Heat acclimation of grapevine leaf photosynthesis: mezo- and macroclimatic aspects

Heat sensitivity of grapevine (Vitis vinifera L. cv. Kékfrankos) photosynthesis was studied in two vineyards (Eger-Kőlyuktető, flat; and Eger-Nagyeged hill, steep slope) with different mesoclimates and water supply conditions in two climatically different years. 2007 was drier and warmer, with higher vapour pressure deficit (VPD) than 2005. Pre-dawn water potential measurements indicated mild water deficit at the steep-sloped vineyard. In July 2005 mild water deficit enhanced the thermostability of grapevine photosynthesis, as reflected in the temperature dependence of optimal quantum yield (F_v/F_m) and in the critical temperature of initial fluorescence (F₀T_c). Decreased F_v/F_m and actual quantum yield (ΔF/F_m') was recorded at most temperatures in September at the water-stressed (steep slope) site. This time, F₀T_cs were also lower due to early leaf senescence. In September 2007, heat sensitivity of F_v/F_m was similar to 2005, and ΔF/F_m' indicated higher thermostability at both sites, but keeping the consistent difference between the two vineyards. The critical points of steady-state fluorescence (F_sT_c) were higher by 3-6°C at both vineyards in 2007 than in 2005. Although, in September thermolabile F₀ signals were measured at the water-stressed vineyard, the heat sensitivity was not decreased in light adapted state, assumingly as a result of enhanced xanthophyll cycle pigment pool size. The higher xanthophyll pigments pool size (V + A + Z) in 2007 (compared to 2005) at the unstressed (flat) vineyard suggests that high temperature and VPD play a role in changing (V + A + Z)/(chl a + b), and, thus, results in higher thermostability under high light conditions.