Will a plant grow faster or slower when affected with less/more gravitational forces Will a plant grow faster when you would apply more upward gravitational force?
and/or
Will a plant grow slower when you would apply more downward gravitational force?
 A: First let us see the impact of gravity on plants: Gravitropism (also known as Geotropism) is a turning or growth movement by a plant in response to gravity.  Roots show positive gravitropism and stems show negative gravitropism. That is, roots grow in the direction of gravitational pull (i.e., downward) and stems grow in the opposite direction (i.e., upwards). Gravity is sensed in the root tip and this information is relayed to the elongation zone so as to maintain growth direction and mount an effective growth responses to changes in orientation and continue to grow its roots in the same direction as gravity. Roots bend in response to gravity due to a regulated movement of the plant hormone auxin known as polar auxin transport. The differential sensitivity to auxin helps explaining why  stems and roots respond in the opposite way to the gravity vector. In both roots and stems auxin accumulates towards the gravity vector on the lower side. In roots, this results in the inhibition of cell expansion on the lower side and the concomitant curvature of the roots towards gravity (positive gravitropism). In stems, the auxin also accumulates on the lower side, however in this tissue it increases cell expansion on the upper side and results in the shoot curving up. Now lets see what will possibly happen if gravity changes.
More upward gravitational force: Roots will tend to be longer and stem shorter.
More downward gravitational force: Roots will tend to be shorter and stem longer. This can make tall trees unstable. 
This is contrary to what our common sense might tell. On the hind sight, I am just beginning to wonder if earth has just the right gravity for plants to grow.
