UNL Professor, Department of Agronomy and Horticulture
As Nebraska’s irrigated soybean crop moves toward stage R7 (physiological maturity — the end of seed-filling), make sure your crop has sufficient water to ensure that it can keep its stomates open to gather CO2 for photosynthesis. Open stomates, of course, result in loss of water from the humid interior of the leaf to the much less humid atmosphere. Hundreds of H2O molecules escape from the open stomatal pore for each CO2 that enters. In fact, plant dry matter production and transpiration are nearly linearly related. In that sense, you want to make sure that your crop can transpire as much water as needed to fix as much carbon dioxide as possible to ensure a high yield potential arising from greater seed size, the last yield component to be “fixed” by your crop.
So, what symptoms reveal that your soybean crop’s rate of transpiration is greater than the amount of soil water being gathered by the water-absorbing root hairs located about an inch behind each growing root tip? One of the earliest leaf symptoms is the inversion of the terminal leaflet of the three leaflets that comprise a soybean trifoliolate leaf found at each soybean stem (and branch) node. This is the legume equivalent of leaf-rolling in cereals. Figures 1a-c show what this symptom looks like on respective scales of a field, an individual plant row, and an individual single trifoliolate leaf (so-called because it has a terminal leaflet flanked by two side leaflets).
The authors of the second paper in the source list below noted that leaflet inversion began whenever the plants had depleted 60% of the plant available water (PAW) in an Arkansas field (where the atmospheric humidity tends to be greater than that in Nebraska). In fields where the temperature is higher, humidity is lower, and wind speed is a factor, soybean crops may show terminal leaf inversion at lower depletion percentages of PAW, particularly from mid day onward.
Figures 1 a-c. Leaf inversion in soybean is similar to leaf-rolling in cereal crops — a signal that your plant is in water stress as its rate of transpiration is greater than the rate of soil moisture uptake. From top to bottom, these figures show the symptom exhibited in a field, in a row, and in an individual plant.
Don't Cut Irrigation Short
As you try to schedule your last irrigation of the season with the hope that the crop will deplete exactly 50% of the PAW in top 3-feet of soil on the actual day of R7 (PM), keep in mind that if weather conditions suddenly turn sunny, hot, windy, with low humidity, you may need to revise the date and/or amount of that last irrigation.
Source : unl.edu