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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.

The effect of fluctuation in soil moisture on the root development of sweetpotato was studied during its establishment period, i.e., from planting to about one month after planting. The number of leaves, shoot dry weight and vine length were suppressed significantly by deficient moisture but the same were markedly increased by excessive moisture regardless of the time of occurrence relative to the initial development of the plant. In terms of its roots, the number and total length of the root system components were either increased or decreased depending upon the soil moisture regime in which they were subjected immediately prior to the time of sampling. Deficient soil moisture reduced the number and total length of the root system components while excessive moisture caused notable stimulation. Continuous exposure to normal soil moisture caused substantial reduction in the number of adventitious roots and consequent depression in total length but this could be attributed to sink competition among adventitious roots. Under fluctuating soil moisture, normal moisture content appeared to restore the aberrant development of the roots caused by deficient or excessive levels. The suppressed formation and elongation of the root system components under deficient soil moisture was alleviated when the soil moisture condition was changed to normal. On the other hand, the stimulated root formation and elongation in the excessive moisture was abated when the normal soil moisture condition prevailed afterwards. The result was the opposite when the soil moisture condition was reversed, i.e., normal then followed by either deficient or excessive soil moisture.

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.

Root zone temperature (RZT) is an important factor that affects the establishment of crops in the field. This study was conducted to determine the effect of high RZT on the root system development of cassava and sweetpotato and to assess the response of each root system component of cassava and sweetpotato to high RZTs. Cassava and sweetpotato are widely grown crops in soils that are prone to reach a high temperature especially during the dry season. The plant root system was the primary object of investigation in this study. Cassava and sweetpotato cuttings were grown in pots for 20 days in a growth chamber where two different soil temperature regimes were maintained : 40°C (high RZT) and 25°C (normal). High RZT significantly decreased the total length of the adventitious roots (ARs) and the number and total length of the first order lateral roots (LRs) in both cassava and sweetpotato. Reduction in these root growth parameters was greater in the latter than in the former. High RZT caused a marked increase in the number of the second order LRs in sweetpotato and tended to enhance the formation of the third order LRs. Under high RZT, in cassava, a higher proportion of the total number of ARs and the first order LRs, and thus, a greater fraction of the total root length came from the lower nodes of the cuttings that were buried deep in the soil at planting. In sweetpotato, a greater proportion of the total number and total length of the ARs and the first order LRs was observed in the nodes of the cuttings that were buried closer to the soil surface than those buried deeper. An opposite trend was evident in both cassava and sweetpotato grown under a normal RZT. In the screenhouse experiment where plants were grown for 12 days, root growth of cassava was significantly reduced by the high RZT that prevailed when the soil was not covered with mulch materials. The root development in the mulched soil was similar to that under normal RZT in the growth chamber.