Purpling of sorghum seedings: What is it and what does it mean?
Purpling of sorghum seedlings during a prolonged period of cool weather is not uncommon. The purple colour is the result of an accumulation of anthocyanin pigment in the leaves of the plant. Generally, this condition is not a cause for alarm and symptoms will disappear once the weather returns to warmer temperatures.
In some cases however, the purpling may be an indication of phosphorous deficiency within the plant caused by inadequate soil nutrition or restricted root growth due to soil compaction or root injury by soil insects.
This is a build-up of pigment. It is the same pigment that causes a reddish-purple coloration in red grapes, red cabbage and leaves in autumn.
Generally it occurs in sorghum seedlings because of a temporary phosphorous deficiency, combined with an accumulation of sugars within the leaves because of restricted root growth and reduced plant respiration induced by cool temperatures.
Some anthocyanin genes are induced by cool temperatures and diminishing levels of phosphorus within the plant. Sorghum varieties vary in terms of the number of genes for anthocyanin production and their potential to turn purple (MR Buster), as opposed to others that contain no genes for anthocyanin and may not turn purple at all. There is no correlation between anthocyanin and performance of a variety.
Anthocyanin occurs in the form of a sugar containing molecule. When plants are able to photosynthesize (make sugars) during the day but temperatures are too cool at night for optimal respiration (the breakdowns of sugars for energy) sugars can build up in the leaves, encouraging anthocyanin pigment formation. Once temperatures warm up, the sugars will be metabolized and plants will begin to take on their normal green appearance.
Will purpling reduce yield potential?
The accumulation of anthocyanin pigment in leaves is not considered to affect yield potential, but the underlying cause of the purpling may impact yields. If purpling is caused by phosphorous deficiency in the soil or long term root growth restriction, it is likely to negatively impact yield potential and a decision should be made on this basis as to replant or remediate.
How do I tell if colouring is cold induced purpling?
Key indicators of cold induced purpling will be if the paddock is affected in a uniform pattern. If there is isolated patches of purpling occurring it is more likely to be an indication of a yield limiting stress. If the weather is about to warm up and daytime and there aren’t large temperature differences between day and night time, the sorghum will correct itself.
If however, the cold snap lasts in excess of a week, the impact of the plant not being able to access enough nutrients and generate enough energy for good root growth is high there is a potential for yield loss.
Generally, there is a higher chance of paddocks of sorghum recovering to produce good yields than needing to be replanted.
New tools for the cupboard
New bio-stimulant and stress inhibitor formulations are being trialled by our Technical teams at Elders. Some of these products are showing signs of providing benefits in the crop protection area as a protective mechanism to mitigate yield loss.
Talk to your Elders Agronomists about some of these options in your planning sessions so that we can advise which product to use in your situation.
Elders’ own sorghum seed varieties have shown strong trial results and may be a good option for plant this season. Visit out Plant Genetics page for more information.
This article was written by Elders Toowoomba’s Technical Service Manager, Maree Crawford. Maree has decades of experience as an agronomist and specialises in seed and grain.