The Effect of Light Intensity and Carbon Dioxide Concentration on Plant Growth

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Plants in natural habitats are subject to progressive variations in light intensity, atmospheric carbon dioxide concentrations and temperature (Yamori et al. 2010). Plants make their own food by the process of photosynthesis. Photosynthesis is based on two reactions, the light reactions and the dark reactions. The light reactions make use of sunlight captured by photosystems in chloroplasts which result in the splitting of water into oxygen; the dark reactions make use of atmospheric carbon dioxide and converts it into carbohydrate in the stroma of the chloroplast. (Johnson,2016).

Photosynthesis is the basis of all life forms on Earth; providing constant supply of oxygen needed for survival and plays a significant role in fulfilling current energy needs for human beings in the form of fossilized photosynthetic fuels (Johnson,2016). Plants are of great significance to human life as they provide us with constant supply of oxygen, different medicines, essential nutrients and help maintain biodiversity (Pimental et al. 1997). If future generations are to live in safe, healthy and fruitful environment then it needs to be understood how plant life is threatened by different human activities and how this can be alleviated by understanding the effect of different environmental conditions on plant growth (Pimental et al. 1997). Growth rates of plants seem to differ even when they are exposed to constant ecological surroundings, so it is important to acquire knowledge on the factors that limit plant’s growth in order to increase crop yield for human benefit (White et al. 2015). This study was conducted to analyse how changes in environmental factors like light and carbon dioxide effect the surface area of the plant leaves.

Eruka satvia plants have been grown in different conditions(High light & low carbon dioxide; low light & high carbon dioxide; low light & low carbon dioxide and high light & high carbon dioxide) for four weeks keeping the nutrient and water supply constant and then students measured the surface area of the leaves exposed to different conditions to evaluate effect of light intensity and carbon dioxide on the plant’s growth. Several studies have been conducted in the past to determine changes in plant growth with changes in carbon dioxide and light as there has been recent climatic changes such as rise in levels of carbon dioxide due to different human activities (Sinha et al. 2011, p-432-436). The results of studies show that high light and carbon dioxide generally lead to a higher plant growth.

Two investigations were carried out in which one of the variables were changed; either carbon dioxide or light. In investigation 1, leaves from E. satvia were exposed to different light levels at ambient carbon dioxide concentrations. The hypothesis would be if plants are exposed to high light and ambient carbon dioxide concentrations then their growth in terms of their leaf area would increase.

In investigation 2, leaves from E. satvia were exposed to different carbon dioxide concentrations at high light leaves. The hypothesis would be if plants are exposed to high light and elevated carbon dioxide concentrations then their growth in terms of their leaf area would increase.

Fig 1: Average leaf surface area of E. Satvia plants in cm2 and with SE bars with respect to change in light intensity.

Fig 2: Average leaf surface area of E. Satvia plants in cm2 and with SE bars with respect to change in carbon dioxide levels.

The average leaf surface area for plants grown in ambient carbon dioxide, high light is 40.29 and for those grown in ambient carbon dioxide, low light is 31.77(Figure 1). The standard error bar for A is longer (11.57) showing that the average value is uncertain while the standard error bar for C (8.25) is smaller indicating that the average values are more accurate.

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The average leaf surface area for plants grown elevated carbon dioxide , high light is 48.6 and for those grown in ambient carbon dioxide, high light is 40.29(Figure 2).The standard error bar for A (11.57) is slightly longer than that for B (10.25) indicating that the average values are more accurate for B. However, there is no big difference in the standard error bars so it can be said that the average values are of similar accuracy.

Statistical results were also used for the two investigations carried out. The results indicate that for both the investigations there is a significant difference between the two treatments as pInvestigation 1: t= 5.991799485, df=198, p= 9.64547 x 10-09

Investigation 2: t= 5.376231498, df=198, p= 2.12923 x 10-07

The results of the study indicate that high light and elevated carbon dioxide levels increase the growth rate of E. Satvia plants as seen by the height of the bar graph in Figure 2. The aim of the study is fulfilled, and the hypothesis can be accepted as the results match with the prediction. This research area is of significant biological importance. Plants help maintain atmospheric levels of carbon dioxide and oxygen and provide us with food so this study would help us understand how changes in environmental conditions can affect plant growth. Moreover, this study’s importance is further denoted by the values of the statistical results as p values are significant for both the investigations which shows that plant growth is affected by different levels of carbon dioxide and light.

In a previous study, Begonia × hiemalis plants were used to analyse the effect of carbon dioxide enhancement on their growth at different light levels and it was seen that increasing the CO2 concentration from 330 to 900 uL L-1 greatly increased the biomass, number of leaves and flowers of the plant ( Mortensen & Ulsaker,1985).This indicates similar results with this study. In another study where elevated carbon dioxide levels were used to determine rate of photosynthesis which drives growth rate of plants, showed positive results but it was noted that long-time exposure to carbon dioxide leads to a decrease in photosynthesis (Thompson et al.2017).However, in a different study it was concluded that for a ‘sun’ plant increase in light intensity results in a swift increase in leaf-area ratio while for a ‘shade’ plant low light intensity is more suitable for increase in growth(Blackman and Wilson, 1951).So it can be said that when determining the effect of environmental conditions on the growth of plants, the type and nature of the plant should also be taken into consideration. In a similar study, seed-grown plants of ryegrass were cultivated in growth rooms set at temperature of 20/15 degrees and was exposed to different carbon dioxide concentrations (340 or 680 uL L-1) and the results concluded that in the elevated carbon dioxide level; the specific leaf area was 13-40% lower in comparison to ambient conditions and also caused a decrease in the ratio of roots and shoots as the experiments progressed (Ryle et al.1992).

The study conducted has some limitations. The main limitation is the use of leaf area as the means to analyse growth of plants. In a model used to analyse plant growth, a non-linear relationship was seen between leaf area and plant biomass due to changes that happened in the use of carbon in different organs of the plant during shift from one growth phase to another (Weraduwage et al., 2015).This indicated that leaf area can only partly signify overall plant growth and growth is more closely associated with specific leaf area which takes into consideration the dry weight of plant than to area based photosynthesis rate (Weraduwage et al., 2015). Another limitation would be that lack of sample leaves that could be used as control to compare results. In this investigation 100 leaves were tested for each different environmental condition; so, a sample size of 100 leaves should have been used to measure growth under ideal carbon dioxide concentrations between 700 and 900 uL L-1 and light intensity. Also, the leaves were taken from only the E. satvia plants; results may vary for different type and species of plants as mentioned earlier.

Plant growth is primarily affected by light, water, temperature, soil nutrients and carbon dioxide levels. In this investigation the carbon dioxide and light levels were varied keeping the temperature, water and nutrient supply constant. Plant growth is limited by the supply of nutrients such as nitrogen and phosphorus in the soil as seen in a previous study that the addition of nitrogen and phosphorous in the soil resulted in an extensive increase in diameter growth of Metrosideros polymorpha plants grown in Laupahoehoe, Hawaii(Vitousek,1996).The availability of proper nutrients in soil such as nitrogen, phosphorus and potassium is essential for plant growth and hence the use of fertilizers is essential for crop yield (Lowler et al. 2001). Therefore, future studies can be conducted where plants will be grown in growth mediums where the soil nutrients would be varied keeping the environmental conditions constant. The effect of temperature on respiration and photosynthesis have been discussed in many studies and it has been concluded that at low temperatures the ratio of photosynthesis and respiration is over 10 while at very high temperatures respiration plays the key role ( Went,1952).Thus, this can somewhat explain the reason behind plants growing better in temperate regions when compared to tropical regions (Went,1952). So future studies can be carried out to determine how growth of E. satvia plant is affected by different extremities of temperature. Moreover, in future studies results should be based on specific leaf area which takes into consideration the plant biomass as it is more precise and will help eliminate limitations.

Plants are of biological importance to humans, so it is very essential to understand plant growth in order to ensure good living environments for our future generation. The findings of this study conclude that elevated carbon dioxide levels and high light increase plant growth which is indicated by the largest average leaf area. However, there are some limitations especially in the methodology used which draws attention towards the need to conduct future research with a different species of plant and with different independent variables such as temperature or soil nutrients.

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