The effect of perturbations and sensor models on the key parameters of growth compared with experimental observations.

Growth rate (dL/dt), tip wall thickness (Lwall), and turgor pressure (ΔP) were computed from the model in response to different perturbations and compared to values measured in this study or observations in the literature. External osmotic pressure Πo and hardening α can be experimentally manipulated under physiological conditions but zone length Lperm cannot. The magnitude of simulated perturbations, whether step changes or sinusoidal, are at least 50% of the steady-state values. Only the model with an osmosensor of (ΔΠ–ΔP) and the output as pectin exocytosis dVwall/dt fits the experimental observations.
1Wall thickness is constant until very low values of Πo (hypotonic challenge) whereupon it becomes unstable and bursting occurs.
2Wall thickness is constant until [Ca] approaches zero when it becomes unstable and bursting occurs; at high concentration ([Ca] = 2 mM) the growth slows to zero during which time the wall thickens substantially.
3Turgor pressure change with Ca changes have not been measured in pollen to our knowledge; however, increased wall plasticity induced in fungal hyphae do lead to a fall in turgor [56] as would be expected.
4If periodic changes in this parameter are occurring during growth.