I think this is worth expanding a bit on. I am not sure if I included the AM/wheat system in the course. The main processes of symbiosis establishment are the same as in a pathogenic interaction. So the partners check each other in a multistep crosstalk for the potential outcome. This crosstalk has been found to break at more and more earlier stages with increased dwarfing of wheat varieties. It is tempting to think that the potential for the this signalling has been lost as it was not a breeding target – and as you may remember from the chickpea example from the course, there is a genetic tendency for root hair length. So this loss of signalling competence may well be the case in some of the interactions.

What I assume as general rule is that the plant simply has no photosynthetic reserves to pay in the symbiosis. I have been talking a lot about root system and nitrogen in the course – if you have the nitrate avoidance root architecture and therewith low water availability plus low mineral nutrition in a dwarfed root system with the genetic imperative e.g. to fill an ear (or another improportional organ), there is a fair chance of downregulation of photosynthesis. Such a plant is not an interesting partner for a fungus. Remember AM-fungi need to be paid in carbs but also in lipids. Where are these supposed to come from if photosynthesis is down and the environment oxidized?

So the system of high density, dwarfing and soluble nutrition is somehow balanced in itself if you seek high yield, but not in terms of soil building. Only with additional pampering (foliars, inoculation…) – if you tune up photosynthesis and inoculate, then you run the system in another gear (but this is not what it was originally designed for). Do the test and get some green beauty Kapuler peas and a Lamborn variety. In terms of root systems and nutrient acquisition the Lamborn varieties are classes behind. They are the only varieties out of 50 that manage somehow to show phosphate deficiency symptoms.

So you may see a better performance of a modern lettuce variety if you work with plugs and such cut the taproot development – I have seen exactly the opposite happen in low irrigation and low input system with long taproots outside. But also the same as you with plug-transplants in the greenhouse (it was strongly burnt leaf edges). And there are really decent modern varieties too!

I think that if only a fraction of the breeding effort would be spent to properly maintain and develop the heirlooms for SRI culture we would see fantastic results. Now it is sometimes hard to begin the work with a bunch of misbehaving unselected “varieties” (and some of them really are complete junk! Esp. if you have to work with 15 seeds from the genebank dating from 1950 to bulk them up…). Still, usually they respond quickly. But the point I really want to make is that we are looking into two completely different systems.