Rice plants root system

Signs Lesions appear on the leaves of rice plants and vary in size. They are usually diamond shaped and have a gray or white center with a brown or reddish-brown border. Crop loss of 50-90% has been reported. Lesions also appear on the rice head but are brown or black in color. Rice grains do not develop properly. In severe neck infections, the stem will break and the head will drop off. The fungus can infect the roots and also invade the plant s vascular system blocking the transport of nutrients and water from the roots. ... 

In wetlands N2 fixation can occur in the water colnmn, in the aerobic water-soil interface, in the anaerobic soil bulk, in the rhizosphere, and on the leaves and stems of plants. Phototrophic bacteria in the water and at the water-soil interface are generally more important than non-photosynthetic, heterotrophic bacteria in the soil and on plant roots (Buresh et al, 1980 Roger 1996). The phototrophs comprise bacteria that are epiphytic on plants and cyanobacteria that are both free-living and epiphytic. A particularly favourable site for cyanobacteria is below the leaf surface of the water fern Azolla, which forms a very efficient symbiosis with the cyanobacterinm Anabaena azollae. This symbiosis and those in various leguminous plants have been exploited in traditional rice prodnction systems to sustain yields of 2 to 4 t ha of grain withont fertilizer for hnndreds of years. 

Kirk (2003) has developed a simple model to compare root requirements for aeration with those for efficient nutrient acquisition in rice. The main features of the rice root system are summarized in Figure 6.4. The model considers roots in the anoxic soil beneath the fioodwater—soil interface, receiving their oxygen solely from the aerial parts of the plant. 

Figure 6.4 Root system of the rice plant (Kirk, 2003). Reproduced by permission of Blackwell Publishing...

Evidence was obtained recently that pesticide vapors may enter the air by still another mechanism, involving plant circulation and water loss (57). Rice plants were found to efficiently transport root-zone applied systemic carbamate insecticides via xylem flow to the leaves, eventually to the leaf surface by the processes of guttation and/or stomatal transpiration, and finally to the air by surface volatilization. Results from a model chamber showed that 4.2, 5.8, and 5.7% of the residues of carbaryl, carbofuran, and aldicarb, respectively, present in rice plants after root soaking vaporized within 10 days after treatment. The major process was evaporation of surface residues deposited by guttation fluid. 

Ferreira, G.A.L. Seiber, J.N. Volatilization of three N-methylcarbamate insecticides from rice plants following root-soak systemic and foliage spray treatments. 

Some early work suggested that symbiotic infection may result in ISR. In that work, a split root system was used and it was discovered that inoculation on one side of the root followed by later inoculation on the second side of the roots resulted in nodules on the first side and inhibition of nodulation of the second side (Kosslak and Bohlool, 1984 Sargent et al., 1987). It is not known if this effect is a result of an LPS caused ISR. However, it has been reported that rice plants inoculated with R. leguminosarum biovar trifolii have an increased production of phenolics... 

Methane is formed in biological processes that occur in low-oxygen environments. Anaerobic bacteria, which flourish in swamps and landfills, near the roots of rice plants, and in the digestive systems of cows and other ruminant animals, produce methane FIQURE 18.14). It also leaks into the atmosphere during natural-gas extraction and transport. It is estimated that about two-thirds of present-day methane emissions, which are increasing by about 1% per year, are related to human activities. 

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