Physiological Indexes of Populus euphratica Leaves from Different Canopy Position in the Lower Reaches of Tarim River

Plants were often easily affected by various environmental stresses under natural conditions, such as high temperatures, drought, and salinity. Among these stresses, drought and salinity were the widespread environment problems, especially in the arid and semi-arid areas. These stresses limit the growth and productivity of most plant species. Understanding the plant responses to the prevailing environmental stresses is important for effective management and rehabilitation of the degenerated ecosystems.

Several environmental factors vary with different positions in a plant crown. These differences of environmental factors also bring different stresses for leaves in different canopy positions. The leaves in the different canopies have changed some physiological indexes to adjust the external stress. So, it is important to study the physiological changes for understanding the cause resulting in the wilt of plants.

Populus euphratica plays a very important role in maintaining ecosystem function and improving environment in arid and semi-arid regions. The distribution area of P. euphratica in the world is about 64.87×10⁴ hm², and the forest in Tarim River Basin account for about 54% of the total area in the world. With the decrease of the groundwater in the lower reaches of Trim River, P. euphratica began to wither from upper canopy to lower canopy.

To understand the adaptation mechanisms of stresses in plants in arid and semi-arid regions, researchers divided the tree crown into 3 canopies (upper canopy, middle canopy and lower canopy) and investigated the difference of physiological indexes between the 3 canopies and their role in osmotic adjustment under the external stresses.

The results showed that the contents of water (RWC), soluble sugar (SS), free proline (Pro) and the activities of peroxidase (POD) decreased from lower canopy to upper canopy, while the content of malondialdehyde (MDA), the activities of superoxide dismutase (SOD) and the ratio of chlorophyll a and chlorophyll b (Chl a/b) increased. These results indicated that the external stress on the upper canopy was higher than the middle and lower canopy. That may be the main reason resulting in the death of the upper canopy. The contents of chlorophyll a (Chla), chlorophyll b (Chlb) and total chlorophyll (Chlt) were slightly higher in the middle canopy than in other canopies, indicating the ability of absorbing light was same in canopies. These findings from this study will contribute to the best management of P. euphratica.

The result has been published on Pakistan Journal of Botany, 2012, 44 (3): 933–938. The paper can be downloaded from http://www.pakbs.org/pjbot/PDFs/44(3)/14.pdf.