Introduction

Introduction

1

Rice is a grass “autogame” a tall crop growing more easily in the tropics. Rice belongs to family “ Poaceae ”. The genus Oryza includes 24 species of which two are cultivated : “Oryza sativa L” which originated in the humid tropic of Asia, and “ O glaberrima” from West Africa. Asian cultivated rice has evolved into three ecogeographic races – “Indica”, Japonica” and

“Javanica”. Rice is

tolerant to desert, hot, humid, flooded, dry and cool conditions and grows in saline, alkaline and acidic soils. Today rice is grown in more then 100 countries, ranging from mountainous Himalayan to lowland delta areas. The

association between rice and human community was

clearly indicated in the exciting excavations at the He – Mu – Du, Luo – Jiao - Jia in China where rice was a principal food plant in the developing human settlements there more then 7,000 years ago. At present rice is an important source of food for more then half of the world population (Anonymous, 1996) and after wheat, rice is the second most important crop of the world more then 90% is currently growing in India.(IRRI,2006). Rice is an important cereal food crop of India. It occupies about 23.3% of gross cropped area of the country. Rice contributes about 43% of total food grain production of 46% of the cereal production of the country ( Kush, 1979). Almost 90% of

the

world’s rice is produced in Asia, China is the world’s leading Introduction

producer, growing 2/5 of the Asia’s total on 32 million hectares. India possesses the largest rice area (44 million hectares) producing nearly a quarter of Asia. In terms of productivity even though the area of rice cultivation is less in Europe ( 0.59 million ha), it recorded higher rice productivity (5.89 t/ha). Due to lack of the high yielding varieties and advance technologies, African country could able to attain less productivity of 1.88t /ha. There are generally several biotic and

abiotic factors that

adversely affect the productivity via. physiology of plant.

Rice is

generally affected by the water deficit, submergence salinity and deficiency of zinc and phosphorus (Wissuwa, 2008). Phosphorus deficiency is one of the major limiting factor that limits crop production in highly wethered soils and in many part of world soils particularly prone to P deficiency are coarse – textured with small amount of the

reserves ( e.g sandy soil in northeast

Thailand, Cambodia), highly weathered with higher

P fixation

capacity , degraded lowland soil ( e.g North Vietnam), calcareous, saline and sodic soils ,volcanic soils with high P- absorption capacity, peat soils, acid sulfate soils (Dahal,1977) Phosphorus after the nitrogen is the key element for plant growth that limits the 40-50% of crop productivity throughout the world. Phosphorus is an essential constituent of the ATP, nucleic acid and phospholipids, it is major function is energy storage and membrane integrity . Phosphorus is mobile with in the plant and Introduction

promotes tillering, root development, early flowering, and ripening. It is most important for the early growth stages of plants. Generally total phosphorus content of most soils is low, averaging only 0.6% phosphorus. This compares to an average soil content of India 0.14% nitrogen and 0.83% of potassium. Although P is widely distributed in nature, it is not found as the elemental form, elemental phosphorus is extremely reactive and combines with oxygen when exposed to air (Fox,1981). In soil it generally exists as orthophosphate

species

( H 3Po4,

H2Po4-, and HPo42- ) depending

upon pH. The availability of phosphate is generally higher above pH 6.5, becomes very low at 6.0, because of fixation. ( Abel et al., 2002) Phosphorus deficiency in general can be corrected by the initial application of large quantity of phosphorus. However, farmers are facing difficulties with increase in the cost of the fertilizers, especially in developing countries. The possibility of exploiting genotypic differences in absorption and utilization of P to improve efficiency of P fertilizers use or to obtain higher productivity on Pdeficient soils has received considerable attention in recent years ( Cakmak,2002) . P fertilizers are manufactured from nonrenewable resources that are increasingly becoming more costly and less available to resource poor farmers, these resources as per some estimates will get depleted by 2060. Plants usually posses many mechanisms to enhance the efficiency of the uptake and utilization of available phosphorus. Introduction

These mechanisms includes mycorrhizal symbiosis (Bagyaraj and Verma, 1995 ), remobilization of internal organic phosphates, exudation of organic compound like “citrates” for efficient utilization of phosphorus from the soil ( Kirk and Santos,1999). In addition plants develop many alternative pathway that are quite different from usual pathways that doesn’t require the phosphorus. Plants generally shows two types of mechanism against the low phosphorus stress i.e an internal mechanism that involves the efficient utilization of internal phosphorus content and an external mechanism that allows greater uptake of the phosphorus from the soil. ( Wissuwa and Ae, 2002). An extensive root system unable the plant to have more uptake of P from the soil. Molecular mechanisms are found to work that has been characterized from various organisms. In case of the E. coli and Brewer's Yeast multigene inducible system called “Pho-regulon ” is found that enable the plant to scavenge any available traces of organic phosphates from the soil ( Torriani and Ludte,1990).Acid phosphatase help in the mobilization of phosphate from the cellular and extracellular organic compounds. QTL analysis had lead to characterization and mapping of one major QTL for phosphorus uptake pup1 isolated from Chromosome 12 of rice variety with tolerance to low phosphorus ( Wissuwa et al,2007).

Introduction

Analysis of this pup1 showed a three times more uptake rate as compaired to non tolerant line that lacks the pup1 ( Wissuwa, yano and Ae.,

2007). In the maize P73 proteins have been

identified by the two dimensional electrophoresis that are expressed under phosphors starvation. Similar factors i.e P95 have been characterized in rice ( Wissuwa and Ae, 2009), these proteins are responsible Pi solublization and Pi absorption. Mutants displayed specialized P efficient system with a higher capacity for mobilization of internal Pi and increased cell division under P starved conditions ( Wissuwa and Ae., 2009). Tissue culture provides an efficient way to produce improved variants against many problems associated with rice, suitable for crop improvement. A number of authors observed variation among the regenerated

plants and their progenies during

embryoid

formation from pollen calli. P.J Larkin and W.R Scowcroft(1981) plant industry

division,CSIRO

Australia

gave

the

name

“Somaclonal

variation” to the genetic variability originated as a result of tissue culture conditions. The mass occurrence of the somaclonal variants increase in resistance, productivity

are vital force of plant

(the

heterotic effects) are explained by the “Carlson” (1983). Screening of Somaclones for various kinds of stresses like drought, salt has lead to development of tolerant lines against different kinds of stresses. The production of the embryogenic calli

from different

parts of plants like seeds, coleoptile and immature embryo highly depends upon the genotype of plant.Common variations among Introduction

samaclones includes tillers per plants, plant height, flag leaf length, panicle length and number of seeds produced (Karp,1995). Keeping the above facts in mind regarding tissue culture the present investigation was carried out with the following objectives . (1) Development of somaclones against low phosphate stress (2) Regeneration and acclimatisation of “P” tolerant samaclones (3) Biochemical and physiological characterization of selected somaclones.

Introduction