Abstract:

The desert plants, Haloxylon ammodendron and Calligonum mongolicum, grow under extreme arid conditions that are associated with high temperatures and intensive radiation. Their true leaves are highly reduced and the young annual shoots become photosynthetic organs. In order to provide a theoretical basis of reference for restoring natural ecosystems of desert regions and maintaining the stability of protective ecosystems that surround oases, the cross sectional anatomical structure, stable carbon isotope ratio (δ13C) and photosynthetic characteristics of the assimilating shoots of these two species were studied. The study area lies in the northern part of the Hexi Corridor Region in Gansu Province at the southern edge of the Badain Jaran Desert. During the growing season (late May to early October in 2001), we collected assimilating shoots and prepared samples for analysis. For light microscopy, samples of assimilating shoots of H. ammodendron and C. mongolicum were cut in late July and fixed in FAA (alcohol∶formalin∶glacial acetic acid; 90∶5∶5). Transverse sections of assimilating shoots of the two desert plants were observed and photographed using a Nikon 1 671 CHR microscope. The stable carbon isotope ratios (δ13C) of assimilating shoots of the two plants were analysed using a MAT-252 mass spectrometer in the State Key Gas Geo-chemical Laboratory of the Lanzhou Institute of Geology, Chinese Academy of Sciences, and compared with other desert plants and the C4 crop, Zea mays. The response of the net photosynthetic rate (Pn) of the in vivo assimilating shoots of the two plants to different photon flux density (PFD) and CO2 concentration (Ca) were measured using a LI-6400 portable photosynthesis system (LI-COR, Nebraska, USA). The assimilating shoots were cut after measurements of Pn and their areas measured with the LI-3100 Area Meter. The difference in δ13C and Δ of leaves or assimilating shoots of different plants and the difference in δ13C values of the same plant determined in different growth periods were analyzed by single factor variance analysis. If there was a significant difference, various levels of multiple comparison tests were made using Duncan's new multiple range test. The results are summarized below. 1) Assimilating shoots of H. ammodendron and C. mongolicum have a layer of hypodermal cells and two layers of chlorenchyma on the stem periphery, i.e. an outer layer of palisade cells and an inner layer of bundle sheath cells. The central portion of the shoot is occupied by water storage tissue with the main vascular bundles located in the center. Central bundles are thus separated from Kranz-type cells by layers of water storage cells. There are some small peripheral bundles that have contact with bundle sheath cells. Mesophyll and bundle sheath cells contain chloroplasts. The chloroplast of bundle sheath cells contains starch grain, but the chloroplast of mesophyll cells does not contain starch grain. For H. ammodendron, some crystal-containing cells existed in mesophyll and water storage tissue. For C. mongolicum, many mucilage cells existed in the mesophyll and water storage tissues. These results demonstrate that these two species have Kranz anatomy. 2) The δ13C values of H. ammodendron and C. mongolicum were -14.3‰ and -14.8‰, respectively, whereas the corresponding values of Caragana korshinskii, Nitraria sphaerocarpa, Hedysarum scoparium and Reaumuria soongorica were -25.8‰, -25.8‰, -26.4‰ and -28.1‰, respectively. The δ13C values of the desert plants H. ammodendron and C. mongolicum were similar and were not significantly different from the δ13C value of maize. Under different growth periods and under high and low water conditions, the δ13C values of these two species varied between -14‰ to -16‰. However, the desert plants C. korshinskii, N. sphaerocarpa, H. scoparium and R. soongorica, and oasis shelterbelt arborous species, such as Elaeagnus angustifolia, Populus gansuensis, and the oasis species, Populus euphratica, all had similar δ13C value which varied between -25 ‰--30‰. The stable carbon isotope discrimination of H. ammodendron and C. mongolicum was 5‰-6‰ which was similar to the C4 crop maize, while those of the other plants mentioned above were 16‰-22‰. 3) The CO2 compensation point (CCP) of H. ammodemdron and C. mongolicum was 2 and 4 μmol•mol－1, respectively, and that of C. korshinskii was 91 μmol•mol－1. 4) The light saturation point (LSP) of H. ammodemdron and C. mongolicum was 1 660 and 1 756 μmol•m－2•s－1, respectively, C. korshinskii was 1　267 μmol•m－2•s－1. 5) The apparent quantum yield of H. ammodemdron and C. mongolicum was 0.044 and 0.057 mol CO2•mol－1 photons and C. korshinskii was 0.02 mol CO2•mol－1 photons.