Purpling of plant tissues in sorghum can become more frequent during the fall. In many cases, this relates to an abundance of photosynthetic sugars and accumulation of a pigment called anthocyanin (reddish-purple pigment) within the plants. Anthocyanin is a sugar-containing glucoside compound. The accumulation of reddish-purple anthocyanin pigment within the plant is primarily due to an imbalance between continued production of photosynthetic sugars by leaves (the “source”) and small demand for those sugars by grain (the “sink”). This results in sugar and anthocyanin buildup within the plants.
From a physiological perspective, such a sugar buildup might be related to biotic/abiotic stresses that resulted in poor pollination, which reduced the number of grains per head. When this happens, the total amount of grain produced by the head is insufficient to utilize all the sugars generated by photosynthesis. Thus, the sugars and anthocyanin accumulate in the leaves and stems.
Figure 1. Purpling in sorghum during grain filling. Photo by Ignacio Ciampitti, K-State Research and Extension.
Typically, symptoms occur in the upper stem and leaves, close to the head (Figure 1). Less frequently, the symptoms occur in lower sections of the stems (Figure 2). Purpling is occasionally found in heads with poor grain formation and when there has been stressful weather conditions around flowering (either before or after this stage), followed by a return to favorable conditions during grain filling.
Figure 2. Reddish-purple sorghum plants during the grain-filling period. Photo by Ignacio Ciampitti, K-State Research and Extension.
To diagnose the cause of purpling in the stem, split the stem open to check for any damage or discoloration inside. If the stem is white with a creamy texture, and without brown spots or lesions, this indicates the stem is still functional and mobilizing nutrients (carbon) and water from the main plant to the head. In that case, we can say that the purpling is related to an accumulation of sugar within the plant due to lower-than-normal demand by the grain.
Regardless of the specific factor causing this buildup of reddish-purple coloration by anthocyanin late in the season, the purpling does not affect plant functionality. Instead, it is a warning sign associated with the occurrence of an earlier stress that affected the plant and reduced grain development.
Purpling in sorghum can also become more apparent as cooler temperatures slow the movement of plant sugars into grain. This results in a lower than normal demand by the grain as described above and the associated accumulation of photosynthetic sugars. This is often apparent in high elevation areas of western Kansas and areas of northwest Kansas that are typically the first to experience cooler temperatures, especially nighttime temperatures, as grain sorghum is completing the grain filling stages.
Will the purpling reduce yields?
Not directly, but whatever stress occurred earlier to reduce grain counts within the head will almost surely affect yields. A reduction in grain counts due to any biotic (insects, diseases) or abiotic (heat, drought) stresses will produce an unbalance of sugar and anthocyanin buildup if weather conditions during the reproductive stages are favorable for good photosynthesis and plant growth (Figure 3).
Figure 3. Purpling in sorghum, September 2016. Photo by Tom Maxwell, K-State Research and Extension.
Remember to continue scouting your acres for early identification of any potential problems affecting your crops before harvest time.
Ignacio Ciampitti, Crop Production and Cropping Systems Specialist
Lucas Haag, Northwest Area Agronomist – Colby