Global change factors, such as variation in precipitation regimes and nitrogen (N) deposition, are likely to occur simultaneously and may have profound impacts on the relative abundance of grasses differing in functional traits, such as C 3 and C 4 species. We conducted an extreme drought and re-watering experiment to understand differences in the resistance and recovery abilities of C 3 and C 4 grasses under different N deposition scenarios. A C 3 perennial grass ( Leymus chinensis ) and two C 4 grasses (annual species Chloris virgata and perennial species Hemarthria altissima ) that co-occur in Northeast China were selected as experimental plants. For both C 3 and C 4 grasses, N addition caused a strong increase in biomass and resulted in more severe drought stress, leading to a change in the dominant photosynthetic limitation during the drought periods. Although N addition increased antioxidant enzyme activities and protective solute concentrations, the carbon fixing capacity did not fully recover to pre-drought levels by the end of the re-watering period. N addition resulted in lower resilience under the drought conditions and lower resistance at the end of the re-watering. However, N addition led to faster recovery of photosynthesis, especially in the C 3 grass, which indicate that the effect of N addition on photosynthesis during drought was asymmetric, especially in the plants with different photosynthetic nitrogen use efficiency (PNUE). These findings demonstrated that nitrogen deposition may significant alter the susceptibility of C 3 and C 4 grass species to drought stress and re-watering, highlighting the asymmetry between resistance and resilience and to improve our understanding about plant responses to climate change.

Whether photosynthetic pathway differences exist in the amplitude of nighttime variations in the carbon isotope composition of leaf dark-respired CO2 (δ13Cl) and respiratory apparent isotope fractionation relative to biomass (ΔR,biomass) in response to drought stress is unclear. These differences, if present, would be important for the partitioning of C3-C4 mixed ecosystem C fluxes. We measured δ13Cl, the δ13C of biomass and of potential respiratory substrates and leaf gas exchange in one C3 (Leymus chinensis) and two C4 (Chloris virgata and Hemarthria altissima) grasses during a manipulated drought period. For all studied grasses, δ13Cl decreased from 21:00 to 03:00 h. The magnitude of the nighttime shift in δ13Cl decreased with increasing drought stress. The δ13Cl values were correlated with the δ13C of respiratory substrates, whereas the magnitude of the nighttime shift in δ13Cl strongly depended on the daytime carbon assimilation rate and the range of nighttime variations in the respiratory substrate content. The ΔR,biomass in the C3 and C4 grasses varied in opposite directions with the intensification of the drought stress. The contribution of C4 plant-associated carbon flux is likely to be overestimated if carbon isotope signatures are used for the partitioning of ecosystem carbon exchange and the δ13C of biomass is used as a substitute for leaf dark-respired CO2. The detected drought sensitivities in δ13Cl and differences in respiratory apparent isotope fractionation between C3 and C4 grasses have marked implications for isotope partitioning studies at the ecosystem level.

We evaluated the effects of light conditions on leaf gas exchange, chlorophyll content, and growth responses in the Oriental lily (Lilium auratum L.) cv. Sorbonne. The experiment involved application on increasing shade densities (0, 60, 75, and 80%) to Sorbonne, and was carried out in the Horqin Sandy Land of northern China. Shade tests showed that growth of the lily was primarily affected by the level of irradiance. Photoinhibition occurred in the 0 and 60% shade treatments, but not in the 75 and 80% shade treatments. Shade treatments led to increase in photosynthetic pigment content, enhancement in photosynthetic effi ciency, and finally increase in the commercial value of the lily. P n , on the other hand, was lowest in the 80% shade treatment. So irradiance less than that achieved in the 80% shade treatment limited carbon assimilation and led to decreased plant growth. Plants grown under 75% shade displayed the optimal traits determining commercial value (plant height, flower length, flower diameter). Trends in P max , AQY, LSP and LCP to shade confirmed that the lily is a shade tolerant plant. Excessive light, therefore, was the primary factors limiting lily quality. Growth under conditions of 75% shade is recommended to improve photosynthetic efficiency and alleviate photodamage, thus increasing the commercial value of the lily grown in the Horqin Sandy Land.Abbreviations: AQY-apparent quantum yield; Chl a-chloro phyll a; Chl b-chlorophyll b; E-transpiration rate; g s -sto matal conductance; LCP-light compensation point; LSPlight saturation point; PAR-photosynthetically active radiation; P max -maximum net photosynthetic rate; P n -net photosyn thetic rate; R D -respiration rate.