrop with cowpea (TC); T6: Sole cowpea (SC); T7: Maize intercrop with cowpea and tomato (MCT); SE: Standard Error; CV: Coefficient of variance
3.5 Interspecific and intraspecific competition stress effects on the plant growth indices of maize

A higher leaf area ratio in T3-plants to other treatments was due to competitive advantage of T3-plants over its companion (tomato plants). This is in correlation with higher increase in the shoot height of T3-plants compared to other treatments. The lower leaf area ratio which was recorded in T7-plants is attributed to low irradiance received by T7-plants. This is in agreement with Peace and Grub (1982) who found that low irradiance reduce the unit leaf rate in Impatiens parviflora. One of the useful indices of plant growth is crop growth rate and relative growth. These plant growth indices were higher in T1-plants compared to other treatments. This can be attributed to the fact that plant growth of T1-plants is quite rapid at the early stages of plant life. The lowest relative growth rate and crop growth observed in T3-plants was due to the influence of its neighbor which is an important factor that influences plant allocation patterns and architecture when plants are in mixed culture (Aarssen, 1995). The reduction in net assimilation rate in T3-plants is in agreement with Boyer (1985) who showed that low seed yield at low water potential can be accounted for by lack of assimilate supply at flowering. In the case of tomato/maize intercropping, tomato may absorb nutrients and water from deeper soil profiles than maize as it has a deeper rooted system, whereas maize could be satisfied from the shallower soil zones as its root system is fibrous. This account for the higher rate of net assimilation, crop growth and relative growth in T1-plants and lower rate in T3-plants compare to other treatments. Greater water extract by T1- and T7-plants could be the major reason for greater tissue water contents in T1- and T7-plants compared to T2- and T3-plants. The result of ANOVA shows that leaf area ratio of T1-, T2-, T3- and T7-plants were significantly different from one another. No significantly different (P>0.05) were observed in other growth indices of maize.

Table 5 Interspecific and intraspecific competition stress effects on the plant growth indices of maize
 Treatments Net assimilation rate Leaf area ratio Crop growth rate Relative growth rate Tissue water contents (%) T1 0.22a 259.08ab 0.63a 0.00086a 81.80a T2 0.043a 250.49ab 1.10a 0.00023a 84.17a T3 0.032a 302.75a 0.34a 0.00004a 84.30a T7 0.16a 189.28b 1.39a 0.00017a 81.80a SE 0.077 3.61 0.56 0.02 0.15 CV% 44.69 22.95 57.74 112.89 1.70 Means with the same letter along the same column are not significantly different at P>0.05, T1: sole maize (SM); T2: Maize intercrop with cowpea (MC); T3: Maize intercrop with tomato (MT); T7: Maize intercrop with cowpea and tomato (MCT); SE: Standard error; CV: Coefficient of variance
3.6 Interspecific and intraspecific competition stress effects on the plant growth indices of tomato

The observed higher net assimilation, crop growth and relative growth rates in T4-plants can be attributed to higher Chlorophyll b content and therefore higher rate of photosynthesis. This is in agreement with McDonald et al. (1992) who found that light and nutrient interactions were significant for relative growth rate, crop growth rate and net assimilation. The lower net assimilation, crop growth and relative growth rates in T7-plants compared to other treatments is attributed to lower rate of photosynthesis. The significant difference in the leaf area ratio of T4- and T7-plants was a result of different patterns of canopy development and different maturation times in the sole and mixed culture (Hailu, 2015). The level of tissue water content in the treatments which was higher in T5-plants is attributed to greater root concentrations or complementary exploration of the soil profile than its companion (cowpea). Intercropping of plants with different rooting patterns create access to mobile and immobile nutrients (Hailu, 2015). The result of ANOVA shows that leaf area ratio and tissue water contents in all the treatments were significantly different from one another. No significantly different (P>0.05) were observed in other growth indices.

Table 6 Interspecific and intraspecific competition stress effects on the plant growth indices of tomato
 Treatments Net assimilation rate Leaf area ratio Crop growth rate Relative growth rate Tissue water contents (%) T3 0.01a 2.24ab 0.13a 0.013a 88.26ab T4 0.026a 1.02b 0.40a 0.02a 92.13a T5 0.0085a 3.67ab 0.15a 0.019a 92.51a T7 0.004a 4.85a 0.14a 0.012a 86.76b SE 0.026 0.66 0.39 0.087 0.30 CV% 30.53 51.94 63.87 100 3.17 Means with the same letter along the same column are not significantly different at P>0.05, T3: Maize intercrop with tomato (MT); T4: Sole tomato (ST); T5: Tomato intercrop with cowpea (TC); T7: Maize intercrop with cowpea and tomato (MCT); SE: Standard error; CV: Coefficient of variance
4 Conclusion

Photosynthetic pigments and growth indices of tomato, maize and cowpea were enhanced in mixed culture as compared with their yield and accumulation of photosynthetic pigments in the sole cultures. This is due to competition of shared resource used in the mixed culture and the same resource incurred in the sole culture. For optimum growth, it can therefore be recommended that maize, cowpea and tomato be grown together.

Acknowledgments:

The authors' sincere appreciation goes to Professor A.A. Adelusi, not only for the part he played as my supervisor but also for his fatherly role, advice and unflinching supports throughout the period of this research. The authors also thank him for making himself available and giving me the opportunity to obtain access to his personal laboratory facilities.

Reference