Effects of Growing Season and Ripening Stages on Transcription Level of Geranylgeranyl Reductase (OeCHL P) and Some Biochemical Properties in Some Important Olive Cultivars (Olea europaea L.)

Transcription level of geranylgeranyl reductase (OeCHL P), and contents of alpha-tocopherol, oleuropein, hydroxytyrosol and tyrosol of seven olive cultivars obtained from Olive Germplasm Collection, Kemalpasa, Izmir were determined in two ripening stages (green and black fruit) in two consecutive years, 2017 and 2018. Transcription level of OeCHL P was significantly affected by year, ripening stage and cultivar. The highest values were detected in green fruit of 'Uslu' in both years. In comparison to 2017, a significant increase in gene transcription was observed in 2018 independent of cultivar and ripening stage. The highest alpha-tocopherol and oleuropein content were obtained from 'Girit Zeytini'. The content of oleuropein decreased with ripening in all cultivars in both years. Tyrosol reached its highest and lowest values in 'Girit Zeytini' at black stage in 2018 and green stage in 2017, respectively. 'Girit Zeytini' stood out for both nutritional value and fruit size. The highest and lowest values of hydroxytyrosol content recorded in 'Girit Zeytini' (2017, black stage) and 'Izmir Sofrahk' (2018, green stage), respectively. We also detected positive correlations between OeCHL P relative transcription level and tocopherol, tyrosol contents. Our overall results indicated that olive CHL P plays an important role in regulation of tocopherol synthesis. A direct relationship was determined between OeCHL P and alpha-tocopherol, while there was an indirect link between others. These results revealed that more than one factor could affect the evaluated parameters.