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Developmental plasticity in lateral roots may be one of the key traits for the growth of rice plants under soil moisture fluctuations. We aimed to examine responses in seminal root system development to changing soil moisture for diverse rice cultivars. Special attention was paid to the two different types of lateral roots ; the generally long, thick L type capable of branching into higher orders, and the non-branching S type. Plants were grown in half-split polyvinyl chloride tubes fixed with transparent acrylic plate for root observation under glasshouse conditions. When plants were grown first under drought conditions, then rewatered, the seminal root system development in terms of dry weight and total length was promoted as compared with plants grown under continuously well-watered conditions in IR AT 109 and Dular, drought tolerant cultivars. Promoted production of L type lateral roots mainly contributed to the development of the longer seminal root system. Plants exposed to soil submergence before they were grown under drought conditions did not show such promoted responses in these two cultivars. However, in KDML 105, a drought tolerant cultivar, the production of especially L type laterals was substantially promoted under drought and rewatered conditions. Honenwase was characterized by the shallow root system and great reduction in root system length when soil moisture becomes limited. These facts show that genotypic variations exist in the plastic response of rice seminal root system and that the L type lateral root plays a key role in manifestation of this plasticity.

Rice plants in the rainfed areas are mostly grown under fluctuating soil moisture. We examined responses in dry matter production, root development and water use to changing soil moisture in diverse rice cultivars. Rice plants were grown in polyvinyl chloride tubes under glasshouse conditions. Progressive drought right after planting greatly inhibited the shoot dry matter production, tiller development, nodal root development and water uptake in all cultivars tested. When the plants experienced soil submergence before being exposed to drought, all the cultivars exhibited higher dry matter production than their well-watered counterparts. Cultivar differences were clearly noted in the growth responses to rewatering after these plants were droughted. With well–watered control as basis, IRAT 109 and KDML 105 plants increased efficiency in converting available dry matter to increase their total root length by means of enhanced lateral root development. In the latter, however, the dry weight of roots also increased and so did root water uptake. In Dular, droughted plants did not show a clear response in terms of root development and water uptake to rewatering while its shoot growth was much more severely inhibited than the other cultivars. These findings suggest that phenotypic plasticity in the root system structure exhibited by promoted lateral root development and new nodal root production play a key role in the growth of rice under changing moisture level in the soil.

The effect of different time and duration of progressive drought on cassava, during the establishment stage, i.e., from planting to about three months after planting, was studied with special attention to root system development. Plants exposed to drought, regardless of whether early (from 9-44 days after planting (DAP)) or late (from 44-82 DAP), in the establishment period had significantly fewer leaves and lower shoot dry weight (DW) than well-watered controls. Late droughted plants manifested similar inhibition of shoot growth with those continuously droughted from 9-82 DAP. With regards to root development, early drought caused reduction in the number and length of adventitious roots, number of first, second and third order lateral roots and total root DW. The value of these parameters generally increased after the plants were rewatered. Late drought caused reduction in the number and length of the adventitious roots, and the number of first and second order lateral roots, but not the third order lateral roots, which appeared to be promoted instead by drought. The total root DW also increased in late-droughted plants due to the thickening of adventitious roots. Overall, the results show that cassava has sensitivity to drought stress during its establishment period.

The length of the seminal root (SR) axis and the number and length of lateral roots (LRs) of sorghum (Sorghum bicolor Moench) were markedly inhibited by taro [Colocasia esculenta (L.) Schott] residues incorporated into a sand growing medium. The sand profile was divided equally into zones with and without residues. Production and elongation of the first-order LRs of the SR axis facing the zone containing taro residues were severely suppressed. On the side facing the zone that was free of residues, production and elongation of LRs was not inhibited. SR and LR growth was drastically impaired and many plants were killed when taro residues were incorporated in large amounts into the uppermost 2 cm of the growing medium. The activity of the allelopathic substances in the root zone appeared to be location-specific.

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.

In order to affect a better management for sweetpotato and cassava, especially during their establishment period, a series of experiments were conducted to determine possible regulation of root development of sweetpotato and cassava as a function of (i) moisture regime in the soil and variety, and, (ii) shifting soil moisture. In both sweetpotato and cassava, the production (formation) and growth (elongation) of the adventitious roots and their contemporaneous lateral roots were greatly affected by soil moisture, a deficiency of which generally suppressed their formation and subsequent growth. Genotype variation likewise influenced the individual root system development but this was more apparent in sweetpotato than in cassava. Fluctuation in the amount of water in the soil brought about variable development in the individual component roots in a root system of both sweetpotato and cassava, with deficient moisture as the most depressive, even though the plants had been exposed previously to well watered (normal) condition. In both crops, the individual root system components' growth under deficient soil moisture were in all instances improved when exposed afterwards to normal soil moisture.