Strawberry (Fragaria xananassa Duch.) growth and productivity as affected by temperature

Thermotolerance of photosynthesis and productivity in 'Chandler' and 'Sweet Charlie' strawberry plants (Fragaria x ananassa Duch.) exposed to three temperature regimes was studied. Net CO2 assimilation rate (A), variable chlorophyll fluorescence (Fv), efficiency of photosystem II (Fv/Fm), relative chlorophyll content, plant growth, and fruit yield and quality were measured. High temperature (40 degrees C day/35 degrees C night) was more detrimental to photosynthesis and productivity than the moderate or low temperature (30/25 or 20/15 degrees C). Net CO2 assimilation rate in both cultivars was markedly reduced by 40/35 degrees C, although there was slight decline in 'Sweet Charlie' at 30/25 degrees C. 'Chandler' maintained significantly higher A rates than 'Sweet Charlie' for at least three weeks of heat stress, indicating that 'Chandler' might tolerate longer exposure to high temperature. In parallel to the decrease in A rate, intercellular CO2 concentration (C-i) and instantaneous water use efficiency (WUE) were significantly decreased at high temperature. 'Chandler' leaves were cooler and transpired more than 'Sweet Charlie' leaves, suggesting that each cultivar adopted different heat resistance mechanisms at 40/35 degrees C. There were changes in Fv and Fv/Fm with increasing temperature, indicating irreversible damage to photosystem II at 40/35 degrees C might have occurred. The trend of reduction in stomatal conductance (g(S)) in both cultivars at high temperature did not coincide with the reduction in A rates. Decline in A rates at high temperature was more related to changes in Fv/Fm than to g(S) activity. The optimal temperature for vegetative growth was 30/25 degrees C. Reduction in A rate at high temperature resulted in reduction in total leaf area (LA), shoot, root, and leaf biomasses. Strawberry roots were more responsive than shoot growth to temperatures above 20/15 degrees C. Fruit yield for 'Chandler' was higher at 20/15 degrees C than at 30/25 degrees C, suggesting that 'Chandler' might have a higher source-to-sink relationship at 20/15 degrees C than at 30/25 degrees C. Fruit skin color was temperature dependent only for 'Chandler'. A quadratic relationship between flower development and duration of exposure to 30/25 degrees C for both cultivars was observed; more than two weeks of 30/25 degrees C can be detrimental to flower development. Regardless of the cultivar and duration of exposure, 40/35 degrees C was the temperature regime most detrimental to fruit set.