The first part of this series described how the growth of crops like corn is driven by heat accumulation. For corn the accumulation of heat determines the growth stage of the plant. Naturally light, water, and nutrients are important but heat accumulation determines the rate of growth. For crops such as soybeans heat is certainly important and the proper temperature range enhances growth but day length, or more precisely night length determine the onset of flowering and therefore bean production. So how is soybean production in Kansas determined by light?
• If you were a summer annual row-crop like soybeans, your primary function is to make sure the species continues. You do this by producing seed before a hard fall freeze comes along and you die. How does the plant sense when it is necessary to start flowering in time so it can produce viable seed for next year’s soybean plants?
• First, air temperature won’t work since a soybean plants flower for an extended period of time, determined by the maturity group, and need approximately two months from the beginning of flowering until seed is maturing. Air temperatures can be summerlike well into September and even October. And soybean plants can essentially shut down when exposed to cool temperatures not that close to freezing.
• Light is the key to initiate flowering or more precisely dark. Day length increases until the summer solstice and then starts to gradually decrease until the winter solstice when day length increases again. Soybean plants have evolved taking this decreasing day length, increasing night length, into account.
• Soybeans are termed short-day plants. They start to flower in response to a critical night length. This critical night length varies by the maturity group and even within it. Simply put, once the night increases to a certain length, it keys the plant to begin flowering and reproduction. This is why when producers double-crop soybeans after wheat they seem to begin flowering almost immediately while those planted in early May take almost two months. How does this work?
• Soybean plants contain a compound termed phytochrome. This compound is found in two forms red and far red. In the presence of light the red form very rapidly turns into the far red form. The far red form turns back very slowly to the red form. So when night reaches a genetically determined length, floral initiation occurs based on the ratio in the plant of far red to red.
• Interestingly even with long enough nights, if you expose the soybean plant to a brief period of light the red turns far red rapidly enough it will prevent flowering. If you did this every night, the plant would never flower.
Finally, wheat is just the opposite. To protect itself from flowering when it is too cold, in addition to vernalization, wheat is a long-day plant and requires an increasing night length in addition to proper temperatures to flower.