中文摘要：

冻融交替是我国北方常见气候现象，其对地衣光合作用的影响尚不清楚。研究了采自雾灵山的卷叶点黄梅Flavopuncteliasoredica和平盘软地卷Peltigeraelisabethae的光合活性（以净光合速率表示，netphotosyntheticrate，Pn）对冻融处理的响应及其与地衣体含水量（干冻组：含水量〈20％干重；湿冻组：含水量〉200％）和物种的关系。结果显示卷叶点黄梅的干冻组Pn经5次冻融后下降至对照的21％，湿冻组经3次冻融后下降至负值，平盘软地卷的干冻组和湿冻组在5次冻融后Pn均为负值；相对净光合速率（relativenetphotosyntheticrate，Rpn）与冻融次数的线性回归分析表明，卷叶点黄梅湿冻组的斜率绝对值（58．06）〉平盘软地卷湿冻组（41．01）〉平盘软地卷干冻组（32．27）〉卷叶点黄梅干冻组（11．44）。结果表明冻融胁迫可显著抑制两种地衣的光合活性，这种抑制作用具有物种差异且和地衣体内水分含量有关：水分含量的增高将增强冻融胁迫对地衣光合活性的抑制作用；干燥状态下，卷叶点黄梅的低温耐性远高于平盘软地卷，但在湿润状态下则低于后者。两种地衣对冻融循环的光合响应的物种差异与其微生境气候有关：生长于较干燥开阔地带的卷叶点黄梅与生于阴湿生境中的平盘软地卷相比，可能已形成了更强的低温干燥适应能力，其低温湿润适应能力则较弱。全球气候变化可能会通过冻融事件的时空格局的改变而对地衣的光合作用和分布造成负面影响。

英文摘要：

The effect of freezing-thawing alternation on lichen photosynthesis is yet unknown in China. To study the responses of photosynthetic activity （indicated by net photosynthetic rate, Pn） to freezing-thawing cycles and their relationships with water conditions, two species of lichens from Mt. Wulingshan of North China, Flavopunctelia soredica and Peltigera elisabethae, were subjected to 5 freezing-thawing cycles under two thallus water conditions （wet-freezing treatment group： thallus water content 〉 200% dry weight; dry-freezing group： thallus water content 〈 20%）. The results show that Pn of F. soredica under dry-freeze decreased to 21% as compared with the control after 5 freezing-thawing cycles, and decreased to negative values after 3 wet-freezing cycles. Pn of P. elisabethae decreased to negative values after 5 wet-freezing and dry-freezing cycles. The linear regression analysis between relative Pn and freezing-thawing cycles shows that the slope absolute values decrease in order of wet-freezing F. soredica （58.06） 〉 wet-freezing P. elisabethae （41.01） 〉 dry-freezing P. elisabethae （32.27） 〉 dry-freezing F. soredica （11.44）. These results indicate that the photosynthetic activity of both lichens is inhibited by freezing-thawing alternation, and the inhibition extent is dependent on species and thallus water conditions. The increase of water content can intensify the inhibition effects The difference of photosynthetic response to freezing-thawing cycles between the two lichens may be due to the physiological adoption to different microclimate conditions. P. elisabethae inhabits the shady and moist habitats, while F. soredico occupies more open and drier habitats. F. soredico has higher tolerance to dry and low temperature than P. elisabethae, and its adaptation ability to wet and cold is weaker. Global climate change may have negative effects on the photosynthesis and distribution of lichens through the impacts of alteration of spatio-temporal patterns of freezing-thawing