Photosynthesis is one of the principle biochemical processes underpinning plant growth and development. Because of its basic nature, it is intimately involved with reproductive growth and determining crop yields.

Photosynthesis of a crop canopy can be broken down into three components; leaf area development, photosynthetic rate per leaf area, and partitioning of assimilates between vegetative and reproductive growth, and/or source-to-sink relationships.

The leaf surface area intercepts the solar radiation and allows for the photosynthetic conversion of that radiant energy into chemical energy.

This production of chemical energy and the subsequent utilization of that chemical energy used to fix CO2 into photosynthetic carbon assimilates constitutes the source side of yield development.

The fruiting buds, flowers, and fruit development constitute the reproductive sink side of the yield equation, although other vegetative growing points can operate as secondary sinks.

All life on earth depends on plants. Plants are autotrophic, meaning they ^can^convert^simple^molecules^like^CO2 ^from^the^atmosphere^andminerals from the soil into the complex carbohydrates, proteins, and fats, forming the basis of living organisms.

The most important set of chemical reactions in plants harness the energy of sunlight in the process of photosynthesis which generates sugar, oxygen, and a molecule called ATP.

ATP is energy in its simplest form and powers the chemical reactions that support life in both plant and animal cells.

The Basics of Photosynthesis

Photosynthesis, Plant Growth and Partitioning of Assimilate

Photosynthesis literally means “to put together with light”. All of the reactions of photosynthesis happen inside chloroplasts. Chloroplasts are small organelles that are green because they contain chlorophyll.

Mesophyll cells in the leaves and stem contain many chloroplasts, each having a highly ordered array of membranes arranged in stacks.

These stacks are called thylakoid membranes and are solar panels with a large surface area that organize chlorophyll and pigments called carotenoids that can collectively absorb and utilize light energy.

Photosynthesis has two distinct parts:

Light reactions: The light absorption part of photosynthesis is referred to as the light reactions.

It relies on energy from the sun, so it occurs only during the day in the thylakoid membrane of the chloroplasts.

In the light reaction, water is split and oxygen released, but more importantly, it provides the chemical energy to fix CO2 into carbohydrate.

The carbon reactions: Carbon or dark reactions occur in the matrix of the chloroplast called the stroma and uses protein types called enzymes. The most important enzyme in this process is called rubisco.

Rubisco is the most abundant protein in plants and therefore, the major consumer of nitrogen.

This is why when plants are deficient in nitrogen, they are not productive and turn yellow because they stop photosynthesizing.

The dark reaction creates three carbon sugar products that leave the chloroplast for use in respiration and sucrose (common table sugar) synthesis.

The sucrose is converted to starch or shipped out to other parts of the plant for storage or growth through the phloem.

Factors Affecting Photosynthesis

The following points highlight the 14 main factors affecting photosynthesis. They are: 1. Temperature 2. Carbon Dioxide Concentrations 3. Light 4. Intensity 5. Quality 6. Duration 7. Oxygen 8. Water 9. Mineral Elements 10. Air Pollutants 11. Chemical Compounds

12. Chlorophyll Contents 13. Protoplasmic Factor 14. Accumulation of Carbohydrates.

Temperature: When CO2, light and other factors are not limiting, the rate of photosynthesis increases with a rise in temperature, over a range from 6°C to about 37°C.

Above this temperature, there is an abrupt fall in the rate and the tissue dies at 43°C. High temperatures cause the inactivation of enzymes and therefore affect the enzymatically controlled ‘dark’ reactions of photosynthesis.

The optimum temperature for the maximum falls between 20- 30°C. Above 25-30°C the maximum rate is not maintained as the time factor begins to operate and the optimum temperature is reduced from 37°C to 30°C.

Given other factors are limiting, the rate of photosynthesis follows Vant Hoffs rule between 6°C-30°C to 35°C i.e., it doubles with each increase of 10°C.

The reason being that all the reactions of the Calvin cycle are temperature ^{dependent and the rate of diffusion of CO}2 ^{to the chloroplasts is}accelerated by high temperature.

Carbon Dioxide Concentrations: Nearly 0.032% by volume of carbon dioxide is present in the atmosphere and at this low level it acts as a limiting factor.

Under laboratory conditions when light ^and^temperature^are^not^limiting^factors,^increaseⁱⁿ^CO2 concentration in the atmosphere from 0.03% to 0.3-1% raises rate of photosynthesis.

With the further increase in the concentration of CO2 progressively the rate of carbon assimilation increases slightly and then it becomes independent of CO2 concentration.

Thereafter, it remains constant over a wide range of CO2 concentrations. Plants vary in their ability to utilize high concentrations of CO2.

In tomatoes, high concentration of CO2, above the physiological range, exerts harmful influence causing leaf senescence. During the early period of the earth, the concentration of CO2 in the atmosphere was as high as 20%.

Light: The photosynthetically active region of the spectrum of light is at wavelengths from 400-700 nm. Green light (550 nm) plays an important role in photosynthesis.

Light supplies energy for the process. Light varies in intensity, quality and duration. A brief account on these three aspects is given as follows:

Intensity: When CO2 and temperature are not limiting and light intensities are low, the rate of photosynthesis increases with an increase in its intensity.

At a point saturation may be reached, when further increase in light intensity fails to induce increase in photosynthesis.

Optimum or saturation intensities may vary with different plant species e.g., C4 and C3. C3 plants become saturated at levels considerably lower than full sunlight but C4 plants are usually not saturated at full sunlight.

When the intensity of light falling on a photosynthesizing organ is increased beyond a certain point, the cells of that organ become vulnerable to chlorophyll catalyzed photo-oxidations.

Consequently, these organs begin to consume O2 instead of CO2 and CO2 is released. Photo-oxidation is maximal when O2 is present or carotenoids are absent or CO2 concentration is low.

Quality: The action spectrum for photosynthesis in leaves shows two major peaks, one in the red and the other one in the blue. In these regions, chlorophylls absorb maximal light.

Most effective wavelengths differ with different plants. It is of interest to note that plants show high photosynthesis in the blue and red light while red algae do so in green light and brown algae in blue light. The blue-green algae have action spectrum peak in yellow or orange light.

Duration: In general, a plant will accomplish more photosynthesis when exposed to long periods of light. It has also been found that uninterrupted and continuous photosynthesis for relatively long periods of time, may be sustained without any visible damage to the plant.

We would also do well to bear in mind that if we remove the source of light, the rate of CO2 fixation falls to zero immediately.

Clearly, no species has evolved and/or has developed a storage battery in its leaves whereby the immediate products of the photochemical reactions can be retained in significant amounts to be utilized for the fixation of CO2 later on.

Oxygen: Oxygen has been shown to inhibit photosynthesis in C3 plants while C4 plants show little effect. It is suggested that C4 plants have photorespiration and high O2 stimulates it.

The rate of photosynthesis increases by 30-50% when the concentration of oxygen in air is reduced from 20% to 0.5% and CO2, light and temperature are not the limiting factors.

Oxygen is inhibitory to photosynthesis because it would favour a more rapid respiratory rate utilizing common intermediates, thus reducing photosynthesis.

Secondly, oxygen may compete with CO2 and hydrogen becomes reduced in place of CO2. Thirdly, O2 destroys the excited (triplet) state of chlorophyll and thus inhibits photosynthesis.

It may be stated that direct effect of O2 on photosynthesis remains to be understood.

Water: Water is an essential raw material in carbon assimilation. Less than 1% of the water absorbed by a plant is used in photosynthesis.

The decrease in water contents of the soil from field capacity to the permanent wilting point results in the decreased photosynthesis.

The inhibitory effect is primarily attributed to increased dehydration of protoplasm and also stomatal closure.

The removal of water from the protoplasm also affects its colloidal state, impairs enzymatic efficiency, inhibits vital processes like respiration, photosynthesis etc. Dehydration may even damage the micro-molecular structure of the chloroplasts.

It is also assumed that primary factor of dehydration in retarding photosynthesis is due to stomatal closure which reduces CO2 absorption.

Water deficiency may cause drying of the cell walls of mesophyll cells, reducing their permeability to CO2. Water deficiency may accumulate sugars and thus increase respiration and decrease photosynthesis.

Mineral Elements: As discussed earlier, several minerals are essential for plant growth. These include Mg, Fe, Cu, CI, Mn, P and are closely associated with reactions of photosynthesis.

Air Pollutants: Gaseous and metallic pollutants decrease ^{photosynthetic}^activity.^These^include^ozone,^SO2^,^oxidants,hydrogen fluorides, etc.

Chemical Compounds: Compounds like HCN, H2S, etc. when present even in small quantities, depress the rate of photosynthesis by inhibiting enzymes.

In addition, chloroform, ether etc., also stop photosynthesis and the effect is reversible at low concentrations. However, at high concentrations the cells die.

Chlorophyll Contents: The rate of photosynthesis in two varieties of barley having normal green leaves and yellow leaves was studied. CO2, light and temperature were not limiting factors.

The rate of assimilation per unit area of leaf surface in the two varieties was the same even though the green-leaved variety contained ten times more chlorophyll than the yellow one.

Clearly, the chlorophyll in the green leaves is surplus. Leaves having high chlorophyll content do not photosynthesize rapidly since they lack the enzymes or co-enzymes to use the products of the light reactions to reduce available CO2.

Protoplasmic Factor: Besides chlorophyll certain protoplasmic factors also influence the rate of photosynthesis. They affect the dark reactions. It has been shown that these factors are absent in the young stage and develop as the seedling becomes old.

That these protoplasmic factors appear to be enzymatic is indicated by the fact that the capacity for photosynthesis is lost at temperatures above 30°C or at strong light intensities in many plants even though cells are green and living.

Accumulation of Carbohydrates: Accumulation of photosynthate in the plant cells, if not translocated, slows down and finally stops the process. The accumulated products increase the rate of respiration.

Sugar is also converted into starch and the accumulation of starch in chloroplasts reduces their effective surfaces and the process slows down.

Photosynthetic Variations of the Carbon Reactions (C3 & C4 plants)

Plants are classified based on how they complete photosynthesis. The dark reactions described above are found in more than 95 percent of the plants on Earth.

They are called C3 plants because the first organic molecule that CO2 is incorporated into is a three-carbon molecule.

Two variations of the carbon reactions have evolved in angiosperm plants as ways to get around the problem of photorespiration.

They are C4 photosynthesis. Plants with C4 photosynthesis include corn, buffalo grass, and many weedy grasses including crabgrass.

It is called C4 photosynthesis because the first organic molecule that CO2 is incorporated into is a four-carbon malate molecule.

These C4 plants minimize photo-respiration and water loss through a specialized cellular architecture in the leaves: light reactions occur in one cell type and the carbon reactions occur in cells called bundle sheath cells not in direct contact with the air.

Plants with C4 photo-synthesis are able to achieve high rates of photosynthesis with their stomata only slightly open which minimizes water loss.

They often look better than C3 plants during hot dry conditions because they are able to protect the plant from high water loss by closing their stomata.

Plants with crassulacean acid metabolism (CAM) photosynthesis, such as cacti, all succulents, and purslane, minimize photorespiration and water loss by keeping their stomata completely closed during the day and so no water is lost.

This also means they cannot take in CO2 during the day. To get around this, CAM plants take in CO2 through the stomata at night and store it as a molecule called malate.

Then during the day, with the sun shining and the stomata closed, rubisco coverts the malate to useful carbohydrate.

Although this enables CAM plants to grow in extreme heat and be extremely water efficient, they have low photosynthetic productivity they grow slowly.

Plant Growth and Partitioning of Assimilate

All the developmental processes occurring in plants involve growth. The growth of plants involves various changes such as the addition of new cells through cell division, an increase in its size and weight, and an irreversible increase in the volume.

Therefore, we can define the term growth as a permanent and irreversible change in the size of a cell, organ or whole organism usually accompanied by an increase in its dry weight.

Partitioning of assimilated carbon among sink organs is a critical factor that controls the rate and pattern of plant growth.

During the sequence of development, different plant parts exert varying demands on the available nutrient and assimilate resources of the whole plant system.

Classical growth analysis has provided a background of quantitative information on the distribution of dry matter in different organs throughout development.

Photosynthate partitioning is the deferential distribution of photosynthates to plant tissues. A photosynthate is the resulting product of photosynthesis. These products are generally sugars.

These sugars that are created from photosynthesis are broken down to create energy for use by the plant. Sugar and other compounds move via the phloem to tissues that have an energy demand.

These areas of demand are called sinks. While areas with an excess of sugars and a low energy demand are called sources.

Many times sinks are the actively growing tissues of the plant while the sources are where sugars are produced by photosynthesis the leaves of plants.

Sugars are actively loaded into the phloem and moved by a positive pressure flow created by solute concentrations and turgor pressure between xylem and phloem vessel elements (specialized plant cells).

This movement of sugars is referred to as translocation. When sugars arrive at the sink they are unloaded for storage or broken down and or metabolized.

The partitioning of these sugars depends on multiple factors such as the vascular connections that exist, the location of the sink to source, the developmental stage, and the strength of that sink.

Vascular connections exist between sources and sinks and those that are the most direct have been shown to receive more photosynthates than those that must travel through extensive connections. This also goes for proximity those closer to the source are easier to translocate sugars to.

Developmental stage plays a large role in partitioning, organs that are young such as meristerm and new leaves have a higher demand, as well as those that are entering reproductive maturity – creating fruits, flowers, and seeds. Many of these developing organs have a higher sink strength.

Those with higher sink strengths receive more photosynthates than lower strength sinks. Sinks compete to receive these compounds and combination of factors playing in determining how much and how fast sinks receives photosynthates to grow and complete physiological activities.

In summary, partitioning of assimilates between vegetative and reproductive growth, or source-to-sink relationships is dependent of demand.

Assimilates are made possible through the simple but complex process in plants called photosynthesis. Virtually in life, both animal and plant depend on photosynthesis.

Photosynthesis is one of the principle biochemical processes supporting plant growth and development.

This is the most important set of chemical reactions in plants where the energy of sunlight is harnessed to generate sugar, oxygen, and a molecule called ATP.

Some plants are classified as either C3 or C4 based on the number of carbon molecules involved in the process. These sugars that are created from photosynthesis are broken down to create energy for use by the plant.

Sugar and other compounds move via the phloem to tissues that have an energy demand. These areas of demand are called sinks.

Sinks compete to receive these compounds and combination of factors playing in determining how much and how fast sinks receives photosynthates to grow and complete physiological activities.