Agriculture Reference
In-Depth Information
demonstrated all of these criteria. In Brazilian Oxisols, upland rice yield is significantly reduced
after 2 or 3 years of consecutive planting on the same area in monoculture due to autoallelopathy
(Fageria and Baligar, 2003a). Fageria and Souza (1995) also reported a yield reduction of upland
rice in the third year when it was grown in rotation with dry bean on a Brazilian Oxisol.
The organic compounds involved in allelopathy are collectively called allelochemicals
(Olofsdotter et al., 1995; Olofsdotter, 2001). Harborne (1977) and Rice (1979) have reviewed the
chemistry of various allelochemic compounds in terrestrial ecosystems. These include simple phe-
nolic acids, aliphatic acids, coumarins, terpenoids, lactones, tannins, flavonoids, alkaloids, cya-
nogenic glycosides, and glucosinolates. Phenolic acids have been identified in an allelopathic rice
germplasm (Rimando et al., 2001) and have previously been described as allelochemicals (Blum
et al., 1999). Most of them are secondary metabolites released into the environment by leaching,
volatilization, or exudation from shoots and roots. Many compounds are degradation products
released during the decomposition of dead tissues. Once these chemicals are released into the
immediate environment, they must accumulate in sufficient quantity to affect other plants, persist
for some period of time, or be constantly released to have lasting effects (Putnam and Duke, 1978).
Abiotic (physical and chemical) and biotic (microbial) factors can influence the phytotoxicity of
chemicals in terms of the quality and quantity required to cause injury (Fageria and Baligar,
2003a). After entering the soil, allelochemicals encounter millions of soil microbes. The accumu-
lation of chemicals at phytotoxic levels and their fate and persistence in soil are important deter-
mining factors for allelochemical interference. After entry into the soil, all chemicals undergo
processes such as retention, transport, and transformation, which influence their phytotoxic levels
(Fageria and Baligar, 2003a).
Maintaining an adequate level of OM in the soil is a useful method for neutralizing toxic chemi-
cals produced in the process of allelopathy by plants. SOM content can be improved through the
application of animal manures and green manuring, use of crop rotation, and conservation tillage
(Fageria and Baligar, 2003a). The beneficial effects of OM in detoxifying chemical substances
depend on the concentration and the type of chemical compounds and also on other soil chemical
properties. SOM may coat mineral surfaces (e.g., Mn
2+
and Fe
2+
), which prevents phenolic acids
from directly contacting mineral ions and, thus, oxidation of phenolic acids.
Guimares and Yokoyama (1998) studied the effects of upland rice-soybean rotation and upland
rice planted in monoculture on a Brazilian Oxisol (Table 4.3). The data in Table 4.3 show that the
upland rice yield was significantly reduced in monoculture as compared to rice grown in rota-
tion with soybean. This illustrates that the negative effects of rice allelopathy on the rice crop can
be reduced by crop rotation. Similarly, Fageria (2001) reported that a substantially good yield of
upland rice can be obtained when grown in upland rice-common bean-corn-soybean rotation on
an Oxisol.
TABLE 4.3
Upland Rice Grain Yield in Rotation with Soybean
and Monoculture Grown on a Brazilian Oxisol
Crop Rotation
Grain Yield (kg ha
−
1
)
Rice after 3 years of soybean
4325
Rice after 1 year of soybean
2577
Rice in monoculture for 5 years
1160
Source:
Adapted from Guimaraes, C. B. and L. P. Yokoyama. 1998.
Technology for Upland Rice
, pp. 19-24. Santo Antonio de
Goias, Brazil: Embrapa Arroz e Feijão. With permission.
