Search

The nature of inhibition of seminal root elongation in sorghum ( Sorghum bicolor Moench) plants grown under different periods of high root-zone temperature (RZT) of 40°C was analyzed by measuring the epidermal cell lengths at fixed points along the growth zone. Cell lengths of roots exposed to high RZT were significantly shorter than those grown at 25°C. Growth in cell lengths became more inhibited as the period of exposure to high temperature was increased. Cell elongation peaked closer to the apex as the period of exposure to high RZT increased. Reduction of the elongation region was 10–16% for every 2 days of growth under high RZT. The total number of epidermal cells added to a cell fine per hour (cell flux), estimated from epidermal cell lengths and root elongation rates, was reduced by 26% for every 2 days of exposure to the high RZT.

Under natural conditions crop plants are often exposed to high soil temperature regimes during growth. To determine the influence of a hot root-zone temperature (RZT) on growth and development of root system components of plants, sorghum (Sorghum bicolor Moench) was grown in nutrient solution with three temperature regimes. Maximum seminal root (SR) elongation and first order lateral root (LR) initiation and elongation occurred at 25°C (control). At 40°C SR elongation and first order LR initiation and elongation were severely inhibited. Less inhibition in root growth occurred at 40/25°C (day/night) RZT than at 40°C. The duration of exposure to 40°C (i.e. from 0 to 6 days) had a profound influence on subsequent root growth and development when the plants were returned to 25°C; the longer the 40°C period the greater the degree of root growth inhibition at 25°C. Nodal roots (NRs) which arose mainly from the first node of the stem were initiated regardless of their previous exposure to 40°C; the number and length increased as the exposure duration at 40°C increased.

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 trends 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.