RESPONSE OF THE DIFFERENT ROOT-SYSTEM COMPONENTS OF SORGHUM TO INCIDENCE OF WATERLOGGING

Crop plants are often exposed to excessive soil moisture conditions during growth. Sorghum plants (Sorghum bicolor Moench) were grown in pots and subsequently subjected to continuous waterlogging during their vegetative stage to determine the growth and development of the different root system components. Plants grown in well-drained pots served as controls. Other plants were subjected to different timings of waterlogging, i.e. early (waterlogged during the early part of their vegetative growth) and late (waterlogged during the late part of their vegetative stage). All plants were kept under glasshouse conditions. Root and shoot development was measured by destructive sampling. Continuous waterlogging caused an immediate increase in the number of nodal root (NR) axes but not their total length. All other root system components such as NR laterals and the seminal root (SR) and its laterals had their number and length markedly restricted by waterlogging. NR production appeared to be an adaptive response of sorghum to waterlogging. In the event that NRs in the lower nodal position of the plant's stem died due to waterlogging injury, new NRs appeared in the next higher nodal position, suggesting a relationship between the death of older NRs and the production of new ones. The trend of NR lateral production followed that of the NR axes. Waterlogged plants generally showed a small root system and limited shoot growth. In early-waterlogged plants the NR and their laterals regrew actively after they were removed from waterlogged conditions and allowed to grow under drained conditions for 9 days. In the late-waterlogged plants NRs continued to increase in number and length while other components were inhibited. The SR and its laterals did not show any evidence of active regrowth in the early or late waterlogging treatments. Plant recovery from waterlogging was brought about mainly by the resumption of growth of the NRs, i.e. elongation of existing functional axes and initiation of new laterals.