Moderate shade environment facilitates establishment of desert phreatophytic species Alhagi sparsifolia seedlings by enlarge fine root biomass

Phreatophytic species (Alhagi sparsifolia Shap.) is commonly found to distribute in open area with full sun light environment without shading, and spontaneous occurrence of seedlings in mixed species communities is rare in arid and semi-arid regions. Seedlings and field-grown perennial plants at the southern rim of the Taklamakan Desert were grown under three light availability, i.e., full sun light, 80 and 20% of full sun light treated as high-light (HL), moderately high-light (MHL), and low-light (LL) groups. Plastic acclimation of leaf morphological and photosynthetic traits induced by low light was evident. Low-light-grown plants exhibited higher maximum photochemical efficiency (Fv/Fm) and relatively low assimilation rate (Pn), while difference in daily Pn between the HL and MHL was not significant. Photosynthetic efficiency at the HL on summer midday was constrained partially due to lower quantum yield of electron transport at the primary quinone acceptors between photosystem II and I. Compared with the HL and LL, seedlings at the MHL group had larger relatively growth rate, net primary production per plant, and longer assimilative branch. Allocation proportions of biomass into root, assimilative branches, leaves, and stem, root/shoot ratio, and specific root length between the HL and MHL were, respectively, similar. However, very fine root biomass (<1 mm) was significantly enlarged by the MHL, which directly contributed to biomass production. Collectively, plant growth of A. sparsifolia favored by the MHL was mainly ascribed to rapid growth rate, photosynthetic efficiency without suffering midday stresses, and importantly to strong root system featured by enlarged fine root biomass.