Antioxidant and photoprotective responses to elevated CO2 and heat stress during holm oak regeneration by resprouting, evaluated with NIRS (near-infrared reflectance spectroscopy)

Photosynthetic, photoprotective and antioxidant responses during high temperaturestress were determined in leaves of evergreen holm oak (Quercus ilex L.), themain species in Mediterranean forests, during resprouting under elevated CO2(750 llÆl)1). Leaf chemicals, chloroplast pigments and non-enzymatic antioxidantswere quantified in a single measurement using NIRS (near-infrared spectroscopy),a rapid and suitable method for ecophysiological purposes. Resprouts from plantsgrown under elevated CO2 (RE) showed photosynthetic down-regulation, higherstarch content and lower stomatal conductance, but similar stomatal density, thanplants grown under current CO2 concentrations (350 llÆl)1) (RA). The photosyntheticsink reduction and need for more antioxidants and photoprotection in REwere reflected in an increased concentration of ascorbate (Asc) and phenolic compoundsand in the contribution of the xanthophyll (Z / VAZ) and lutein epoxidecycles to excess energy dissipation as heat, and also reflected in chlorophyll fluorescencemeasurements. CO2 assimilation parameters were stable from 25 to 35 Cin RE and RA, declining thereafter in RA in spite of a 2.3 C lower leaf temperature.RE showed a more marked decline in photorespiration above 35 C and lesssensitive stomatal responses to high temperature stress than RA. During heatstress, RE had higher Asc, Z/VAZ and phenolics content, together with delayedenhancement of chloroplast lipophilic antioxidant compounds (carotenes and tocopherols).The high contribution of photoprotective systems and high temperaturetolerance in resprouts developed under elevated CO2 would mitigate the effect ofphotosynthesis acclimation during the regeneration of Q. ilex plants under climatechange.