Purple Corn … Should I Worry?

During the late 1980’s, one of the seed corn dealers stated that, “The first week of June should be declared ‘National Purple Corn Week!’” Purple corn seedlings do not seem to be as large a problem today. But that leads us to ask “what was happening then, what has changed now, and are we likely to see it again?”

There can be numerous triggers for purpling of corn tissue, but the process in the plant is simply an accumulation of sugars with no-where to go. Some of these sugars are converted to anthocyanin, which has a purple colour. In normal growth, the sugars are incorporated into amino acids and complex carbohydrates as the plant grows. When some stress on the plant removes or reduces this sink while photosynthesis continues, the sugars build up.

Purpling is often associated with phosphorus (P) deficiency, and it is true that this is one symptom of a severe shortage of P. It was never completely clear how much, or whether, P deficiency contributed to purple corn seedlings. We do know that root growth, and P transport through the soil to the roots, are both greatly slowed by cool conditions. Purple corn was almost always associated with a change to cool weather while the corn was in the 3-6 leaf stage. The timing also corresponded to the change-over in the plant from the primary roots, which came out of the seed, to the nodal roots that formed closer to the soil surface. If the corn growth was slowed for any reason, there was a lag in the growth of the nodal roots while the primary roots continued to die off, leaving the plant with fewer resources to absorb nutrients from the soil.

Other conditions that contributed to purpling were genetics (some hybrids that were very popular at the time showed purpling at the least stress), soil compaction that reduced root growth, insect damage, or temperature shock as a sudden change to cold weather stalled the corn growth.

The one common factor was that the purpling itself did not seem to hurt the corn yields. A few days of good growing conditions, and the crop grew out of the symptoms and never looked back from there. The exceptions, of course, were the fields where the underlying cause of the discolouration was more serious than simply backwards weather.

So what has changed that we don’t see as much purpling today? The biggest change has been in the genetics used, with a move away from the lines that were most susceptible to purpling, and also breeding for improved stress tolerance in the plants. It is much more common today to hear complaints about poor colour or unthrifty corn, than about purpling. There has also been an increase in the amount of seed-placed starter fertilizer, which improves the early growth of the corn plant. So while the cause of the purpling may not have been phosphorus deficiency, the enhanced vigour of the plant helps it to out-grow the lag period. Finally, it has been a few years since we have had ideal conditions in May for early growth, followed by an extended cool spell in early June. We may see purple corn again, if the weather conditions favour it!

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