The experiment was conducted in two consecutive years (۲۰۱۵ and ۲۰۱۶) at the Faculty of Agriculture of Guilan University, Rasht, Iran. Cultivation was done in polyethylene pipes in both years. In the first year, the experiment was conducted with ۱۶ (aerobic, upland and lowland) rice genotypes under optimal irrigation conditions, using a completely randomized design with three replicates. Based on cluster analysis results four cultivars Vandana, Neda, Tarom Mahalli, and one upland (IR۸۳۱۴۱-۱۱) genotype were selected for the second year experiment. The second-year experiment was a factorial design conducted with complete randomization and three replications. The factors were included four rice genotypes and soil water conditions (full irrigation and drought stress). The results indicated that drought stress reduced both the dry matter and growth rates of shoots and roots in rice cultivars. The difference in shoot dry matter between the two irrigation conditions was greater in the Tarom Mahalli (۱۶.۲ g.plant-۱) compared to the upland genotype (۶.۷ g.plant-۱) and other genotypes. The shoot was more affected by drought stress than the root. The difference in root dry matter between the two irrigation conditions was greater in the Tarom mahalli (۱ g.plant-۱) and upland genotype (۰.۰۹ g.plant-۱) and other genotypes. The root dry matter variation between the two irrigation conditions was greatest in the Tarom Mahalli (۱ g.plant-۱) and lowest in the upland (۰.۰۹ g.plant-۱). Drought conditions led to a shorter timeframe for the Vandana cultivar to reach maximum growth rate and achieve maximum dry matter accumulation, with growth rate accelerating by three days and accumulation occurring six days earlier. Due to drought stress, the time to reach maximum root dry matter decreased in Neda and Vandana cultivars (with a difference of ۴.۶۵ and ۰.۱۳ days) and increased in upland genotype and Tarom Mahalli cultivar (۲.۵ and ۶.۵ days). Drought stress caused a five-day acceleration and a seven-day delay in reaching the maximum accumulation of root dry matter in Neda and Tarom Mahalli cultivars, respectively. Aerobic and upland genotypes displayed a greater root-to-shoot ratio compared to lowland genotypes, particularly during the initial growth stages, suggesting a more extensive root system in these genotypes. Except for the Tarom Mahalli, drought stress increased root dry weight in other genotypes at most soil depths compared to full irrigation. These traits play a crucial role in breeding programs, particularly those focused on drought tolerance, in order to develop resilient cultivars that can withstand challenging conditions. In this experiment, it is suggested that further research could be conducted on the Vandana, upland, and Neda genotypes due to their superior root traits.
