Why is it necessary to correct the readings of the respirometers containing seeds with the readings taken from respirometers containing only glass beads? your answer should refer to the concepts derived from the general gas law: pv = nrt

So, the fact that it's a specific instrument doesn't change why you need to correct the data.
However, for this question it is important to know the instrumentation.
Since respirometers are measuring the CO2:O2 relation, clearly having biotic material and abiotic material will change the measurements.
You should be able to figure out that the general gas law is coming into effect because of the act of cellular respiration. In general terms, you are correcting your instrument to a known blank (you do this with scales and every other instrument).
By determining the reading for the blank (marbles) you can know the relative error of the instrument.
Taking a scale for example: You 0 the scale to a mass of importance (the amount of error you're okay with) and then use it for your measurements so you can be adequately confident of the mass you're measuring .

Region b indicates the occipital lobe