Figure 6.

Effect of light intensity on leaf xylem pressure. In the typical experiment shown, light intensity was 150 μmol m⁻² s⁻¹, when a xylem vessel of the leaf was probed (arrow). It was then increased in steps to 200 and 260 μmol m⁻² s⁻¹ (arrows). It can be seen that there was a substantial and rapid response of P_x to light intensity that correlated with a higher transpiration rate as measured by weighing the plant (data not shown). Note that the half-time for attaining a steady P_x after puncturing was about 5 s and about 20 to 50 s following a change in light intensity.