Inhibition of flowering 'Arbequina' olives from chilling at lower temperatures

Authors: Malik, Nasir; Bradford, Joe

Submitted to: Scientia Horticulturae
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/7/2009
Publication Date: 4/30/2009
Citation: Malik, N.S., Bradford, J.M. 2009. Inhibition of flowering 'Arbequina' olives from chilling at lower temperatures. Scientia Horticultureae. 7(2):429-431.

Interpretive Summary: In the 50s through 70s, a series of studies on temperature induced regulation of flowering in olives (Olea europaea L.) concluded that optimal inflorescence production occurred when diurnal temperature varied between a minimum of 2°C to a maximum temperature of 15°C. Later they extrapolated their finding to suggest that for optimal temperatures for olive flowering range from a minima of 2-4°C to a maxima of 15.5-19°C. In 1983, Denney and McEachern proposed a revised version of “optimal temperatures” with minimal temperatures down to 1.7- 4.4°C to maximal temperatures of 15.6-18.3°C. However, they conceded that “there is as yet no way to set a lower limit for the minimum temperatures”, and therefore, proposed that further research was required to determine the lower limit of minimum or nighttime temperatures beyond which floral induction in olives may begin to decrease. Although 7.2°C was considered a minimal chilling temperature required for induction of flowering in olives, we obtained extensive flowering in ‘Arbequina’ cultivar even at minimal temperatures of 8.3°C. In more recent studies, we found that inflorescence production in ‘Koroneiki’ cultivar was 28% higher in trees that were given minimal temperature of 8.3°C as compared to trees that were given a minimal temperature of 2.5°C; in both treatments the tree were given maximal temperature of 18.3°C. These findings, and concerns of Denney and McEachern (1983) regarding minimal temperatures as mentioned above, prompted us to study a series of minimal temperatures to determine where optimal inflorescence in ‘Arbequina’ cultivar is produced and also at which chilling temperatures the inhibition of inflorescence development occur. We found that while maximal flowering occurs at minimal temperatures ranging from 4.4 - 7.8°C, the inhibition of flower induction in ‘Arbequina’ olives starts with chilling stress even at 2.2°C; i.e. at temperatures that were previously considered optimal for olive flowering. Inhibition of flowering increases with increased chilling temperatures below 2.2°C, and is strongly inhibited below freezing temperatures. The appearance of inflorescence is also progressively delayed with increased chilling.

Technical Abstract: The effect of four nighttime chilling temperatures on the induction of flowering in ‘Arbequina’ olives was investigated. Daytime temperature was kept at 17.5 ± 0.8°C (8 hrs) while nighttime temperatures (8 hrs ) were maintained at 7.8 ± 0.5, 4.4 ± 0.5, 2.2 ± 0.5, or -1.2 ± 0.6°C; transition from daytime temperatures to nighttime temperatures, and vice versa, occurred stepwise in 4 hrs periods. Maximal flowering occurred in trees that were given nighttime temperatures of 4.4 ± 0.5°C. Increasing nighttime chilling by lowering temperatures to 2.2 ± 0.5 and -1.2 ± 0.6°C significantly reduced inflorescence production by 71 and 90%, respectively. Our results indicated no significant difference in inflorescence production in trees that were kept at nighttime temperatures of 7.8 ± 0.8 or 4.4 ± 0.5°C. Different levels of chilling had no significant effect on number of flowers per inflorescence. Time of initiation of first inflorescence was progressively delayed with increased chilling temperatures; i.e. first inflorescence appeared after 98, 107, 124, and 131 days from the start of induction period in trees that were given nighttime temperatures of 7.8 ± 0.5, 4.4 ± 0.5, 2.2 ± 0.5, or -1.2 ± 0.6°C respectively.