Effects of direct sowing cultivation on the rhizosphere of lowland rice

Abstract

Abstract This report describes the effects of direct sowing cultivation on the rhizosphere of lowland rice compared with tbose of transplanting cultivation. Paddy soils in pots (planted or non-planted) were kept under upland conditions for 20, 40 or 60 days and then turned to submerged conditions to obtain different degrees of reduction. Soil Eh, Fe2+, sugar content and microorganisms in the rhizosphere soil or non-rhizospbere soil were determined at successive times 0, 20, 40, 60, 80 and 100 days after sowing. At the same time, crown roots were separated from the main stem according to each ‘shoot units.’ Observations were made on the precipitation of iron oxides, and the colonization of bacteria and Polymyxa graminis-like organisms on crown root surfaces under the microscope at a magnification of 200-400 × . The results obtained were as follows. 1) In the non-rhizospbere soil, the level of reduced eondition inereased in the plot witb earlier submergence. In contrast, the reduction of the rhizospbere soil was mainly controlled by the difference among the 3 plots. In the tillering stage, the roots were in the oxidative state and physiological activity of the rice plant, and there was no serious the rhizosphere soil was in a condition as oxidative or more oxidative than the non-rhizospbere soil, but after the maximum tillering stage (or panieie initiation stage) the Eh in the rhizosphere decreased to a very low level. 2) The main difference between the two eultivation systems (direct sowing cultivation and transplanting cultivation) in the number of rhizosphere mieroorganisms was that in the ease of direct sowing cultivation, the inftuence of the rhizospbere environment obtained during the upland nurstry stage remained longer after submergence. 3) In the tillering stage, no iron oxide precipitation was observed on any of the crown root regardless of the plots. But after the maximum tillering stage on the 100th day, in all the plots abundant precipitation of iron oxides occurred on the crown roots. The microbial colonization supported these results; that is, Polymyxa graminis-like organisms (Polymyxa is astriet aerobic organism) colonized on/in rice roots during the tillering stage. but after the maximum tillering stage they disappeared completely. 4) Little iron oxide was precipitated on the youngest crown root surfaces. Crown roots which belonged to the 3rd younaest ‘shoot unit’ showed the highest level of precipitation on their surfaces, and as the roots became older, the amount of iron oxide precipitation decreased. General conclusions abaut the importance of the physiological activity of the rice plant wbich controls the rhizospbere environment of lowland rice are also presented from the results of aseries of authors' experiments.