ABSTRACT
Irrigation water stress and low water use efficiency (WUE) or water productivity (WP) both influence crop growth and fruit quality. Therefore, the corresponding relationship between amount of water supply and synthesis of bioactive compounds requires to be understudied. In this regulated deficit irrigation research study, three irrigation treatments were imposed on two commercial garden egg cultivars (Solanum aethiopicum and Solanum gilo) to determine their effects on several plant’s growth and fruit indicators as well as their effects on the phytochemical composition of garden egg fruits. The study was conducted in open field (uncontrolled condition) with the garden egg plants fed solely by rain water, which served as control and in a greenhouse where plants were subjected to three water supply treatments – 100FC, 70FC and 50FC and treatment effects were compared with the control. Antioxidant activity was evaluated using DPPH, FRAP, H2O2 and NO assays. Deficit irrigation had no significant effect on some plant growth and fruit yield indicators amongst treatments, but was significant when compared with the control. This suggests that sunlight influences garden egg growth than water supply, as the solar radiation recorded outside the greenhouse was thrice higher than inside the greenhouse. Deficit irrigation significantly increased antioxidant contents in garden egg fruits, but the degree of increment was found to be cultivar-dependent. Overall, the choice of appropriate cultivars subjected to deficit irrigation strategies can significantly influence the bioactive compounds in garden egg, particularly total phenolic contents (TPC) and total flavonoid contents (TFC).
TABLE OF CONTENTS
Title Page i
Declaration Page ii
Certification Page iii
Dedication iv
Acknowledgement v
Table of Contents vi
List of Tables ix
List of Figures x
List of Plates xi
Abstract xii
CHAPTER I: INTRODUCTION
1.1 Background of Study 1
1.2 Statement of Problem 2
1.3 Objectives of Study 3
1.4 Justification 4
CHAPTER II: REVIEW OF RELATED LITERATURE
2.1 Irrigation 5
2.1.1 Surface Irrigation 6
2.2 Sub – Surface Irrigation 8
2.3 Agronomic approaches for enhanced nutritional/bioactive contents in fruit crops 10
2.3.1 Irrigation water management approach 12
2.3.2 Fertilization approach 10
2.3.3 Cultivation on substrates approach 13
2.4 Deficit irrigation for improved water productivity 15
2.4.1 Water use efficiency/crop water productivity under deficit irrigation 16
2.4.2 Crop growth and yield components under deficit irrigation 16
2.5 Crop water requirement measurement 17
2.5.1 Factors affecting evapotranspiration
2.6 Deficit irrigation and main techniques 22
2.6.1 Stage-based deficit irrigation/regulated deficit irrigation 22
2.6.2 Partial root-zone irrigation 24
2.6.3 Subsurface irrigation or infiltration movement 26
2.7 Chemical and hydraulic signaling in deficit irrigation 28
2.8 Physiological and biochemical responses of plants under deficit irrigation 29
2.8.1 Physiological basis 29
2.8.2 Biochemical basis 31
2.9 Gas exchange (co2 and h2o vapour) in deficit irrigation 32
2.9.1 Gas exchange modelling in deficit irrigation 34
2.9.2 Root development and water uptake in deficit irrigation 36
2.10 Field and greenhouse studies on deficit irrigation 38
2.10.1 Deficit irrigation on maize 38
2.10.2 Deficit irrigation on wheat and hot pepper 39
2.10.3 Deficit irrigation on tomato and sorghum 41
2.10.4 Deficit irrigation on beans and potato 42
2.10.5 Deficit irrigation on cotton and pear 44
2.10.6 Deficit irrigation on apple, grape and olive 45
2.11 Production of garden egg in Nigeria 48
2.11.1 Importance/benefits of garden egg 50
2.12 In-vitro methods of evaluation of antioxidants activity 52
CHAPTER III: MATERIALS AND METHODS
3.1 Study Area 56
3.1.1 Preliminary Test 56
3.2 Experiment 1 (greenhouse experiment) 58
3.2.1 Plant Materials and Growing Conditions 58
3.2.2 Irrigation treatments 59
3.2.3 Measurements during crop growth 60
3.2.4 Measurement of fruit yield and quality 61
3.3 Measurement of antioxidant activity 61
3.3.1 DPPH scavenging activity 61
3.3.2 Hydrogen peroxide scavenging (h2o2) assay 62
3.3.3 Nitric oxide scavenging activity 62
3.3.4 Ferric reducing-antioxidant power (frap) assay 63
3.4 Experiment 2 (field experiment) 63
3.4.1 Plant materials and growing conditions 63
3.4.2 Measurements during crop growth 64
3.4.3. Statistical analysis 64
CHAPTER IV: RESULTS AND DISCUSSIONS
4.1 Dynamics of soil moisture content and irrigation water use 66
4.2 Plant growth indicators 67
4.3 Plant fruit yield indicators 69
4.4 Bioactive compounds and antioxidant activity 70
4.5 Correlation between bioactive compounds and antioxidant activity 76
4.6 Statistical comparison of greenhouse and field plant growth and fruit yield indicators 79
CHAPTER V: CONCLUSIONS AND RECOMMENDATIOS
5.1 Conclusions 87
5.2 Recommendations 88
References 90
Appendices 102
LIST OF TABLES
2.1: Popular models applied in the simulation of stomatal conductance for field crops. 36
4.1: Plant growth indicators at harvest of garden egg cultivars (solanum gilo and solanum aethiopicum) subjected to irrigation treatments based on soil field capacity. 67
4.2: Plant fruit yield indicators at harvest of garden egg cultivars (solanum gilo and solanum aethiopicum) subjected to irrigation treatments based on soil field capacity. 69
4.3: Antioxidant activity (DPPH, FRAP, H2O2 and NO) and antioxidant components (TFC and TFC) in fruits of garden egg cultivars (solanum gilo and solanum aethiopicum) subjected to irrigation treatments (100FC, 70FC and 50FC). 70
4.4: Pearson correlation coefficients for individual antioxidant components and the antioxidant activity determined by different assays 76
4.5: Results of MLR analysis showing the contribution of bioactive compounds to the antioxidant activity determined by different assays 76
4.6: Statistical comparison of plant growth and yield parameters of two varieties of garden egg (S.aethiopicum and S.gilo) for 100FC irrigation treatment and open field (control). 79
4.7: Statistical comparison of plant growth and yield parameters of two varieties of garden egg (S.aethiopicum and S.gilo) for 70FC irrigation treatment and open field (control). 80
4.8: Statistical comparison of plant growth and yield parameters of two varieties of garden egg (S.aethiopicum and S.gilo) for 50FC irrigation treatment and open field (control). 81
4.9: Proximate analysis of the experimental garden egg cultivars 85
4.10: Physical soil properties of the experimental site 85
4.11: Analysis of Variance for Plant Height Data 85
4.12: Analysis of Variance for Plant Stem Diameter Data 86
4.13: Analysis of Variance for Leaf Area Data 86
4.14: Analysis of Variance for Fruit Yield Data 86
LIST OF FIGURES
2.1: Physiological and molecular bases of drought stress tolerance. (Shao et al., 2008). 26
2.2: Main deficit irrigation approaches. (Chai et al., 2016). 28
2.3: Typical relationship between photosynthesis rate (An) and stomatal conductance (gs). The arrow shows the point where An decreases sharply due to severe drought stress. 34
3.2: Layout of 3×2 RCBD factorial experimental design 65
4.1: Changes in soil water content (%vol.) measured by electronic moisture meter of full irrigation (100FC), mild deficit irrigation (70FC), and moderate deficit irrigation (50FC). 66
4.2: Some plant growth and fruit yield indicators of two garden egg cultivars for greenhouse and field plants. 72
4.3: Total phenolic contents in garden egg cultivars, S.aethiopicum and S.gilo subjected to 100FC, 70FC and 50FC irrigation treatments. Bars represent standard deviation (n = 5). 73
4.4: Total flavonoid contents in garden egg cultivars, S.aethiopicum and S.gilo subjected to 100FC, 70FC and 50FC irrigation treatments. Bars represent standard deviation (n = 5). 73
4.5. Plant growth indicators showing the vegetative growth of greenhouse and field plants for the two cultivars. 83
LIST OF PLATES
1: Garden egg (solanum gilo) fruits in decreasing order of age. 49
2: Garden egg (solanum aethiopicum) fruits harvested from experimental site. 50
3: Air-dried soil mix 58
4: Drip irrigation set-up in the Green House 60
CHAPTER 1
INTRODUCTION
1.1 BACKGROUND OF STUDY
Agriculture has been the major contributor to Nigeria’s economy, with the massive exportation of cash crops for foreign exchange in the past decades, until the discovery of petroleum. The huge economic returns during the oil boom had a serious adverse effect on the nation as the government focused its attention and resources on the oil sector, thereby presenting Nigeria as a mono-sectorial economy. But the recent continuous downward fall of oil prices in the international market has sent the government scampering for other sources of income and a seemly return to agriculture. This recent development and implementation of agricultural friendly policy by successive government in recent times and also the embargo placed on massive importation of certain agro-products, will likely mean that farmers may have to cultivate crops all year round to be able to feed the growing population.
This may not be achieved if farmers fail to incorporate irrigation in their crop cultivation practices, but with fluctuations in rainfall pattern observed in recent years, there has to be a paradigm shift in the way irrigation water is managed, otherwise, fresh water resources withdrawals will experience increased pressure due to extensive industrial and domestic demand. One way to conserve irrigation water is through the application of deficit irrigation strategies. Deficit irrigation, commonly defined as an irrigation technique whereby water is artificially supplied to a plant at an amount lower than the full crop water requirement desired for optimum growth. This aims to cut down on the amount of water used for irrigating crops, improve plants response to some degree of water deficit in a positive way, and enhance the crop’s water use efficiency (WUE) (Chai et al., 2016). Although, several investigations have reported that deficit irrigation techniques had suppressive effects on crop growth and productivity, but have also shown to have positive effect on the quality and nutritional composition of fruit crops, with few exceptions (Bogale et al., 2016).
Notwithstanding, deficit irrigation remains a prime tool towards actualizing high water savings in areas and situations of water scarcity. It has also been shown that adoption of deficit irrigation by Nigerian farmers during dry season cultivation of crops, does not only guarantee tremendous water savings, but also has the potential advantage of improving phytochemical and nutritional quality of the farm product as a result of the physiochemical responses of plant to water stress (Sepaskhah and Ahmadi, 2010). Although, little reduction in crop yield has been reported in literature for some crops, and in other cases, no significant yield reduction occurred for other crops subjected to deficit irrigation. Nevertheless, the impressive advantage that this irrigation practice offers to farmers outweighs its demerits.
Additionally, increasing consumer demand for nutritionally-rich foods justifies the drive to study suitable strategies that can be applied by farmers at little or no cost, which have the potential of improving the concentration of phytochemical or bioactive compounds in fruit crops. For this purpose, agronomic methods provide a practical approach for enhancing the nutritional and commercial quality of fruits and vegetables. It has also been reported that manipulating or modifying these agronomic practices can positively affect the concentrations of beneficial health-promoting compounds in vegetables and fruits (Martinez-Ballesta et al., 2008). These agronomic approaches include; fertilization, irrigation, use of soilless medium (substrates) and so on. Amongst these methods, irrigation management seems to be outstanding because of its wide acceptance and adoption by researchers.
In summary, irrigation water and fertilization have been reported to be the popular cultivation practices that are applied in manipulating the biosynthesis of antioxidant compounds in fruit crops (Dumas et al., 2003), with some exceptions (Krumbein et al., 2006). There are sufficient available literatures on the application of deficit irrigation techniques on horticultural and tree crops such as tomato (Kirda et al., 2004; Kaman et al., 2006), hot pepper (Dorji et al., 2005), potato (Ahmadi et al., 2010), and grape (de la Hera et al., 2007; Keller, 2005), but from literature scan, no data was found on the effects of these techniques on garden egg. Additionally, Sepaskhah and Ahmadi (2010) in their review recommended that continued research be carried out to determine the beneficial effects of deficit irrigation on other horticultural and tree crops in order to identify and document a wide list of crops which exhibit positive responses to water stress. Also, with the recent campaign and demand by nutrition experts for increased consumption of foods, fruits and vegetables rich in antioxidants by the Nigerian populace and also considering the high consumption of garden egg in Nigeria, the present study aims to investigate and evaluate the effects of deficit irrigation and varietal factor (cultivar type and water relations) on the physiochemical properties in two local garden egg varieties commercially cultivated in Nigeria.
1.2 STATEMENT OF PROBLEM
Numerous investigations have demonstrated the application of deficit irrigation as a strategy for improving crop quality with its resultant effects on several vegetative and reproductive crops (Sepaskhah and Ahmadi, 2010), however, a search through literature revealed complete lack of information with respect to the effects of deficit irrigation and varietal factor on the physiochemical composition of garden egg. Furthermore, sufficient information is available on the possible benefits of deficit irrigation in boosting water use efficiency (WUE) in crop production (Chai et al., 2016), but results relating to the impact of deficit irrigation, DI on the nutritional quality and antioxidant compositions in garden egg seems to be lacking.
Also, understanding the influence of genetic and environmental factors and their interactions on the natural antioxidants in garden egg fruits is needed if aiming for quality garden egg fruits rich in health-promoting substances.
Considering this fact and the high consumption of garden egg in Nigeria, the present study aims to investigate the influence of deficit irrigation on the physiochemical properties of two local garden egg varieties commercially cultivated in Nigeria, and also identify any advantages it may have over rain-fed irrigation.
1.3 OBJECTIVES OF STUDY
The main aim of this study is to determine the effect of deficit irrigation (DI) treatments on the growth, yield and bioactive compounds in two local garden egg varieties.
The specific objectives are to:
i. Determine the effect of DI treatments on vegetative growth, yield, fruit quality and water use efficiency of garden egg.
ii. Establish if the influence of these deficit irrigation techniques on garden egg is cultivar dependent.
iii. Compare the growth and yield performance and determine the influence of environmental factors on garden egg cultivated in open field and green-house using statistical analysis.
1.4 JUSTIFICATION
Sepaskhah and Ahmadi (2010) noted that a crop that is imposed to deficit irrigation as a water-saving technique, exhibits diverse responses to water stress in terms of three (physiological, biochemical and molecular) response levels, which modifies the physiochemical composition of the crop, depending on the severity of the water stress. Therefore, since water stress and low water use efficiency (WUE) both impacts fruit quality of crops, the corresponding relationship between water supply and synthesis of bioactive compounds requires to be understudied. In this study, the effects of deficit irrigation on physiochemical properties of two local garden egg (solanum gilo and solanum aethiopicum) will be investigated.
The positive outcome of this research work and when adopted by dry season farmers of garden egg, will go a long way in improving the diet nutrition of Nigerian populace, being a high demand fruit and relatively cheap and affordable for an average Nigerian, while saving considerable amount of irrigation water. Also, indigenous research studies on this important crop is scanty; therefore, the findings from this research work may spur researcher’s interest on this seemly orphan crop, which will help beef-up available data on garden egg in scientific literatures.
1.5 SCOPE OF STUDY
This research study was limited to evaluation of the influence of deficit irrigation on growth, fruit production and phytochemical contents of garden egg cultivated in controlled and uncontrolled environments. DPPH (2, 2-diphenylpicrylhydrazyl), FRAP (Ferric reducing antioxidant power), NO (Nitric oxide) and H2O2 (Hydrogen peroxide) were the assays employed in determining the antioxidants activity in garden egg fruits.
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