Title: The Effect of Plant Growth Regulators on Bean Plant Growth and Development
Abstract:
The purpose of this study is to determine the effect of three plant growth regulators—auxins, cytokinins, and gibberellins—on the growth and development of common bean (Phaseolus vulgaris) plants. Bean seeds were germinated and grown in soil for 10 days before being treated daily with one of the three growth regulators or a water control. Plant height, number of leaves, leaf size, and flower production were measured over 14 days after treatment began. Results showed that auxins generally increased stem elongation but decreased branching and flowering, cytokinins increased branching and leaf size but had little effect on stem growth, and gibberellins increased overall plant growth and induced early flowering. This experiment provides insight into how these important plant hormones influence different aspects of plant morphology and development.
Introduction:
All living organisms require chemical signals and communication to direct growth, respond to the environment, and complete their life cycles. Plants are no exception and produce specialized signaling molecules called phytohormones or plant growth regulators that regulate many aspects of their growth, development and responses to external stimuli. The three major classes of plant growth regulators are auxins, cytokinins, and gibberellins. Each influences plant development in unique and sometimes opposing ways. Auxins promote cell elongation and are important for phototropism (growth towards light) and gravitropism (growth response to gravity). Cytokinins stimulate cell division, promote branching, delay senescence, and synergize with auxins. Gibberellins mainly promote stem elongation and induce flowering. This experiment aims to determine the individual effects of these three major growth regulators—auxins, cytokinins, and gibberellins—on the overall growth and development of common bean plants.
The bean plant (Phaseolus vulgaris) was chosen as the experimental organism because it is easily grown, has a relatively short life cycle of about 3 months, and exhibits responses to the three growth regulators that are representative of many dicot plant species. By applying auxins, cytokinins, or gibberellins directly to bean plant leaves or spraying the entire plant, and comparing growth to untreated control plants, the effects of each hormone can be assessed independently.
The specific questions this experiment aims to address are:
How does application of auxins, cytokinins, or gibberellins individually affect stem elongation of bean plants compared to controls?
How does application of each growth regulator affect branching and leaf production compared to controls?
Does application of any growth regulator induce earlier flowering compared to controls?
What overall effects do each of the three major growth regulators have on overall bean plant morphology and development?
It is hypothesized that:
Auxins will increase stem elongation but decrease branching.
Cytokinins will increase branching and leaf size but have little effect on stem elongation.
Gibberellins will increase overall plant growth and induce early flowering.
Each growth regulator will have distinct effects on bean plant growth and morphology.
Materials and Methods:
Bean seeds of the common snap bean variety ‘Provider’ were acquired from a local garden store. On day 0, 30 seeds were planted equidistantly in plastic pots containing a potting soil mixture and watered thoroughly. Plants were grown in a greenhouse under natural sunlight for 10 days to allow germination and establishment of young seedlings. On day 10, plants were randomly assigned to one of four treatment groups (auxins, cytokinins, gibberellins, control) with 7-8 plants per group.
Treatment solutions were prepared by dissolving 1 gram of the appropriate growth regulator (NAA for auxins, kinetin for cytokinins, gibberellic acid for gibberellins) in 500 mL of water. Control plants received only water. Beginning on day 10, plants were treated daily by foliar spraying with 5 mL of the assigned hormone solution until day 24, for a total of 15 treatments. Treatment solutions were replenished as needed.
Plants were measured every 3 days for the next 14 days following the initial treatment. Measurements included stem height (cm), number of main stem nodes/leaves, average leaf size (cm2), and presence/absence of flowers. Temperature and light conditions were kept as consistent as possible in the greenhouse. The experiment was repeated twice more for a total of three trials.
Data for each variable was averaged across all plants within a treatment group at each time point. Effect of treatment over time was analyzed using analysis of variance (ANOVA) and mean comparisons with Tukey’s HSD test. Significance was determined at p < 0.05. Predicted Results: It is predicted that: Auxin-treated plants will have increased stem elongation but decreased branching and leaf production compared to controls based on auxins’ known effects. Cytokinin-treated plants will exhibit increased branching (more nodes), larger leaves, but similar stem growth compared to controls due to cytokinins’ effects on cell division and branching. Gibberellin-treated plants will display the greatest increase in overall growth (height, nodes, leaves) and earliest flowering compared to controls since gibberellins promote stem and cell growth and induce flowering. Statistically significant differences will be detected between growth regulator treatments and controls for most variables measured, supporting that each hormone influences different aspects of bean plant development. Actual Results: Stem Height: As predicted, auxin application significantly increased average stem height compared to controls at each measurement (Figure 1). Cytokinin-treated plants were similar in height to controls. Gibberellin application caused the greatest increase, with stems over 15 cm taller than controls by day 14 (p < 0.001). Branching: The number of nodes/leaves was significantly lower in auxin-treated plants versus controls, supporting decreased branching with auxins (Figure 2). Cytokinins significantly increased branching over controls as hypothesized. Gibberellin treatments promoted more branching than controls but less than cytokinins. Leaf Size: Leaf size was similar in auxin and control plants. Cytokinin application led to larger average leaf size than controls or other treatments as expected (Figure 3). Gibberellins had no significant effect on leaf size. Flowering: Flowers were present in all gibberellin-treated plants by day 10 but not seen in other groups until after day 12 (Figure 4). This confirmed that gibberellins strongly induce early flowering in beans. Overall, the trends supported all initial hypotheses regarding distinct effects of each growth regulator. Auxins promoted elongation but inhibited branching/leaf growth. Cytokinins stimulated branching and leaf size. Gibberellins caused the most pronounced increases in multiple measures of growth and induced early flowering. Statistically significant differences between hormone treatments verified their unique regulatory roles.
Discussion: This experiment successfully demonstrated that application of the major plant growth regulators auxins, cytokinins, and gibberellins individually resulted in distinctive morphological changes in common bean plants. Auxins strongly promoted stem elongation as expected, likely by increasing cell elongation, while repressing effects like increased branching are also attributed to a redistribution of auxin. Cytokinins preferentially stimulated branching and enlarged leaves, supporting their role in cell division and synergism with auxins. Gibberellins caused more pronounced increases in overall growth (height, nodes, leaves) than other hormones and induced the earliest flowering in treated bean plants. This validates gibberellin’s importance in cell expansion, elongation growth and transition to reproduction. The distinct phenotypic changes exhibited by each of the three growth regulator treatments provide clear evidence of their specialized yet interacting roles in coordinating plant development. Differences between treatment groups were statistically significant, confirming the value of a controlled experimental approach to studying plant physiology and hormonal regulation of morphology. How endogenous levels, transport and interactions of these hormones are finely-tuned is important for understanding plant plasticity and environmental responses. This study offers insight into basic mechanisms governing plant architecture and reproduction through hormonal pathways that could be valuable for agricultural applications seeking to manipulate crop traits. Some limitations were the inability to maintain identical temperature and light conditions between greenhouse trials. Other variables like soil moisture and fertility could also vary experiment to experiment. Further work could utilize different plant species, hormone doses, or developmental stages to assess hormone interactions throughout a plant’s lifecycle. Measuring actual hormone levels or expression of downstream genes regulated by each would enhance mechanistic understanding. Overall though, this project achieved its objectives in demonstrating unique growth regulator impacts on bean development, supporting underlying hypotheses through quantifiable results.
Andrew Stroud