Quantifying uncertainties in leaf thermal thresholds and their implications for photosynthetic recovery

Understanding plant heat tolerance requires assessing their thermal
thresholds, but commonly used methods have rarely been compared. Moreover,
whether the photosynthetic machinery is irreversibly damaged past these
thresholds remains unclear. We determined the critical
temperature (Tcrit), the temperature causing a 50% reduction (T50), and
the maximum tolerable temperature (Tmax) of photosystem II in
Mediterranean cypress, Aleppo pine, and Scots pine saplings using 15- or
30-min heat exposure curves performed on living plants (in-vivo), excised
needles (ex-vivo), and excised needles continuously exposed to each rising
temperature (ex-vivo continuous). Dark-adapted fluorescence (Fv/Fm) and
gas exchange were recorded for four days post-heat stress to track
recovery. Longer heat exposure (30 vs. 15min)
consistently led to lower Fv/Fm, T50, and Tmax. T50 and Tmax were reduced
in both ex-vivo conditions compared to in-vivo ones. Conversely, Tcrit
remained consistent between species, exposure durations, and methods. Gas
exchange and Fv/Fm recovery mainly occurred before reaching T50 values
(about 45°C). Our work highlights the importance of exposure
duration and method selection when measuring and comparing thermal
thresholds. Moreover, while Tcrit appears to be a reversible threshold,
the photosynthetic machinery of studied species appears irreparably
damaged past their T50.

解析植物耐热性需明确其温度阈值，但现有常用测定方法鲜少被系统比较。此外，当温度突破这些阈值后，光合机构是否会发生不可逆损伤，目前仍不明确。本研究针对地中海柏木、阿勒颇松与欧洲赤松的幼苗，分别采用15分钟或30分钟热暴露曲线法，通过三种实验体系——活体植株（in-vivo）、离体针叶（ex-vivo）、持续升温处理的离体针叶（ex-vivo continuous），测定了其光系统II（photosystem II）的临界温度（Tcrit）、致50%活性下降的温度（T50）以及最大耐受温度（Tmax）。热胁迫后连续四天监测暗适应荧光（Fv/Fm）与气体交换参数，以评估光合恢复进程。结果显示，30分钟热暴露相较于15分钟组，均显著降低了Fv/Fm值、T50与Tmax数值；与活体植株实验体系相比，两种离体实验体系下测得的T50与Tmax均有所降低；相反，Tcrit在不同物种、暴露时长与实验方法间均保持一致。气体交换与Fv/Fm的恢复过程主要发生在温度达到T50（约45℃）之前。本研究结果凸显了在测定与比较温度阈值时，热暴露时长与实验方法选择的重要性。此外，尽管Tcrit属于可逆性温度阈值，但本研究涉及物种的光合机构在突破T50后，似乎会发生不可逆的损伤。