ROOT TURNOVER OF GROUNDNUT (ARACHIS-HYPOGAEA L) IN SOIL TUBES

Five groundnut cultivars were grown in transparent tubes of pasteurize d loam compost in growth-chamber conditions. Weekly tracings were made of all the roots visible through the walls of the tubes. White roots were assessed as living, and brown or decayed roots as dead; this corr elated with microscopical assessments of root viability based on cytop lasmic staining with neutral red followed by plasmolysis. For all five cultivars, root laterals began to die 3-4 weeks after plants were sow n. Death of root laterals progressed down the soil profile with time, while new roots were produced successively deeper from the extending t aproot. The half-life of individual roots was calculated as 3.7-4.4 we eks for all cultivars, based on assessments of the roots that died up to plant maturity (14-20 weeks, depending on cultivar). At maturity, 7 3-83% of the cumulative length of root systems had died. The onset and rate of root death were not related to onset of flowering or pod-fill ing; instead, the peak times of root death at different distances down the root system were related to earlier (3-5 week) peak times of root production in those regions. The net result of root turnover was that , despite continued new root production, the maximum length of living (white) roots of each cultivar was recorded at 2-4 weeks after sowing. Death of the earliest formed root laterals was also observed in the f irst five weeks after sowing of groundnut in an experimental field plo t in Malawi. Progressive root turnover is considered to be a normal fe ature of groundnut, perhaps representing an energy-economy strategy.