Allelopathic Interactions and Allelochemicals: New Possibilities for Sustainable Weed Management

Weeds are known to cause enormous losses due to their interference in agroecosystems. Because of environmental and human health concerns, worldwide efforts are being made to reduce the heavy reliance on synthetic herbicides that are used to control weeds. In this regard the phenomenon of allelopathy, which is expressed through the release of chemicals by a plant, has been suggested to be one of the possible alternatives for achieving sustainable weed management. The use of allelopathy for controlling weeds could be either through directly utilizing natural allelopathic interactions, particularly of crop plants, or by using allelochemicals as natural herbicides. In the former case, a number of crop plants with allelopathic potential can be used as cover, smother, and green manure crops for managing weeds by making desired manipulations in the cultural practices and cropping patterns. These can be suitably rotated or intercropped with main crops to manage the target weeds (including parasitic ones) selectively. Even the crop mulch/residues can also give desirable benefits. Not only the terrestrial weeds, even allelopathy can be suitably manipulated for the management of aquatic weeds. The allelochemicals present in the higher plants as well as in the microbes can be directly used for weed management on the pattern of herbicides. Their bioefficacy can be enhanced by structural changes or the synthesis of chemical analogues based on them. Further, in order to enhance the potential of allelopathic crops, several improvements can be made with the use of biotechnology or genomics and proteomics. In this context either the production of allelochemicals can be enhanced or the transgenics with foreign genes encoding for a particular weed-suppressing allelochemical could be produced. In the former, both conventional breeding and molecular genetical techniques are useful. However, with conventional breeding being slow and difficult, more emphasis is laid on the use of modern techniques such as molecular markers and the selection aided by them. Although the progress in this regard is slow, nevertheless some promising results are coming and more are expected in future. This review attempts to discuss all these aspects of allelopathy for the sustainable management of weeds. Referee: Dr. Amrjits S. Basra, Central Plains Crop Technology, 5912 North Meridian Avenue, Wichita, KS 67204     

Biochemical effects of pure isomers of hexachlorobiphenyl: fatty livers and cell structure.

The effects of a single oral dose (50 mg/kg body wt.) of 3,4,5,3',4',5'-hexachlorobiphenyl (HCB), 2,4,5,2',4',5'-HCB or 2,3,5,2',3',5'-HCB for a period of 72 h have been studied in the male rat. Only 3,4,5,3',4',5'-HCB caused necrosis of thymocytes. 3,4,5,3',4',5'-HCB caused marked pathological changes in the liver with less marked effects being caused by 2,4,5,2',4',5'-HCB and 2,3,5,2',3',5'-HCB. Total lipid content was increased by all the isomers studied, but 3,4,5,3',4',5'-HCB had more pronounced effect on total lipid content. Lipid accumulation was pericentral in the livers obtained from rats treated with 3,4,5,3',4',5'-HCB, but midzonal in the liver obtained from the rats treated with the other two isomers. Analysis of various lipid fractions showed that triacylglycerols were increased seven-fold only by 3,4,5,3',4',5'-HCB, while phospholipids were increased slightly by 2,4,5,2',4',5'-HCB or 2,3,5,2',3',5'-HCB. Only 3,4,5,3',4',5'-HCB increased the level of total and esterified cholesterol. These results show that the fatty livers caused by 3,4,5,3',4',5'-HCB were qualitatively and quantitatively different from those caused by the other two isomers at the same dose. For the first time a hexachlorobiphenyl unchlorinated in the para position, 2,3,5,2',3',5'-HCB has been shown to be a specific inducer of cytochrome P-450. The effects of 2,3,5,2',3',5'-HCB on cell structure and phospholipid content were quantitatively similar to those caused by 2,4,5,2',4',5'-HCB. Thus, chlorination at para (4,4') positions in chlorobiphenyls is not necessarily required for biological activity. It is hypothesized that net stereoelectronic properties of the isomers or resistance to metabolism may be the underlying factor in determining structure-activity relationships.     

Induction of mixed function oxidases on oral administration of dieldrin

The administration of dieldrin (30 mg/kg body weight) caused an increase in the liver weight of rats. The metabolism of aflatoxins B₁ and G₁ by the microsomes obtained from the liver of dieldrin-treated animals was enhanced significantly as compared to the controls showing that dieldrin increased the activity of mixed function hydroxylases. Dieldrin caused an increase in the activity of liver microsomal NADPH oxidase and a decrease in the lipid peroxidation. Dieldrin brought about an increase in the phosphatidylcholine content of rat liver.     

Autotoxic impact of essential oil extracted fromLantana camara L.

The effect of crude volatile oils extracted from the young leaves ofLantana camara var.camara was studied on the parent plant itself. The contents of water and chlorophyll, of leaves apart from seed germination, seed vigour and length of seedlings of the parent plant were adversely affected with increasing concentration of theLantana oils; this indicated autotoxic potential of the oil. The correlations between oil concentration and the studied parameters were rather strong and obeyed the concentration response relationship, typical of chemical bases. The cell respiration, however, increased with increasing concentration of the oil vapours. Compared to control, less than 13 % of the seeds of the parent plant exposed to 30 cm³ m^-3 of oil showed incipient germination with no growth of radicle beyond immeasurable protuberance, and no plumule emergence.     

Calculated microwave absorption of double-helical B-conformation poly(dG)·poly(dC)

We have estimated the absorption of microwave radiation by the four acoustic modes of poly(dG)·poly(dC) in B conformation. The eigenvectors used in this calculation are those obtained from a refined normal coordinate analysis by fitting the velocity of the longitudinal acoustic mode to that observed by Brillouin scattering. The acoustic modes couple to the radiation field when translational invariance is broken such as for finite segments of helix and as may be the case in vivo. Calculations for various helix lengths show that the estimated absorption is greater than that of water by at least two to three orders of magnitude in some cases.