Investigating the Influence of Light Intensity and CO2 Concentrations on the Growth Rate of Eruca sativa

Investigating the Influence of Light Intensity and CO₂ Concentrations on the Growth Rate of Eruca sativa

Introduction

The current evidence for climate change is unequivocal. A scientific consensus has been reached that “human activities are the primary driver of global climate change” (Linden et al. 2015, Anderegg et al. 2010). However, despite this agreement among scientists, uncertainties present themselves as research is conducted into the ecosystem’s response to climate change, specifically, how the growth rates of plants are influenced by the changing environment.

It is well known that even in constant environmental conditions, relative growth rates of plants can vary considerably among species (Grime & Hunt, 1975). Thus, in a shifting global environment, the need to understand the impacts that environmental conditions will have on growth rates becomes vital. However, there has been a noticeable gap in knowledge regarding the long-term effects of global climate change on agricultural productivity, with crop yield changes at the forefront of these uncertainties (Adams et al. 1998). Many species of plants are yet to be studied to observe the influence of climate change on their growth rates. Understanding this influence will begin to set the foundations for the adaptation of crop farming in the future.

Currently, research has shown that crops are extremely vulnerable to climate change (Nelson et al. 2009), most noticeably through atmospheric CO₂ and light concentrations. Interestingly, the mechanisms for allowing CO₂ to diffuse into the plant – the opening of the stomata – and the absorbance of light interact with each other. Thus, as the average atmospheric CO₂ concentration is expected to increase, the interaction between light and CO₂ is suspected to vary (Murray, 2006). This study hypothesises that if the concentration of atmospheric CO₂ is increased, then the rate of photosynthesis is expected to increase accordingly. Similarly, if the amount of light intensity is increased, then the rate of photosynthesis is also expected to increase.

These variables will be compared via the growth rates of Eruca sativa leaves grown in climate-controlled conditions, aiming to provide a cursory glance into the impact of climate change on plant growth.

Results

Influence of varied light conditions on surface area of leaves

Light conditions were varied while maintaining the atmospheric CO₂ concentrations – ambient level (400ppm) – and graphed against the average surface area of E. sativa leaves (fig. 1). A noteworthy observation is the significant difference in standard errors between high and low light conditions (SE_High = 1.2, SE_Low = 0.89). The results of the t-tests show a significant difference between the 2 light treatments (t = 5.44, d.f. = 198, p = 1.52 x10^-7).  

Fig. 1 Influence of varied light conditions on average surface area of leaves, in cm² with SE bars.

Influence of varied CO₂ concentrations on surface area of leaves

Atmospheric CO₂ concentrations were varied while maintaining a stable light condition – high level (800µmol m²s¹) – and graphed against the average surface area of E. sativa leaves (fig. 2). It should be noted that the percentage difference between the average leaf surface area in the CO₂ treatments is less than the difference found in the light treatments. Finally, the results of the t-tests do show a significant difference between the 2 CO₂ treatments
(t = 4.90, d.f. = 198, p = 1.95 x10^-6).  

Fig. 2 Influence of varied CO₂ concentrations on average surface area of leaves, in cm² with SE bars.

Discussion

The results display a direct correlation between an increase in light conditions and atmospheric CO₂ concentrations with a relative increase in the average leaf surface area of the E. sativa plant. The high light growing condition provides a growth rate of approximately 1.3x the ambient light condition, while a 1.2x growth rate was identified in the high CO₂ vs ambient concentration. These results are thus in full support of the initial hypotheses with both sections of the experiment showing significant differences through the statistical analyses (p < 0.05). Through additional analysis, it is evident that there is a higher statistical correlation between the average surface area of leaves and the increased light conditions.

This result was observed by a previous study conducted on plant growth responses to light, temperature and CO₂ (Hayman, 1974). The study used a highly similar method to the one used in this study, that is, growing plants in highly controlled conditions then measuring the dry weight of leaves. Hayman (1974, p. 71) concluded that the highest light conditions “simulated growth to the same extent as the addition of soluble phosphate” – a fertilizer that improved plant growth significantly – supporting the results of this experiment.

In a biological context, this highlights that the amount of light available to E. sativa holds more influence on the growth rate than the CO₂ concentration. Further, this implies that in an agricultural setting, it is more beneficial to grow crops that are similar to E. sativa in areas of high light due to the noticeable surface area differences observed (Fig. 1). A high crop yield would thus be expected when utilising this growing condition. The results also show that a high CO₂ concentration would be beneficial, however, this growing condition is significantly harder to control and maintain than searching for areas of high light.

Another study conducted in 2016 provides an alternative approach to understanding the interaction between light intensity, CO₂ concentration and growth rate. By investigating the variables that limit plant growth, this study identified components with the strongest control over growth in major crops (White et al. 2016). Elevated CO₂ concentrations were found to stimulate photosynthetic rates, which typically translates into faster growth. However, a certain trade-off exists, in that, the acclimation of photosynthetic rates could not be achieved without defoliation due to the limiting factor of light (Bryant et al. 1998, as cited in White et al. 2016). Despite the differences in methodology and investigation, this supports the results of this study, highlighting that while elevated atmospheric CO₂ concentrations will positively impact photosynthetic rates, light intensity is still necessary for the translation into faster growth.

Evidently, from the results of this study and others, conclusions can be drawn about the short-term impacts of variable conditions on the growth rate of E. sativa plants. However, an obvious limitation of this study is the lack of ability in observing impacts over longer time periods – the main reason being the economic sustainability of the study. To critically analyse the viability of crop farming in future environments, gathering data over substantial periods of time in order to extrapolate with accuracy is essential (Adams et al. 1998, Nelson et al. 2009). Studies to rectify this limitation have been carried out, depicting logarithmic relationships between growing conditions and plant growth, suggesting that after a certain increase in either light intensity or CO₂ concentration, photosynthetic rates will begin to plateau (Eilers & Peeters, 1998). This is due to the plant reaching the point of maximum CO₂ fixation.

This however, highlights another limitation in the methodology of this study – an inability to observe the rate of plant growth. This study restricts itself to the results of each growing condition and not the rate at which E. sativa grows. In the future, this study could be improved by taking photosynthetic measurements for each plant in each growing condition with a portable photosynthesis system – very similar to the method used by Eilers and Peeters (1998). Measurements should be taken daily for the duration of the study to allow for the comparison of growth rates as well as the final leaf surface area. This would enable a progressive study of plant growth changes correlated with the changing climate.

A third and final limitation of this study concerns the accuracy of the measurements being taken, namely, the surface area of the selected E. sativa leaves. Leaf surface area is naturally expected to increase at a rate that is correlated with the growth of the plant. However, while this may be true for overall photosynthetic levels, net photosynthesis per leaf weight is actually found to decrease with increasing light intensity (B. Chabot & J. Chabot, 1997). This observation was attributed to a massive starch accumulation in the chloroplasts which slows the photosynthesis process. Although this observation does not present itself significantly in the current study, the systematic error still exists. In the future, a more accurate method for measurement of plant growth is required – possibly an average of multiple measurement techniques including leaf thickness, density and mesophyll cell surface area. Using multiple measurements in the methodology will reduce errors and allow for more accurate data to be collected in future studies.

This study aimed to investigate the influence of climate change on the growth rate of E. sativa plants via the environmental variables of light intensity and atmospheric CO₂ concentration. This study has identified that despite CO₂ being a controlling factor in plant growth, light intensity holds the most influence over growth. These results were highlighted as being short-term findings, with more research required to understand the long-term sustainability of crop farming in our society. However, in the immediate term, farmers can begin to expect higher crop yields due to our changing climate.

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