Water Restriction Effect on Pear Rootstocks: Photoprotective Processes and the Possible Role of Photorespiration in Limiting Carbon Assimilation

Under water restriction, stomatal closure limits photosynthesis (Pn) leading to a dangerous energy excess. Non-photochemical quenching (NPQ) and non-net carboxylative transports (NC: alternative electron transports and photorespiration) cooperate in dissipating the energy surplus by consuming energy and CO2. Leaf and fruit behaviour were evaluated on 'Abbe Fetel' pear, grafted on Adams and Sydo and irrigated with 100 (T100), 50 (T50), 25 (T25) and 0% (T0) of the estimated evapotranspiration, during the whole season. At fruit cell expansion Pn and stomatal conductance (gs) decreased in T25 and T0, in Adams, where the energy surplus was dissipated via NC. In Sydo, Pn and gs reductions occurred already in T50, indicating its higher drought susceptibility. The energy surplus was dissipated via NPQ in addition to NC. At the end of fruit cell expansion, the difference between the two rootstocks was maintained. Water-limited treatments (T25 and T0 for Adams; T50, T25 and T0 for Sydo) dissipated the energy surplus increasing NPQ and NC, indicating the pivotal role of these photoprotective pathways under drought stress. At this stage, water limitation reduced gs but did not decrease the internal CO2 concentration. This suggests that photosynthesis was not completely carbon limited and stomatal closure reduced leaf thermo-regulation raising leaf temperature, causing heat inhibition and affecting the Rubisco photorespiratory activity. The higher drought susceptibility of Sydo was observed also in fruit yields which were lower than in Adams.