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Sorghum seedlings may exhibit a purple coloration of the leaf sheaths, tips, and margins, or entire leaf blades during a prolonged period of cool weather. Generally, this condition is not a cause for alarm and symptoms will disappear after favorable weather returns. The most common causes of purple sorghum seedlings do not translate into reduced yield potential. However, it may be prudent to identify if there are yield-limiting factors contributing to purple sorghum seedlings, such as soil fertility issues.
The purple coloration occasionally seen in sorghum seedlings results from the buildup of a pigment called anthocyanin. This is the same pigment that causes a reddish-purple coloration in red grapes, red cabbage, autumn leaves, and many other plants. In some plants, anthocyanin production is associated with stresses including cool temperatures, nutrient deficiencies (namely phosphorous), and pest and pathogen attack.1
Cold-induced gene expression. Some anthocyanin genes are induced by cool temperatures (a stress response).2 Anthocyanin production is also associated with diminishing levels of phosphorus within the plant.3 Sorghum products vary in terms of the number of genes for anthocyanin production and their potential to turn purple. Some sorghum genotypes contain no genes for anthocyanin and may not turn purple.
Sugar accumulation. 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 breakdown of sugars for energy) sugars can build up in the leaves, further encouraging anthocyanin pigment formation. Once temperatures warm up the sugars will be metabolized and plants will begin to take on their normal green appearance.
Restricted root growth. In the early stages of growth plants rely on nutrients in the seed for growth and development. When the seed reserves are depleted plants must rely on their root system to obtain the nutrients they need. Young seedlings that experience restricted root growth, as can happen in cool, wet soils, may have not yet developed sufficient root systems to obtain enough phosphorus from the soil. Phosphorus deficiency within the plant will affect sugar metabolism since phosphorus is required for sugar transport and respiration.
Inadequate soil fertility, compaction, and root injury. Inadequate soil phosphorus, compaction, and root injury from pests, diseases, or herbicides can limit a plant’s ability to reach and absorb enough phosphorus for normal growth. Sorghum plants that are deficient in phosphorus will often take on a dark green appearance and be stunted with thin stems. In more severe deficiencies, leaf tips, margins, and sheaths of older leaves will have a reddish-purple pigmentation. Newer leaves are not affected.
Yellow sugarcane aphid feeding. Purple discoloration is the first symptom of yellow sugarcane aphid feeding in seedling sorghum. The aphids prefer to feed on the underside of the lower leaves and may not be noticed without closer inspection.
The accumulation of anthocyanin pigment in leaves is not considered to affect yield potential, but the underlying cause of the purpling may impact yields.
In general, when the distribution of purple plants is uniform throughout the field it is likely due to the cool temperatures. If purpling is isolated to certain areas of the field it may be an indication of a yield-limiting stress.