Delayed Inhibition of Photosynthetic Performance—Three Linden Species in an Urban Environment

The presented study concerns the delayed response of photosynthetic performance to summer drought. Increasing drought events are thought to be one of the most detrimental and expensive natural disasters. For that reason, we studied the condition of urban trees, which are especially susceptible to water deficit. Three species of linden trees (T. tomentosa, T. × europaea and T. cordata) growing in the city area of Krakow, Poland, were investigated for two consecutive years to compare their response to a precipitation deficit. For the first time, the physiological traits of linden urban trees were compared in years with and without a natural summer drought. For evaluation of the photosynthetic performance, we used measurements of the chlorophyll a fluorescence concerning the OJIP transients and derived parameters. The contents of photosynthetic pigments, such as chlorophylls and carotenoids, were analysed, and stress-sensible parameter ratios, such as chl a/b and chl/carot, were calculated. The most common method of research conduction assumes examination close to the presence of a stressor. We chose an innovative method of prolonged investigation after relief from the stress conditions. While there was no evident reaction of PSII to meteorological drought during the stress occurrence, all of the investigated species of linden revealed delayed reaction of PSII to precipitation deficit in summer. Changes in PSII were connected with the decrease of active reaction centres (RCs), which was evident in the increase of the ABS/RC, DI0/RC and TR0/RC, and the decrease of ET0/RC. T. × europaea demonstrated a more intense decrease of certain parameters during late senescence compared with other species. Despite the close relation of T. cordata and T. × europaea, species showed different physiological traits during senescence after a summer drought. Our results underline the need for monitoring tree conditions not only during stress but also in the wider perspective.