Photosynthetic Physiological Characteristics ofGazania rigensL. Under Drought Stress

Leaf chlorophyll parameters and photosynthetic characteristic variations with stand age in a typical desert species (Haloxylon ammodendron)

“…Haloxylon ammodendron may consume much soil water and soil nutrients in the early stage of rapid growth, resulting in a stressed growth environment for the middle-aged H. ammodendron (22-year-old). Soil water has an important effect on the activity and quantity of photosynthetic enzymes ( Yang et al., 2008 ); thus, water deficit limited the photosynthetic capacity and LUE ( Gao et al., 2016 ). When subjected to water stress, H. ammodendron regulated the physiological characteristics by reducing stomatal conductance, decreasing transpiration ( Figure 5 ), and reducing respiratory consumption, to realize the optical protection process ( Demmig-Adams and Adams, 2000 ).…”

“…The P nmax and LUE max increased in the older H. ammodendron (34- and 46-year-old), and there was a decreasing trend with growth. In response to this phenomenon, we believed that there are three main reasons for this phenomenon: firstly, the growth speed of the belowground part of H. ammodendron was prior than that of the belowground part ( Wei et al., 2007 ), the main depth that H. ammodendron used soil water increased as stand age increased ( Zhu and Jia, 2011 ), and water stress was alleviated by the formation of a more powerful root system in the growth of H. ammodendron (34- and 46-year-old), in order to adapt to the arid environment; secondly, degradation of the older H. ammodendron (34- and 46-year-old) has occurred, and the ratio of the numbers of live to dead branches of individual H. ammodendron decreased, providing opportunities for photosynthesis by live branches which have larger stomatal conductance; thus, the live branches of the older H. ammodendron had higher P nmax and LUE max ; thirdly, H. ammodendron has developed its own extreme drought tolerance mechanism and photosynthetic regulation strategy in long-term adversity ( Gao et al., 2016 ), and the older H. ammodendron plants are able to use a greater range of light and maintain a higher photosynthetic capacity and light energy use efficiency. However, with the increase of stand age, plants have a tendency to senesce, physiological activity decreases, and photosynthetic capacity gradually decreases ( Schoettle, 1994 ).…”