Photosynthetic Activity of Six Hop (Humulus lupulus L.) Cultivars under Different Temperature Treatments

The Pacific Northwest grows the majority of hops in the United States; however, the region is experiencing an increase in the number of days with high heat. In addition, there is an increased interest in growing hops in other warmer regions of the United States. To understand how hop plants respond to high temperatures, we measured several physiological traits of six hop cultivars under a range of temperatures from 15 to 45 degrees C. We found that hop plants achieved maximal carbon assimilation at temperatures of 21 to 39 degrees C when given sufficient water. At temperatures of 41 degrees C and higher, all cultivars experienced declines in carbon assimilation. This was likely due to multiple effects on the cell, including damage to photosystem II (PSII), as reflected in declines in FV/FM, damage to membrane integrity as reflected in electrolyte leakage at high temperatures, and declines in Rubisco activity likely due to degradation of Rubisco activase, as reflected in declines in V-c,V-max. 'Cascade', 'Willamette', and 'Southern Brewer' may be good candidates for growing in warm climates because all experience relatively high rates of carbon assimilation at high temperatures and did not experience significant declines in FV/FM or increases in electrolyte leakage. 'Chinook' appeared susceptible to extreme heat stress and exhibited evidence of irreparable damage to PSII and membrane integrity at 45 degrees C.