Under natural conditions, plants constantly encounter various biotic and abiotic factors, which can potentially restrict plant growth and development and even limit crop productivity.  Among various abiotic factors affecting plant photosynthesis, light serves as an important factor that drives carbon metabolism in plants and supports life on earth.  The two components of light (light quality and light intensity) greatly affect plant photosynthesis and other plant’s morphological, physiological and biochemical parameters.  The response of plants to different spectral radiations and intensities differs in various species and also depends on growing conditions.  To date, much research has been conducted regarding how different spectral radiations of varying intensity can affect plant growth and development.  This review is an effort to briefly summarize the available information on the effects of light components on various plant parameters such as stem and leaf morphology and anatomy, stomatal development, photosynthetic apparatus, pigment composition, reactive oxygen species (ROS) production, antioxidants, and hormone production.

 

The impact of increased shading stress on agronomic traits, photosynthetic performance and antioxidants activities in leaves of two soybeans cultivars (D16 and E93) was studied.  Soybean seedlings were grown in pots and exposed to no shade (S0), slight shade (S1), moderate shade (S2), and heavy shade (S3).  Our findings showed that under the S3 in both cultivars, leaf fresh weight (LFW), specific leaf area (SLA) and leaf thickness decreased significantly, accompanied by a reduction in photochemical parameters including the maximum quantum yield (F_v/F_m) and electron transport rate (ETR).  Furthermore, compared to S0, S1 significantly increased the ETR, sucrose content and the activity of catalase (CAT) in both D16 and E93 cultivars while S2 and S3 decreased the activity.  However, under all treatments of shading stress, the antioxidant activities of superoxide dismutase (SOD) and peroxidase (POD) were lowered in both cultivars.  Such morphological and physiological plasticity to adapt S1 compensates for the decrease in biomass and leads to seed weight compared to that obtained with an amount of normal light.  Through configuring the space in the intercropping systems, S1 could be helpful for optimum growth and yield.  Redesigning photosynthesis through S1 for the intercropping systems could be a smart approach.

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.

 

Lodging is the most important constraint for soybean growth at seedling stage in maize-soybean relay strip intercropping system.  In the field experiments, three soybean cultivars Nandou 032-4 (shade susceptible cultivar; B1), Jiuyuehuang (moderately shade tolerant cultivar; B2), and Nandou 12 (shade tolerant cultivar; B3) were used to evaluate the relationship between stem stress and lignin metabolism in the stem of soybean.  Results showed that the intercropped soybean was in variable light condition throughout the day time and co-growth stage with maize.  The xylem area and cross section ratio played a main role to form the stem stress.  The B3 both in intercropping and monocropping expressed a high stem stress with higher xylem area, lignin content, and activity of enzymes (phenylalanine ammonia-lyase (PAL), 4-coumarate: CoA ligase (4CL), cinnamyl alcohol dehydrogenase (CAD), and peroxidase (POD)) than those of B1 and B2.  Among the soybean cultivars and planting pattern, lignin content was positively correlated with stem stress.  However, a negative correlation was found between lignin content and actual rate of lodging.  In conclusion, the shade tolerant soybean cultivar had larger xylem area, higher lignin content and activities of CAD, 4CL, PAL, and POD than other soybean cultivars in intercropping.  The lodging in maize-soybean intercropping can be minimized by planting shade tolerant and lodging resistant cultivar of soybean.  The lignin content in stem could be a useful indicator for the evaluation of lodging resistance of soybean in intercropping and activities of enzymes were the key factors that influence the lignin biosynthesis.

To clarify how shade stress affects lignin biosynthesis in soybean stem, two varieties, Nandou 12 (shade tolerant) and Nan 032-4 (shade susceptible) grew under normal light and shade conditions (the photosynthetically active radiation and the ratio of red:far-red were lower than normal light condition).  Lignin accumulation, transcripts of genes involved in lignin biosynthesis, and intermediates content of lignin biosynthesis were analyzed.  Both soybean varieties suffered shade stress had increased plant heights and internode lengths, and reduced stem diameters and lignin accumulation in stems.  The expression levels of lignin-related genes were significantly influenced by shade stress, with interactions between the light environment and variety.  The gene of 3-hydroxylase (C3H), cinnamoyl-CoA reductase (CCR), caffeoyl-CoA O-methyltransferase (CCoAOMT), and peroxidase (POD) attributed to lignin biosynthesis under shade stress, and the down-regulation of these genes resulted in lower caffeic, sinapic, and ferulic acid levels, which caused a further decrease in lignin biosynthesis.  Under shade stress, the shade tolerant soybean variety (Nandou 12) showed stiffer stems, higher lignin content, and greater gene expression level and higher metabolite contents than shade susceptible one.  So these characteristics could be used for screening the shade-tolerant soybean for intercropping.