THE POLLEN TUBE GROWTH TEST (PTGT)
The pollen grain is used by plants to carry male sperm from one flower to another. Once transported to another flower (via bees or wind) the pollen lands on the style and grows a pollen tube, which grows down through the stigma to the ovary at the base of the flower. The sperm then travels down this pollen tube, where it can fertilise the eggs inside the ovary. The diagram below shows a few pollen grains, and the tubes they produce. (fig1)

We use the growth of the pollen tube to tell us about the effect our formulations can have on living cells. The addition of a chemical formulation can slow the growth of the tube; the more formulation is added, the slower it grows. The typical effect of a chemical on a living cell can be described using this chart (pdf), which shows the decrease in pollen growth (100% = no growth) and has four crucial points:

• The bottom of the curve. The chemical has no effect on the living cell below this point
• The Hill slope. The chemical effects the growth of the cell in a linear fashion. Usually the more toxic the chemical, the steeper the hill slope will be
• The EC50. This is the level of chemical (the concentration) at which 50% of the observable effect has occurred. (usually 50% growth)
• The Top of the curve. Adding more chemical above this point will have no more effect on the living cell.

Very well, you ask, but how is this relevant to people? The pollen tube growth test has been compared against the most common other types of toxicity testing, and it shows the best correlation against the Draize rabbit eye test. The graph (pdf) shows a comparison between the Draize and the Pollen tube Growth test. The Draize test involves inserting drops of the chemical into the eyes of rabbits, then looking for signs of irritation, including redness and weeping. When compared with this, the Pollen Tube Growth test does not use animals, is much quicker to provide results, and is less likely to be influenced by individual bias.

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