Rice plant roots

Despite increased citrate accumulation in roots of Zn-deficient rice plants, root exudation of citrate was not enhanced. However, in distinct adapted rice cultivars, enhanced release of citrate could be observed in the presence of high bicarbonate concentrations in the rooting medium, a stress factor, which is frequently associated with Fe and Zn deficiency in calcareous soils (235) (Hajibo-huid, unpublished). This bicarbonate-induced citrate exudation has been related to improved Zn acquisition in bicarbonate-tolerant and Zn-efficient rice genotypes (Fig. 9) (23S). Increased exudation of sugars, amino acids, and phenolic compounds in response to Zn deficiency has been reported for various dicotyledonous and monocotyledonous plant species and seems to be related to increased... 

Yoshida, S., K. Tazaki, and T. Minami-kawa. Occurrence of shikimic and quinic acids in angiosperms. Phytochemistry 1975 14 195-197. Kanamori, T., and H. Matsumoto. Role of glutamine in asparagine biosynthesis in rice plant roots. Z Pflan-zenphysiol 1974 74 264. 

NAD-GDH is rapidly induced in rice plant roots by NH4, whereas in shoots the enzyme is less affected. In roots NAD-GDH is found in both the soluble fractions and in mitochondria. Induction by NH4+ causes an increase of enzyme mainly in the mitochondria. A new band of GDH activity was detected on the zymograms of polyacrylamide gel electrophoresis, and this inducible enzyme is active with both NAD and NADP 254). 

In plants this reaction is the first step for the introduction of nitrogen from ammonia into organic compounds and is therefore of major importance for nitrogen metabolism in general (225, 224). A partially purified preparation of the enzyme catalyzing the reaction has been isolated from a number of different plant sources, including peas 46, 267, 345), soybean root nodules 205), rice plant roots 124), pea leaves... 

Kakimoto, Y., a. Kanazawa, T. Nakajima, and I. Sano Isolation of y-L-glutamyl-L-p-aminoisobutyric acid from bovine brain. Biochem. Biophys. Acta 100, 426 (1965). Kanamori, T., and H. Matsumoto Glutamine synthetase from rice plant roots. Arch. Biochem. Biophys. 125, 404 (1972). 

One must however note that SRB could also have some benefits ranging from assistance in the Evolution [14, pp. 17-19] to contribution to ititrogen-fixing capacity of the soil and kilUng nematodes which infest the rice plant roots by sulphide toxicity [ 14, Chapter 8, pp. 205-206]. 

For most crops, other than rice, urea in the soil must first undergo hydrolysis to ammonia and then nitrification to nitrate before it can be absorbed by plant roots. One problem is that in relatively cool climates these processes are slow thus plants may be slow to respond to urea fertilization. Another problem, more likely in warmer climates, is that ammonia formed in the soil hydrolysis step may be lost as vapor. This problem is particularly likely when surface appHcation is used, but can be avoided by incorporation of the urea under the soil surface. Another problem that has been encountered with urea is phytotoxicity, the poisoning of seed by contact with the ammonia released during urea hydrolysis in the soil. Placement of urea away from the seed is a solution to this problem. In view of the growing popularity of urea, it appears that its favorable characteristics outweigh the extra care requited in its use. 

Inabenfide. [4-Chloro-2-(a-hydroxybenzyl)]-isonicotonanilide) [82211 -24-3] (Inabenfide) (32) is not for use in the United States, but is used in other countries to inhibit the growth of rice plants. The compound is appHed to the soil 40—60 days prior to the heading up of plants, where it is absorbed through the roots and translocated throughout the stem. It inhibits the elongation of the lower intemodes and this stops lodging. It is extremely toxic to fish. 

Hosono T, Nouchi I. The dependence of methane transport in rice plants on the root zone temperature. Plant Soil, 1997 191 233-240. 

Lu Y, Wassmann R, Neue HU, Huang C. Impact of phosphorus supply on root exudation, aerenchyma formation and methane emission of rice plants. Biogeochemistry. 1999 47 203-218. 

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. 

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

Kirk and Solivas measured the time course of N uptake by soil-grown rice plants and the simultaneous changes in soil solution NH4+ and root length density, and then compared the results with the calculated minimum root length densities required to explain the uptake. The calculation was based on the following picture of events. 

Figure 6.13 Influx of N into roots of intact rice plants grown on 100 p,M N as either N03 or NH4 (a) concentration dependence of and NH4 influx in 4-week-old plants (b) induction of uptake in 3- or 4-week-old plants deprived of N for 24 h before re-supply at 100 p,M for the indicated periods (Kronzucker et al., 2000). Reproduced by permission of Blackwell Publishing...

Three compounds have been developed that utilise the inhibition of melanin biosynthesis as the basis of disease control (Figure 4.18). The first and most valuable of these is tricyclazole. Tricyclazole is readily absorbed by the foliage and roots of rice plants and redistributed acropetally. On treated plants, conidial germination of P. oryzae and... 

Exactly the opposite problem may occur for plants whose roots are growing in anaerobic media. In Hooded soils the roots may be exposed to high levels of irontll). posing potential problems of iron toxicity. Rice plants and water lilies with roots in anaerobic soils transport dioxygen (from the air or photosynthesis, or both) to the periphery of the roots where it oxidizes the iron(II) to irondll). In (his case the insolubility of Irondll) hydroxide is utilized to protect the plant from iron poisoning.113 A similar problem from too much iron occurs in parts of sub-Saharan Africa. 

Related to amino acids is picolinic add (8), which is present at high concentrations in dormant tissues such as plant seeds and whose involvement in the quiescent state may involve iron chelation. Also mugineic acid (9) possesses strong Fe111 chelating properties. This amino acid derivative has been isolated from the roots of rice plants and has been found to have a strong stimulatory effect on Fe uptake. The crystal structure of a complex of Co111 shows that the anion of (9) can function as a hexadentate chelate.38... 

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. 

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