Gce Model Questions Vol1 2018 Biology Question 21

Two desert plants are compared for water‑use efficiency (WUE). Plant C is a cactus that uses CAM photosynthesis; its net photosynthetic rate is \(5 \;\mu\text{mol CO}_2\,\text{m}^{-2}\,\text{s}^{-1}\) and its stomatal conductance is \(0.05 \;\text{mol H}_2\text{O}\,\text{m}^{-2}\,\text{s}^{-1}\). Plant S is a shrub that uses the C\(_3\) pathway; its net photosynthetic rate is \(12 \;\mu\text{mol CO}_2\,\text{m}^{-2}\,\text{s}^{-1}\) and its stomatal conductance is \(0.15 \;\text{mol H}_2\text{O}\,\text{m}^{-2}\,\text{s}^{-1}\). Both plants experience the same ambient temperature of \(35^{\circ}\text{C}\) and relative humidity of \(30\%\). The saturation vapour pressure (kPa) at temperature \(T\) (°C) can be approximated by \(e_s = 0.6108\,\exp\!\left(\dfrac{17.27T}{T+237.3}\right)\). Transpiration rate \(E\) (mol m\(^{-2}\) s\(^{-1}\)) is given by \(E = g_s \times \text{VPD}\), where VPD = \(e_s - e_a\) and \(e_a = \text{RH}\times e_s/100\). Calculate the WUE for each plant (\(\text{WUE}=A/E\)) and state which plant has the higher WUE.