Since the report of H i l t n e r 5 that micro-organisms were more abundant in the soil nearer to plant roots than in the soil farther apart, there has been considerable speculation on the availability of nutritive substances near the root region and their exudation by the plant roots. K n u d s o n s brought out the first conclusive evidence that the roots of peas and maize grown under aseptic conditions exuded reducing sugars. L y o n and W i l s o n 9 found that organic nitrogen was exuded by maize roots. Since then the exudation of different spectra of chemicals by plant roots has been reported by several workers and literature on the subject has been reviewed by R o v i r a la and S c h r o t h and H i l d e r b r a n d 14. Nevertheless, the nature and quantity of these exuded materials at different stages of plant growth is little understood. It is also known now that the nature of the root exudates could be altered to some extent by foliar application of chemicals ( V r a n y et a/.la).The exudation pattern of amino acids and sugars at different stages of plant growth in four crop plants, viz sorghum (Sorghum vulgare Pers.), sunnhemp (Crotalaria ju~cea Linn.), ragi or finger millet (Eleusim coracana Gaertn.) and tomato( Lycopersico1, escu/enturn Mill.) and the influence of foliar application of N and P on the spectra of chemicals exuded by the root are reported in this paper.--2 1 0 --
For the purpose of examining the correlations, if any, between the root and shoot growths of three Citrus species and their rhizosphere microflora, studies were made with 6-year-old plants in the Annamalai University Experimental Orchard. The rhizosphere population was analyzed quantitatively. Bacteria were about 40 to 90 times, actinomycetes 2 to 6 times, and fungi 3 to 6 times more abundant in the rhizosphere than in the soil, in rhizosphere populations there was no significant difference between the three Citrus species. Comparisons of populations on the growing and non-growing roots of the three Citrus species revealed that the growing roots harbored 2 to 3 times more bacteria and actinomycetes than the non-growing roots. There was only slight increase in the fungal population in the rhizosphere of the citrus variety Pummelo but not in the other two varieties.
The studies on the activities of the enzymes hydrolysing sucrose and starch in the rhizosphere of finger millet plant (Eleusine coracana Gaertn.) showed significantly greater activity of both the enzymes in the rhizosphere than in the soll away from the influence of plant roots. The activity of the enzyme hydrolysing sucrose was always greater than that hydrolysing starch and a constant ratio in the activity of these two enzyme systems was maintained all through plant growth. A highly positive correlation between the bacterial and fungal populations of the rhizosphere and the enzyme activities was also observed.Foliar application of N as ammonium sulphate, P as disodium phosphate and K as potassium chloride, seemed to inerease the activities of the enzymes in the rhizosphere from the 45th to 75th day of plant growth, such an effect being relatively more with K than N or P.Application of Dimecron 100, a systemic insecticide and Blastin, a systemic fungicide, on the foliage signifieantly altered the populations of bacteria and fungi as well as the enzyme activities at various stages of plant growth. Although the enzyme activities were reduced initially due to the first spray treatment, a subsequent significant inerease in the enzyme activities was recorded. The increase in the enzyme activities due to the foliar treatments appeared to have been retained in the rhizosphere even 15 days after the last spray.
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