Search

Key message: Mg deficiency affected only the differentiation zone in Citrus sinensis roots. Phloem impairment in Mg-deficient leaves resulted in cell wall lignifications of both vascular cambium and spongy parenchyma cells. Decrease of Mg content in the roots hampered secondary lignification of endodermal and protoxylem cell walls.Abstract: Magnesium (Mg) is a vital nutrient for plant photosynthesis and biochemical reactions. Mg deficiency is a widespread problem in citrus production, directly affecting yield and quality. With the objective of investigating the relationship among root and leaf anatomy, photosynthetic efficiency and mineral nutrient uptake and transport in Mg-deficient citrus, the effects of Mg deficiency in 'Xuegan' (Citrus sinensis) seedlings on plant biomass, leaf gas exchange and chlorophyll, nutrient contents, root and leaf anatomy and lignin biosynthesis were studied. Mg deficiency decreased root and shoot dry weight, leaf CO2 assimilation, chlorophyll and Mg, phosphorus (P), boron (B), cupper (Cu) and iron (Fe) concentration in roots, stems and leaves, and increased potassium (K) and calcium (Ca) concentration. Mg deficiency in the leaves resulted in phloem disorder that blocked phloem transport, leading to carbohydrate accumulation in chloroplasts and secondary lignifications of both vascular cambium and spongy parenchyma cell walls. In contrast, decrease of Mg content in the roots hampered lignin deposition on the protoxylem and endodermal cell walls at the root differentiation zone. Poorly lignified Casparian strip and xylem cell walls in Mg-deficient root tips might decrease xylem loading efficiency of nutrients, lowering their contents in the above ground parts, which ultimately inhibited whole plant growth. [ABSTRACT FROM AUTHOR]

The foliar stable N isotope ratio (δN) can provide integrated information on ecosystem N cycling. Here we present the δN of plant and soil in four remote typical tropical rainforests (one primary and three secondary) of southern China. We aimed to examine if (1) foliar δN in the study forests is negative, as observed in other tropical and subtropical sites in eastern Asia; (2) variation in δN among different species is smaller compared to that in many N-limited temperate and boreal ecosystems; and (3) the primary forest is more N rich than the younger secondary forests and therefore is more N enriched. Our results show that foliar δN ranged from −5.1 to 1.3 ‰ for 39 collected plant species with different growth strategies and mycorrhizal types, and that for 35 species it was negative. Soil NO had low δN (−11.4 to −3.2 ‰) and plant NO uptake could not explain the negative foliar δN values (NH was dominant in the soil inorganic-N fraction). We suggest that negative values might be caused by isotope fractionation during soil NH uptake and mycorrhizal N transfer, and by direct uptake of atmospheric NH/NH. The variation in foliar δN among species (by about 6 ‰) was smaller than in many N-limited ecosystems, which is typically about or over 10 ‰. The primary forest had a larger N capital in plants than the secondary forests. Foliar δN and the enrichment factor (foliar δN minus soil δN) were higher in the primary forest than in the secondary forests, albeit differences were small, while there was no consistent pattern in soil δN between primary and secondary forests. [ABSTRACT FROM AUTHOR]

Unfortunately, the 9th author family name was incorrectly published in the original publication. The complete correct name should read as follows. [ABSTRACT FROM AUTHOR]