There wasn't a question. The Physics and Maths tutor who wrote about PES being -1.33 doesn't understand Economics. Of course the supply of wheat can fall (due to poor weather conditions, blight of the crop etc...) but this is not in response to a rise in the price of wheat. Remember, PES measures the response of the supply to a change in the price. Hence, my assertion that a rise in price will always lead to a rise in supply because of the profit motive.