Variation of photosynthesis and pigment concentration relative to irradiance and nitrogen content for two coexisting desert shrubs

Variation of light within canopies directly influences foliage photosynthetic acclimation and hence is important for determining the fitness of plants growing in fragile arid ecosystems. This study was conducted at Gurbantünggüt desert in the hinterland of the Eurasian continent, Xinjiang, China. Gas exchange, pigment concentration and nitrogen (N) content were measured in the sun-lit and shaded assimilating branches/leaves of two coexisting desert species during an annual growing season. Photo-saturated photosynthesis was higher in Tamarix ramosissima but light compensation points were lower compared to Haloxylon ammodendron. T. ramosissima exhibited a higher efficiency and sustained highest maximum net photosynthesis (PN) toward the end of the growing season, while H. ammodendron had a higher efficiency earlier in the growing season. Both species exhibited significantly higher (P < 0.05) PN in sun-lit branches compared to shaded branches throughout the growing season. T. ramosissima recorded higher pigment concentration and N content compared to H. ammodendron during the growing season. Although pigment concentrations decreased with seasonal courses regardless of light conditions, they were significantly higher (P < 0.001) in sun-lit branches compared to shaded branches of T. ramosissima but completely opposite results were obtained in H. ammodendron. Seasonal N content was influenced by variation in precipitation with age-related changes in assimilating branches affecting the allocation of N content along light gradients in both species. The observed differences in PN between the sun-lit and shaded canopy fractions were caused by N import of the leaves, which varied slightly with the gradient in irradiance due to N reallocation within photosynthetic apparatus. T. ramosissima revealed a higher potential for irradiance acclimation through plasticity in photosynthetic apparatus, and hence an important attribute for colonizing wider ecological range compared to H. ammodendron.