Responses of leaf photosynthesis, pigments and chlorophyll fluorescence within canopy position in a boreal grass (Phalaris arundinacea L.) to elevated temperature and CO<sub>2</sub> under varying water regimes

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The effects of elevated growth temperature (ambient + 3.5°C) and CO2 (700 μmol mol-1) on leaf photosynthesis, pigments and chlorophyll fluorescence of a boreal perennial grass (Phalaris arundinacea L.) under different water regimes (well watered to water shortage) were investigated. Layer-specific measurements were conducted on the top (younger leaf) and low (older leaf) canopy positions of the plants after anthesis. During the early development stages, elevated temperature enhanced the maximum rate of photosynthesis (P max) of the top layer leaves and the aboveground biomass, which resulted in earlier senescence and lower photosynthesis and biomass at the later periods. At the stage of plant maturity, the content of chlorophyll (Chl), leaf nitrogen (NL), and light response of effective photochemical efficiency (ΦPSII) and electron transport rate (ETR) was significantly lower under elevated temperature than ambient temperature in leaves at both layers. CO2 enrichment enhanced the photosynthesis but led to a decline of NL and Chl content, as well as lower fluorescence parameters of ΦPSII and ETR in leaves at both layers. In addition, the down-regulation by CO2 elevation was significant at the low canopy position. Regardless of climate treatment, the water shortage had a strongly negative effect on the photosynthesis, biomass growth, and fluorescence parameters, particularly in the leaves from the low canopy position. Elevated temperature exacerbated the impact of water shortage, while CO2 enrichment slightly alleviated the drought-induced adverse effects on P max. We suggest that the light response of ΦPSII and ETR, being more sensitive to leaf-age classes, reflect the photosynthetic responses to climatic treatments and drought stress better than the fluorescence parameters under dark adaptation.