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Steve Dixon wrote that the red color in Eusteralis, as seen on the cover of
Kasselmann's book may indicate impending leaf senescence.
I did research on nitrogen and phosphorus deficiencies in various aquatic
plants back in the 60's, and I found that the plants I studied could
tolerate even extreme phosphate deficiency without any die-back. The
plants just got small and dark colored. They never lost any leaves.
Tissue contents expressed as % of dry weight were as low as 0.07 in highly
deficient plants. Critical values (where growth just begins to be
phosphorus limited) were right around 0.15%, Luxury consumption was as high
as 0.71%. Red or purple coloration is well known in phosphorus deficient
plants if the plants make anthocyanins. Some species get bright red or
purple when P-deficient. Others do not and just stay small and green.
Tomatoes, for example, get purple leaves when phosphorus deficient. It is
hard to kill a plant with phosphorus deficiency. They just stop growing
and stay alive for a very long time. Phosphorus is one of the most
commonly limiting nutrients in nature, and most plants are quite tolerant
of P deficiency. Limiting P is a way to stop that sword from pushing the
top off of your tank, while still keeping it looking nice.
Paul Krombholz, in dry central Mississippi, starting on our fifth week
without rain and none in sight.