Rice (Oryza sativa L.) is one of the world's most important cereal crops where it is a staple food for more than three billion people (Cantrell and Reeves, 2002). It is said to be cultivated on about 148 million hectares annually which is equivalent to 11 percent of the world's cultivated land (Khush, 1997). Rice is produced under a wide range of climatic and conditions and altitudes (Maclean, Dawe, Hardy and Hettel, 2002). Each year an estimated 408,661 million metric tons of rice is consumed, supplying 20% of the world's total caloric intake (Londo et al., 2006). It accounts for 35 to 60 percent of the calories consumed by three billion Asians according to Khush (1997).
Origin of rice
Rice belongs to the genus Oryza which consists of 20 wild species and two cultivated species; O. sativa and O. glaberrima (Chang, 2003). O. sativa is of the Asian origin and cultivated globally while the African O. glaberrima is grown in restricted areas of West Africa (Londo, Chiang, Hung, Chiang and Schaal, 2006). New rice varieties named ''New Rice for Africa'' (NERICA), a cross between O. glaberrima and O. sativa have been developed. They combine the hardiness of the African species with the productivity of the Asian species. Thus, the new rice holds great promise for a region in desperate need of decreasing hunger and increasing food security (Linares, 2002). The genus Oryza is further reported to have originated probably 130 million years ago in Gondwanaland and different species are distributed into different continents following the breakup of Gondwanaland as shown in figure 1.1 (Khush, 1997).
Cultivation of rice
Rice is grown between 550N and 360S latitudes under diverse growing conditions such as irrigated, rainfed lowland, rainfed upland and floodprone ecosystems (Khush, 1997). More than half of the world's rice is grown under irrigated conditions (Figure 1.2). Since the time of its initial domestication, Asian cultivated rice has been moved across the globe with migrating human populations. Rice cultivation can now be found on all continents except Antarctica and feeds more than half of the world's population (Londo et al., 2006).
Rainfed lowland rice
Rainfed lowland rice is grown in one-fourth of the total rice cultivated area in the world (Figure 1.2) in bunded fields that are flooded for at least part of the cropping season to water depths that exceed 100 cm for less than ten consecutive days (Maclean et al., 2002). It comprises of approximately 37 million hectares (harvested area) an equivalent of 25 percent of the world rice area. With a total of 92 million t year-1 it produces 17 percent of the global rice supply (Wopereis, Kropff, Maligaya and Tuong, 1995).
Upland or dryland rice
Upland rice is always direct seeded and grown in unbunded fields of often naturally well drained soils without surface accumulation of water. It covers about 19 million hectares of land and contributes four percent to the world total rice production with an average of one t ha-1 (Wopereis et al., 1995). It is grown on three continents (Asia, Latin America and Africa) mostly by small scale subsistence farmers in the poorest regions of the world (IRRI, 1975).
PROBLEM STATEMENT AND JUSTIFICATION
The world's population is said to be growing inexorably yet harvests worldwide are threatened by climate change. Currently the 30% of the two million hectares of winter wheat produced annually in the UK is grown on drought-prone soils. It is projected that by 2050 heat stress is likely to increase due to global warming and this will lead to increased evapo-transpiration (BBSRC, 2009). Water deficit (drought) has been defined by Cabuslay, Ito and Alejar (2002) as the absence of adequate moisture necessary for a plant to grow normally and complete its life cycle. This lack of adequate moisture for proper plant growth and development has been reported to be a major abiotic threat to rice production under rainfed ecosystems (Asch, Dingkhuhn, Sow and Audebert, 2005; Price, Steele, Gorham, Bridges, Moore, Evans, Richardson and Jones, 2002). It has significantly reduced rice yields to an average of only 1.5 t ha-1 in the rainfed lowland ecosystem in South and Southeast Asia (Cabuslay et al., 2002).
Root characteristics such as root length density, root thickness, and rooting depth and distribution have been established as constituting factors of drought resistance, with deep rooting cultivars being more resistant to drought than the shallow rooted (Asch et al., 2005). Thus root morphological characteristics significantly contribute to drought resistance in rice (Price et al., 2002; Azam-Ali and Squire 2002). In spite of the numerous reports on water deficit and recent advances in molecular biology techniques, drought tolerance remains poorly understood in comparison with grain quality and disease resistance which are governed by major genes. This demonstrates the complexity of rainfed ecosystems exacerbated by unpredictable moisture supply (Cabuslay et al., 2002; Azam-Ali and Squire 2002).
This study seeks to understand the different root characteristics deployed by upland rice in comparison with lowland rice in order to circumnavigate the adverse effects of water deficit on its productivity. Also to further quantify the effects of different levels of drought on root development of selected upland and lowland rice cultivars. In addition, a deeper understanding of the mechanisms of rice tolerance to water deficit is necessary for breeders to be able to identify heritable traits which will make plants adapt to growth conditions in rainfed areas (Cabuslay et al., 2002). Literature further reveals that indigenous upland rice cultivars are more drought tolerant compared to lowland cultivars (Asch et al., 2005). Also information about root distribution is important for characterization and modelling of water and nutrient uptake, biomass, and yield (Buczko, Kuchenbuch and Gerke, 2009). Since these root modifications during drought are a reflection of plant response to soil water and nutrient status (Price et al., 2002).
To evaluate the tolerance of selected upland and lowland rice cultivars to water deficit (drought)
- To compare the total root dry matter and length (a) among the upland varieties, (b) among the lowland varieties and (c) between selected upland and lowland rice varieties.
- To determine whether plant water potential and soil moisture content are directly related in both upland and lowland varieties
- There is no difference in the total root dry matter and length among selected upland and lowland rice varieties
- The total root dry matter and length are higher in upland rice varieties than lowland rice varieties
- The high total root dry matter and length in upland rice varieties is influenced/induced by drought conditions
- Whether plant water potential influences soil moisture content
MATERIALS AND METHODS
Irrigated upland and lowland rice cultivars will be used in this study, the seeds will be selected basing on their viability. The two upland rice cultivars to be used include: O. glaberrima and Moroberekan whilst the lowland cultivars are IR43 and IR72. All the seeds will be obtained from the International Rice Research Institute (IRRI), Philippines. Before seeds are germinated, they will be thoroughly washed with distilled water to remove any seed dressing and then placed in damp petri dishes containing a filter paper. The seeds will be left for ten days in the shade (growth room) to test for their viability and determine the germination percentage as well.
Description of the plant materials
It was developed at IRRI and released in the Philippines in 1978 with the aim of increasing multiple disease and insect resistance (Peng and Khush, 2003). It is resistant to rice blast and bacterial blight diseases and green leafhopper insects. The variety is also said to be suited to upland areas and tolerant to salinity and zinc and phosphorous deficiency (Khush and Virk, 2005).
It was released in the Philippines in 1988 with the aim of increasing multiple to diseases and insect resistance (Peng and Khush, 2003). The variety is resistant to bacterial blight, Tungro and grassy stunt diseases and green leafhoppers and various strains of brown planthopper. It is suited to irrigated and rainfed lowland areas and tolerant to iron toxicity and zinc deficiency (Khush and Virk, 2005).
Moroberekan is tropical upland japonica variety of long stature and is also said to be drought resistant and tolerant to saline soil conditions (Haq, Akhtar, nawaz and Ahmad, 2009). It has its origin in West Africa and is considered to confer durable resistance to rice blast (Girish, Gireesha, Vaishali, Hanamareddy andHittalmani,2006).
Oryza glaberrima (V4)
Oryza glaberrima (African rice) is native to sub-Saharan Africa and is thought to have been domesticated from the wild ancestor Oryza barthii by people living in the flood plains at the bend of the Niger River some 2,000-3,000 years ago. At the present time the cultivar is being replaced in West Africa by the Asian species (O. sativa). O. glaberrima plants have luxurious wide leaves that shade out weeds and the species is more resistant to diseases and pests. This African rice is said to be tolerant to fluctuations in water depth, iron toxicity, infertile soils, severe climates, and human neglect. Some O. glaberrima types are reported to mature faster than Asian types, making them an important emergency food (Linares, 2002).
Experimental set up
A complete randomised block design (RCBD) will be used in the study to avoid variability in the light intensity by blocking. Four rice varieties will be used in this study; two lowland V1 (IR43) and V2 (IR72) and two upland V3 (Moroberekan) and V4 (O. glaberrima).
The experiment will comprise of two treatments; treatment one will be under a uniform water regime and well watered (22% soil moisture content) throughout the experiment. In treatment two the plants will treated to a uniform water regime like in treatment one up to 30 days after sowing (DAS). This will be followed by slow soil drying to a given soil moisture content from 22 to 18, 15, 12 and 9% by withholding irrigation and subsequent compensatory irrigation to keep the soil moisture level constant. These treatments will be replicated three times thus a total of 48 columns will be used in the study.
The glass house experiment will be set up at the, University of Nottingham, School of Biosciences, Sutton Bonington Campus where the plants will be exposed to 12 hours of light at 300C. Polyvinyl chloride (PVC) columns of diameter 0.2m and a height of 0.6m containing 25kg of sandy loam soil will be used in proportions of 25:75. The base of the columns will be sealed with heavy duty tape with a drainage hole at the bottom. In order to ensure a homogenous distribution of irrigation water in the soil column, the columns will be irrigated according to the treatment via a perforated silicon tube inserted in the soil diagonally over the entire soil length of the column. The soil moisture content will be monitored on a twice a week using a Theta Probe Soil Moisture Sensor-ML2x and any soil moisture losses due to evapo-transpiration compensated through irrigation. The soil field capacity, pH and nutrient status will be assessed and the N-P-K fertilizer requirement needed at the beginning of the experiment also determined. Harvesting will be done 90 days after sowing (DAS) for each treatment and the following parameters measured.
Root total dry matter
This destructive sampling technique will be measured at each harvest using randomly chosen representative root samples from each of the treatments for each variety. The fresh weight of all the samples will be taken using a precision balance and then oven-dried to a constant weight at 700C. The total root dry matter will be calculated as the difference between the fresh and dry weights.
Plant water potential
Water movement within plants occurs along gradients of free energy which depends on the plant water potential which is a sum of osmotic potential, matric potential and hydrostatic or turgor pressure. For plants to extract water their water potential should be less than the soil water potential (Azam-Ali and Squire, 2002). This will be measured twice a week using randomly chosen representative shoot samples from each of the treatments for each variety using the pressure chamber technique as described by Turner (1988).
Soil moisture content
There is an inverse relationship between the amount of water in the soil and the tenacity at which it is held. The soil moisture content measures the amount of water available in the soil (Azam-Ali and Squire, 2002). The soil moisture content with in the soil column will be monitored using a calibrated Theta Probe Soil Moisture Sensor-ML2x (Delta T Devices) inserted through the holes on the columns at 0.2, 0.4 and 0.6m depths and measurements taken twice a week.
Total root length
This destructive sampling technique will be measured at each harvest using randomly chosen representative root samples from each treatment for each variety. Root samples will be taken at 0.2, 0.4 and 0.6m depths Roots will be washed using a hydropneumatic elutriation device (Gillison's Variety Fabrications, Benzonia, MI, USA). The equipment employs a high kinetic energy first stage in which water jets erode the soil from the roots followed by a second low-kinetic-energy flotation stage which deposits the roots on a submerged sieve. A digital image of the roots will be taken with a scanner (Hewlett Packard 3CX). The root length will be determined from the digital image using the Newman method based on the number of intercepts of randomly placed lines with roots spread over a surface (Asch et al., 2005).
The analysis of variance will be used to determine the significance of the differences and interactions in the data obtained using statistical package GenStat Release 12.1 (VSN International, 2009. Differences will be considered significant at P 0.05 and where differences are significant, least significant difference (LSD) multiple comparison test and t-test will be performed.
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