On 10/06/2017 14:23, Clive George wrote:

I know the answer is "depends on the plant", but I was wondering how
long it would take for the energy investment a plant makes in growing eg
a new leaf or stalk to be recouped.

Depends on the plant *and* the climate.

I thought here might be a good place to ask - any ideas?

Slowest growing desert plants like lithops manage to save enough energy
in a very good year to add another leaf pair and/or flower. But they are
restricted to only growing in a brief period by the very arid climate.

Common or garden plants the true leaves are pretty much self sustaining
from the moment they first appear. That is why you have to persecute
couch grass or ground elder so aggressively to get rid of it. Any leaves
that can see sunlight will be storing energy back into the roots.

Orchids and ferns are probably amongst the most tenuous plants where
payback has to be almost instant since their seed is basically dust with
no reserves for the young plant to fall back on.

If the ambient temperature is about 30C and humid then C4 photosynthesis
allows even faster payback for less water use eg maize and sugar cane.

https://en.wikipedia.org/wiki/C4_car...the_C4_pathway

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Regards,
Martin Brown