Leaf Nitrogen Allocation and Partitioning in Three Groundwater-Dependent Herbaceous Species in a Hyper-Arid Desert Region

The desert plants are typical components of Groundwater-dependent vegetation (GDV), and must have access to groundwater to maintain their growth and function. GDV is useful as an indicator of water table depth and water availability in north-western China. Although water is essential for plant growth, Nitrogen (N) availability has also been determined as a critical factor limiting plant growth in arid regions. N is an essential limiting resource for GDV growth, especially in the desert regions. Leaves accumulate most of N in the plant, and about half the total leaf N is used for photosynthetic activities. The fraction of the total leaf N allocated to the photosynthetic apparatus is a factor that influences photosynthetic N-use efficiency (PNUE). Studying leaf N allocation and partitioning patterns in GDV species is important for understanding how GDV adapst to the N-limited environments.

To evaluate leaf photosynthetic response to environmental change, N allocation and PNUE in GDV, and elucidate how N content and N allocation in leaves influence leaf photosynthesis and PNUE, three typical GDV species, namely Alhagi sparsifolia Shap., Karelinia caspica (Pall.) Less. and Peganum harmala L., were selected for the study, and their leaf photosynthesis and N allocation were investigated at the southern fringe of the Taklamakan Desert.

The results showed that K. caspica and P. harmala had lower leaf N content, and allocated a lower fraction of leaf N to photosynthesis. However, they were more efficient in photosynthetic N partitioning among photosynthetic components. They partitioned a higher fraction of the photosynthetic N to carboxylation and showed higher PNUE, whereas A. sparsifolia partitioned a higher fraction of the photosynthetic N to light-harvesting components. For K. caspica and P. harmala, the higher fraction of leaf N was allocated to carboxylation and bioenergetics, which led to a higher maximum net photosynthetic rate, and therefore to a higher PNUE, water-use efficiency (WUE), respiration efficiency and so on. In the desert, N and water are limiting resources; K. caspica and P. harmala can benefit from the increased PNUE and WUE. These physiological advantages and their higher leaf-area ratio may contribute to their higher resource-capture ability.

The main finding has been published on Australian Journal of Botany, 2012, 60: 61–67. The paper is also archived at http://www.publish.csiro.au/?paper=BT11181.