Depends on which leaves. The older leaves or newer leaves. If it's from the base of the plant it may be wilt. If it's the new leaves it may be a nitrogen deficiency and if it's the whole plant it may be a potassium or magnesium deficiency. It may be a lack of iron or it could be that the above nutrients are there but the Ph of the soil is too low or high for the plant to access the required nutrients.
Therefore, i would first recommend a Ph test, then, if the Ph is OK. IE between 6-7, have a look at these pictures for more info.
http://www.plantphys.net/article.php?ch=t&id=289