Abstract
Climatic warming and drying are having profound impacts on terrestrial carbon cycling by altering plant physiological traits and photosynthetic processes, particularly for species in the semi-arid Mediterranean ecosystems. More effective methods of remote sensing are needed to accurately assess the physiological responses and seasonal photosynthetic activities of evergreen species to climate change. We evaluated the stand reflectance in parallel to the diurnal and seasonal changes in gas exchange, fluorescence and water contents of leaves and soil for a Mediterranean evergreen shrub, Erica multiflora, submitted to long-term experimental warming and drought. We also calculated a differential photochemical reflectance index (ΔPRI, morning PRI subtracted from midday PRI) to assess the diurnal responses of photosynthesis (ΔA) to warming and drought. The results indicated that the PRI, but not the normalized difference vegetation index (NDVI), was able to assess the seasonal changes of photosynthesis. Changes in water index (WI) were consistent with seasonal foliar water content (WC). In the warming treatment, ΔA value was higher than control in winter but ΔYield was significantly lower in both summer and autumn, demonstrating the positive effect of the warming on the photosynthesis in winter and the negative effect in summer and autumn, i.e., increased photosynthetic midday depression in summer and autumn, when temperatures were much higher than in winter. Drought treatment increased the midday depression of photosynthesis in summer. Importantly, ΔPRI was significantly correlated with ΔA both under warming and drought, indicating the applicability of ΔPRI for tracking the midday depression of photosynthetic processes. Using PRI and ΔPRI to monitor the variability in photosynthesis could provide a simple method to remotely sense photosynthetic seasonality and midday depression in response to ongoing and future environmental stresses.
Highlights
Droughts have occurred frequently under global warming around the world [1,2], prominently disturbing terrestrial ecosystemic services and functioning, such as water cycles [3], terrestrial production [4,5], ecosystemic respiration [4], biodiversity [6] and plant survival and mortality [7,8]
We used photochemical reflectance index (PRI) to detect the seasonality of photosynthesis in E. multiflora under long-term drought and warming conditions
water index (WI) provided a simple method for non-destructively detecting changes in plant water content in this drought-tolerant shrub
Summary
Droughts have occurred frequently under global warming around the world [1,2], prominently disturbing terrestrial ecosystemic services and functioning, such as water cycles [3], terrestrial production [4,5], ecosystemic respiration [4], biodiversity [6] and plant survival and mortality [7,8]. Plants respond to warmer and drier conditions mainly by downregulating photosynthesis due to stomatal limitation and lack of soil water [8,18,24,25,26] or to electron-transport limitation and Rubisco (ribulose 1,5-bisphosphate carboxylase/oxygenase) deactivation [27,28]. Such decreases in photosynthesis are accompanied by decreases in the maximum photochemical efficiency of photosystem II (PSII, FV/FM) [29]. PRI tracks the rapid physiological changes that are generally difficult to follow in evergreen species using indices of greenness and canopy structure, such as the normalized difference vegetation index (NDVI) [35,36,37]
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