According to the Potash Development Association (PDA), potassium affects both the yield and quality of grain, as well as the general health and vigour of the plant.
Cereal crops need at least as much, if not more, potassium than any other nutrient, including nitrogen.
Potassium is needed in such large amounts because it is the major regulator of solution concentrations throughout the plant.
It controls cell sap content to maintain the turgor of the plant, and supports the movement of all materials within the plant.
Root development
Potassium supply is therefore essential for all nutrient uptake by the roots and movement to the leaves for photosynthesis, and for the distribution of sugars and proteins made by the green tissue for plant growth and grain fill.
Cereals convert natural resources, including water, solar energy and carbon dioxide into grain.
With light being one of the most common limitations for Irish crops, in most seasons, the more light intercepted, the higher the yield.
As cereals are annual crops, they are not able to capture sunlight all year. It has been suggested that crops grown in the UK and Ireland have the ability to capture up to 60% of the season-long energy.
Most of the ‘lost’ solar energy occurs later in the season, as crops begin to senesce.
Senescence
The canopy starts to senesce from June onwards, but the speed of senescence will depend on a variety of factors, including moisture availability.
While there is little that can be done about the weather, one of the variables that can be controlled is the supply of potassium to a crop.
The level of the nutrient available to a crop can have a dramatic impact on the speed of crop development from flowering through to ripening.
Measurements taken from soils with varying levels of potassium confirm that crops, which were well supplied with the nutrient, took 29 days longer to fully senesce than crops that were deficient.
This represented a 64% increase in relative terms according to the PDA.
Moisture stress
With moisture limitations becoming more probable towards the end of the growing season, one of the reasons for the big difference in speed of ripening from potassium deficient crops is likely to be related to the nutrient’s impact on stress.
Moisture stress significantly reduces growth and accelerates leaf senescence, due to the reduced time to translocate metabolites from leaves to grain, which ultimately affect grain yield and quality.
Delayed leaf senescence can therefore facilitate plants in remobilising nutrients from old senescing leaves, to young leaves and the developing grains.
The potash association has said that the maintenance of a favourable water status is critical for plant survival under drought stress.
Osmotic adjustment is a major trait that is associated with maintaining cell turgor pressure and water retention in response to drought stress.
As the plant preferred ion, potassium plays a key role in maintaining the water content and hence turgor pressure of each cell, even under drought conditions.
Stomata
Plants also depend on potassium to regulate the opening and closing of leaf stomata (tiny apertures on the underside of leaves).
These are important for allowing the movement of carbon dioxide (CO2) into the plant, while releasing oxygen, the PDA said.
The plant regulates the opening and closing of the stomata through movement of potassium into or out of the guard cells.
When potassium moves into these cells, they accumulate water as a result of their higher salt concentrations (through osmosis) and therefore swell, opening the pores.
During drought stress, quick stomatal closure and internal moisture preservation are essential for plant adaptation to drought conditions.
If potassium levels within the plant are low, the stomata become slow to respond and do not close as quickly – resulting in the loss of water vapour which if not corrected, could lead to premature crop senescence.
Grain number and weight
The yield of a grain crop depends on three key factors: the number of ears per unit area; the number of ripe grains per ear; and the weight of the grain.
There is significant interaction between all three elements, and although increasing all three may theoretically result in an increased yield, the reality is that increasing one will inevitably lead to a reduction in another, the PDA added.
E.g., high ear numbers can result in shading, which leads to fewer grains in each ear.
Due to its influence on photosynthesis and assimilate transport, potassium is particularly effective in the improvement of grain number and grain weight.
According to the Agricultural and Horticultural (AHDB) Wheat Growth Guide, the benchmark for grain number per ear is 48.
With 460 surviving fertile shoots/m2, this gives 22,000 grains/m2. Cool, bright conditions – especially one or two weeks before flowering – can prolong or enhance the ear formation period and increase grain number per ear.
However, inclement weather at flowering, such as heavy rain, heat or drought, can impair pollination and reduce the number of fertilised florets.
Adequate potassium levels in plants leading up to anthesis can reduce the shading stress in wheat, potentially through changing the ability of chlorophyl to intercept light, and therefore enhance photosynthesis.
Grain filling
Grain filling starts when flowering is complete, and continues until grain reaches about 45% moisture.
Grains accumulate more water than dry matter (DM) for about four weeks after flowering, when water content is at its maximum.
Water enables cells first to divide, then expand. Severe drought or disease can significantly reduce grain size. As water uptake stops, DM accumulation accelerates.
Rapid dry weight growth continues with starch and protein deposition in expanded grain cells; these are supplied by both photosynthesis and redistribution of reserves.
Sub-optimal photosynthesis during the first two or three weeks of grain growth reduces the cell number and potential weight of each grain.
More carbohydrate is produced during the construction phase than is required by the plant, therefore these stored reserves are utilised during grain fill through redistribution within the plant.
Adequate potassium levels are required to maximise the production of these carbohydrates. This is linked to the impact on stress and photosynthetic rates within the plant.
However, potassium also acts to help translocate these carbohydrates to the grain.
