The value of root pressure is about __________ to __________ bars...

Root pressure is a positive hydrostatic pressure that develops in the roots due to the active absorption of ions from the soil.

The typical value of root pressure is about $+1$ to $+2$ bars.

Since $1$ bar of pressure can support a water column of approximately $10$ meters, a root pressure of $+1$ to $+2$ bars is sufficient to pump water to a height of $10$ to $20$ meters in plants.

Answer: $+1$ to $+2$