Effect of Chilling and Accumulative Photo-Thermal Units on Flowering of Strawberry (Fragaria × ananassa Duch.)

The influence of chilling period hours and accumulative photo-thermal unit at different base temperatures, 10, 11, 13, 15, and 18 °C, on the flowering of strawberry was examined in Sana’a University-Yemen during two seasons: 2006 and 2007. The randomized complete block design (RCBD) field experiment was designed with 5 chilling hours (CHs) periods (0, 360, 750, 1080, and 1440 CHs under 2 ± 1 °C), with four replicates each. Accumulative photo-thermal unit (PTU) was calculated for every treatment at different base temperatures for two stages: (i) from planting date to the flower opening (THA) and (ii) from planting date until the end of 2 months (THB). The results revealed that the plants treated with 1080 h of chilling produced flowers with about 55.12% and 61.97% earlier when compared with control (without chilling) in both the seasons, respectively. Stepwise multiple regression analysis showed that both CHs and the accumulative PTU THA13 (in the first season) and CH plus THA10 (in the second season) have significant effect on days until flower opening, in which the majority effect was related to the chilling period. The effect of chilling period and accumulative PTU at 18 °C (THB18) had s significant influence (p < 0.05) on the number of early flower per plant, with about r2 = 0.250 and r2 = 0.536 in both the seasons, respectively, and r2 = 0.531 and r2 =0.740 for the total effect of both CH and THB18, respectively. However, the exposure of plants to the long period of chilling (1080 and 1440 h) led the plants to produce runners and break up the flowering stage after 9 and 11 weeks in both the seasons, respectively. Meanwhile, plants that received 360-h chilling produced significantly highest (p < 0.05) total number of flowers (24.83 per plant), with about 71.2% higher than that produced by control plants. Moreover, in the second season, plants treated with 750-h chilling produced the significantly higher total number of flowers per plant, with about 50.3% higher than that of the control. Meanwhile, the 360-h and 750-h chilling periods have similar effect on the number of flowers per plant in the second season. The result showed that the starch level in the crown significantly positively correlated with the days to flower opening (r2 = 0.415, p = 0.05) and negatively correlated with the number of early flower (r2 = 0.587, p = 0.01). The data from this study might be used for the management of strawberry production.