traits such as yield can be facilitated via molecular marker technology Khush Virk 2002.
Although each marker system is associated with some advantages and disadvantages, the choice of marker system is dictated to a large measure by the
intended application, convenience and the cost involved. High throughput approaches have also been developed, thus making it possible to scale-up the
use of some of these markers.
2.5. The Physiology of Iron and Zinc in Rice Plants
In green leaves, 80 of the iron is localized in the chloroplast regardless of the iron status of the plant. In plant cells, iron is located and accumulated in the
stroma of plastids as phytoferritin. It is characterized by high metabolic activity. Iron also has important function in solute transportation e.g. phloem loading.
The rhizodermal transfer of iron in deficient roots are most likely the sites of H
+
- efflux pumps and the release of phenolic compounds. After the supply of iron is
restored, the transfer cell degenerates within 1 or 2 days. Iron deficiency is a worldwide problem in crop production in calcareous soil. Iron toxicity is the
second most severe yield-limiting factor in wetland rice Marschner 1986. Iron is an important component in many plant enzyme systems, such as
cytochrome oxidase electron transport and cytochrome terminal respiration step. Iron is a component of protein, ferridoxin. Iron is required for nitrate
NO and sulfate SO reduction, nitrogen N
3 4
2
assimilation, and energy NADP production. Iron is a catalyst or part of an enzyme system associated with
chlorophyll formation. Iron affects to protein synthesis and root tip meristem growth. The majority of plant iron is in the ferric Fe
3+
form as ferric phospoprotein, although the ferrous Fe
2+
ion is believed to be the metabolically active form Jones 1998.
Zinc is known to have various physiological functions in higher plants. There are appoximtely 300 enzymes in which Zn is an integral component. In
these enzymes Zn has catalytic e.g. carbonic anhydrase, carboxipeptidase, alkaline phosphatase, and phospholipase or structural e.g. alcohol
dehydrogenase, Cu-Zn superoxidase dismutase, and RNA polymerase function Marschner 1986.
13
Beside Zn-containing enzymes, Zn is either essential for the activity, or modulates the activity of some enzymes, including dehydrogenases, aldolases,
isomerases, transphosphorilases, and Zn-dependent inorganic pyrophosphatase Zn-IPPase.
The physiological basis for micronutrient include iron and zinc efficiency in crop plants and the process controlling the accumulation of micronutrient in
seed is not understood with any certainty. There are several barriers to overcome in genetically modifying plants to accumulate more micronutrient in their edible
parts. These barriers to micronutrient uptake and distribution in plants are the result of tightly controlled homeostatic mechanism that regulate micronutrients
uptake and distribution in plants assuring adequate but non toxic levels of these nutrient to accumulate in plant tissues. The first and most important barrier to
micronutrient uptake reside at the root-soil interface. To increase micronutrient metal uptake by roots, the available levels of the micronutrient in the rhizosphere
must be increased to allow for more absorption by root cell such as by stimulating root-cell H
+
, metal chelating compounds and reductants release rates, and increasing root absorptive area such as number and extent fine roots and roots
hairs. Second, the root cell plasma membrane absorption mechanism must be sufficient and specific enough to allow for the accumulation of micronutrient
minerals once they enter the apoplast of root cell from the rhizosphere. Third, once taken up by root cells, the micronutrients must be located efficiently to
edible plant organs. For seed and grain, phloem sap loading, movement and unloading rates are important characteristics to consider increasing micronutrient
metal accumulation in seed and grain Welch Graham 2002. Increasing the micronutrient stored in seeds results in increased seedling
vigor and viability enhancing the performance of seedlings, when the seed is planted in micronutrient-poor soil. Finally, it leads to improved yields compared
to seed with low micronutrient stores Welch Graham 2002.
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III. MATERIALS AND METHODS 3.1.
