The use of varieties that are able to adapt well to extreme environments is one strategy to overcome the challenges of decreasing production in sub-optimal land. Indonesian tropical rice varieties (Jatiluhur, IPB 9G, IPB 3S, Hipa 19, Mentik Wangi, Ciherang, Inpari 17, and Mekongga) have been tested and established as water-used-efficient varieties in an optimal environment. However, to date, these varieties have not been examined in the suboptimal area, in particular, drought stress conditions. Therefore, this study aimed to evaluate the adaptation response of production, morphological, and physiological character of several water-efficient rice varieties under drought stress in the field. The study was designed in a split-plot with two factors and 4 replications, where the first factor (main plot) was drought stress stages i.e. vegetative (Dv), reproductive (Dr), generative (Dg), and control (Dc). The second factor was rice varieties, consisting of eight varieties, i.e., Jatiluhur, IPB 3S, IPB 9G, Hipa 19, Mentik Wangi, Ciherang, Inpari 17, and Mekongga. The experiment was conducted from May to December 2018 in Muneng Kidul Village, Probolinggo Regency, East Java Province. The experimental variables were morphology, production, leaf scrolling score during drought stress, drought sensitivity index, water use efficiency, physiology and root anatomy. The result showed that upland rice varieties were more tolerant to drought stress and had a higher water use efficiency than lowland rice varieties. This shows that Jatiluhur and IPB 9G which are indicated to be adaptive to drought stress, and have the ability to regulate water use more efficiently when drought stress occurs. Therefore, water use efficiency could be used as selection characters under drought conditions in rice particularly tropical upland rice. Moreover, morphological characters, i.e., grain yiled per plot, weight of pithy grain, weight of shoot biomass and weight of roots could be the selection characters to predict drought tolerant tropical rice. According to physiological characters, photosynthesis rate, transpiration rate, proline content, malondialdehyde content, leaf water potential and leaf greenness could be used as a selection tool to predict water use efficient genotypes in rice. However, further studies are needed to understand the complex mechanisms of water use efficiency by combining various approaches.
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