Responses of two strawberry cultivars to severe high temperature stress at different flower development stages

- A one-time severe high temperature stress caused low fruit set in 'Nyoho' and 'Toyonoka' strawberry cultivars.
- Earlier and later flower development stages were heat sensitive, whereas intervening stages were heat tolerant.
- The decrease in fruit set percentage, however, varied by flower development stage, flower position, and cultivar.
- Severe high temperature stress did not necessarily lead to an overall decrease in fruit fresh weight and fruit size. The effects varied by flower development stage, flower position, and cultivar.

Prolonged heat stress negatively affects fruit set and fruit development in strawberry (Fragaria x ananassa Duch.), but the effect of a one-time severe heat stress at different stages of flower development is poorly understood. We hypothesized that strawberry is negatively affected by acute and severe heat stress but the response varies by floral development stage and by cultivar. Two Japanese cultivars, 'Nyoho' and 'Toyonoka', which were previously reported to differ in their tolerance to prolonged high temperatures, were heat stressed at 42 °C for 4 h at 12, 9, 6, 3, and 0 days before anthesis (DBA) of the primary flower. Data on the percentage fruit set and fresh weight, diameter, and length of all developed fruits were then collected. In terms of fruit set from primary to tertiary flowers, two heat-sensitive floral development stages were observed in 'Nyoho': at 12 DBA and 0 DBA (or at anthesis). In 'Toyonoka', the heat-sensitive floral development stages were at 9 DBA and 0 DBA. Fresh weight and fruit size were larger in primary and secondary fruits of 'Nyoho' when the heat stress was applied during the earlier floral development stages but tertiary fruits were larger when heat stress was applied during the later floral development stages. For 'Toyonoka', the fresh weight and fruit size of primary fruits were smallest when the heat stress was applied at 9 and 0 DBA. These findings confirm our hypotheses and will be helpful in predicting yield losses in open-field strawberry production when a sudden heat wave occurs during the crop's flowering and fruiting season from late winter to early summer.