Potassium works in multiple ways to maintain photosynthesis during
periods of moisture stress. This allows plants to continue growing,
albeit at a slower rate, but at a rate that ultimately explains the
improved yields as compared to crops with less potassium. For
photosynthesis to continue, the plant needs to be able to maintain
and increase water uptake and retention as well as maintain the
cellular membranes that protect the chlorophyll structures. This is
achieved by increasing the osmotic (salt) concentration of the
cells and through the rapid closure of the stomata (pores in the
leaf surface). Increasing the osmotic concentration of the cells
has the effect of increasing the activity of aquaporins (channels
in the membranes of the plant cells) which facilitate the uptake of
water. The increase in osmotic concentration also assists the plant
in extracting water from the diminishing supply in the soil and
helps to keep cells turgid. Potassium plays a crucial role in
changing the turgor of the guard cells needed to close the stomata
and prevent water loss. When potassium is in short supply, the time
taken by the guard cells to close increases from a few minutes to
over an hour. As a result the plants continue to lose moisture and
are more susceptible to drought. Plants that experience moisture
stress produce more oxygen radicals which are very reactive
chemical compounds that, if left, unchecked will destroy the
cellular membranes thereby disrupting photosynthesis. To counteract
this, plants produce NADPH oxidase, an enzyme that reacts with the
oxygen radicals and reduces them to neutral compounds. NADPH
oxidase activity is regulated by potassium.
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