Potassium is one of the three primary macronutrients that are essential for plant growth. A deficiency in potassium is known to reduce photosynthesis and inhibit growth. However, it plays a far more complex role in photosynthesis than previously understood, according to a new study published March 9 in The Plant Journal.

Researchers from the Xishuangbanna Tropical Botanical Garden (XTBG) and the Kunming Institute of Botany of the Chinese Academy of Sciences have demonstrated its role extends far beyond its classic function as a simple "stomatal osmoticum", or a substance that controls water pressure in guard cells. It actively regulates the genetic machinery that drives photosynthetic efficiency under both stable and fluctuating light conditions.

Using the tomato plant (Solanum lycopersicum) as a model, the researchers combined gas exchange measurements, cellular anatomical analysis, and transcriptomic profiling to investigate how potassium deficiency constrains photosynthesis.

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

In terms of diffusion limitations, potassium deficiency markedly down-regulated the expression of genes encoding plasma membrane aquaporins and carbonic anhydrase, thereby 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 caused a significant delay in stomatal opening in response to increasing light intensity, while accelerating stomatal closure during transitions to darkness. This temporal lag created a substantial bottleneck for CO₂ fixation, reducing overall carbon assimilation capacity.

Transcriptomic data identified the molecular basis for this sluggish response, revealing altered expression of potassium transporters, anion channels and sugar transporters acting as "molecular brakes" that prevent stomata from responding efficiently to change light intensity.

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