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Damage and capacity to recover of photosystem II (PSII) from long exposures to heat stress were investigated in grapes using chlorophyll fluorescence. Two wine grapes,  Vitis aestivalis  Michx. cv. ‘Cynthiana’ and French-American hybrid ‘Vignoles’ ( Vitis  L. hybrid), were exposed to a sudden heat shock (SHS) and a gradual heat shock (GHS) at 40/35 °C. After heat stress, plants were moved to a greenhouse to allow PSII to recover from heat treatments. Changes in maximum quantum efficiency of PSII, indicated by the ratio of variable fluorescence and maximum fluorescence (Fv/Fm), were observed after 3, 6, and 12 days of heat stress and after 3, 7, 14, and 21 days recovery periods of damage to PSII. Total leaf area (LA) and leaf, shoot, and root biomass were determined at the end of the experiment. Regardless of the heat treatment, increasing duration of exposure to high temperature caused a decline in Fv/Fm in both cultivars. Heat stress treatments also caused a progressive decline in LA as well as leaf and shoot biomass. Maximum quantum efficiency of PSII was observed after 3 days of exposure in both cultivars, regardless of the heat stress treatment. ‘Vignoles’, however, showed higher PSII photochemical efficiency 12 days after heat exposure. GHS was less detrimental to PSII compared with SHS heat treatment. The damaged PSII of ‘Vignoles’ recovered faster than that of ‘Cynthiana’. A positive relationship was observed between Fv/Fm and LA of plants exposed to heat treatments. Based on Fv/Fm values, this study indicates that PSII of ‘Vignoles’ is more thermostable and can recover faster than that of ‘Cynthiana’ leaves, regardless of the heat treatment. These results suggest that ‘Vignoles’ is generally more heat-tolerant than ‘Cynthiana’ and changes in Fv/Fm ratio under heat stress conditions could be a good indicator for screening heat-resistant grape cultivars.

Photochemical Efficiency and Recovery of Photosystem II in Grapes After Exposure to Sudden and Gradual Heat Stress