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.

 

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.