Temperature, evapotranspiration and primary photochemical responses of apple leaves to hail

The objective of this work was to examine immediate physiological plant responses to hail and subsequent recovery in terms of evapotranspiration, leaf temperature and primary photochemical processes using apple as a model crop. Thermal emission pictures were taken in darkness to avoid interference from stomatal movements; temperature gradients were identified in concentric rings around sites of hail injury, with a distinct drop in temperature of up to 2.3 °C in the center immediately after the induced hail injury. This was due to enhanced evapotranspiration from the injured tissue. Six to twelve minutes after hail injury, the initial decrease in leaf temperature partially reversed. Chlorophyll fluorescence kinetics of light-adapted leaves showed a dramatic decrease in effective photosynthetic electron transport rate (ETR), from 20.5 to 9.0 μmol electron m−2 s−1 within 5 min from hail injury, and a rapid recovery to 14.1 μmol electron m−2 s−1 within the next 5 min. After 7 h, ETR partially recovered to 17.4 μmol electron m−2 s−1. An initial drop in non-photochemical efficiency (NPQ) from 1.07 to 0.90 units within 5 min after hail injury was followed by a sharp increase to 1.67 units after another 5 min. During the next hour, NPQ gradually decreased to the initial level. This indicates increased thermal dissipation in photosystem II (PS II) as a protective mechanism against incident excessive energy in the leaves with closed stomata for 1 h after hail injury. In contrast to the fluorescence kinetics of light-adapted leaves, maximum quantum yield Fv/Fm of PSII in the dark-adapted state remained unchanged at 0.79–0.81 relative units for the first 5 min after hail injury. Thereafter, Fv/Fm slowly declined to 0.75 within 1 h, and to a trough of 0.73 at 3 h. Seven hours after hail injury, Fv/Fm values were at 0.76, indicating partial recovery of PS II efficiency. The discrepancy in the dynamics of ETR and Fv/Fm responses may be explained by the formation of alternative electron sinks such as reactive oxygen species, particularly superoxides, which withdraw electrons from the photosynthetic transport, resulting in apparently higher values of calculated ETR.