Abstract
BackgroundNitrogen (N) is an essential component of photosynthetic apparatus. However, the mechanism that photosynthetic capacity is suppressed by N is not completely understood. Photosynthetic capacity and photosynthesis-related genes were comparatively analyzed in a shade-tolerant species Panax notoginseng grown under the levels of low N (LN), moderate N (MN) and high N (HN).ResultsPhotosynthetic assimilation was significantly suppressed in the LN- and HN-grown plants. Compared with the MN-grown plants, the HN-grown plants showed thicker anatomic structure and larger chloroplast accompanied with decreased ratio of mesophyll conductance (gm) to Rubisco content (gm/Rubisco) and lower Rubisco activity. Meanwhile, LN-grown plants displayed smaller chloroplast and accordingly lower internal conductance (gi). LN- and HN-grown individuals allocated less N to light-harvesting system (NL) and carboxylation system (NC), respectively. N surplus negatively affected the expression of genes in Car biosynthesis (GGPS, DXR, PSY, IPI and DXS). The LN individuals outperformed others with respect to non-photochemical quenching. The expression of genes (FBA, PGK, RAF2, GAPC, CAB, PsbA and PsbH) encoding enzymes of Calvin cycle and structural protein of light reaction were obviously repressed in the LN individuals, accompanying with a reduction in Rubisco content and activity. Correspondingly, the expression of genes encoding RAF2, RPI4, CAB and PetE were repressed in the HN-grown plants.ConclusionsLN-induced depression of photosynthetic capacity might be caused by the deceleration on Calvin cycle and light reaction of photosynthesis, and HN-induced depression of ones might derive from an increase in the form of inactivated Rubisco.
Highlights
Nitrogen (N) is an essential component of photosynthetic apparatus
low N (LN)-grown leaves were dramatically reduced in the thickness of upper epidermis, lower epidermis, spongy tissue and palisade tissue, and biomass of leaf was significantly reduced in LN and high N (HN) treatments (Table 1)
LN significantly decreased the size of chloroplasts accompanied with a reduction in chloroplast exposed to intercellular air space per unit leaf area (Sc), and correspondlingly an increase in the size of chloroplasts was observed in P. notoginseng under excessive N supply (Fig. 1 c; Table 2)
Summary
Nitrogen (N) is an essential component of photosynthetic apparatus. The mechanism that photosynthetic capacity is suppressed by N is not completely understood. Photosynthetic capacity and photosynthesis-related genes were comparatively analyzed in a shade-tolerant species Panax notoginseng grown under the levels of low N (LN), moderate N (MN) and high N (HN). N-deficient crops show the premature of leaves, and reduce leaf area expansion, plant height and yield of their own [5,6,7,8,9]. N surplus in plants receives relatively little attentions in comparison with N deficiency over the past decades
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