Shading-induced canopy cooling alleviates waterlogging damage during flowering by disrupting heat synergism in field-grown cotton

Abstract

Context

In major cotton-growing regions like China's Yangtze and Yellow River valleys, flowering (July-August) frequently coincides with cloudy-rainy weather, causing concurrent waterlogging (W) and shading (S). This period also experiences high temperatures (H), creating damaging compound stresses. While W combined with H causes severe synergistic damage, the impact of the W+S combination remains unclear. We hypothesized that S-induced canopy cooling disrupts the W-H synergism, converting the interaction into antagonism and reducing yield loss.

Method

Two-year field trials (2023–2024) applied individual (W, S) and combined (W+S) stresses at flowering. We measured agronomic (yield, biomass partitioning, boll density), physiological (photosynthesis, chlorophyll, canopy temperature), and stress response markers (oxidative stress: H₂O₂, MDA; anaerobic metabolism: ADH activity) were quantified. A growth chamber experiment decoupled light reduction from cooling to isolate temperature effects under W, H, W+H, and simulated S.

Results

Field data revealed antagonism between W and S: combined stress (W+S) caused significantly less yield loss (35.3 %) than predicted additivity or W alone (44.6 %). This mitigation was strongly associated with S-induced canopy cooling (reductions of 4.0°C in 2023, 3.6°C in 2024). Shading attenuated oxidative stress as evidenced by H₂O₂ accumulation being 34.8 % below additive predictions, and suppressed anaerobic metabolism as indicated by ADH activity reduced by 60.7 % relative to waterlogging. Crucially, growth chamber experiments confirmed temperature's pivotal role: W+H caused synergistic damage (photosynthesis and biomass loss exceeding additive predictions), while simulated S (light reduction without concomitant cooling) failed to mitigate W damage. Canopy cooling under field shading suppressed the induction of heat shock proteins (e.g., HSP21 expression reduced to 0.4-fold of control), disrupting the heat-amplified induction of heat shock proteins typically triggered during waterlogging.

Conclusions

Under high-temperature at flowering, shading antagonizes waterlogging damage primarily through canopy cooling. This cooling reconfigures stress interactions by suppressing the heat-dependent component of waterlogging injury, specifically mitigating oxidative stress and anaerobic metabolism. It thereby converts potential W-HT synergism into a W-S antagonism. Canopy-mediated thermal amelioration is a vital mechanism for compound stress resilience in cotton.

Implications

This work highlights canopy temperature management as a critical strategy alongside drainage for mitigating waterlogging damage. Agronomic practices (e.g., optimizing planting density for self-shading, temporary shade nets) or breeding for cooler-canopy traits promoting cooler canopies (e.g., leaf angle, reflectance) could enhance resilience in flood-prone, high-temperature environments.