Potassium is one of the three primary macronutrients essential for plants. Its deficiency is known to reduce photosynthesis and inhibit growth. However, the underlying physiological mechanisms by which potassium regulates photosynthesis remain unclear.

In a study published in The Plant Journal, researchers from Xishuangbanna Tropical Botanical Garden (XTBG) and Kunming Institute of Botany (KIB) of the Chinese Academy of Sciences have demonstrated that potassium’s role extends far beyond its classic function as a simple "stomatal osmoticum" (a substance controlling water pressure in guard cells). Instead, potassium actively governs the genetic machinery that drives photosynthetic efficiency under both stable and fluctuating light conditions.

Using tomato (Solanum lycopersicum) as a model, the researchers integrated gas exchange measurements, cellular anatomical analysis, and transcriptomic profiling to elucidate how potassium deficiency limits photosynthesis.

Contrary to previous assumptions, the study revealed that the significant reduction in mesophyll conductance under potassium deficiency was not attributable to changes in leaf anatomical structure. Instead, transcriptomic analysis uncovered key regulatory pathways.

In terms of diffusion limitations, potassium deficiency markedly downregulated the expression of genes encoding plasma membrane aquaporins and carbonic anhydrase, directly impairing carbon dioxide (CO₂) transmembrane transport efficiency and the interconversion rate of bicarbonate to CO₂, which collectively reduced mesophyll conductance.

Under fluctuating light conditions, potassium deficiency significantly delayed stomatal opening in response to increasing light intensity while accelerating stomatal closure during darkening transitions. This temporal lag created a substantial bottleneck for CO₂ fixation, diminishing overall carbon assimilation capacity.

Transcriptomic data identified the molecular basis for this sluggish response, pointing to altered expression of potassium transporters, anion channels and sugar transporters that act as "molecular brakes" preventing stomata from responding efficiently to changing light.

"Our findings demonstrate that potassium mediates photosynthetic efficiency through genetic regulation rather than anatomical variation. This provides a new perspective on the central role of potassium in optimizing both steady-state and dynamic photosynthesis," said HUANG Wei of XTBG.

First published: 09 March 2026