Plant Breeding in Rice
Rice is the one of the most important crop as a food source of more than half of the world's population. It has really large percentage in total cereal grain production. Rice is the most commonly used grain product after the wheat in human nutrition. For most of the countries in the world rice is primary staple food, especially in Asia and India. It contains fiber, vitamins, mineral and little bit protein, for nutrition and even the starch ratio in rice is higher than in wheat. Rice is the only grain species that can germinate in water soaked lands, in full of water because its anatomy provide this oxygen utilization that dissolved in water can utilize dissolved oxygen in the water rice, which has an important place in the cultivation of many countries including Turkey in terms of being able to grow in salty and alkaline land. This plant is also effective improving economic efficiency of these lands. However, most of the world's paddy production is done in tropical and temperate regions that provide the conditions for growth. A great deal of paddy planting sites in the world are located in Asia. The time of sowing rice in our country varies according to regions. In the Marmara-Thrace and Black Sea regions, rice planting should be done in the first half of May. In the Aegean and Mediterranean regions, paddy can be planted from April to late June. In the Southeastern Anatolia region, planting between April 20 and May 15 is the most appropriate result. The planting of rice should start a little later in the Central Anatolia region. In this region, the paddy should be planted between May 10- 20.
The origin of rice is not exactly known. Wild species of rice occur across South and East Asia, including India and South China. Rice was domesticated in the ‘fth millennium BC. The cultivated species of rice Oryza sativa is believed to have derived from annual antecedents found in the area extending from India to China. Based on the genetic evidences it is founded out that rice firstly domesticated near to Pearl River valley region of China. Then planting rice is spread to South and Southeast Asia from East Asia. Some of the ancient sites also found along Yangtze River. Twenty species of the genus Oryza are known, with a basic chromosome number of 12. The genus has six genome groups ‘ A, B, C, D, E, and F. The cultivated species O. sativa (2n=2x=24) has the genome formula AA.
21 different wild varieties are found in the world and there are three distinct cultivated species
Japonica Type: Oryza sativa japonica, domesticated in central China .This group of rice varieties grown extensively in some areas of the world from northern and eastern China. This varieties can be grown in little bit cooler climate zones of the subtropics and in the temperate zones. Grains are generally short, roundish and spikelets can change awnless to long-awned
Javanica Type: Oryza sativa javanica, domesticated around Indonesia, Phillipines and Mayanmar. The species that are cultivated around Indonesia, Phillippines, Taiwan and Japan are intermediate types. Javanica rice varieties have light green and tough broad leaves. This types usually taller than other types and needs low-tillering. Grains of this type are usually long and thick. Spikelets can be awned or awnless.
Indica Type: Oryza sativa indica is domesticated around Indian subcontinent. It is tropical type rice and extensively grown around India, Srilankha, Thailand, Malaysia and neighboring countries. They are usually tall plants with weak stem, they have photoperiod sensitivity. Their leaves can be broad to narrow leaves and their grains have dormancy. Indica plants need amply of tillering. Grains can be long to short, slender to somewhat flat. And spikelets are usually awnless.
General botany
An annual grass rice has bristle-like erect culms. It produces about ‘ve tillers. Rice is a self-pollinated plant its in’orescence is terminal panicle which consists of spikelets. Spikelets can grow up to 60-150 cm, stem or culm is cylindrical and consists of jointed solid nodes and hollow internodes, buds found in axil of basal leaves and give rise to tillers. Leaves are located one after another and linear with parallel venation. Inflorescence found terminal panicle. Each spikelet has complete flower that consist of two outer glumes, vessel shaped pelia and flattered lemma. Lemma and pela from husk enclose grain after fertilization. Lemma and pelia that surrounded with six stamens and slender filament, pistil.During early stage spikelets are usually white with green anthers and has closed style. On maturity spikelets turn green and anther become yellow and style separate from each other. Depending on the variety, the lemmas can be fully awned, partly awned, tip-awned, or awnless.
Reproductive biology
Rice has terminal panicle with branches that emanate singly or in whorls. It is primarily self-pollinated crop but natural cross pollination take place up to 3% depending upon varieties. Rice has perfect flowers which has male and female part in the same flower. Flower consists of two lodicules, six stamens, and there is two plumose stigmas on two styles that are surrounded by floral petals. The time of pollination or anthesis is influenced by environmental factors. The blooming starts from tip of the panicle and go on downward and continues for 7 to 10 days. Most of the flower blooms within 2 to 4 days from emergence of panicle. Anthers burst and distribute pollens just before the flower opens. Anthers and pollination occurs at the same time flower opening.
Common Breeding Objectives
An increase in productivity and yield is always one of the main goals of any crop breeding. A long list of goals can be identified for rice vary from region to region. A few examples are given below.
‘ Increase in grain yield: It is the major goal of all rice breeders around the world. Grain yield in rice depends on yield potential, yield stability across seasons, and crop productivity.
‘ Grain quality: Grain quality traits of interest vary from one region to another. They include grain size and shape, color, aroma, stickiness, and protein content.
‘ Disease resistance: Host resistance to many major diseases is very important in rice. IT and insect pests have been identi’ed in rice.
‘ Resistance to environmental stresses: Breeding resistance and tolerance to various environmental stresses are important in rice breeding. Drought and ‘ooding frequently alternate with each other. Regarding moisture stress, the escape and avoidance mechanisms are important in dryland culture, while tolerance and recovery mechanisms are important under rain-fed wetland culture
Emasculation
As we know rice is self-pollinating crop with perfect flowers or bisexual flowers. When we want to do cross-pollination we need to take precaution. This is generally practiced by removing the stamens from recurrent parent which is high yielding and widely adopted before they shed their pollens. Remove them with using small fine pointed forceps (tweezers), or by suction, or may be killed by chemical agents, alcohol, heat, cold. And these emasculated flowers are covered with bags to protect them from other foreign pollens. Cytoplasmic male sterility can be utilized for hybrid seed production without emasculation. The ears that is just emerged should be selected and then all spikelets that are already opened should be clipped. Spikelets which are ready to open selected and remove six anthers from each spikelet with needle and fine pointed forceps. Examine the emasculated ear to be sure then cover with perforated paper bag and labelled.
Pollination
The stigma remains receptive for a few days so pollination should be made in this period. Pollination should be handled during the period of anther dehiscence. Pollen can gathered before anther dehiscence. The male panicles and the ‘ag leaf can be removed to make easy control. These panicles should be observed for anther clasp and used in pollination. The pollen that are collected put into female plant or shake over it and then it cover female parts with another bag to avoid from unwanted pollination. Other techniques also can be used for pollination.
‘
Cross-pollination
Rice is highly self-pollinated. In commercial F1 hybrid seed production, natural cross pollination often doesn’t occur enough. So sometimes natural pollination should be done with hand pollination. To help the release of pollen the ‘ag leaf and the leaf sheaths that enclose the panicle torn can be cut.
Pollination is done in this way: First matured anthers are collected from protected male parent in petri dish and dusted on the stigma of emasculated flower with brush and forceps or collect ripe anthers and then put pollens and stigma in the same bag and shake it little bit and cover with paper bag to protect natural cross pollination then label it.
Breeding Methods
Being a self-pollinated species, rice can be improved by any of the methods of breeding for self-pollinated species. Field tests may be conducted to evaluate introductions from various parts of the world to identify adapted varieties for commercial production.
Introduction Breeding: The seed of varieties that are released in other countries are imported in another country and tested, evaluated along with native varieties of that country. If this variety found superior it released for commercial cultivation. This is called introduction breeding.
Selection: Outstanding lines are selected from land races and evaluated along with existing varieties and when the superior types are found they multiplied and released for commercial cultivation.
Inbreeding and Hybrid Production: Hybrid breeding is one of the most refined and productive methods of developing cultivars. It is based on observation of hybrid vigor (heterosis) in F1 progeny. In use of inbred lines as parents results in uniform F1 hybrids. This method used mainly in cross-pollinated crops and some high value self-pollinated crops. Methods of producing inbreds for hybrid production:
a) Self-pollination
i. Self-individuals from selected populations
ii. Testing inbreeds for combining ability
b) Sib-mating
c) Backcrossing to convert an inbred to include a gene for a desired trait
d) Doubled haploids
i. Haploids derived by various methods often double spontaneously or can be colchicine doubled to produce completely homozygous lines
ii. Naturally occurring haploid seed
iii. Chromosome elimination in wide crosses
iv. Anther culture
Backcross Breeding: This method involves initial cross between two or more parents followed by repeated backcrossing of progeny to one parent and used mostly with self-pollinated species or with inbreeds of cross-pollinated species. It is used to introduce one or a few genes from donor parent into otherwise desirable parent (recurrent parent). Results in backcross line that resembles recurrent parent except that it contains desired gene(s) from donor parent
‘
Pedigree Method: It is the most common method of rice breeding. It is the method of breeding in which ancestral lineage is recorded. Recording of parents and selections from progenies. This method is used primarily with self-pollinated species. Record keeping is very important. Recorded lineage does not allow one to recreate a selected line, but provides information on relationships of lines. Parent selection depends on breeding objectives. It is often interested in improving a cultivar for one or more traits in other words combining genes for desirable characteristics from two or more parents. Crosses made between cultivars that complement each other for traits of interest by use of two, three or four initial parents. Individuals are selected starts with F2 and onwards generations and superior progenies are carried forward and tested with existing check variety and if found superior released for commercial cultivation. New parents can be introduced later in pedigrees and the number of crosses made depends on resources, objectives, parents used and philosophy of breeder.