Kinetics of upward bending response of gravistimulated snapdragon shoots
Discussion

Angular position transducer

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With some shoots, especially the smaller ones, it was observed that when the lever or the shoot was touched, the shoots released its stored tension and bent upward. This phenomenon was described previously in cocklebur plants (Xanthium strumarium L.) by Wheeler and Salisbury (1981). When cocklebur plants were restricted, the bending energy as well at the gravitropic stimulus were stored. When released, the shoot bent upwards within seconds due to the stored bending energy and continued to bend afterwards (stored stimulus).

With all snapdragon shoots, an early net downward bending was observed during gravistimulation. This bending of the shoot occurred immediately after the tip of the shoot was hooked up to the APT and the recording started. This phenomena could been have caused either by actual elongation of the upper half and thus positive gravitropism or by sagging of the tip due to its weight. Harrison and Pickard (1989) have shown, that this net downward bending in tomato seedlings was active rather than passive. However, the net downward bending in snapdragon shoots occurred often quicker than the one observed with tomatoes (5 min). This can be explained by the time needed to hook up the shoot and adjust the recorder, in wich case, the shoot was already placed in a horizontal position without recording the curvature. Due to the relatively large weight of the buds and flowers, snapdragon shoots are more likely to bend downwards from sagging, although this has not been tested. Proof for positive gravitropism of the shoot can be found by placing the shoot in an upright position in a clinostat after they have been in a horizontal position for few minutes. If sagging is active, the shoot will bend toward the predicted direction, while no bending will occur when sagging is passive.