Freezing temperatures are a major limitation to strawberry production in temperate regions, and protected-cultivation strategies such as the use of tunnels and row covers are used to overcome this limitation. In order to optimize management under protected cultivation, it is necessary to understand the damage thresholds for strawberry plant tissues. The effects of freezing temperatures (−3, −5, and −7 °C) on leaf CO2 assimilation were evaluated for ‘Chandler’, ‘Seascape’ and ‘Jewel’ strawberry (Fragaria × ananassa). Growth chambers were used to expose plants to freezing temperatures under carefully defined conditions. Net assimilation was then measured on the cold-exposed leaves after the plants had been returned to 10 °C. Exposure to −3 °C did not significantly reduce CO2 assimilation when compared to plants maintained at 10 °C day/5 °C night. However, leaves exposed to −5 °C for one night had a net CO2assimilation rate that was 49% of the control. When leaves were first exposed to a conditioning night of −3 °C and then exposed to −5 °C, the net assimilation rate was 62% of the untreated control. Repeated exposure to −5 or −7 °C night temperatures resulted in a further decrease in net assimilation after each successive exposure. Leaves exposed to −7 °C for one night had a net assimilation rate of 6% of the control. Leaves exposed to −5 °C or −7 °C did not show any recovery over a 28-d monitoring period. There was no significant difference among cultivars in the sensitivity of leaves to cold temperatures. These results indicate that protected cultivation systems should be managed to maintain strawberry leaf temperatures above −5 °C in order to preserve full photosynthetic activity of existing leaves which would extend the growing season of the crop.
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