The pressure of water vapour inside and outside leaves can be measured. The difference between these pressures is known as the leaf vapour pressure deficit (LVPD). LVPD is one of the factors that influences the rate of transpiration. Scientists measured the effect of changing the LVPD on the rate of transpiration in several species of flowering plant that live in a variety of different habitats. Two of these species were: • Nerium oleander, a species that is adapted to grow in hot, dry conditions • Helianthus annuus, a species that is not adapted for survival in hot, dry conditions. Fig. 5.1 shows the effect of increasing the LVPD on the transpiration rates of the two species. All other factors were kept constant. key Helianthus annuus Nerium oleander [Figure 5.1] Fig. 5.2 shows part of a plant of N. oleander. [Figure 5.2] Fig. 5.3 shows a cross-section of part of an oleander leaf. [Figure 5.3] Fig. 5.4 is a drawing of a high-power view of region N on Fig. 5.3. [Figure 5.4]

Mark scheme

Marking points from the official mark scheme (May/June 2024). The mark or mark type is shown in brackets.

1. [3] both units must be used at least once
2. [3] any three from:
3. [3] 1 no transpiration when LVPD is 0 kPa ;
4. [3] 2 (both) increase in transpiration rate until 2.5 kPa ;
5. [3] 3 H. annuus rate remains constant between 2.5 and 3.0 kPa, N. oleander rate decreases slightly;
6. [3] 4 H. annuus has higher rate of transpiration (at all LVPDs) ; ora for N. oleander
7. [3] 5 H. annuus has a higher rate of increase (between 0 and 2.5 kPa) ; A steeper gradient
8. [3] 6 any comparative data quote from Fig. 5.1 ; must include figures and units from both axes and data must come from both species
9. [4] if a correct adaptation does not match the figure, allow ecf for the explanation Fig. 5.3
10. [4] any four from:
11. [4] 1 three layers of thick-walled cells at, upper / lower, surface; A two layers of cells under epidermis / epidermis and (two layers of) hypodermis
12. [4] 2 reduces, diffusion of water vapour to atmosphere / cuticular transpiration ; A increases the distance for water vapour to diffuse
13. [4] 3 no stomata on upper surface / stomata only on lower surface ;
14. [4] 4 stomata not exposed to direct sunlight / AW ;
15. [4] 5 (stomata in) pits / depressions / cavities / chambers / crypts / infoldings / grooves / AW (on lower side of leaf);
16. [4] 6 creates humid atmosphere in, pit / AW or reduces the, diffusion / water potential, gradient (for water vapour) ; A vapour pressure gradient A minimises effect of, external air currents / wind
17. [4] Fig. 5.3 or Fig. 5.4
18. [4] 7 thick walled epidermis ;
19. [4] 8 reduces diffusion of water vapour through lower epidermis ;
20. [4] 9 thick (waxy) cuticle (on epidermis / surface);
21. [4] 10 reduces diffusion of water vapour through cuticle / reduces cuticular transpiration; A (waxy) cuticle is waterproof
22. [4] 11 (epidermal) hairs / trichomes (around stomata / fill pits);
23. [4] 12 reduces air movement/traps still air/creates humid atmosphere / traps water vapour or reduces the, diffusion / water potential, gradient for water vapour ;