Thermostability of photosynthesis of Vitis aestivalis and V. vinifera

High temperature adversely affects photosynthetic rates and thylakoid activities in many species, but photosynthesis response to heat stress is not well defined in grapes (Vitis L.). Genotypes within species respond differently to high temperatures, indicating a genetic variability for the trait. The objective of this study was to determine the physiological responses of two grape species to high temperature, at the whole-plant level and at the cellular level. Gas exchange, relative chlorophyll content, and chlorophyll fluorescence of intact leaves and thermostability of extracted thylakoids of the American (V. aestivalis Michx.) 'Cynthiana' and European (V. vinifera L.) 'Semillon', 'Pinot Noir', 'Chardonnay', and 'Cabernet Sauvignon' wine grapes were evaluated. One-year-old vines were placed in controlled environmental chamber held at 20/15, 30/25, or 40/35 degrees C day/night for 4 weeks. Net CO2 assimilation (A) rate, stomatal conductance (g(s)), transpiration (E) rate, chlorophyll content, and chlorophyll fluorescence of intact leaves were measured at weekly intervals. Chlorophyll fluorescence of thylakoids extracted from V. aestivalis 'Cynthiana' and V. vinifera 'Pinot Noir' subjected to temperatures ranging from 20 to 50 degrees C was measured. Optimal temperatures for photosynthesis were 20/15 degrees C for 'Cynthiana' and 'Semillon' and 30/25 degrees C for the other three V. vinifera cultivars. The A, g(s), E, chlorophyll content, and chlorophyll fluorescence values at 40/35 degrees C were lower in 'Cynthiana' than 'Pinot Noir'. In general, reduction of A coincided with decline in g(s) in 'Cynthiana', whereas no strong relationship between A and g(s) was observed in V. vinifera cultivars. Variable chlorophyll fluorescence (Fv) and the quantum efficiency of photosystem II (Fv/Fm) of intact leaves for all the cultivars decreased at 40/35 degrees C, with severe decline in 'Cynthiana' and 'Cabernet Sauvignon,' moderate decline in 'Semillon' and 'Chardonnay', and slight decline in 'Pinot Noir'. A distinct effect of high temperature on Fv and Fv/Fm of 'Cynthiana' was exerted after 2 weeks of exposure. Prolonged-exposure to 40/35 degrees C led to 78% decrease in Fv/Fm in 'Cynthiana', compared with 8% decrease in 'Pinot Noir'. In general, Fv and Fv/Fm of extracted thylakoids declined as temperature increased, with more decline in 'Cynthiana' than in 'Pinot Noir'. Based on A rates and Fv/Fm ratios, results showed that 'Cynthiana' has lower optimal temperature for photosynthesis (20/15 degrees C) than 'Pinot Noir' (30/25 degrees C). Chlorophyll fluorescence responses of intact leaves and extracted thylakoids to high temperatures indicate that 'Pinot Noir' possess higher photosynthetic activity than 'Cynthiana'. Results of this work could be used in selection programs for the development of heat resistant cultivars in the warmest regions.