Daytime and nighttime thermal effect

Thermal performance curves show that increases in temperatures below optima have positive effects whereas increases in temperatures above optima have negative effects. Organisms often experience adverse thermal environments caused by daytime extreme high temperatures while they may also recover or repair themselves during nighttime moderate temperatures. Thermal effects of daily fluctuating temperatures may thus be divided into two opposite processes, i.e. negative effect of daytime heat stress and positive effects of nighttime recovery. Despite recent progress on the consequences of increased daily temperature variability, the independent effect of daytime high temperature and nighttime mild temperature on organism performance and their combination in determining thermal effect of daily temperature fluctuations still remains unclear. By independently manipulating daily maximum and minimum temperatures, we tested how changes in daytime heat stress and nighttime recovery affect development and survival of eggs and the critical thermal maximum (CTmax) of first-instar larva in a lady beetle species Propylea japonica. We find that thermal effects on development rate and survival differed between daytime and nighttime temperatures, with daytime high temperatures have negative effects whereas nighttime mild temperatures have positive effects. The extent of daytime heat stress and nighttime recovery also affect egg development and survival and CTmax of first-instar larva, indicating independent and combined effects of daytime and nighttime temperatures in determining thermal performances. Our findings provide an insight into the thermal effect of day-to-night fluctuating temperatures and have important implications for predicting the impacts of daytime and nighttime warming under climate change.

热性能曲线（thermal performance curves）显示，当温度处于最适温度以下时，升温会产生积极影响；而当温度高于最适温度时，升温则会带来负面影响。生物体往往会遭遇日间极端高温所引发的不利热环境，同时也可在夜间的适度温度条件下完成修复与自我恢复。因此，每日波动温度的热效应可被划分为两个相反的过程：即日间热胁迫的负面效应，与夜间恢复的正面效应。尽管当前关于每日温度变异性增加的研究已取得一定进展，但日间高温与夜间温和温度对生物体性能的独立效应，以及二者共同决定每日波动温度热效应的机制，仍未得到明确阐释。本研究通过独立操控每日最高与最低温度，探究了龟纹瓢虫（Propylea japonica）的卵发育、存活率，以及其一龄幼虫的临界高温上限(CTmax)。研究发现，温度对发育速率与存活率的影响存在日间与夜间的差异：日间高温会产生负面影响，而夜间温和温度则具有积极作用。日间热胁迫的程度与夜间恢复的程度同样会影响卵的发育、存活率以及一龄幼虫的临界高温上限，这表明日间与夜间温度对生物体热性能存在独立及联合效应。本研究结果为解析昼夜波动温度的热效应提供了新的视角，同时对预测气候变化背景下日间与夜间增温的生态影响具有重要的参考价值。