The influence of exogenous monoterpene treatment and elevated temperature on growth, physiology, chemical content and headspace volatiles of two carrot cultivars (Daucus carota L.)

We investigated the effects of application of aqueous solution (50 ml l−1) of limonene spray (LS) on growth, physiology, foliar and root chemistry and volatile emissions of two carrot cultivars (cv Parano and Splendid) growing at control temperature (CT) of 25 °C and at high temperature (HT) of 35 °C. The reduction in net photosynthesis induced by HT tended to be counteracted by LS 2 h and 24 h after the first spraying in cv Parano, but the long-term effect was not significant. LS counteracted temperature-induced reductions seen in chlorophyll fluorescence after 21 days from the start of the experiment. A significant increase in the levels of monoterpenes α-pinene, camphene, sabinene, β-myrcene and limonene emission from foliage was observed at HT. HT elevated the concentrations of monoterpenes and sesquiterpenes (E)-β-farnesene and β-caryophyllene, in roots of both cultivars. The concentration of total phenolics in foliage was not affected by HT as expected by a protein competition model of phenolic allocation. However, the proportion of the single phenylpropanoid compound, methyl isoeugenol, was reduced by HT. HT increased the concentration of total soluble proteins in cv Parano, whereas in cv Splendid the concentrations of chlorophyll and carotenoids decreased. The dry mass of main root was significantly reduced at HT in both cultivars, probably reflecting reduced photosynthesis and carbon allocation costs to defence as a result of thermal stress. Limonene treatment was not sufficient to protect carrot root growth from thermal stress.