Students investigated the relationship between two environmental factors and the rate of cavitation in a Scots pine tree, Pinus sylvestris, over a 50-hour period. The two factors they investigated were: • photosynthetically active radiation (PAR), which is the light energy available to plants that they absorb and use in photosynthesis • wind speed. The cavitation rate was estimated by recording the number of clicks detected by the sensor. The results are shown in Fig. 3.1. [Figure 3.1]

Mark scheme

Marking points from the official mark scheme (Oct/Nov 2022). The mark or mark type is shown in brackets.

1. [1] ref to, difference in water potential / water potential gradient ;
2. [1] (continuous) columns of water (in xylem of, tree trunk / stem) ;
3. [1] transpiration pull / cohesion-tension ;
4. [1] hydrogen bonding / cohesion, between water molecules ;
5. [1] hydrogen bonding / adhesion, between water and, the lining of the xylem / cellulose (in walls of xylem) / hydrophilic parts of lignin ;
6. [1] cavitation (only) occurs when, light is available / PAR is available / the trees photosynthesise ;
7. [1] water movement / transpiration, (only) occurs when, light is available / PAR is available / the trees photosynthesise ;
8. [1] no clear relationship between values for, light energy / PAR, and cavitation rate ;
9. [1] peaks of cavitation does not coincide with peaks for, PAR / wind speed ;
10. [1] no cavitation when the wind speed is below 1.25 m s-1 ;
11. [1] PAR affects, cavitation (rate) / water movement/transpiration (rate), more than wind speed ;