The influence of humic substances (HS) formulations derived from sedimen tary and compost sources w as studied on plant growth at cell level and chlorophyll retention during accelerated senescence of leaf tissue. The direct effect of HS formulations was studied on cell expansion using cucumber and radish cotyledon expa nsion test. The cucumber hypocotyl elongation test was used to study the effect on cell elongation. Chlorophyll pigment retention in excised leaf tissue incubated in dark with high temperature was assessed to study the effect on leaf senescence. Explant tissues were incubated directly in the solu tions of the formulations at the concentration recommended for foliar application to the crop plants. HS formulations showed significant variations in their direct bio-stimulatory effects. Formulations derived from compost sources were found superior in terms of inducing a direct stimulatory effect on cell expansion and cell el ongation and in maintaining chlorophyll pigment retention during accelerated senescence. HS from sedimentary sources stimulated cell expansion and delayed chlorophyll degradation to a lesser extent compared to HS from compost. However, HS formulations derived from sedimentary sources used in this study were not effective in inducing cell elongation in the cucumber hypocotyl elongation test. The direct bio-stimulatory effect of HS formula tions differed significantly between the formulations that were evaluated.
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