Factors Essential for Photosynthesis | Crop Plants | Botany

Light, CO₂ and the proper temperatures, the factors directly affects the photosynthesis in crop plants. Water and mineral elements also influence photosynthesis.

Factor # 1. Light:

Light response curves for leaves are illustrated in Figures 1.19 and 1.20. If there is no light, there is dark respiration, which for a leaf is usually 5 to 10% of CO₂ uptake in bright light. As the light level gradually increases, photosynthesis increases to the light compensation level, which is the light level at which CO₂ uptake is equal to CO₂ evolution (the carbon exchange rate, or CER = 0).

If the light level continues to increase, there is less increase in CER for each unit increase in light level until the light saturation level is reached. Any increase in light level after this level will not significantly increase CER; therefore leaves are more efficient at utilizing light energy at low irradiance levels.

Species differ in their responses to light levels. Most C₄ species (Fig. 1.20, curve A) are able to increase photosynthesis even at light levels equal to full sunlight, whereas most C₃ species reach light saturation before full sunlight. Figure 1.20 illustrates that usually the lower the maximum CER, the lower the light level at which light saturation occurs.

It should be noted that even though C₄ species often do not become light saturated and do use- high light levels better than C₃ species, they use dimmer light more efficiently (CO₂ uptake per unit of light) than bright light. For example, at 50 and 10% of full sun the CER is approximately 72 and 17%, respectively, of that at full sun; the most efficient use of light by CER is always at the lowest light levels. Efficiency is the slope of the light response curve.

Factor # 2. Carbon Dioxide:

Concentration in the Atmosphere:

Carbon dioxide is a gaseous component of air. Dry air contains 78% nitrogen (N₂), 21% oxygen (O₂), 0.93% argon (Ar), 0.034% (340 ppm) CO₂, and traces of other gases. Although CO₂ is at a low concentration, 85 to 92% of a plant’s dry weight is derived from CO₂ uptake in photosynthesis.

Because of the burning of fossil fuels (which represent photosynthetic production millions of years ago) and the burning of forests, there has been an increasing CO₂ concentration in the atmosphere. The projections for fossil fuel (primarily coal) utilization indicate an even larger increase in this concentration in the future.

Since CO₂ causes a greenhouse effect by absorbing infrared bands of light, the higher concentration will cause the earth to retain more heat, which could increase mean global temperatures. Such an increase could influence the global weather pattern enough to change rainfall patterns and crop productive capabilities in many regions of the earth.

Under high-light conditions, most crop species show a linear response for leaf photosynthesis to CO₂ levels above the current atmospheric concentration of 340 ppm. Crop yields could be increased considerably in an atmosphere enriched with CO₂ up to 1500 ppm.

Although there is no current practical means of doing this under field conditions, CO₂ enrichment has shown great benefits in greenhouses, not only increasing dry matter yield but also hastening plant development. It would be interesting to know how much the increase in CO₂ concentration in the atmosphere in the past 100 years (from approximately 290 to 340 ppm) has increased crop yields and influenced crop maturation.

Leaf Resistances to CO₂ Assimilation:

Carbon dioxide gets to the chloroplast by diffusion from the air through the stomata to the cell and then to the chloroplast.

Impediments to CO₂ movement into and through the leaf do occur, and scientists have termed them resistances and quantified them.

r_CO2 = r_a + r_s + r_m …(1.1)

Where, r_CO2 — CO₂ exchange rate, r_a = laminar resistance; r_s — stomatal resistance; and r_m = mesophyll resistance. Laminar resistance (r_a) is the CO₂ concentration at the leaf surface (also called the boundary layer effect), the lower the concentration, the higher and the resistance. Since ambient CO₂ concen­tration is between 300 and 360 ppm, the factors that cause reduced concentra­tion would increase r_a.

In the field, turbulence is the major factor influencing r_a. If no air movement occurs, CO₂ uptake by the leaf causes a CO₂ diffusion gradient that reduces CO₂ concentration at the leaf surface. As wind turbu­lence increases, it will eventually reduce r_a to the minimum level at most leaf surfaces within a plant canopy.

Stomatal resistance (r_s) is the resistance of CO₂ diffusion from outside the leaf through the stomata. Crop leaves generally have the stomatal frequency necessary for efficient CO₂ diffusion. The major factor affecting r_s is the degree to which stomata are open.

To calculate r_s, crop physiologists measure the water loss from the leaf, which is a measure of stomatal impedance and diffusion. The r_s is easily measured assuming that the relative humidity inside the leaf remains near saturation and any water loss is due to stomatal opening and r_a.

Mesophyll resistance (r_m) is calculated as residual resistance to CO₂ uptake by the leaf:

r_m = r_CO2 – r_a – r_s

Mesophyll resistance is a measure of everything about the leaf that affects CO₂ uptake except for r_a and r_s. This is because anything that influences CO₂ fixation will affect CO₂ concentration at the chloroplast, which in turn in­fluences the total diffusion rate of CO₂ from air to chloroplast.

The resistance formula (1.1) is used by crop physiologists as a method to determine if CO₂ uptake by a crop plant is affected by resistance to CO₂ diffusion into the leaf (r_a and r_s) or by CO₂ fixation in the leaf (r_m).

Factor # 3. Temperature:

Photosynthesis must be separated into its component parts to establish its response to temperature. The light reaction, or photophosphoryla­tion, is independent of temperature in the temperature range in which plants grow. Carbon dioxide fixation is an enzymatically controlled reaction and increases at an increasing rate with increases in temperature until temperature reaches a level that favors enzymatic denaturization.

Respiration rates will continue to increase as temperature increases. Measurement of net CERs shows a minimal response of CER to temperature. Photorespiration also increases with temperature, since it is also enzymatically controlled, resulting in lower CER rates for C₃ species than for C₄ species at the higher tempera­tures of plant growth.

Factor # 4. Water:

Water is a substrate for photosynthesis, but only about 0.1% of the total water is used by the plant for photosynthesis. Transpiration accounts for 99% of the water used by plants; approximately 1% is used to hydrate the plant, maintain turgor pressure, and make growth possible. The primary in­fluence of water stress on CER is an increase in r, due to stomatal closure. If water stress becomes severe, r_m will also increase because of permanent dam­age to the photosynthetic apparatus.

Factor # 5. Leaf Age and Mineral Status:

Leaf age has an effect on photosynthesis- senes­cence causes a reduction in the process. The major factor that influences the rate of senescence is the mineral nutrient status of the leaf. Adequate mineral nutrient supplies allow both old and young leaves to meet their nutrient needs. However, limited nutrients are preferentially distributed to young leaves and reduce the photosynthetic rate of older leaves.

In maize, Peaslee and Moss (1966) measured lower photosynthetic rates for lower leaves. The lower rates were correlated with lower levels of potassium, phosphorus, magnesium, and nitrogen.

Apparently if these nu­trients are in short supply they are translocated from older to younger leaves, causing what appears to be more rapid ageing in lower leaves. Other nutrients that are less mobile in the plant (e.g., calcium and iron) can reduce photosyn­thesis in younger leaves, while photosynthesis increases in older leaves due to the steady increase in calcium and iron content over time.

Reduced nutrient levels influence photosynthesis primarily by influencing the photosynthetic apparatus. For example, chlorophyll contains both nitro­gen and magnesium; if they are limited in supply, chlorophyll may not form. Precursor molecules for chlorophyll synthesis include iron, and if it is not present, chlorophyll cannot be synthesized.