The greenhouse as a possible source of experimental error
The greenhouse is probably the most common installation for performing convenient in-house controlled experiments while it can be at the same time a most variable environment. Here reference is made to the standard greenhouse which is not a fully regulated phytotron but rather designed to be cooled in the summer and heated in the winter while having some form of ventilation. The structure might consist of glass or any kind of plastic materials. It is usually located on the campus between other buildings.
There are some widespread problems typical of greenhouse experiments which drive variability in ambient conditions within the greenhouse. These problems are often accentuated in experiments involving drought stress with plants grown in the ground or in containers.
A. When cooling or heating is applied there is usually a gradient of temperature from the source, downstream. If replications and experimental blocks do not account for this gradient it can introduce serious variability into the experiment.
B. Open sides or side fans used for forced ventilation can also develop a gradient of atmospheric conditions inside the greenhouse.
C. It is very common to build the greenhouse close to a main building, as matter of convenience or economy. It is not uncommon for this building to shade the greenhouse differentially; namely shade only part of the greenhouse every day (see Figure). This is a serious fault, especially with plant water relations studies.
Greenhouse partially shaded by an adjacent building
The greenhouse should be inspected several times during the day for possible gradients in ambient conditions inside the structure and try to design the experiment to account for this variability, if possible. The perfectionists use various systems to rotate the pots inside the greenhouse in order to reduce the effect of spatial variability. Advanced systems use special conveyors which permanently rotate the potted plants inside the installation. Never take it for granted that a greenhouse is a "controlled environment".