Photosynthesis Research On The Predominant Legume Species In Hunshandak Sandland

Nitrogen is one of the decisive factors in controlling plant production. In sandland ecosystems, nitrogen is especially deficient. Therefore the elucidation of the relationship between leaf nitrogen content (Nleaf) and the maximal photosynthetic rate (Pmax) and the comparison of nitrogen use efficiency will be helpful to understand the different resource use efficiency among plant functional types (PFTs). In Hunshandak Sandland, we have investigated some 80 plant species largely distributing in different habitats, e.g., sanddune, lowland and wetland. Those species belong to different PFTs. The results showed that Pmax significantly correlated with Nleaf, expressed on both mass and area basis on all date pool. However, there was a significant variation in photosynthetic nitrogen use efficiency (PNUE) among both habitats and plant functional types. PNUE was consistently greater in wetland than in sanddune or lowland, with a series of grasses>shrubs>trees, C4>C3 species, non-legumes> legumes.Due to the large dependence of photosynthesis on leaf nitrogen content, it is supposed that the legumes might have higher Pn than the non-legume. To test such hypothesis, we have compared the maximal photosynthetic rate (Pmax) and Nleaf of three typical legumes (Caragana microphylla, Hedysarum fruticosum var. lignosum and Thermopsis lanceolata) and two non-legumes (Leymus chinensis and Salix gordejevii). Though C. microphylla and T. lanceolata had significantly higher Nleaf than L. chinensis and S. gordejevii, the values of Pmax among four species were not significantly different, in which PNUE played an important role. Four main factors determining PNUE were analyzed further. It is, thereafter, concluded that notchlorophyll content, photochemical efficiency and CO2 pressure but Rubisco activity determined the PNUE of the five species.H. fruticosum van lignosum is the predominant species, somewhere even the pioneer species in the sanddunes in Hunshandak Sandland. Beside its higher nitrogen content, we wanted to find out whether or not it adapts to the severe conditions with special photosynthetic mechanism. Assimilation-light response and diurnal gas exchange curves were conducted on H. fruticosum var. lignosum and S. gordejevii. The results indicated that PN, water use efficiency and PSII chemical efficiency were significantly higher in H. fruticosum var. lignosum than in S. gordejevii. However, the midday depression was much gentle in H. fruticosum var. lignosum. In addition, H. fruticosum var. lignosum showed the higher light saturation point and lower light compensation point.In order to understand the photosynthesis adaptation of H. fruticosum var. lignosum to drought condition, the effect of simulated rainfall on water stressed H. fruticosum var. lignosum was compared. It was noted that gas exchange and chlorophyll fluorescence greatly increased with the elevation of soil moistures. Both stomatal and non-stomatall factors determined the decline of Pn under drought conditions. However, gs recovered more quickly than the PSII potential activity and PSII light energy transform efficiency once the water was supplied in eliminating drought. All the above mentioned traits have been improved at the rainfall from 0 to 15mm but maintained steady status when the rainfall was above 15mm. Such results indicated that H. fruticosum var. lignosum might be an efficient water use species.Photosynthetic enzymes in H. fruticosum var. lignosum were determined in order to probe into its photosynthetic mechanisms. C4 photosynthetic enzymes, e.g., PEPcase, NAD-ME, NADP-ME, NAD-MDH, NADP-MDH and PPDK had significantly higher activities in H. fruticosum var. lignosum, indicating that C4 pathways might exist in H. fruticosum var. lignosum. Nevertheless, H. fruticosumvar. lignosum possessed the stable carbon isotope values of C3 species (-23%). Therefore, it is likely that H. fruticosum var. lignosum is either a C3-C4 intermediate species, or a C3 species displaying C4 metabolic characteristics under the sanddune conditions. The involvement of C4 me...