You will probably run into the issue that plants that are good at scavenging Fe in a Fe scarce environment is responding through a specific mechanism, that is only triggered during Fe starvation.  And when those lines are moved to Fe sufficient soils you may increased Fe accumulation on a mass basis, but you may not see an increase in Fe assimilation. Fe is a very tightly regulated element, and taking up more iron than is required by the plant is also detrimental. Here's a good review on the subject.

http://www.annualreviews.org/doi/full/10.1146/annurev-arplant-042811-105522?url_ver=Z39.88-2003&rfr_id=ori:rid:crossref.org&rfr_dat=cr_pub%3dpubmed&

Chlorophyll meter may work but you may need to test it with leaves of known Fe concentration.

Fe is correlated with greenness, as measured with a chlorophyll meter. However, greenness can also be affected by other nutrients, such as, N, S, Mg, etc. Leaf greenness can also be affected by leaf morphology and growing conditions (light level, etc.). 

Unless if you dont analyse the leaf samples you cannot come to perfect conclusion. Fe in soil and plant is very essential for proper justification. Further there are two kind of Fe in soil viz., Fe2+ and Fe3+. if plant takes more Fe3+ it will show deficient symptoms. Because ferrric iron is not much useful for the plant functionally. In that case you cannot come to proper conclusion. Hence, proper soil and leaf analysis is must for coming to perfect conclusion. For qualitative research what you mentioned is acceptable but for quantitative research all the analysis is must

No.  Iron uptake is under very strict control and so vascular plants and algae normally do not hyperaccumulate iron.  Leghaemoglobin may possibly act as an iron store in legumes.

There may be plants with similar leaf greenness but different leaf iron concentration. If you are looking to find plants with higher leaf iron concentration you need to grow them in a normal condition and then measure total and active iron concentration of leaves.

Thanks for the answers. So the only "proper" way to do this is pay $20 per leaf sample...how exactly are plant breeding programs in the developing world supposed to afford such expensive analysis? Especially when they are the ones who need most nutrient enhanced food?

Yes Leaf analysis is the key tool for assessing iron uptake. However the presence of Iron in the analysis results does not necessarily mean that the iron is active, for when much nitrate fertilizer is used the conversion to amino-acids causes an increase in the cytoplasm pH and ferrous iron can be converted to the ferric phase and thus not utilizable by the plant. Thus causing the IRON PARADOX.

For your to know how efficient your plant are, you need to know the amount of Fe made available for the plant and how much of it is absorbed by the plant. The plant that absorbs more of the Fe and grows well will be more efficient assuming all everything been equal. However, it will be difficult to tell which plant is more efficient without determining the Fe uptake because is not all Fe made available that is absorbed by the plant. The question now is, are you going to use the same plant? if not, note that different plant have different nutrient requirement. Assuming you use the same plant, how sure are you that the plant you grow on low Fe condition actually requires a low amount of Fe to grow well? Again you have to be careful with Fe deficiency when you grow lettuce on low Fe condition, those that will die might be due Fe deficiency.

I am not a plant breeder but if really you don't have time and money, I will recommend, you should re-design your experiment and give it a try, who knows, you might come out with something new that will save cost and time.

Nutrient efficiency would have different definitions. If  we take dry matter accumulation in relation to nutrient supply,  two things: uptake efficiency and internal utilization would be involved. In case of N efficiency for instance utilization efficiency is more imporant under high N and that of uptake efficiency is more imporant under limited N supplies. Indeed, both efficiencies are usually not correlated,  and it is usually difficult to obtain a genotye  efficient both in uptake and utilization.  So, it is advisable to measure efficiency under both low and high( or adequate) nutrient supplies

As for the Fe question, it depends on what you are asking. If it is the same plant, it probably will acquire more Fe as soil Fe increases until the process reaches saturation. It will be expected that the process will saturate at very low levels of Fe. If compared to a sister plant adapted to high Fe, then you low Fe-adapated plant will likely have less Fe at high Fe levels (but more at low Fe levels). It is just a matter of saturation curves as with many other micro and macronutrients.

As for your SPAD-Fe relationship, the best will be for you to build the allometry by growing the plant under different conditions (Fe, water, nutrients) and see if the relationship is established and holds. For that you need both set of measurements. It may work only for your plant, under certain conditions or a given developmental stage. If it does great, you no longer need Fe data. If it doesnt you are stuck with measuring your nutrient content every time.

In any case yield cannot be compromised. Identify useful types then screen limited ones for fe content.

You Have asked a very interesting question Jacob.

In my opinion the effect will be significantly variable between plant species. My response to your question is based solely on my experiences with Kiwifruit Actinidia. This plant species is reasonable well adapted to extracting and up taking iron from low ferrous iron environments with soil well drained and sandy volcanic ash with soil pH in the 6.0 to 7.2 range (artificial due to lime application) under those conditions we expect to see leaf iron concentrations of between 45 & 70 ppm. How ever on wetter lowland soils with high ferrous iron we will see leaf iron concentrations of between 80 & 200 ppm and with significant manganese inhibition by the excess iron.

We use the SPAD (Opti sciences) meter as a routine on about 300 orchards over the last ten years and although we find the tool useful in a broad context I doubt that it will be sensitive enough for your experiment.

I hope this information is of help.

Philip

Its an interesting question , which we debated at length ,  ideally we need a plant which is both nutrient responsive as well as nutrient efficient . It means , it can perform optimum , even under nutrient stress conditions , but , at conditions where soil nutrient level is higher, it will surely perform much better ...But , it will not be a thumb rule..We have seen in crop like citrus , Khasi mandarin grown under toxic level of exchangeable-Fe in kaolinitic Alfisols/Ultisols , when grown under Fe-deficient conditions of Vertisols , leaf Fe concentration does not measure that toxic level , but at the same  time , yield level and quality both are not of that magnitude.. 

Suggestions by Philip is worth considering ..