Objective Under global climate change, water scarcity has emerged as a major constraint on traditional rice production, prompting the increasing adoption of dry direct-seeded rice (DDSR) as a key cultivation system. However, one of the primary challenges in direct sowing is poor seedling emergence, particularly under deep-sowing conditions where seedlings struggle to penetrate the soil surface. Therefore, screening for DDSR germplasm resources and revealing the adaptative mechanisms of key traits has become crucial for breeding improved DDSR cultivars.

Method Four deep-sowing-tolerant upland rice varieties (‘Liandaogu’ ‘Luyin 46’ ‘Milihaotu’ and ‘Aihongzhi’) and four non-tolerant lowland rice varieties (‘Suijing 15’ ‘IR64’ ‘Huanghuazhan’ and ‘Zhonghua 11’) were evaluated under 8 cm deep-sowing conditions to assess key agronomic traits such as the coleoptile length, mesocotyl length, shoot length, and root length, along with cytological characteristics and relevant physiological indicators.

Result The deep-sowing-tolerant varieties exhibited significantly longer coleoptiles than their non-tolerant counterparts, with a highly significant positive correlation between coleoptile length and shoot length (r = 0.441, P < 0.01), reflecting enhanced soil penetration capacity. Paraffin sectioning revealed distinct coleoptile cellular dynamics during emergence, and tolerant varieties suppressed lateral cell expansion in the apical region while promoting longitudinal cell elongation in the basal region, resulting in a slender, elongated structure that effectively reduced mechanical resistance. Physiological and biochemical analyses further demonstrated that tolerant genotypes maintained higher levels of soluble sugars and total proteins, exhibited elevated activities of antioxidant enzymes (SOD, CAT, and POD), and accumulated lower levels of reactive oxygen species (H₂O₂, O₂^·−), collectively contributing to improved stress resilience under deep-sowing conditions.

Conclusion This study preliminarily reveals dynamic physiological and cytological changes in the upland rice coleoptile in response to deep direct sowing, providing an in-depth analysis of physiological mechanisms underlying deep-seed tolerance. It offers a theoretical basis for identifying and utilizing key traits in direct-seeded rice breeding.