N₂-induced hypoxia and waterlogging elicit distinct whole-plant transpiration and adventitious-root responses in tomato (Solanum lycopersicum)

Waterlogging constrains terrestrial plants by limiting gas diffusion and altering the hydraulic and chemical environment of roots. Yet it remains unclear which whole-plant responses arise from oxygen limitation alone and which require the broader physical context of excess water. Using high-resolution gravimetric lysimetry in tomato, we compared N₂-induced hypoxia under near-field-capacity conditions with root-zone waterlogging. N₂ injection reduced root-zone O₂ from approximately 18%-19% to below 1% and altered pH, redox potential, and mineral relations, but whole-plant transpiration declined only after sustained exposure. Waterlogging caused a faster, genotype-dependent transpiration decline in M82, IL11-4, and IL8-1. Adventitious-root emergence at the soil-air interface coincided with transient partial recovery of transpiration, whereas stronger adventitious-root development occurred in plants with larger transpiration losses. The renewed decline in transpiration after drainage was consistent with a partial contribution of surface-associated adventitious roots, although restoration of drainage also altered root-zone aeration, water availability, and primary-root conditions and therefore did not isolate their specific contribution. Together, the distinct response kinetics and root phenotypes show that rapid N₂-induced hypoxia did not reproduce the full waterlogging response. Adventitious roots were induced most strongly under severe stress and were temporally associated with partial, but not complete, recovery of whole-plant transpiration.