Don't you just panic when you see signs like these:
|Cucumber seedling with yellow edges
Yellowing leaves!!! What does that mean? Too much water, too little water, not enough light, too much light too suddenly, lack of nutrients (magnesium deficiency?) . It's murder making up your mind. Thank goodness it's only the first two leaves (cotyledons) and the true leaves are coming in. I think on reflection it's cold that has caused this in my cucumber seedlings, but that is one factor not referred to in any of the books I consulted.
And thank goodness the courgettes seem to be a bit more robust: