By Ignacio Ciampitti
Purpling of plant tissues in sorghum can become more frequent during the fall. In many cases, this is related 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 weak demand for those sugars by grain (the “sink”). Basically, this results in sugar and anthocyanin buildup within the plants during the late-reproductive period.
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.
Symptoms are typically seen 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 sometimes found in sorghum heads when there is poor grain formation and when there has been stressful weather conditions around flowering, followed by a return to favorable conditions during grain filling.
Figure 2. Reddish-purple sorghum plants during the grain-filling period.
To properly 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 biotic or abiotic stress that affected the plant and reduced grain development.
Will the purpling reduce yields?
Not directly, but whatever stress that 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.