Effects of combined high temperature and water stress on soybean growth and physiological processes in a temperature gradient chamber

Abstract

Context or problem

Under global warming and growing water scarcity, soybean frequently faces concurrent high temperature stress and water stress, which together impair biomass accumulation and physiological performance. Although the separate effects of high temperature stress or water stress are well documented, their combined impact across realistic temperature gradients remains poorly defined, making it difficult to predict crop performance under simultaneous stresses.

Objective or research question

This study assessed how combined high temperature stress and water stress affect soybean above-ground dry matter accumulation, growth dynamics, physiological processes, and water use efficiency to elucidate the mechanisms combined stress responses.

Methods

A two-year experiment (2023–2024) was conducted in a temperature gradient chamber divided into four thermal zones—low, medium-low, medium-high, high—and two irrigation regimes: normal irrigation and water stress. Above-ground dry matter was determined at harvest, and periodic measurements of leaf area and dry weight were used to derive growth rate and net assimilation. Key physiological processes were monitored throughout development. Relationships among physiological and growth parameters were analyzed using regression and multivariate methods. Water use efficiency was defined as dry matter produced per unit of irrigation water.

Results

Water stress reduced above-ground dry matter by 39–40 % compared with normal irrigation, and the high-temperature zone produced 35–36 % less dry matter than the low-temperature zone. Combined high temperature stress and water stress caused over a 60 % decline in dry matter relative to the low-temperature zone under normal irrigation. Before beginning pod stage, net assimilation rate and crop growth rate were strongly correlated, indicating that assimilation drives early biomass accumulation. Under simultaneous high temperature and water stress, reductions in leaf water potential led to lower stomatal conductance and photosynthetic rate, which in turn suppressed net assimilation, growth, and water use efficiency. Although water stress alone increased water use efficiency in 2023, elevated temperature stress in 2024 reversed this effect under water stress, demonstrating that high temperature stress exacerbates water stress’s negative impact on water use efficiency.

Conclusions

Simultaneous high temperature stress and water stress severely limit soybean biomass by disrupting leaf water potential, stomatal regulation, and photosynthetic performance. The water stress-induced improvement in water use efficiency is negated under elevated temperatures, amplifying biomass loss.

Implications or significance

These findings clarify the physiological interactions under combined high temperature and water stress, informing breeding of stress-tolerant cultivars and adaptive irrigation strategies that account for temperature variability to sustain soybean productivity under future climate extremes.