Football and Photosynthesis

[Law Student]

One for the botanists:

Most football stadia are used so regularly that the pitches are marked out continuously from September to May. As the whitewash used must reflect almost almost all of the available light; why does the grass underneath it not die?

[theprof]

A student of mine asked me this question last year when I was press-ganged into attending a rugby match of students v lecturers.

It turns out that there are two different types of materials used to mark out these lines in stadia.

The budget option is basically whiting (finely ground chalk) mixed with a resin material. The resin is used as the carrier for the whiting and because of the resins ability to adhere to the grass fairly tenaciously, the whiting remains in situ for a good few months. All the same, it does eventually wear off due to the big boots pounding on it and needs to be resprayed more than once a season.

The other option used at Twickers etc, is a specialised paint made from water-soluble polymers. The polymers used are soluble in water during manufacture, but uniquely are insoluble in water once the paint dries. The paint is more rainfast than whiting applications and it hardly wears off at all. When they do want to remove it, they've even developed a harmless solvent. It also comes in a wide range of colours to design logos etc.

So back to the question, neither of the substances reflect all the available light - sufficient light gets through for photosynthesis to continue. Neither whiting nor the paint are detrimental to gaseous exchange in the grass blade (lamina) as they do not block the stomata and they are both non-toxic to all vegetation.

In simple terms, you can regard these substances as being similar to the whiting solutions that gardeners spray inside their greenhouses during a hot summer to prevent tomato plants etc being burned by the sunlight. These substances provide excellent reflection of shortwave, "burning" infrared rays whilst allowing the rest of the light through the glass for the plants to thrive.