Plants ’ responses to drought and shade environments

Water and light are the most important environements for plants’ growth. These environemts are critical factors needed not only for the survival of plants but also their production. When plants are exposed to drought condition, they change in their anatomical, physiological and biochemical properties. Drought affects plants wildly from their cell structure to growth. It causes higher plastoglobuli, lower starch grain, distortion of thylakoids, disrupted grana and swelling of chloroplast. Plants grown under enhanced light, have increased palisade parenchyma, thicker leaf, higher biomass, increased photosynthesis, lower contents of chlorophyll, carotenoid and nitrogen.


INTRODUCTION
Plants undergo several environmental stresses such as abiotic (drought, salt, temperature, cold and light) and biotic (fungi, bacteria, insects and viruses).Among them, drought is widely known as the main factor that limits plants' growth, productivity and development (Reddy et al., 2004;Shao et al., 2008;Li et al., 2009).Recently, drought occured frequently all over the globe due to climate changes (Khaine and Woo, 2015).
Light and shade are very important environements for plants' growth in the ecosystem.These environmental factors affect plants' growth, morphology, physiology and biochemistry.Many research works have shown that light plays key role in changing the traits of plants (Lee et al., 2013;Ranade and García-Gil, 2013;Adam and Cavaleri, 2014;Kuehne et al., 2014;Lavinsky et al., 2014;Sun et al., 2014).Plants' morphological and physiological differences in growth reactions to shade or low light intensity are exceedingly important in all types of forests in the world.
This mini review covers a brief effect of light and soil moisture on plants.Especially, the morphological, physiological and biochemical changes caused by different light intensity and water contents in the soil are mentioned.

MORPHOLOGICAL, PHYSIOLOGICAL AND BIOCHEMICAL RESPONSES UNDER DROUGHT
When plants are exposed to drought condition, anatomical, physiological and biochemical changes occur in them.Drought affects plants' growth terribly (Kivimäenpää et Chartzoulakis et al., 2002;Zellnig et al., 2004;Villagra and Cavagnaro, 2006;Olmos et al., 2007).Also, it makes the root systems of plants develop more than the stem system in order to facilitate water absorption from soil.To avoid water loss and evaporation by expanded leaf area, plants decrease individual leaf area, total leaf area per plant and the number of leaves per plant.Eventually, reduction of leaf area by drought decreases plants' growth and photosynthesis (Reddy et al, 2003;Shao et al., 2008).
The limitation of photosynthesis under drought is due to stomatal limitation.To keep water evaporation, plants decrease transpiration rate through stomatal closure, and intercellular CO 2 concentration is reduced (Farquhar and Sharkey, 1982).Similar results were observed in Populus, Picea, Querqus seedlings (Lopez-Carbonell et al., 1994;Ren et al., 2007;Yang et al., 2007).
Plants exposed to drought might cause oxidative stresses by the formation of reactive oxygen species, but, it can overcome oxidative stresses by synthesizing antioxidants (carotenoid, ascorbate and glutathione) and producing antioxidant enzymes (APX, SOD, POD and GR) and osmoprotectants (Egert and Tevini, 2002).Proline is known as osmolyte and performs osmotic adjustment, which is an important mechanism under drought condition (Faës et al., 2015).In addition, it acts as free radical scavenger to protect the cells from reactive oxygen species.Proline accumulation was reported in wheat, Helianthus, Populus (Yang et al., 2007).As mentioned previously, plants might adapt to or avoid those changes caused by environmental influences (Givnish, 1988).

MORPHOLOGICAL, PHYSIOLOGICAL AND BIOCHEMICAL RESPONSES UNDER SHADE
Sunlights are indispensable to all living things.Especially, lights are one of the essential resources used for photosynthesis and plants' development (Zhong et al., 2014).It determines not only morphological traits such as seed germination, leaf arrangement and stem growth, but also physiological activities such as photosynthesis or rubisco activity (Lee et al, 2013;Cheung et al., 2014).The demand of lights might be different depending on species, environment and location of leaf.Some species can grow well under either high light or low light.
Plants, grown under enhanced light, have increased palisade parenchyma, thicker leaf, large leaf area, higher biomass, increased photosynthesis, lower contents of chlorophyll, carotenoid and nitrogen (Je et al., 2006;Yang et al., 2007;Huang et al., 2008;Volkova et al., 2010).However, the enhanced light might be unfavorable to growth of seedlings.Photoinhibition regarded as photosynthetic apparatus is damaged by stronger lights.It often occurs when plants accept excessive lights than required (Demming at al., 1987;Valladares and Pearcy, 1997;Niyogi, 1999;Kwon and Woo, 2015).

MORPHOLOGICAL, PHYSIOLOGICAL AND BIOCHEMICAL RESPONSES UNDER DROUGHT AND SHADE
Light and water are the main factors that determine plants' growth and distribution.Many other studies reported morphological, physiological and biochemical changes caused by the combination of drought and shade (Sack, 2004;Aranda et al., 2005;Yin et al., 2013).There are various opinions related to the effects caused by drought and shade.First, shading alleviates the drought condition on the seedlings by lowering leaf and air temperature (Sack, 2004;Dai et al., 2009).Second, another hypothesis is that shading aggravates growth of seedlings which are exposed to drought.Allocating to more shoot, leaf area than root reduces ability to capture water from soil and diffuse water loss due to expanded leaf area (Abrams and Kuniske, 1990;Valladares and Pearcy, 1997).

CONCLUSION
Reduction of irradiance that causes thinner leaf thickness is confirmed.Leaf thickness increases due to larger palisade and spongy parenchyma.Gas exchange (photosynthesis, transpiration rate, stomatal conductance and water use efficiency) increases in response to an increase in water contents.High light and water contents in the soil have severe impacts on the morphological, physiological and biochemical characteristics of several plants.Though only few stuies have reported the interaction between light and water, these environmental factors are considered so important.Probably, most plants might adapt to or avoid those changes caused by environmental influences.
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