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We aimed to assess rice yield components as function of water stress imposed at distinct crop growth stages under greenhouse, in randomized block design under factorial scheme 3 × 4 + 1, with four replications. The factor "A" was the growth stage in which water stress was imposed, being (a) vegetative, (b) reproductive 1, and (c) reproductive 2; factor "B" was four levels of water stress (0-200 kPa). There was also an additional treatment consisting of a flooded check. Water was replenished to saturation every time the threshold stress was reached. At the end of the cycle, all panicles were counted and collected, per plant, for further analysis. In the lab, grains per panicle were counted, being classified either as whole kernel or aborted grains, whose results were used for obtaining sterility percentage. Whole kernel grains were weighted for obtaining the 1000 grains weight for each treatment, and the consequent plant grain yields. Number of panicles was not affected when stress was imposed after panicle initiation, but when imposed at tillering it was reduced; at grain filling, water stress promoted grains sterility higher than 90%; grain weight was only reduced when carbohydrates were directed to root formation in detriment of grain filling; lower rice grain yield per plant was observed even when treatments were maintained above 10kPa all along the cycle, compared to the flooded treatment.

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Abstract. Levees are small land dikes made every rice-cultivation season that allow for flood irrigation in rice fields. Currently, levees are demarcated by utilizing a laser technology (LT) system. However, with current technological advances, the demarcation of levees with the Global Navigation Satellite System (GNSS) and real-time kinematics (RTK) correction has been highlighted in rice production systems in southern Brazil. The objective of this study was to compare the performance between LT and GNSS-RTK systems applied in the demarcation of levees that are used in flooded rice fields. To this end, an experimental area of approximately 27 ha located in Jaguarão, Rio Grande do Sul, Brazil, was used. From a digital elevation model, the area was subdivided into three subareas according to the mean slope: flat (0.16%), intermediate (0.36%), and gently undulated (1.3%). The total length of the levees for the three subareas was 8 km. The relative performances of both demarcation systems were evaluated by analyzing the vertical and horizontal behavior of the levees and the water layer spatial distribution. The results indicated that the demarcation of levees by GNSS-RTK systems is more accurate than that by the LT system, especially in flat areas. In these areas, the GNSS-RTK demarcation system permits a reduction in the total number of levees, as well as an increase in the vertical equidistance between levees and/or an increase in the height of the levee itself. The length of the levee is shorter than in a demarcation using the GNSS-RTK system. Keywords: Contour line, Irrigation, Lowlands, Precision agriculture.

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