Rice, (Oryza sativa) is the staple food for 2.5 billion people (Situation and Labour, 2000) around the world and it covers 9% of arable lands on the earth (Situation and Labour, 2000).From the total rice production of the world, 85% used for direct human consumption and rest of used for production of snack foods, brewed beverages, flour, oil, syrup etc. According to the Cago, (2017) major rice consuming and producing countries are India, China, Indonesia, Bangladesh, Thailand, Vietnam, Burma, the Philippines, Cambodia, and Pakistan.
Rice is the second largest and primary food crop in Sri Lanka.
It is cultivated as a wetland crop in the all districts during Yala and Maha seasons and, at present 708,000 hectares of land devoted for paddy cultivation (Department of Census and Statistics Sri Lanka, 2015).Whole area devoted for paddy is not being cultivated due to number of reasons such as water shortages, prevailing unsettle conditions on the ground etc. About 70% of the cultivable land on the island is used for growing rice (Anon a, 2017).
Demand for rice will increase at 1.1% per year and to meet the rice production should grow at the rate of 2.9% per year (Central BankSri Lanka, 2016). Therefore, national average yield of paddy should be increased to feed the increasing population.
Paddy yield is determined by several factors: (a) climatic factors (b) agronomic and water supply factors, (c)effect of biological agents (d) fertilizers used and, (e) Agronomic traits (Kaur and Attwal, 2016). When consider the agronomic traits, Tillering is one of important trait for rice quality and grain production (Counce and Wells, 1990; Ling, 2000).
Excess tillers provide dense canopy and it cause favorable environment for pest and disease (Cu et al., 1996). But few tillers will result insufficient numbers of panicles (Wang et al., 2017). Rice germinates as a single culmed seedling, but soon after the seedling stage it produces primary, secondary and tertiary tillers. Tillering potential of rice is an important character and it mainly depends on time duration and morphology (Mohanan and Mini, 2008). The seedling stage of rice is followed by tillering stage. The first tiller starts from the axil of one of the lowermost nodes. Tillers emerging from the mother tiller or clumpare the primary tillers. After the initiation of a few primary tillers, secondary tillers emerge from early primaries and then tertiary tillers emerging from the secondaries (Wang et al., 2017) .
Primary and secondary tillers are produced by almost all varieties of rice but tertiary tillers are produced only by high tillering varieties (Wang et al., 2017). Modern rice verities which were developed by Rice Research and Development Institute (RRDI) produce 20-25 tillers including primaries, secondaries and tertiaries under favorable conditions. But from these tillers 14-15 of will produce panicles and remaining become unproductive (Khush,2000).
Phosphorus (P) availability is the major effective factor for the growth of tillers in rice. P plays an important role in plant structural and functional processes, such as formation of cell membrane, synthesis and transfer of genetic material, respiration, photosynthesis, nutrient movement within the plant and cellular energy transfer (Wiedenhoeft, 2006: Abolfazli et al., 2012). P deficiency mainly affects the shoot growth in rice resulting reduced tiller production, and reduced leaf elongation rate. Result is reduction of the rice yield (Luquet et al., 2005). Therefore, it is necessary to maintain soil P availability to ensure rice productivity.
Finding of present study will help to identify high tillering traditional and hybrid verities with P deficiency, and seasonal effect for the growth of tillering in different verities. This can be used to increase the rice yield effectively by selecting best verities for cultivation in two different growing seasons.
1.2 Objectives of the Study
To confirm and validate the tillering variations of different verities in two different growing seasons
To screen Sri Lankan Old improved, New improved and, traditional rice varieties for P deficiency tolerance (PDT)
To identify tillering variation at early vegetative, Late vegetative and, flowering stage
2.1 History of Rice
Rice is believed to have been first cultivated in China or possibly somewhere else in eastern Asia around 10,000 years ago (Rice history, 2002). Beginning in China and the surrounding areas, its cultivation spread throughout Sri Lanka, and India (Rice history, 2002). It was then passed onto Greece and areas of the Mediterranean (Rice history, 2002). Rice spread throughout Southern Europe and to some of North Africa. From Europe rice was brought to the New World and from Protugal it was brought into Brazil and from Spain to Central and South America (Rice history, 2002).
Sri Lanka’s legendary harvests once brought it fame as the Granary of the East (Samara, 2013). Historical records tell us that paddy was cultivated in Anuradhapura in 161 BC and flourished there until 1017 AD (Samara, 2013). Today, it is cultivated across the Island. As society evolved, activities and people close to the heart of paddy cultivation rose to prominence. By keeping the Island fed, the paddy farmers ascended the hierarchy of the Sinhalese cast system, raised by royal patronage because, after all, they satiated the people’s hunger and so were deserving of respect (Samara, 2013) .
2.2 Scientific Classification
2.3 Morphology of Rice
Rice is an annual grass with round, hollow, jointed culms, flat leaves, and a terminal panicle (Corps, Collection and Volunteers, 1998). Among the cultivated cereal plants, only rice were adapted to growing in both flooded and non-flooded soils and it can be grown under a wide range of climatic and geographical conditions.
Rice plant can be divide for four main parts such as: (a) roots (b) stem and leaves (c) reproductive organs (d) grain (Corps, Collection and Volunteers, 1998)
The roots serve as support to absorb water and nutrients and, the root system of rice is relatively shallow (rice hub, 2011). Stem is made of a series of nodes and internodes it bears a leaf and bud. From the lowermost node this leaf and bud will grow into tillers. According to the leaf born angle rice has two leaf types such as auricles and ligules and it will helpful for differentiation of rice from grasses. The fag leaf is the most important energy providing source at the reproduction stage. The reproductive organs of the rice plant contains the panicle and it born at the top of the uppermost node of the stem. The grain of rice plant containing a live embryo capable of germinating to produce a new plant.
2.3.1 The growth stages of rice
Depending on the Varity normally with good environmental conditions and inputs rice plants take 3-6 months to grow from seeds to mature plant (Rice Production (Peace Corps): tChapter 2 – The growth stages of rice, no date). They undergo four general growth phases; (a) Germination (b) vegetative phase (c) reproductive phase and, (d) Ripening phase.
(a) Germination Phase
Rice plant start to grow, when the first shoots and roots start to emerge from seed and in flooded soil the shoot will first emerge and then emerge the roots. But in non-flooded soil, roots will emerge first and then shoots will emerge.
(b) Vegetative Phase
Typically vegetative phase takes between 55-85 days and it is depending on the rice varity. After first root and shoot emerge the seedling stage begins until the first tiller appears and This is the early vegetative phase. When tillering begins late vegetative phase will start and it continue until maximum number of tillers is reached. Just before the panicle initiation plant stops growing in height and it is the signal for the end of the vegetative phase.
(c) Reproductive stage
When the panicle initiate from the stem and continues to grow, rice become to booting stage and when the panicle is fully visible rice is name as heading stage. A day after heading complete, flowering is begins and it continue for about 7 days. As the flower open pollination can occur.
(d) Ripening phase
At the end of the flowering, the ripening phase starts and it will take 30 days. Rainy days or low temperature may lengthen ripening phase and sunny days may shorten it. Based on the texture and color of the ripening grains this phase follows milky, dough, yellow, ripe and, mature stages.
2.3.2 Tillering potential and optimum tillering of Rice
In conventionally rice cultivated by seeds and seeds germinated asa single culmed seedling and soon after seedling stage it produces primary, secondary and tertiary tillers (Mohanan and Foundation,2016). Tillering potential of rice is a varietal character and it mainly depends upon duration and morphology (Mohanan and Mini 2008).
The first tiller start from the axil of one of the lowermost nodes and tillers emerging from the mother tiller are called primary tillers (Kekulandara et al., 2017) . After initiation of primary tillers secondary tillers emerge from early primaries (Mohanan and Foundation, 2016) (plate 2.1).
In some varieties tertiary tillers are emerge from secondary tillers and modern rice varieties produce 20-25 tillers and among them only 14-15 of tillers produce panicles and remaining become unproductive (Mohanan and Foundation, 2016).
Plate 2.1: A rice plant growth of primary roots, a secondary tiller and a leaf from a nodeinternode segment of the base of the culm (primary stem/tiller)
2.4 Paddy Cultivation and Production
Rice provides of the dietary caloric up to 50%, 15% of the protein and 21% of the energy requirements of human beings (Kennedy et al, 2002). It is also the primary source of income and employment for more than 200 million households across countries in the developing world (Muthayya et al., 2014).
2.4.1 Global scenario
Rice is the staple food for more than 50% of the world population (Maclean et al, 2002), and over 90% of rice is produced and consumed by asia-pasific region. Rice is grown in approximately 158 million hectares of area within more than a hundred countries, and producing more than 700 million tons annually(Rice productivity – Ricepedia, no date). China and India are the worlds largest rice producers and after them other rice producing countries are Indonesia, Bangladesh, Vietnam, Myanmar and Thailand. These countries produce 30million tons of paddy and together account for more than 80% of world production (Rice productivity – Ricepedia, no date).
2.5 Sri Lankan scenario
Rice is the staple food in Sri Lanka. Around 807,763 hectares of land cultivated for paddy.
Around 879,000 farmer families are engaged in paddy cultivation. They are 20% of the
countrys population and 32% of the employment. Per capita consumption of rice in Sri Lanka is reported as 100 kg/year/head (Anon, 2017).
Total contribution of rice is GDP 1.8% (Central bank, 2010). As the staple food, it reaches
every household and it is popularly consumed by the people and it remains the major source of calories (45%) and protein source (40%) for Sri Lankan (Mendis, 2006).
There are two cultivation seasons namely, maha and yala which are synonymous with two
monsoons. Maha season falls during north east monsoon from September to March and Yala season is effective during the period from May to end of the August(Amarasinghe, 2009). The total land cultivated for paddy is estimated to be about 1,065,000 ha and average 646,000 ha cultivated during maha season and 419,000 ha cultivated in yala season (Weblet Importer, no date). Average yield recorded as 4528 kg/ha (Central bank, 2016). Rice is a one of the crops that need high quantity of water. It is estimated that 3000 to 4000 1 of water required producing 1 kg of rice (FAO, 2011).
2.5.1 Problems of Paddy cultivation in Sri Lanka
In Sri Lanka the agriculture sector plays an important role of the economy. Paddy cultivation employed 1.8 million farmers and it is about 32% of labor force in the country (Thiruchelvam, 2005). Due to the population increasing, demand for the rice continuously increasing annually at the rate of 1.2%. In Sri Lanka 50% of the total annual production come from Ampara, Polonnaruwa, Kurunegala and, Anuradhapura (Thiruchelvam, 2005).
At present paddy cultivation of Sri lanka faced mainly Biotic stress and abiotic stress which are directly affect the growth, reproduction and hence the productivity of rice(Aluwihare et al., 2016).
Many abiotic factors like salinity, drought, submergence, extreme temperatures, low soil fertility, nutrient deficiency and iron toxicity, and biotic factors like pests and diseases, weeds reduce the rice production. Climate change, by inducing variations in the climate patterns (increasing incidence of drought, extreme temperatures, flooding and increasing levels of salt stress) is expected to aggravate these constrains, thereby affecting rice yields (Setter et al., 1996). The greatest threats to rice production due to climate change are anticipated in the rain fed rice production systems due to their often total dependence on rainfall as a source of water (Setter et al., 1996).
Amongst, a wide range of nutrient deficiency causing yield reduction by effecting several ways and P deficiency consider as one of major.
2.6 P deficiency of Rice
Phosphorus is one of the essential macronutrient for plant functional and structural processes, such as photosynthesis, cell membrane formation, respiration, synthesis and transfer of genetic material, nutrient movement within the plant and cellular energy transfer (Kekulandara et al., 2017).
P deficiency symptoms such as formation of small and curly leaves, growth retardation, dark green to purple coloration of leaves (Aluwihare et al., 2016) and these will cause to the reducing shoot growth and it results reduced tiller production and leaf elongation rate (Kekulandara et al., 2017).
According to MacDonald et al, (2011) one thirds of cultivable lands are lack of P and Kumaragamage and Indraratne (2011) founded, in Sri Lankas rice growing soil also have not adequate P levels. To overcome this problem P is applied as an artificial fertilizer.
When apply fertilizer, it is not practiced based on recommended levels and growers apply more fertilizer. Therefore, the excessive amounts get washed away from agricultural lands and getting accumulated in water bodies, and it cause water pollution and eutrophication(Aluwihare et al., 2016). In other hand P fertilizer is very expensive and it imported to country with purchasing 1.5% of GDP (Aluwihare et al., 2016). Because of the rapid rate of depletion due to extensive consumption and the non-renewable nature of P reserves price of P fertilizer will continue to increase (Cordell et al, 2009). In rice farming, the development of P efficient rice varieties, which are adapted to low P soils, would be a best solution for this problem(Aluwihare et al., 2016).
Under low P conditions the currently recommended rice cultivars in Sri Lanka, cannot be grown profitably. These recommended rice cultivars and some of traditional cultivars have higher harvesting indices and they efficiency use low nutrient (Aluwihare et al., 2016).
2.6.1 Recommended rice varieties
Age group Varieties recommended by RRDI
Five to Six month age group H-9, Bg 3-5, Bg 407, Bg 745 ,Bg 38
H-7, Bg 34-6, Bg94-2, Bg 350, Bg 352, Bg 357
Bg 358, Bg 359, Bg 360,Bw 266-7, Bw 267-3
Bg 351 ,At 16 ,At 353, At 35,Ld 355, Ld 356,
H-10, Bg 34-8, Bg 276-5, Bg 300, Bg 301,
Bg 304, Bw 272-6B, Bw 302 ,At 303, Bg 305
2.7 Development of traits in cultivars
The high tillering capacity is considered as a desirable trait in rice production, because number of tillers per plant is closely related to number of panicles per plant (Wang, Cheng and Zhang, 2007). Li et al., (2003) reported some mutant gene called MOC 1, is important in the control of rice tillering and plant with this mutation only has main clum and it grows without any tiller. Based on this information this gene can used for the controlling tillers in rice when develop new cultivars .
Also when consider the PDT screening in rice, selectable genotypes can be used as parents in the breeding programs (Wissuwa et al, 1998; Chin et al, 2011). Chin et al (2011) repo rted, to improve of cultivars need to identify the genomic regions and then they can be used in marker assisted selection in molecular breeding.