Maize-soybean relay intercropping is an effective approach to improve the crop yield and nutrient use efficiency, which is widely practiced by farmers in southwest of China.  To elucidate the characteristics of different planting patterns on crop nutrient uptake, soil chemical properties, and soil bacteria community in maize-soybean relay intercropping systems, we conducted a field experiment in 2015–2016 with single factor treatments, including monoculture maize (MM), monoculture soybean (MS), maize-soybean relay intercropping (IMS), and fallow (CK).  The results showed that the N uptake of maize grain increased in IMS compared with MM.  Compared with MS, the yield and uptake of N, P, and K of soybean grain were increased by 25.5, 24.4, 9.6, and 22.4% in IMS, respectively, while the N and K uptakes in soybean straw were decreased in IMS.  The soil total nitrogen, available phosphorus, and soil organic matter contents were significantly higher in IMS than those of the corresponding monocultures and CK.  Moreover, the soil protease, soil urease, and soil nitrate reductase activities in IMS were higher than those of the corresponding monocultures and CK.  The phyla Proteobacteria, Acidobacteria, Chloroflexi, and Actinobacteria dominated in all treatments.  Shannon’s index in IMS was higher than that of the corresponding monocultures and CK.  The phylum Proteobacteria proportion was positively correlated with maize soil organic matter and soybean soil total nitrogen content, respectively.  These results indicated that the belowground interactions increased the crop nutrient (N and P) uptake and soil bacterial community diversity, both of which contributed to improved soil nutrient management for legume-cereal relay intercropping systems.

Shades caused by neighboring tall plants in intercropping systems and weak sunlight are constraints in yield optimization.  Shade influences many aspects of plant growth and development, leading to weak stems and susceptibility to lodging.  The plant cell wall is composed of certain proteins that allow the walls to stretch out, a process called cell wall loosening.  Shade affects anatomical, morphological, and physiological traits of plants, thus reducing the physical strength of the stem in crops by changing the loosening of cell walls.  Flexibility of cells facilitates further modifications such as wall loosening.  In addition, shade stress causes increased internode length, and reduced xylem synthesis and photosynthesis.  In shaded plants, lignin deposition in vascular bundles and sclerenchyma cells of stems is decreased.  Lignin is a light sensitive phenolic compound and shading decreases the transcript abundance of several phenolic compound (flavone and lignin) related genes.  Shading significantly influences the metabolic activities of phenylalanine ammonia-lyase (PAL), peroxidase (POD), 4-coumarate: CoA ligase (4CL), and cinnamyl alcohol dehydrogenase (CAD) involved in lignin biosynthesis.  Furthermore, suppression of lignin biosynthesis activities by abiotic stresses causes abnormal phenotypes such as collapsed xylem, bent stems, and growth retardation.  In this review, the underlying mechanisms illustrate that under shading conditions reduced lignin content results in slender, weak, and unstable stems.  The objective of this review is to elaborate lignin biosynthesis and its variability under stressful environmental conditions, especially in shade stress environments.  The effects of shade on stem lignin metabolism are discussed on the morphogenetic, physiological, and proteomic levels.