5.5Summary: Synthesizing What We Kn

5.5Summary: Synthesizing What We Know and Predict into a Conceptual Model of HeatEffects on Roots and Plant---Soil LinksUsing the available literature observations and our own data from the studiesdiscussed here, we have developed a tentative model of how acute heating affectsroots, plant–soil links, and shoot–root interactions (Figure 5.4). We hypothesizethat the mechanisms underlying heating effects differ for moderate versus severe temperature increases or stress. For moderate heating, effects are predicted to bedriven primarily by heat-induced increases in plant water use, which stimulate rootgrowth and allocation of plant C to roots, rather than heat-related damage (i.e., byindirect effects). In contrast, under severe heating, significant cell damage occurs toboth roots and shoots, and this leads to decreases in transfer of shoot C to roots anddeath of fine-roots, a decrease in water uptake capacity and transpirational cooling,and increases in leaf temperature and damage. In both cases, heating effectsshould have an impact on soil resources utilized by microbes (water, and plant Cand N), and thus, should have effects on soil microbes and subsequent soil nutrientlevels. Implicit in this model is the prediction that, under a given level of heating,heat-sensitive and -tolerant species should differ in the extent and often themechanism of heat effects. Heat-induced changes in soil water, root mass, plant Closs to soil, microbial activity, and so on, should then have predictable effects on theperformance of plants after heating and on the rate of recovery from heating. Ifheat effects on roots and plant–soil links are indeed mediated by effects on waterflux from soil to plant to air and C flux from shoots to roots and roots to soil, then itfollows that drought and atmospheric CO2 levels should strongly influence the effects of heating. Finally, preheating growth temperature should have a strongbearing on heat effects (e.g., by causing severe stress at supra-optimal preheatgrowth temperatures versus moderate stress at optimal growth temperatures forthe same absolute increase in temperature during heating).In closing, we emphasize that there are several important knowledge gapsregarding our understanding of root responses to heat stress, including a limitedunderstanding of: (i) root responses to acute, compared to chronic, heat stress, (ii)interactive effects of heat stress with other aspects of global environmental change(especially drought) on roots, (iii) recovery of roots from heat stress, (iv)mechanisms underlying heat effects on root nutrient relations, and (v) heat effectson fine roots and root water status. Further, we suggest that future studiesemphasize investigations with intact plants (rather than detached roots) and thatplants experience realistic heat treatments (shoot and root heating, shoot-heatingonly, or root-heating, as appropriate to the research question), since plant and rootresponses to heat stress may vary with method of heating and intact plants oftenrespond differently than detached roots.