What I suspect is you may have been a little light handed on N during veg stages before you started flowering. It is normal for lower leaves to yeloow in flower it is part of the natural process of senescence. But to have it only 1 week into flower seems rather soon.
What you dont want to do is start blasting them with a bunch of N now that your in flower. Go easy on the N and hope that there is enough stored N in the leaves to make it all the way to harvest. Or add small doses of N for another couple of feeds. There is a lot of information out there that claims that N in flower is a no no and I dont agree totally. Just dont overdue it. Flower is a energy intensive process and while they need more p & K having enough, but not too much N is important as well unless the plant has enough stored N to make the finish line.
I like to load them up with N all thru veg almost to the point of N toxicity and then let them ride all the way to harvest on the stored N from this process. The yellowing of lowers is called abscission and is a normal part of the senescence process
I will qualify tho that unless you know what your doing using my method can and will cause N tox and then thats another can of worms. So be careful on your next run and bring your gurls to the party with more N upfront.
I am going to paste some info from a bros thread and hope this helps you out with some additional information on this topic. Good luck with the remainder of your grow.
"I agree on the point that insufficient N fertilisation leads to poor growth and as such poor yields.
I guess adequate/sufficient is the magic word. Since these are subjective terms, it's down to the individual’s interpretation. With that being said, I like to fertilize with enough nitrogen to keep all but the lowest leaves from yellowing and falling off for as long as I can. The yellowing and dropping of the leaves is known as abscission and is partially due to nitrogen being relocated to higher (apical) parts of the plant. This is easily observed during oncoming and peak of flowering.
The hormone Ethylene is responsible for the promotion of abscission and effectively ‘tells’ the plant (for lack of better words) to do so, by overriding other hormones such as Auxin and Cytokinin. Those hormones are responsible for splitting and elongating cells (increasing mass of the plant as well as any recognizable growth). Auxins and Cytokinins also force or promote the storage and concentration of nutrients in the apical (Auxins) and lateral (Cytokinins) leaves, buds etc. For example, think about why the tops of plants are always greener and more vibrant.
Auxin’s, which actually work against abscission and have proven to completely stop it in some plants. They are produced and found in the highest concentrations at the very tops of plants. Auxin’s promote every type of growth (besides lateral) at certain concentrations, as well as forcing nutrients to the respective areas that they are most concentrated. When the concentration of Auxin weakens (starting from the very lowest points of the plant), cells become more sensitive to Ethylene (a natural gas-hormone responsible for ripening/rotting fruit), which triggers the acceleration of senescence (aging) and in term abscission.
As for a quick note on Cytokinin: It is a direct rival of Auxin in that the balance between the two stimulates different types of growth. If the Cytokinin: Auxin ratio is more prominent on the Cytokinin side, lateral growth is allowed and your lower branches will all grow freely. If the Auxins overpower Cytokinin, you’ll notice your plants will have little leaves/shoots known as ‘dormant buds’ that don’t start growing.
So if you’re still reading, you will be wondering what any of this has to do with nitrogen during flowering. Basically I just wanted to lay down a foundation before elaborating on nitrogen's role in all of this. You’ve probably heard that high levels of nitrogen can ‘prolong the onset of flowering’, which to me makes sense.
Here are a few facts about nitrogen and plants:
• Nitrogen combines with other elements such as Carbon, Hydrogen and Oxygen etc. to form ‘Amino Acids’.
• Amino acids form protoplasm, which facilitates “cell division” and consequently the potential for plant growth and development
• Amino acids are considered the ‘building blocks’ of protein
• All plant enzymes are made up of protein. Enzymes are considered catalysts for all chemical conversions within a plant, therefore the successful functioning of the plants ‘digestive system’ , hormone synthesis and general function
• Nitrogen is a crucial part of chlorophyll and necessary for photosynthesis (your buds might be coated white, but they’re still green underneath
)
So with the previously listed points in mind, I’d like to point out that the most common and important “growth promoting” hormones, Auxins and Cytokinins, all contain nitrogen in their chemical structure. If a plant is lacking nitrogen, it will have to disperse its limited supply amongst all of its metabolic functions and may not be able to keep up supply of crucial Auxins and Cytokinins. This is why I agree with the proposal that high levels of nitrogen can delay flowering, because a large supply would likely attribute to the production of more Auxins and Cytokinins, which would delay Ethylene production/dominance.
It’s worth noting that neither Ethylene nor Gibberellin (which is responsible for stretchy, long-distance inter-node growth in marijuana, as well as being used as a foliar spray for producing male flowers (think:hermies) contain any nitrogen in their composition. So, nitrogen is not responsible, at least directly to the synthesis of these two hormones. This is why I am skeptical about the idea that excess nitrogen causes ‘hermies’.
