Photosynthesis and antioxidant defense system were investigated under chilling stress without (Ch, 25-4 °C) and with acclimation (AcCh, 14-4 °C) in winter (Sabalan) and spring (Zagros) wheat (Triticum aestivum L.) cultivars. Maximum quantum efficiency of photosystem II and CO2 assimilation rate decreased in AcCh ‘Zagros’ but not in ‘Sabalan’, and in contrast, an increase in non-photochemical quenching was observed in ‘Sabalan’ but not in ‘Zagros’. Reduction of leaf starch content was observed in both cultivars while total soluble carbohydrates increased only in ‘Sabalan’ under both Ch and AcCh treatments. Activity of superoxide dismutase was significantly higher in Ch plants and activity of ascorbate peroxidase and catalase was slightly higher in Ch and AcCh plants of both cultivars compared with control. Activity of peroxidase increased in Ch and AcCh plants of ‘Zagros’ while phenylalanine ammoialyase (PAL) activity increased in AcCh ‘Sabalan’. Increase in the leaf content of H2O2 and malondialdehyde (MDA) was more prominent in ‘Zagros’ than in 'Sabalan'. According to our results, chilling tolerance in winter cultivar was associated with greater thermal dissipation, higher soluble carbohydrates content, greater PAL activity and lower H2O2 and MDA content. Furthermore, acclimated plants were not more protected against chilling injury compared with non-acclimated ones. Fotosinteza in antioksidantna obramba sta bili raziskovani v razmerah hladnega stresa brez aklimatizacije (Ch, 25-4 °C) in z aklimatizacijo (AcCh, 14-4 °C) pri ozimni pšenici Triticum aestivum L. (cv. Sabalan) in jari pšenici (cv. Zagros). Maksimalna učinkovitost fotosistema II in asimilacije CO2 sta se zmanjšali pri AcCh za ‘Zagros’, toda ne pri kultivarju ‘Sabalan’. Nasprotno od tega se je povečalo nefotokemično gašenje pri kultivarju ‘Sabalan’, a ne pri ‘Zagros’. Pri obeh kultivarjih je bilo ugotovljeno manj škroba v listih, medtem ko se je vsebnost celokupnih topnih ogljikovodikov povečala pri ‘Sabalan’ pri obeh tretiranjih, Ch in AcCh. Aktivnost superoksidne dismutase je bila značilno večja pri rastlinah Ch, aktivnost askorbatne peroksidaze in katalaze je bila pri obeh kultivarjih malo višja pri razmerah Ch in AcCh v primerjavi s kontrolo. Aktivnost peroksidaze se je povečala pri rastlinah Ch in AcCh pri kultivarju ‘Zagros’ medtem ko se je aktivnost fenilalanin ammoialiase (PAL) povečala pri AcCh cv. ‘Sabalan’. Povečanje koncentracije H2O2 in malondialdehida (MDA) je bilo bolj izrazito pri kultivarju ‘Zagros’ v primerjavi s kultivarjem ‘Sabalan’. Glede na naše rezultate je toleranca na mraz pri ozimnem kultivarju povečana z večjim termalnim trošenjem in večjo vsebnostjo večjih topnih ogljikovodikov. Odpornost na mraz je pri ozimnih kultivarjih povezana z večjo aktivnostjo PAL ter nižjo vsebnostjo H2O2 in MDA. Nadalje, aklimatizirane rastline niso bile nič bolje zaščitene pred poškodbami zaradi mraza v primerjavi z neaklimatiziranimi.