Physiological Maturity – The End Of The Line (Almost).

DR. DENNIS B. EGLI 

LEXINGTON, KENTUCKY

Physiological maturity (PM) in grain crops occurs when seeds or kernels reach their maximum dry weight. Physiological maturity represents the end of the line as far as the production of yield goes.  Environmental stress or disease and insect infestations after PM will not affect yield per se, but they might influence harvestable yield (and grain quality) and it is the harvestable yield that puts money in the producer’s pocket.

The vascular connection between the mother plant and the seed is severed at PM, so the seed no longer receives water or other raw materials (mostly sucrose and a few amino acids) for growth (to keep it simple, ‘seed’ refers to both soybean seeds and corn kernels). The seed is isolated from the mother plant at PM and is essentially in storage on the plant. 

Seed moisture concentrations at PM are stable across environments and specific for each crop. Soybean seeds reach PM at 55 to 60% moisture. Corn seeds contain 34 to 38% moisture at PM compared with 37 to 44% for wheat. Interestingly, the moisture concentration at which seeds germinate is the same as the concentration at PM.

Seeds are not ready to harvest at PM, moisture levels are too high, so they must dry to a harvestable water concentration before harvest begins. Water loss after PM is primarily a function of environmental conditions (temperature, wind speed, solar radiation, and water vapor content of the air). If its hot and dry the seeds will dry much faster than if its cool and wet. Water levels in the seed may actually increase in extremely wet conditions. Soybean seeds dry much faster than corn kernels, probably because the soybean pods are more exposed to the environment and are less of a barrier to water loss than the husks covering corn kernels.

All seeds on a plant do not reach PM at the same time, which is not surprising, given that they did not start growing at the same time. The occurrence of PM is more uniform, however, than the beginning of seed growth. Consequentially, determinations of PM are usually based on the proportion of seeds that have reached PM, rather than waiting until all seeds on the plant are at PM. Estimating PM a few days early does not make much difference in seed dry weight (yield), because seed growth slows as the seeds approach PM, so very little yield is accumulated in the last few days before PM.

There may also be variation in the timing of PM in a field, often as a result of variation in water availability. Plants on eroded hill tops, for example, which experience more water stress, often reach PM before those in lower areas that are not stressed.

Seed moisture concentration or individual seed dry weight can be used as an indicator of PM, but they are not practical in the field, given that daily estimates are needed to pinpoint PM. Fortunately, research has identified visual seed characteristics of PM that are easy to use in the field.

Soybean: A soybean seed reaches PM when it first turns yellow and yellow seeds are usually found in yellow or brown pods. A plant would reach PM when all the seeds on the plant are completely yellow. Growth stage R7 (one normal pod on the main stem has turned brown or tan) is an acceptable indicator of PM. In our research with several varieties, 96% of the seeds were either completely or partially yellow at growth stage R7. We could not detect any difference in yield between plants harvested at growth stage R7 or at full maturity (growth stage R8). Physiological maturity is usually taken as when 50% or more of 10 consecutive plants in the row have reached growth stage R7.

Corn: The appearance of a black layer at the base of the corn kernel signifies that there will be no more movement of water or raw materials into the kernel – the kernel is at PM. When the milk line that marks the division between the solid and liquid endosperm in the kernel nears the base of the kernel (75% of the kernel’s length contains solid endosperm), the kernel is at PM. Growth stage R6 is an acceptable indicator of PM in corn.

Physiological maturity is an important growth stage in grain crops because it represents the end of the grain-filling period. Yield is made at PM, so management after PM will not increase yield, although stress after PM can reduce the harvestable yield.   

 Adapted from Egli, D.B. 2021. Applied Crop Physiology: Understanding the Fundamentals of Crop Management. CABI. 178 pp.   ∆

DR. DENNIS B. EGLI: University of Kentucky