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Effects of Auxin on Defoliated Bean Petioles

Examination of the effects of auxin on bean plant petioles suggests that Auxin speeds up the loss of the petiole. The leaves were removed and auxin was applied daily. Qualitative determination was made regarding the petiole. Numerical data produced by the number of plants used is insufficient to make any significant generalizations.


Auxin is a plant hormone more formally a group of chemicals including indoleacetic acid (IAA). IAA is similar to tryptophan, an amino acid. IAA in particular is actively transported and is only transported in one direction. IAA promotes cell elongation.

Leaf abscission, or the shedding of leaves, is controlled by age, environmental stimuli, health of the plant, and hormones, ethylene and auxin.

I hypothesized that the auxin exposed petiole would stay on longer because auxin promoted cellular elongation and more "growth." Literature predicts the opposite. Low auxin levels give rise to leaves thus high auxin levels will hasten the shedding of leaves.

Materials and Method

The Auxin used was found in a white, oil based paste. The established procedure suggested that 3 drops be used each day. Drops could not be used because of the medium in which the Auxin was delivered.

Control plants also recieved a tap with another applicator which had adsorbed some water. Leaves were removed from four plants using metal scissors. One control plant and one experimental plant had a young "terminal" leaf removed. Similarly, one control plant and one experimental plant had an old mature leaf removed. Since only four plants were provided, quantitative data is limited to time data and percentage of the plants exhibiting the effects.


                  Experimental                         Control                      

                  Mature            Immature           Mature            Immature   

Day 1                                                                               

Day 2             opposing leaf                                                     

Day 3                                                  slight withering             

Day 4                                                                               

Day 5             -opposing leaf                       opposing leaf                
-generally        fell                                 yellow                       
droppy leaves                                                                       

Day 6             -petiole fell                                                     
                  -new leaves                                                       
                  apparent at the                                                   
                  base of the                                                       

Day 7                               -new leaves                                     
                                    apparent on                                     
                                    opposing side of                                
                                    auxin petiole                                   

Table 1: Observations

Please look at the accompanying pictures for progress on the plants. The ruler shown is 15.2 cm (6.00 inches). The white dots denote the experimental side. The auxin applications appear white on the petioles.

Quantification of the data is not easily possible. Various destructive testing mechanisms could be used to measure related quantities. The use of many more plants would allow percentages to be compared.

The results are mainly inconclusive. One experimental's petiole did fall off earlier than the control plant. Auxin thus may play a role in accelerating the loss of the petiole. It should be noted that yellowing of the leaf opposite the experimental site was detected very early. Given the method that auxin is transported, in a single direction and a single side, it is harder to believe that auxin had a role in making the opposing leaf yellow.

The second experimental's plant shows no changes and neither does the control. Because leaf age, plant health, temperature, soil quantity and content, and bugs were not actively controlled, the lack of additional data prevents a more definate interpretation of the data.


Auxin's role is definately inconclusive regarding its role on the acceleration of the loss of the petiole. Although the experimental plant exhibited that the petiole would fall off faster than the other control plant, other factors, especially the age of the leaves, cannot be overlooked. Until more data is available, the data collected is insufficient to draw a conclusion.