What is “Climate”?

“Climate” is defined as the weather conditions prevailing in an area in general or over a long period of time which includes components such as temperature, atmospheric pressure, wind, solar radiation, humidity and precipitation. These factors play a major role in determining the type of vegetation that can thrive and survive in that particular area.

What are “crops”?

A “crop” is described as a cultivated plant, grown in large scale commercially. As harvest it may give a type of cereal, fruit or vegetable. Humans worldwide depend on the yields of these crops, for they serve as their staple food and satiate their hunger. Few examples for the major crop plants in the world are maize- Zea mays (also known as ‘corn’), wheat – Triticum aestivum, Rice – Oryza sativa, Cooking banana – Musa acuminata and Sorghum – Sorghum bicolor.

Where “C3” and “C4” comes in…

Crop plants can be categorized into two groups based on their carbon fixation pathway in photosynthesis. A plant that utilizes the C3 carbon fixation pathway as the sole mechanism to convert CO₂ into an organic compound, yielding a three-carbon compound, Phospho Glyceric Acid (PGA; also called 3-phospho glycerate and Glycerate-3-phosphate), as the first stable product in the Calvin cycle are known as “C3 plants”. “C4 plants” follow the C4 carbon fixation metabolic pathway where a four-carbon compound, Oxalo Acetic acid (OAA) is produced as the first stable product of the Calvin cycle. This is an alternative pathway that has evolved in plants to reduce negative impacts due to photorespiration.

Examples for C3 crop plants are rice, wheat and potatoes whereas corn and sugarcane are C4 plants. C3 plants are better adapted to cool, wet environments while C4 plants are better adapted to hot, sunny environments. Changes in climate can affect the yield of these plants, for the process of photosynthesis, in relation to both C3 and C4 pathways, is affected depending on the temperature, humidity and concentration of gases such as CO₂ and O₂ either adversely or in a positive manner.

How climate change intervenes..

In hot, dry climatic conditions the photosynthetic efficiency of C3 crop plants decrease because of a process that causes wasteful loss of carbon, known as photorespiration. Photo-respiration occurs when the environmental temperature is very high. In order to conserve water, the plants close their stomatal pores. This causes the concentration of O₂ to increase within the leaves. At higher temperatures the affinity of RuBISCO enzyme, (which normally binds to CO₂ and takes it along the Carbon fixation pathway) to O₂ is higher. Hence it binds with O₂ leading to a couple of carbon containing products; one  is PGA- the usual product and phospho-glycolate, which is a 2-Carbon compound that cannot enter the Calvin cycle. Although plants have a mechanism to partially retrieve carbon lost by this process, only three-fourths could be retrieved. Therefore, the amount of Carbon fixed is reduced, resulting in a lesser amount of stored food within the plant. Thereby the yield of the crop will reduce significantly in hot seasons.

Is it always a negative intervention?

No.

It certainly does have a negative impact on C3 plants as described above, but not when it comes to C4 plants! Rise in temperature, (especially in the temperate zone) has a different effect on them.

The C4 pathway is better adapted to function at very low CO₂ concentrations. The efficiency of RuBISCO enzyme in capturing CO₂ is increased via a special leaf anatomy, whilst keeping their stomata closed and thereby preventing damage to the plant by dehydration. In crops like corn, sugarcane, an alternate enzyme called PEP carboxylase is used for the first step of carbon fixation. It has no oxygenase activity and has a much higher affinity for CO₂. Leaves of these plants consist of two types of photosynthetic cells: mesophyll cells on the exterior of the leaf, near stomata and bundle sheath cells in the interior of the leaf, away from stomata).  RuBISCO is present in bundle sheath cells, but not in mesophyll cells. This separation minimizes photorespiration and thereby a good harvest can be obtained even in hot climatic conditions.

How human activities threaten the crops

Climate is a phenomenon, best controlled by nature, for it has a perfect balance of its own. This balance has been disrupted by humans. Owing to human activities leading to increased CO₂ emission and other gases which deplete the ozone layer, climatic changes have been brought about. This has affected the rate of photosynthesis and survival of crop plants, according to the results of numerous studies that have been carried out.

Impacts

Increased carbon dioxide concentration, rise in global temperature and drought stress are key factors causing climatic change. Water related climatic factors and water retention capacity of the soil o n which these crops grow are altered by the above factors. Hence they collectively impose a significant impact on crop production, especially in semi-arid regions.

Increased CO₂ emission, through vehicle exhaust, industries, deforestation etc., has resulted in a rise of temperature globally. This has an adverse effect on C3 crops such as paddy, wheat and potatoes, for it leads to photorespiration.

Temperature rise has also posed a threat upon the water availability on earth. Lack of water affects the growth of crops. A good example is paddy. Paddy plant cultivation, which requires an ample supply of water would be severely affected.

Rise in global temperature has led to changes in rainfall patterns. Harvest of these plants decreases, following prolonged dry seasons whereas ceaseless rain could destroy crops entirely, if it is during a critical period in their growth.

Most of the rice is currently cultivated in regions where temperatures are above the optimum temperature for its growth (28/22 °C). Any further increase in temperature or exposure to drought periods during sensitive stages of plant growth and development may reduce rice yields drastically. In the tropical zone, high temperature is already a major environmental stress limiting rice productivity, causing reductions in grain weight and quality.

Heavy rainfall or unwelcome rainy seasons meanwhile interrupt the growth and development of crop plants mainly due to two reasons. One is nutrient leaching. Heavy rain makes the nutrients come out of the soil and flow away from the fields, to accumulate in water bodies. As a result, the plants face nutrient deficiencies and show stunted growth. The other reason is waterlogging in soil.  Low levels of oxygen in the root zone trigger the adverse effects of waterlogging on plant growth. It mostly affects germinating seeds and young seedlings. Established plants are most affected when they are growing rapidly. Therefore farmers tend to put off planting till the rain ceases and this leads to an overall delay in getting harvest.

When considering the effect of temperature on C4 plants, it is not as worse as for C3 crops. However, temperatures exceeding 37◦C causes poor emergence of maize seedlings, due to high soil temperatures, resulting a major limitation of crop potential in the lowland tropics. Only some seeds show resistance to high temperatures and complete germination successfully. Wheat, at high temperatures, show a low rate of photosynthesis and the rate of respiration continues to increase. Basically, the plants begin to use more sugars than they can produce by photosynthesis, leading to a drastic decrease in yield.

In addition to the effect of CO₂, exposure of crop plants to harmful radiation from the sun due to the depletion of the ozone layer has impacted the quality and quantity of yield. This radiation is capable of altering the plant productivity due to increased UV-B radiation and tropospheric ozone formation. It can cause diseases in plants and ultimately result in the loss of crop.

Thus, the impact of climate change upon plant life is quite significant and this adversely affects human life via the damage caused to crop plants. Irony lies where we ourselves have become a threat to us! It is vital to take precautionary measures for food security and environmental protection without waiting for long. It all lies upon our hands!

References:

https://www.ars.usda.gov/southeast-area/raleigh-nc/plant-science-research/docs/climate-changeair-quality-laboratory/ozone-effects-on-plants/
https://www.researchgate.net/publication/307879151_Climate_Variability_Impact_on_Wheat_Production_in_Europe_Adaptation_and_Mitigation_Strategies
https://www.sciencedirect.com
https://www.southernstates.com/Blog/index.aspx?topic=How-Can-Heavy-Rainfall-Impact-Crop-Production%3F