5.2.4Chronic versus Acute Heat Stre

5.2.4Chronic versus Acute Heat StressMost past studies investigating the relative sensitivity of shoot and root function toheat stress grew plants at different temperatures for extended periods (i.e., chronicheat stress); few studies have examined effects of abrupt heat stress events on shootversus root function, and fewer still (if any) have compared acute versus chronicheat stress in the same study (Tables 5.1–5.3). However, several past studiesincluded regular measurements of plants through time and these studiesdemonstrate that plant responses early in the heat treatment can be quite differentfrom later responses. For example, Xu and Huang [45] observed an increase in rootrespiration within the first few days of heat treatment, but later, heating decreasedroot respiration. We are currently investigating effects of chronic versus acute heatstress on roots in tomato (S. lycopersicum), and have found that an abrupt heat wavecan be more detrimental to root mass and have larger effects on R/S mass thanchronic heating, and these effects can persist for many days after heat stress is over(Figure 5.2). Notably, we can find few past studies that have examined recovery ofroots from heat stress (Tables 5.1–5.3).In addition to the tomato results above, we recently completed a study, comprisedof companion lab and field experiments, investigating the effects of heat stress onroot and soil/soil-microbe function using A. gerardii, a warm-season dominant oftallgrass prairie [80]. We imposed 5-day “heat waves,” by increasing daytime airtemperature from 30 (optimal) to 35 or 40
C in the lab and from about 30 to 40
Cin the field, and then monitored root and soil processes (including root and microbial mass and respiration, carbon (C) flux from plant to soil and nitrogen (N)flux from soil to plant). Heat treatments did not decrease shoot mass, stomatalconductance, shoot %C, or translocation of shoot C to roots, and net photosynthesisand leaf water potential decreased only slightly (and non-significantly) with heating(about 12.5% and 0.3 MPa or less, respectively). In contrast, heating significantlyincreased root mass and R/S (data included in Figure 5.1), and also increased theloss of root C to the soil (likely via increased fine-root turnover rather thanexudation), decreased root N uptake rate and root respiratory (R) capacity (R at20
C). Collectively, the available evidence indicates that in some regards, acute andchronic heat stress may have similar effects on roots or roots versus shoots, but inother regards may have different effects.