|MadSci Network: Botany|
Your question qualifies as biochemistry and plant physiology. The biochemical effect of caffeine on plants (and animals) is well known, so much so that caffeine is often used as a tool to investigate processes affecting a variety of cell functions. Caffeine is a calcium release inducer. That means that adding caffeine to cells causes internal calcium to be released. This can result in a wide array of effects as calcium is used in plant cells for a number of purposes. The well-defined action of caffeine makes it useful, because basically, if you see an effect when caffeine is added, the effect is presumed to involve calcium release and/or membrane permeability. For example, recent literature investigating haptonema coiling and cellular differentiation used caffeine to identify calcium efflux and permeability changes as playing a role in these plant processes. Another apparent affect of caffeine on plants may be a role in UV protection. This effect may be mediated through calcium as well (I am not familiar with the mechanism of action for caffeine in UV photoprotection). I am not aware of a direct effect of caffeine on photosynthesis, but read-on.
All plants have a requirement for calcium which affects the permeability and organization of membranes. Calcium is also required by alpha-amylase, an enzyme involved in the hydrolysis of starch. Calcium can be used for detoxifying oxalic acid, which becomes insoluble and non-toxic to the plant protoplasm when calcium is bonded onto the acid. Some algae deposit calcium externally, as a by-product of obtaining carbon dioxide for photosynthesis, and are responsible for a large proportion of reef-building. Adding caffeine in appropriate doses would lead to symptoms similar to calcium deficiency including stunted growth. (I will only mention briefly that adding caffeine, or anything else, in high doses could result in more immediate toxic effects. For example, caffeine solutions may be overly acidic, so that the result seen is a pH effect, rather than a direct effect of caffeine. These potential problems must all be taken into consideration in designing your experiment.)
Calcium deficiency is seen in plants as a general disorganization of cells and tissue, which is consistent with its role in membrane organization. Growing tips may be particularly affected resulting in stunted growth of leaves and roots because calcium is poorly transported from older to younger shoots. Both effects are reflected in the general health of the plant which may succumb to a variety of diseases as secondary infections take hold. For example, blossom-end rot in tomato fruit is often attributed to calcium insufficiency. Caffeine application is likely to mimic these effects.
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