Weeds and crops that grow together often confront similar types of environmental stress, especially drought stress. Weedy rice (Oryza sativa f. spontanea) and cultivated rice (O. sativa L.) provide a unique pair consisting of a weed and a conspecific model crop that can be used to study the drought tolerance of plants across a large distributional range. The investigation on weedy rice's damage to paddy fields showed that it was more serious in dry direct seeding than water direct seeding. Compared with water direct seeding, the seeds of cultivated rice and weedy rice in dry direct seeding will absorb water and germinate under the condition of insufficient soil moisture. Our hypothesis is that weedy rice seeds have evolved stronger germination ability than coexisting cultivated rice under water stress, so that they can obtain more growth space in the early stage in dry direct seeding and thus obtain higher fitness. Seeds of weedy rice populations and coexisting rice cultivars were collected from 61 sites across China and were germinated with 20% polyethylene glycol-6000 to simulate drought stress. Two drought response indices, which assessed germination rate and germination index, plus one germination stress tolerance index, indicated significantly greater drought tolerance in weedy rice populations than in coexisting rice cultivars (P < 0.01). Drought tolerance for the three indexes were indica weedy rice > indica rice cultivars, japonica weedy rice > japonica rice cultivars, and indica weedy rice > japonica rice cultivars. These results indicate that weedy rice populations show stronger drought stress tolerance than coexisting rice cultivars at various sites, specifically during the seed germination period. Furthermore, Pearson's correlation found that drought response of weedy rice populations and coexisting rice cultivars were significantly different with these environmental factors: latitude, altitude, annual mean precipitation, mean annual temperature, mean precipitation in the sowing month, mean temperature in the sowing month, and sowing methods. Weedy rice shows different patterns of drought tolerance variation across geographical (latitude and altitude) and environmental (precipitation) gradients compared to coexisting rice cultivars. This study suggests that weedy rice might have evolved new drought tolerance and could provide a useful source of genetic resources for improving drought tolerance of crop cultivars and breeding direct seeded cultivars to reduce the usage of seeds in direct seeding.

Weedy rice (Oryza sativa f. spontanea) is a conspecific weedy relative of cultivated Asian rice (O. sativa L.) that occurs in paddy fields worldwide. The mechanism underlying the persistence of weedy rice in rice fields remains unclear. We tested for evidence that southern Chinese weedy rice has originated through pollen flow from weedy rice that has resulted in hybridization with three‐line hybrid rice cultivars, which have been extensively cultivated in southern China for the last 40 yr and now account for over half the total rice cultivation in China. To test the hypothesis, we screened for the presence of the maternally inherited WA352 gene in weedy rice populations in southern China. WA352 is a mitochondrial gene that controls wild abortive cytoplasmic male sterility (CMS‐WA) and is present in three‐line hybrid rice cultivars. Screening of 48 weedy rice populations (939 accessions) indicates that 36 of 48 populations (75%) and 325 of 939 accessions (34.6%) harbor the WA352 gene. The rate is significantly higher than the 1% outcrossing rate between weedy rice and rice cultivars. Further analysis indicates significance between the frequency of the WA352 gene in weedy rice populations and depth of plowing. Our results suggest that outcrossing with rice cultivars is one of the important mechanisms underlying the persistence of weedy rice in southern China and that deep plowing may be conducive to promoting the persistence of weedy rice derived from hybridization (three‐line hybrid rice × weedy rice).