A literature summary about photosynthesis

1. Seminar on Plant Development Biology
Literature Summary
on
Environmental factors affecting photosynthesis
and future prospect for enhancing crop yield
Submitted by:
Ripon Kumar Sikder, PhD Fellow
Institute of Cotton Research, CAAS
ID No. 2017Y90100144
Graduate School of Chinese Academy of Agricultural Sciences
12, Zhongguancun Nadajie, Haidian District, Beijing 100081, P. R. China

2. 1
Abstract
The photosynthetic productivity of plants, especially those growing under natural conditions,
is strongly influenced by factors of the environment. For example, photosynthetic tissues may
experience wide variations in temperature which affect the rate and integrity of many
component reactions of photosynthesis. The availability of essential resources for
photosynthesis (light, water, CO2, and nutrients) also varies with time and habitat.
Environmental stresses such as those imposed by drought, salinity, nutrient deficiency,
pollutants, or excessively high or low temperatures have direct effects upon photosynthetic
capacity. The complex interactions between plants and their environments are approached
primarily from the viewpoint of identifying the mechanisms that underlie the plant responses.
In addition, here is also discussed the progress in producing transgenic lines of different C3
crops with enhanced photosynthetic performance, which was reached by either the
overexpression of C3 enzymes or transcription factors or the incorporation of genes encoding
C4 enzymes into C3 plants.
Introduction
Although the plant growth is controlled by a multitude of physiological, biochemical, and
molecular processes, photosynthesis is a key phenomenon, which contributes substantially to
the plant growth and development. The chemical energy expended in a number of metabolic
processes is, in fact, derived from the process of photosynthesis, which is capable of converting
light energy into a usable chemical form of energy. This key process occurs in all green plants,
whether lower or higher, occurring in oceans or on land as well as in photosynthetic bacteria
(Taiz and Zeiger 2010, Pan et al. 2012). However, stressful environments, including drought,
salinity, and unfavourable temperatures, considerably hamper the process of photosynthesis in
most plants by altering the ultrastructure of the organelles and concentration of various
pigments and metabolites including enzymes involved in this process as well as stomatal
regulation.
In the process of photosynthesis (Fig. 1), two key events occur mandatorily; light reactions, in
which light energy is converted into ATP and NADPH and oxygen is released, and dark
reactions, in which CO2 is fixed into carbohydrates by utilizing the products of light reactions,
ATP and NADPH (Lawlor 2001, Taiz and Zeiger 2010, Dulai et al. 2011).
There are two main pathways of CO2 fixation, C3 and C4. Plants have been categorized into C3,
C4,or C3-C4 intermediate plants depending on the spatial distribution of these two pathways
within leaf tissues or as crassulacean acid metabolism (CAM) plants with a temporal

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distribution (Doubnerová and Ryšlavá 2011, Freschi and Mercier 2012). Plants possessing the
different types of photosynthetic mechanisms are adapted to specific climatic zones.
Fig. 1. Light and dark reactions of photosynthesis
For example, C3 plants, representing over 95% of the Earth plant species, thrive well in cool
and wet climates, with usually low light intensity. In contrast, C4 plants occur in hot and dry
climatic conditions with usually high light intensity.
Generally, C4 and CAM plants are the best adapted to arid environments, because they have
higher water-use efficiency (WUE) than that of C3 plants. C4 plants have higher photosynthetic
efficiency than C3 plants, namely in arid, hot, and under high-light conditions, because they
possess an additional carbon fixation pathway and characteristic anatomy to limit
photorespiration. Furthermore, CAM plants can effectively save metabolic energy and water
during harsh environmental conditions by closing their stomata during the day (Taiz and Zeiger
2010).
In this paper, the role of abiotic stresses such as light, temperature, drought, humidity, soil
quality, flooding, salinity, wind, insect, diseases emphasized in various aspects of
photosynthesis, mainly of agricultural plants.
Description
In nature, plants grow under fluctuating environmental conditions, requiring frequent
acclimation of photosynthesis and other metabolic processes for optimal growth. Chloroplasts
are sensors of the environmental changes, fulfilling key roles in the regulation of plant growth
and development in relation to environmental cues. Photosynthesis varies widely in response
to natural fluctuations in available sunlight over time and among different environments.

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Photosynthesis largely depends on different environmental factors such as light, temperature,
drought, humidity, soil quality, flooding, salinity, wind, insect, diseases etc.
Proper responses to external biotic and abiotic stimuli, together with coordinated adjustments
in photosynthesis and growth, have provided significant evolutionary competitiveness to plants
and support their survival in ever-changing environments.
Light and rate of photosynthesis
a. Light Intensity:
Fig.2. Relation between light intensity and photosynthesis
At low light intensities, as light intensity increases, the rate of the light-dependent reaction, and
therefore photosynthesis generally, increases proportionately (straight line relationship). The
more photons of light that fall on a leaf, the greater the number of chlorophyll molecules that
are ionized and the more ATP and NADPH are generated. Light dependent reactions use light
energy and so are not affected by changes in temperature. As light intensity is increased further,
however, the rate of photosynthesis is eventually limited by some other factor. So the rate
plateaus. At very high light intensity, chlorophyll may be damaged and the rate drops steeply
(not shown in the graph).
Fig. 3. Absorbance spectrum of photosynthesis in a green plant
Reaction center chlorophylls absorb maximally at 680 and
700 nm. Longer wavelength light does not have sufficient
energy to drive photosynthesis in plants

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Chlorophyll a is used in both photosystems. The wavelength of light is also important. PSI
absorbs energy most efficiently at 700 nm and PSII at 680 nm. Light with a higher proportion
of energy concentrated in these wavelengths will produce a higher rate of photosynthesis.
b. Light Quality
The light consists of rays of different wavelengths. Only red and blue light are effective for
photosynthesis. Green light is reflected or transmitted. Therefore, it does not play role in
photosynthesis. Light of wavelength longer than 700nm run is not effective for photosynthesis
for green plants.
Fig. 4. Properties of different quality light and wavelength
Temperature and rate of photosynthesis
Although the light dependent reactions of photosynthesis are not affected by changes in
temperature, the light independent reactions of photosynthesis are dependent on temperature.
They are reactions catalyzed by enzymes. As the enzymes approach their optimum
temperatures the overall rate increases. It approximately doubles for every 10 °C increase in
temperature. Above the optimum temperature the rate begins to decrease, as enzymes are
denatured, until it stops.
Fig. 5. Relation between temperature and rate of photosynthesis

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Carbon dioxide and rate of photosynthesis
CO2-exchange characteristics have been regarded an important indicator of the growth of
plants, because of their direct link to net productivity (Ashraf 2004, Piao et al. 2008). An
increase in the carbon dioxide concentration increases the rate at which carbon is incorporated
into carbohydrate in the light-independent reaction, and so the rate of photosynthesis generally
increases until limited by another factor.
Fig. 6. Relation between CO2 concentration and rate of photosynthesis
As it is normally present in the atmosphere at very low concentrations (about 0.04%),
increasing carbon dioxide concentration causes a rapid rise in the rate of photosynthesis, which
eventually plateaus when the maximum rate of fixation is reached.
Fig. 7. Effects of CO2 levels on photosynthesis

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Water and rate of photosynthesis
Water effects the rate of photosynthesis because water is one of the reactants in the
photosynthesis reaction. If not enough water is being pulled out of the ground via the roots and
up the plant through the xylem, then the leaves and plant might become dehydrated. If this
happens, then the stoma on the leaves of the plant will close shut in order to conserve the water
in the plant, as water is constantly exiting the plant through the stoma. When the stoma of the
plant are shut, this also prevents CO2 in the air from entering the plant, and as a result, the rate
of photosynthesis plummets. Also, if there is too much water in the soil, the roots will become
rotten and die, causing the plant to die.
The availability of water heavily affects the rate of photosynthesis in plants that live in dry
areas such as desert plants and conifers. These plants have a waxy coating on their leaves that
reduces water loss so that more is available for the photosynthetic reaction to occur. An
example of such adaptations is the Joshua tree (picture in link) which has the ability to survive
with little water.
Humidity and rate of photosynthesis
The effect of humidity on the rate of photosynthesis in a plant is very similar to that of water.
If there is a lot of humidity in the air around the plant, less water from the plant evaporates.
This allows the plant to open its stoma wider because there is no risk of losing excessive
amounts of water. Because of this, the rate of photosynthesis increases as the humidity
increases. Another factor of the humidity in the air is that the ground can be moister, so the
plant's roots can extract more water from the ground.
It was hypothesized that since sub-stomatal carbon dioxide concentrations are often saturating
to photosynthesis at ambient external concentrations in C4 plants at high light, photosynthesis
might be insensitive to partial stomatal closure caused by large leaf-air water vapor pressure
difference. The response of stomatal conductance and photosynthesis at high irradiance to
vapor pressure difference was determined under uniform conditions in C4 plants grown under
controlled conditions, and outdoors. In several cases, photosynthesis was less sensitive to
stomatal closure than it would have been if photosynthesis had a linear response to sub-stomatal
carbon dioxide concentration.
Salt stress and photosynthesis rates
Different stressful environments have been reported to reduce the contents of photosynthetic
pigments. For example, salt stress can break down chlorophyll (Chl), the effect ascribed to
increased level of the toxic cation, Na+ (Pinheiro et al. 2008, Li et al. 2010, Yang et al. 2011).
Reduction in photosynthetic pigments, such as Chl a and b has been reported in some earlier
studies on different crops. The salt-induced alterations in a leaf Chl content could be due to

8. 7
impaired biosynthesis or accelerated pigment degradation. Although salt stress reduces the Chl
content, the extent of the reduction depends on salt tolerance of plant species. For example, it
is generally known that in salt tolerant species, Chl content increases, whereas it decreases in
salt-sensitive species under saline regimes (Hamada and El-Enany 1994, Khan et al. 2009,
Akram and Ashraf 2011).
Drought stress and photosynthesis rates
As salinity stress, drought stress causes not only a substantial damage to photosynthetic
pigments, but it also leads to deterioration of thylakoid membranes (Huseynova et al. 2009,
Anjum et al. 2011, Kannan and Kulandaivelu 2011). Thus, a reduction in photosynthetic
capacity in plants exposed to drought stress is expected. The decrease in Chl content is a
commonly observed phenomenon under drought stress (Bijanzadeh and Emam 2010,
Mafakheri et al. 2010, Din et al. 2011). It is generally known that under drought stress the
reduction of Chl b is greater than that of Chl a, thus, transforming the ratio in favor of Chl a
(Jaleel et al. 2009, Jain et al. 2010).
Effects on activities of key photosynthetic enzymes
One of the most prominent effects of various stresses is the stomata closure, which leads to a
lower concentration of intercellular CO2, which in turn causes deactivation of Rubisco as well
as other enzymes such as sucrose phosphate synthase (SPS) and nitrate reductase (Chaves et
al. 2009, Mumm et al. 2011). Increased levels of Na+ and Cl– (above 250 mM) in the leaf tissue
may substantially perturb the metabolic processes of photosynthesis (Tavakkoli et al. 2009,
Biswal et al. 2011). Furthermore, the salt-induced, osmotic effect can adversely affect the
activities of a number of stroma enzymes involved in CO2 reduction (Kaiser and Heber 1981,
Xue et al. 2008).
Improvement in photosynthetic capacity under stress by engineering photosynthesis-
related genes
Two approaches are currently used to improve the photosynthetic capacity in different plant
species. Firstly, an effort is devoted to developing transgenic C3 plants with over-expression of
Rubisco genes or other enzymes of the C3 pathway. Secondly, attempts are underway to transfer
the genes of a key C4 photosynthetic pathway to C3 plants. Considerable progress has been made
during the last two decades in engineering the photosynthetic genes using advanced protocols
of the recombinant DNA technology. Using the first strategy, Feng et al. (2007) produced a
transgenic line of japonica rice by overexpressing OsSbp cDNA from an indica rice cultivar.
This transgenic line showed beside an enhanced activity of the C3 enzyme sedoheptulose l,7-
bisphosphatase (SBPase, EC 3.1.3.37), which is involved in the regeneration of RuBP, also the
enhanced photosynthetic capacity as well as the increased tolerance to salt stress. They
suggested that the enhanced rate of photosynthesis under salt stress might have been due to the

9. 8
enhanced supply of RuBP to Rubisco by the activation of SBPase. In another study, Tanaka et
al. (2005) assessed the role of the overexpression of chloroplastic glutamine synthetase (a key
enzyme of photorespiratory pathway involved in the utilization of NH3 released as a result of
conversion of two molecules of glycine into serine in mitochondrion), in salt tolerance of rice
(Table 3). They showed that the overexpression of glutamine synthetase significantly reduced
the tissue Na+ content under saline regimes, suggesting the putative role of this enzyme in salt
tolerance of rice. The enhanced salt tolerance in this transgenic rice was suggested to be
associated with increased re-assimilation of NH3 in the photorespiratory process. Similarly to
this study, Kozaki and Takeba (1996) showed enhanced protection from photo inhibition of
transgenic tobacco plants overexpressing glutamine synthetase.
Transfer of C4 genes/traits to C3 plants
Several attempts have been made during the past three decades to improve capacity of C3 plants
by transferring C4 traits to C3 plants. Initially, classical hybridization of C4 plants with those of
C3 was done, but most of the resultant C3-C4 hybrids were infertile (Brown and Bouton 1993).
Fig. 8. Strategies to improve C4 photosynthesis
However, with the advent of recombinant DNA technology, it is possible now to transfer and
express substantially the enzymes of C4 pathways in appropriate places within the leaves of C3
plants (Miyao 2003, Begonia and Begonia 2007, Kajala et al. 2012). For example, Häusler et
al. (2002) reported that the first cDNAs of the C4-PEPC sequences for cloning were derived
from maize (Izui et al. 1986) and Flaveria trinervia (Poetsch et al. 1991). Later on, a tobacco
transgenic line was developed overexpressing maize PEPC gene under the control of its own
promoter (Hudspeth et al. 1992) or by using the constitutively expressing Cauliflower mosaic
virus (CaMV) 35S promoter (Benfey and Chua 1990, Kogami et al. 1994). However, in all
these transgenic lines, although the activity of C4-PEPC increased two-fold compared with that
in non-transformed plants, the transgenic and non-transformed lines did not differ significantly
in the net CO2 assimilation rate.

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In addition, although photosynthesis-related genes have been transferred from C4 to C3 plants
and their photosynthetic capacity has been examined under non stress conditions, no attempts
have been reported to test the effectiveness of this approach under stressful conditions.
Role of transcription factors in regulation of genes involved in photosynthesis
Transcription factors are found in all organisms, because they are essential for the regulation
of the gene expression. Different types of transcription factors exist and an organism with a
larger genome usually contains more transcription factors than one with a smaller genome.
Elucidation of the mechanism of the gene expression for a particular trait is a major focus of
molecular biologists to find out how different types of transcription factors are involved in the
gene expression. Recently, Saibo et al. (2009) have described the role of a number of
transcription factors involved directly or indirectly in the regulation of genes involved in
photosynthesis. For example, a transcription factor LONG HYPOCOTYL 5 (HY5), a bZIP-
type, was reported to be mainly involved in the regulation of CAB gene expression by light,
although it may also exhibit a significant role in abiotic stress tolerance (Maxwell et al. 2003,
Saibo et al. 2009). This transcription factor, despite controlling the expression of Chl a/b
binding protein 2 (CAB2) (Maxwell et al. 2003), regulates the expression of the gene for the
Rubisco small subunit (RbcS1A) (Chattopadhyay et al. 1998, Lee et al. 2007).
Conclusion and future prospects
It is now evident that stresses lead to the considerable reduction in photosynthetic performance
mediated through stress-induced stomatal or nonstomatal limitations (Athar and Ashraf 2005,
Rahnama et al. 2010, Taiz and Zeiger 2010). However, it is not easy to discriminate between
the effects of these limitations on overall photosynthetic capacity of a plant. Certainly, it
depends on the species to what extent the process of photosynthesis under different
environmental abiotic conditions are controlled by stomatal or nonstomatal factors.
Nevertheless, knowledge of the proportion of the stomatal and nonstomatal factors controlling
the process of photosynthesis is vital for appointing the future research on photosynthesis.
It is also evident that stressful factors depending on their intensity and duration can differently
down- or up regulate the genes involved in the mechanism of photosynthesis in plants. Thus it
could be useful to know expression patterns of such genes for understanding plant
photosynthetic or other metabolic responses to various stresses and to develop transgenic lines
of different crops with enhanced photosynthetic capacity under stressful conditions as well as
the crop yield improvement.

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References
Class Lectures of Professor Dr. Wenbin Zhou
Ashraf, M., & Harris, P. J. C. (2013). Photosynthesis under stressful environments : An
overview, 51(2), 163–190. https://doi.org/10.1007/s11099-013-0021-6
Horton, P. (2000). Prospects for crop improvement through the genetic manipulation of
photosynthesis : morphological and biochemical aspects of light capture, 51(February),
475–485.
http://www.rsc.org/Education/Teachers/Resources/cfb/Photosynthesis.htm
https://www.ncbi.nlm.nih.gov/books/NBK22345/
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https://www.sciencedirect.com/science/article/pii/B9780122943027500173
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