Physiological Response of Non-acclimated Spinach to Repeated Freeze–Thaw Cycles

While high tunnels offer growers a way to extend the local growing season, winter production limits productivity when plants are repeatedly exposed to freezing temperatures. During winter production, plants experience both sub-optimal growing temperatures and extreme diurnal temperature fluctuations.  The focus of this study was to evaluate the effect of repeated freeze/thaw cycles on photosynthetic rates and efficiency in spinach.  Spinach (cv. Space) was exposed over three days to various diurnal temperature regimes. After each cycle, photosynthesis (Ps) and chlorophyll fluorescence (Fv/Fm) were monitored for four hours with a Li-COR 6400.  Non-cold acclimated plants were exposed to either a mild (10/0 °C day/night) or severe (10/-5 °C) freeze cycle that were compared to the controls (10/5 °C). Single or multiple freeze/thaw cycles (mild or severe) did not significantly lower steady state Ps rates, when compared to the controls. However, the rate of Ps recovery was significantly slower as freezing severity increased.   Fluorescence ratios for cold exposed plants were not significantly different from the unstressed controls (0.832 ± 0.004) indicating no reduction in photosynthetic efficiency. Results support the finding that biomass production in winter grown spinach is governed more by how quickly photosynthetic rates recover rather than by an actual reduction in photosynthetic rate after exposure to freeze/thaw events.