In Huanshandak Sandland, China, net photosynthetic rate (PN), transpiration rate (E), stomatal conductance (gs), intercellular CO2 concentration (Ci), water use efficiency (WUE), photochemical efficiency of photosystem 2 (Fv/Fm), and leaf nitrogen content were compared for Hedysarum fruticosum var. mongolicum (H.f.m.), a nitrogen fixing shrub, and Salix gordejevii (S.g.), a nitrogen non-fixing shrub. PN, E, and gs of the two shrubs were similar in trends, i.e. two peaks were observed in diurnal courses. However, except Ci, other parameters of H.f.m. were higher during the measured days than those of S.g. The midday depression of PN was mainly due to decrease in stomata conductance and to reduction of Fv/Fm at midday. The higher PN of H.f.m. was consistent with the higher leaf N content and there was a positive relation between them. In addition, several C4 traits were found in H.f.m., i.e. high saturation irradiance and WUE, low dark respiration rate, and Ci, which partly resulted in higher PN. This seems to indicate that the C3 plant H.f.m. may have C4 photosynthesis pathway or C4 enzymes. and S. L. Niu ... [et al.].
Field studies of gas exchange and chlorophyll fluorescence of three desert shrub species, Hedysarum fruticosum var. mongolicum, Artemisia ordosia, and Salix pasmmophylla, showed different patterns under different leaf temperature (T1) and incident photosynthetic photon flux density (PPFD). H. fruticosum var. mongolicum and A. ordosia exhibited higher PN and gs than S. pasmmophylla, especially under very high T1 (>46 °C) and high PPFD (>2 100 µmol m-2 s-1) in hot summer. The decreases of PN with the diurnal course were due mainly to stomata closure. However, PN of S. pasmmophylla was seriously depressed by very high temperature from midday to evening as shown by the negative PN in hot summer, whereas none of such depression was found in spring. Maximal spring photochemical efficiency of photosystem 2 (PS2), i.e, Fv/Fm, was the lowest at 16:00, indicating the injury of PS2 by heat at this stage. In hot summer again, all the three shrubs underwent pronounced midday depression of PN and gs, while in spring they showed a one-peak response. And the first peak appeared 2 h earlier in hot summer than in spring for all the three shrubs. It was the high temperature that led to the different patterns of gas exchange and the serious depression of PN in S. pasmmophylla. H. fruticosum var. mongolicum and A. ordosia were much more tolerant to heat and high irradiance than S. pasmmophylla, which fixed most of CO2 at the fast growing stage in spring. Nevertheless, in hot summer it had to survive the severe hot environment through strong respiration and storage of CO2 only in the early morning. and G. M. Jiang, G. J. Zhu.