Final answer:
In an experiment to test the effect of day length on plant growth, plants would be divided into an experimental group with extended daylight and a control group with standard daylight, with variables like water and fertilizer controlled. Longer daylight may cause growth in long-day plants, while shorter periods might induce blooming in short-day plants.
Step-by-step explanation:
The most likely conclusion reached by the agronomist regarding the effect of day length on plant growth is that plants exhibit photoperiodism, which means they respond to the length of the day or night. In an experiment to test the hypothesis that plants will grow more when the day is longer, we would have two groups of plants: an experimental group that receives extended daylight hours and a control group that receives the standard amount of daylight. This would help to isolate the independent variable (day length) and measure the dependent variable (plant growth). Variables that should be controlled include the amount of water, fertilizer, the size of the containers, and the type of plant used to ensure that the observed effects are due to changes in day length and not other factors.
The experiment described could be conducted in a greenhouse where light exposure can be accurately manipulated. For instance, agronomists could set up multiple sets of plants and subject them to different photoperiod treatments while keeping other conditions constant. As evidenced by the scenario in which one set of shrubs bloomed under shorter light periods, we might conclude that the shrubs subjected to longer periods of light were long-day plants, which do not flower under such conditions, as opposed to the other set which can be concluded to be short-day plants that thrived under limited light exposure.