If you want to extract DNA, it is not a problem, DNA must be the same. But for RNA and protein, the conditions is important if you plant to compare your samples with another species/conditions.
Exactly, I agree with dr Taras, meatabolism of plant depends on time of prelevment biotic anf abiotic conditions especiallty photosyntetic periode. So you have to work in conditions (time of prelevment) already used and published by an other author, or you can take your samples at differents time and choose the best one to achieve your goals...