1165 DOI: 10.32615/ps.2019.099 PHOTOSYNTHETICA 57 (4): 1165-1175, 2019 Ecophysiological study of the impact of SiK ® fertilization on Castanea sativa Mill. seedling tolerance to high temperature A. CARNEIRO-CARVALHO *,+ , R. ANJOS * , A. AIRES * , T. MARQUES ** , T. PINTO * , and J. GOMES-LARANJO * Centre for the Research and Technology of Agro-Environmental and Biological Sciences (CITAB), University of Tras-os-Montes and Alto Douro (UTAD), Vila Real, Portugal * University of Trás-os-Montes and Alto Douro (UTAD), Vila Real, Portugal ** Abstract The aim of this work was to evaluate the impact of Si fertilization on the resilience capacity of chestnut plants growing under high air temperatures and its recovery capacity after returning to optimal temperatures. Castanea sativa plants were supplied with 0, 5, 7.5, and 10 mM potassium silicate (SiK ® ) and exposed for a month at each temperature, 25, 32, and 25°C. The results demonstrated that phytoliths were accumulated in the leaf tissues, both on the cell wall and xylem vessels, suggesting their role in the plant's tolerance to heat. Under high temperature, Si fertilization in chestnut plants increased the gas exchange and the photochemical efciency of the PSII as the increase of 50% on performance index suggests. The presence of Si also induced higher contents of photosynthetic pigments and promoted a better adaptation of chloroplasts to high temperatures. The present study suggests that the application of Si may be used to enhance the high temperature tolerance of chestnut plants. Additional key words: chloroplast activity; fuorescence transient; heat stress; lipids; silicon. Received 20 March 2019 , accepted 7 June 2019. + Corresponding author; e-mail: andreiamcc_500@hotmail.com Abbreviations: ABS/RC – light absorption fux (for PSII antenna chlorophylls) per reaction center; BSA – bovine serum albumin; Car – carotenoids; Chl – chlorophyll; Chl/Car – chlorophylls to carotenoids ratio; CS – excited cross section; DI0/RC – dissipation energy fux per PSII reaction center; E – transpiration rate; EDTA – ethylenediamine tetraacetic acid; ET0/CS – electron transport fux per cross section; ET0/RC – maximum electron transport fux (further than QA − ) per PSII reaction center; Fd – ferredoxin; Fv/Fm – maximal quantum yield of PSII photochemistry; gs – stomatal conductance; OEC – oxygen-evolving complex; OJIP – fast chlorophyll fuorescence transients; Pht – phytoliths; PI – performance index; PN – net photosynthetic rate; PQ – plastoquinone; PVP – polyvinylpyrrolidone; QA – primary quinone acceptor of PSII; QB – secondary quinone acceptor of PSII; RC – reaction center; RC/ABS – density of reaction centers per PSII antenna chlorophyll; SiK ® – potassium silicate; SM – (Area)/(Fm – F0), representing energy necessary for the closure of all reaction centers; TR0/RC – fux of energy trapping per reaction center of PSII; UI – unsaturated index; WUE – water-use efciency (= PN/E). Acknowledgements: The authors thanks to Mrs. Ana Fraga and Mrs. Cesaltina Carvalho for technical assistance. This work was supported by National Funds by FCT – Portuguese Foundation for Science and Technology, under the project UID/AGR/04033/2019. Introduction Agriculture is one of the main activities afected by climate change, namely by the increase in temperature, changes in precipitation, and increase in the occurrence of heat and drought waves (Das et al. 2016). In Portugal, eight of the ten warmest years of the last 100 years occurred during the last 20 years, with Portugal being considered a climate change hotspot (Carvalho et al. 2014). Until the end of the 21 st century, an increase in the mean summer season temperature of about 3–7°C is expected, afecting the northern and central part of Portugal in particular (APA 2009). Castanea sativa is classifed as a thermophilic species, well adapted to ecosystems with an annual mean tempera- ture ranging between 8–15°C and monthly mean tempera- tures for six months, corresponding to the vegetative cycle, over 6–8°C (Gomes-Laranjo et al. 2008a). It is classifed as a mesophyll species of the Mediterranean temperate climates, growing preferentially in the coastal area at sea level or in mountainous area of the interior regions. In these regions, crops are found between 400 and 1,000 m (Gomes-Laranjo et al. 2007), where typical rainfall must be between 600–1,200 mm without a long dry season (Fernández-López and Alía 2003). This dim-light species prefers the cool north-facing slope localities with mean annual temperatures between 8 and 15°C, and daily maximum values during the vegetative cycle temperature around 27°C (Gomes-Laranjo et al. 2007, 2008b). In such regions, during the summer time, a combination of heat stress and water stress is more and more frequent, causing many problems in chestnut crops, which leads to a reduction in plant vigour, plant health, and consequently the reduction of chestnut production (Gomes-Laranjo et al. 2004, 2008b; Mota et al. 2014). High air temperatures may constitute one of the main restrictive factors afecting the