The effect of high daytime temperatures on inhibition of flowering in Koroneiki olives (Olea europaea L.) under chilling and non-chilling nighttime temperatures

Abstract: Regulation of flowering in 'Koroneiki' olives by various regimes of daytime and nighttime temperatures was investigated. The trees flowered profusely under chilling (2.5?C; 569 inflorescences tree^-1) and non-chilling nighttime temperatures (8.3?C; 729 inflorescences tree^-1) when daytime temperatures were kept optimal (18.3?C). Chilling nighttime temperatures (2.5?C) did not produce any greater number of inflorescence than non-chilling temperatures of 8.3?C. High daytime temperatures (26.6?C) strongly inhibited flowering at both chilling and non-chilling nighttime temperatures (i.e., 0.5 and 0.0 inflorescences tree¹ under chilling and non-chilling temperatures, respectively). Mildly high daytime temperatures (23.9?C) also inhibited flowering but there were significantly more inflorescences per tree at 23.9?C (220 and 127 inflorescences tree^-1 under chilling and non-chilling nighttime temperatures, respectively) than at 26.6?C. There was no significant difference in the number of inflorescences tree^-1 between chilling and non-chilling nighttime temperatures at both inhibitory daytime temperatures; i.e. 23.9?C and 26.6?C. The trees that were kept vegetative by high daytime temperatures (26.6?C), but given flower inducing nighttime temperature for three months, when returned to optimal flower inducing conditions did not flower before the normal induction period (70-80 days), indicating that inhibitory daytime temperatures canceled any effects of nighttime flower inducing temperatures. Surprisingly, trees kept vegetative in growth chamber at a high daytime temperature (26.6?C) produced fewer inflorescences compared to trees kept vegetative in the greenhouse where temperatures were less controlled but generally, with a few exceptions, remained between 15-20?C in the night and 25-30?C during day.