Attribution of allelopathy in tea soils on bush debilitation and yield decline in old tea cultivar TRI 2026 grown in mid elevations of Sri Lanka

Abstract

An experiment was conducted at Meddakanda Estate, Balangoda, Sri Lanka (Elevation: 1050 m amsl) during 2008 – 2010 to investigate the underline causes for bush debilitation and yield decline in tea (Camellia sinensis L.), of 36 yr old cultivar TRI 2026. The tea land was divided in to three categories as severely affected, moderately affected with above problem and unaffected and a 10 x 10 m land block was demarcated from each category. Soil samples were obtained beneath randomly selected 15 tea bushes, from each category at 3 depths (0 – 15, 15 – 30 and 30 – 45 cm) and in-vitro bio assays were performed by germinating Lettuce seeds (Lactuca sativa) (variety Grand rapids) directly on these soils. Furthermore, soils at 30 cm depth were extracted for allelochemicals using a neutral EDTA solution and in-vitro bio-assays were performed using Lettuce seeds treated with these extracts. Distilled water was used as control. Tea yield in severely affected block was zero but the moderately affected and unaffected tea blocks recorded 119 and 92 g made tea/bush, respectively, over 7-month period. The healthy seedling number of test plants was significantly (P>0.01) reduced with soils of all three depths from severely and moderately affected tea blocks compared to that of the unaffected tea block. Germination percentage was also significantly (P>0.01) reduced with soil extracts of severely and moderately affected blocks compared to that with extract of unaffected field and control. Hypocotyl length and radical length of Lettuce were lower and growth inhibition percentage of both were significantly (P>0.01) higher with soil extracts of severely and moderately affected fields compared to those of unaffected tea block. Allelochemicals exudates from old tea itself and residues of fertilizers, pesticides specially herbicides etc. added to the soil may have caused phytotoxicity in test plants when treated with soil extracts of affected tea fields and there in bush debilitation and yield decline in tea.