This could be accounted for - for example, if the weight of everything drops by 100 grams, then at least 100 grams of water has been used or evaporated. Eventually any increase in weight from the plant could probably be calculated, but for many plants, (like bush beans), it's a relatively small amount, probably less than a pound.

OTOH indeterminate tomato plant, for example, weighs a lot, and the weight would also be affected by picking the fruit.