Beyond Photosynthesis: How Nitrogen Storage in Lower Leaves Protects Corn Yield

Bob Gunzenhauser, Director of Agronomy

Maintaining a sufficient “solar collection system” in the form of healthy and developed leaves for corn is important. Solar energy is converted to biomass through photosynthesis, and maximizing this photosynthesis process is what drives yield.

As the corn crop reaches its reproductive stage, the plant is no longer developing additional leaves to collect energy – what it has is what it will use to perform grain fill. A corn plant will typically develop 17-20 leaves by the time reproduction starts, but usually the first 5-6 leaves developed will have served their purpose and senesced by this time. The remaining 13-15 leaves are left to continue capturing solar energy in the second half of the plant’s life.

However, the lower leaves do not contribute as much to the overall photosynthetic development as the upper leaves do. Research from Iowa State University and Pioneer in the 1990’s to 2010’s found that V8-V10 leaves contribute less than 10% of the total seasonal dry matter by maturity. Most of that contribution occurred before silking, and during grain fill they contributed nothing or were slightly negative.

University of Nebraska and USDA –ARS (Lindquist et al., 2005 and Archontoulis, et al., 2018) found that in modern hybrids at typical seeding rates (30-34,000 plants per acre), leaves in the lower half of the canopy accounted for < 15% of total canopy photosynthesis from R1 to R5, and the bottom third was usually < 5% contribution.

Upper canopy leaves, defined as the ear leaf and leaves above, typically capture 80% of the light present and generate 80-90% of the total photosynthesis. They are the workhorses, so to speak, that drive grain fill towards physiological maturity.

This research would suggest that the lower 1/3 leaves of the canopy during reproductive contribute little towards photosynthesis, but do they serve another purpose: to be a nearby storage of nitrogen and synthase that the crop can tap into to fill grain?

During reproduction, a corn plant will continue to pull nitrogen from the soil for grain fill development. However, if it experiences low nitrogen availability due to drought or low soil N, it will start to translocate nitrogen from itself, typically from lower leaves and the stalk to the developing ear(s). If this condition continues for a lengthy time, yields will be depressed, typically due to lower seed mass; the ears will still have the same number of kernels established by R2, but each kernel will be lower in mass than if it had sufficient levels of water and nutrients.

A study by Nasielski, et al. 2019 focused on luxury nitrogen uptake in corn. Two nitrogen treatments were made during the vegetative growth phases, low and high. Additional low and high nitrogen applications were made during the reproductive phase to simulate nitrogen stress, and water was also provided or withheld to simulate water stress.

Plants that received the low vegetative nitrogen treatment early on but received additional nitrogen during the reproductive phase yielded as much as plants with the high vegetative nitrogen treatment. However, plants with the same low vegetative nitrogen treatment and did not receive sufficient reproductive nitrogen yielded significantly less than those plants with high vegetative nitrogen treatment and the same reproductive treatments. A similar pattern was displayed whether the plant was well-watered or water-stressed.

Except in the reproductive treatment of high nitrogen and well-watered, the proportion of nitrogen in the grain derived from remobilization from elsewhere in the plant was significantly greater in the high vegetative nitrogen treatments than in the low.

This study showed that having an increased or luxury level of nitrogen in the plant going into the reproductive phase buffered yield against water or nitrogen stress than if the plant had a lower level of nitrogen during the vegetative phase. If the plant had lower nitrogen during vegetative phase, it could yield at parity with the higher vegetative phase nitrogen plant if higher nitrogen and no water stress was present during the reproductive phase; otherwise, the lower vegetative phase nitrogen plants suffered vs those with higher levels.

In a few words, luxury nitrogen content in plants buffers against yield loss, but a higher reproductive nitrogen application can maintain yield when lower vegetative nitrogen content exists, and water is sufficient. This would imply that fertigation or foliar applications during the reproductive phase may be beneficial in maintaining yield, especially if nitrogen was lower or sufficient during vegetative growth and water stress is reduced.

While this study did not specifically investigate nitrogen concentration in lower canopy leaves, it may be assumed that under a luxury nitrogen application program, lower leaves will be natural “stopping points” for nitrogen traveling through the plant and thus will provide a storehouse for later remobilization of the nitrogen to the grain.

Summary:

Fertigation in Tasseled Corn

Proper nitrogen management during the reproductive phase of corn development is somewhat more challenging and detailed than simply managing the vegetative phase only. Building up the solar collection system is key to maximizing yield potential, but having enough nitrogen to maintain grain fill is equally important. Remobilization of nitrogen through the plant is the most likely pathway for grain fill, but if reserves from lower leaves and stalks run short, yields will suffer. Applying additional nitrogen during reproduction may keep corn going that otherwise would have experience premature senescence, and under sufficient water conditions (e.g. irrigation) the potential for success is greater. Farmers should consider how the lower leaves should be used, not so much for solar collection but for nutrient storage for later-season needs to optimize yield.