Comparison of Protein Concentration in Leaves Between Sun and Shade Plants

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Table 2: Results of silverbeet leave suspensions grown in Sun and Shade under 2 procedures.

Discussion:

Protein concentration is measured in a quick and easy way with the use of Biuret reagent and spectrophotometer. The relationship between absorbance and protein level is not absolute linear; therefore the line is best fit and cannot connect all the data points.

The first absorbance measured in original leaf samples exceeds the maximum figure shown by standard protein solution (1.684>1.451>0.621). This can lead to unreliable estimation for protein concentration because the standard cure is only an assumed model for data within the known range.

To ensure that the absorbance reading stay within the range (0.000-0.621), I carried out a modification procedure which is diluting the original sample 5 times by adding 0.8 ml buffer to only 0.2 ml suspension. Another way to modify the experimental procedure is to use BSA protein solution with higher concentration, i.e: 40 mg ml-1. Prepare this solution similarly to the original procedure in Table 1. A different standard cure can be constructed with higher maximum figure. For leaf suspensions, we do not need to add buffer but only use 1.0 ml reacting with 4.0 ml Biuret as the reading already falls in the range of the new standard cure.

The results show that Sun leaf extract has slightly higher protein concentration than Shade one. This agrees with the hypothesis. In fact, the same results were also recorded in a study by Boardman (1977) stating that N, Rubisco, and soluble protein content/mass in Shade is slightly lower than Sun.

During practise, parallax error could happen when more than one person involved in reading the pipette. Different reading angles possibly make the volume pipetted higher or lower than required. In addition, random errors such as reading from spectrophotometer fluctuate during measurement and variation in incubation time might occur.

The sample size is too small (only 1 absorbance is read for each suspension). More readings should be obtained by diluting the extract by various factors such as 3, 4, 6 times. The mean value of concentrations can then be calculated and compared to provide more reliable results.

Rubisco, as an enzyme is synthesised through transcription and translation of an encoding gene. Therefore the amount of Rubisco may not directly link to light level but the availability of mRNA (Vicente, Morcuende & Babiano 2011). In addition, the synthesis of rubisco requires an influx of nitrogen into leaf from soil (Vicente et al. 2011). Shade leaves can have more rubisco due to higher nitrogen component in soil where the plant grows. Another possible negative result is that both leaves from one species have the same rubisco level due to same amount of encoding mRNA.

The relationship of light level and rubisco concentration is correlation, not causation. Other variables in growing conditions such as soil pH, nitrogen availability, and temperature were not known to be controlled.

Reference:

Boardman, N.K. (1977). Comparative photosynthesis of sun and shade plants. Annu. Rev. Plant Physiol. 28, 355-77.

Jurik, T.W., Chabot, J.F., and Chabot, B.F.(1979). Ontogeny of photosynthetic performance in Fragaria virginiana under changing light regimes. Plant Physiol. 63, 542-7.

Vicente, R., Morcuende, R., Babiano, J., (2011). Differences in Rubisco and Chlorophyll content among tissues and Growth stages in Two tomato (Lycopersicon esculentum Mill.) Varieties. Agronomy Research 9 (Special Issue II), 501-507.