FOR YOUR EARS ONLY Stephen D. Strachan - Pioneer Hi-Bred

Proceedings of Indiana Crop Adviser Conference 2004

DROUGHT –STRESSED KERNEL SECRETS: FOR YOUR EARS ONLY Stephen D. Strachan - Pioneer Hi-Bred

The size, placement, and amount of substantially alters or disrupts water and kernel set on a corn ear reveals clues to nutrient flow will cause these the environmental history the corn plant meristematic cells to die. Once these experienced during ear growth and cells die, this particular plant part will development. Understanding how corn not recover and the plant itself may only ears respond to stress can help determine partially recover from this stress event, when stress was present and the severity even if this stress is later removed and of this stress. Four critical stages of ear subsequent environmental conditions are development significantly affect the ideal for corn growth. The expression of number and weight of harvestable the ear response indicates which kernels and subsequent grain yield: (1) developmental stages of meristematic approximately V7, when the corn ear is tissues were in progress when the stress setting the maximum number of kernel occurred. rows around the ear; (2) just before Stress early in ear development pollination, when the ear is establishing results in fewer kernel rows around the the maximum number of ovules along ear. Depending upon CRM, the corn the length of the ear; (3) at pollination, plant determines the maximum number when the maximum number of ovules of rows around the ear at approximately are pollinated to form developing stage V5-V8. Figure 1 (left side) shows a embryos; and (4) approximately R3-R5, picture of a developing corn ear at stage when the ear sets maximum kernel size V9. The meristematic dome is present at during the latter portion of grain fill. the tip of the ear, indicating the This presentation illustrates corn ear developing ear is still producing new responses to some of the more common rows of ovules along the length of the stresses that occur during these stages ear. The upper two-thirds of the ear and explains why corn ears respond as show a series of single rows of they do. Corn developmental stages used developing ovules. These ovules in this article are based upon the Iowa eventually divide to produce a pair of State publication How a Corn Plant rows of ovules from each single row. Develops1 . This paired formation is visible near the A critical point to remember is base of the ear. This division explains that meristematic cells — such as why a corn ear always has an even developing ovules, embryos, and kernel number of kernel rows around the ear. cells during grain fill — are living in a constant state of near starvation for Placement of the primary ear water and nutrients. These cells must be varies with corn genetics. The corn fed and watered every day for normal parent line in Figure 1 is 103 CRM and cell growth, division, and development the primary ear (the ear to be harvested) to occur. An environmental stress that is located on the V14 node. In general, corn lines varying from approximately

1 103 to 118 CRM will produce the S. W. Ritchie , J. J. Hanway, and G. O. Benson. primary ear on the V13 or V14 node. 1997. How a corn plant develops: Special Report no. 48. Iowa State University Cooperative Parent lines of earlier maturity will Extension Service, Ames, Iowa.

sometimes place the primary ear on a Ovule growth and development between lower node, such as the V12 node. the stages of ear initiation and pollination is a dynamic, two-step The node of primary ear process. The first step is the initiation of placement is an excellent reference point ovules as previously explained. The to determine when ear initiation starts. A second step is the cell differentiation and general guideline is to determine the cell division that must occur to prepare node containing the primary ear, then these ovules for fertilization. At any subtract seven. This V stage is moment between ear initiation and approximately when the number of pollination, ovule formation differs kernel rows around the ear is being along the developing corn ear’s length. established. For example, the corn line in Ovules near the ear’s base develop first Figure 1 positions the primary ear at the and newer ovules will continue to form V14 node; thus, the number of kernel as development progresses toward the rows around the ear is being established ear’s tip. After the corn plant has at or very near the V7 stage. established the maximum number of Establishment of the number of ovules, the three essential elements of kernel rows around the ear is a critical energy, nutrients, and water to sustain event in a corn plant’s life cycle. If a these developing ovules must be particular corn line normally has 16-18 supplied. If all resources are adequate, kernel rows around the ear, and the ear ovules along the entire ear will develop in question has less than the normal sufficiently to produce silks and be number, then some sort of stress was receptive to pollen. Each ovule has the present at or just before this critical potential to produce a harvestable kernel. stage. From a diagnostic perspective, if From a diagnostic perspective, if an ear an ear has 12 kernel rows around instead has the proper number of kernel rows of the normal 16, then the stress factor around, but the ear has fewer than that caused this event was present at normal kernel rows along its length, then approximately V7. This information sufficient stress of some sort caused this helps to establish a “time window” in event while the corn plant was looking for the environmental event that progressing through the later stages of caused this ear response. ovule development. A second critical time of The third, and most critical, time environmental stress on ear development of environmental stress on ear occurs approximately one-two weeks development occurs during pollination. before pollination. During this interval, There are two basic parts to the the maximum number of ovules that the pollination process. First, viable pollen entire corn ear will produce is must land on receptive silks. Second, the determined. Figure 1 (right side) shows a silks must support formation of pollen primary ear harvested when the corn was tubes to allow male gametes to fuse with at V12. The meristematic dome is no female gametes inside the ovule. Corn longer present, and paired ovule anthers usually release a large portion of formation is apparent along nearly the mature pollen in mid-morning, entire length of the ear. Maximum depending upon environmental potential ovule formation is established conditions. Pollen loses viability within sometime shortly after this growth stage. a few minutes if air temperature is high

(approximately 104° F or 40° C) and the potential for successful fertilization water deficit stress is present. Pollen decreases. grains contain about 80 percent water Environmental stress during when first shed, but die when the water pollination can have substantial effects content decreases to about 40 percent. A on grain yield. For a specific hybrid, lot of corn is successfully pollinated approximately 85 percent of grain yield under higher temperature conditions. If correlates to the number of kernels soil moisture is adequate and the corn produced per acre with the remaining plant can transpire rapidly enough to 15percent being the weight of individual supply the necessary water to the pollen, kernels at harvest (Figure 4). The then the pollen remains viable long amount of water available for silk enough to properly shed and complete growth substantially influences when the fertilization process. However, if silks emerge, their rate of growth, their water supply is inadequate, the pollen length of receptivity, and their ability to will die prematurely. supply water and nutrients to support The second part of successful pollen tube growth and gamete fusion. ovule fertilization is the formation of the From a diagnostic perspective, corn pollen tube and deposition of male plants under stress during pollination gametes inside the ovule. This process produce ears with portions of the cobs relies heavily on the female portion of barren (examples shown in Figure 4). the plant because the silks must supply Portions of the cob are barren because all necessary nutrients and water for mature ovules were not properly pollen tube growth. Based on all of the fertilized. These unfertilized ovules pictures we have seen to date, viable begin to disintegrate and disappear pollen grains adhere to silk trichomes, before the ear reaches physiological not directly to the silks, to start the maturity. fertilization process (Figure 2). The fourth critical time for Trichomes are hair-like projections that environmental stress on ear development extend from a silk’s main stem much occurs during grain fill. A successfully like root hairs extend from a plant root. fertilized kernel goes through two phases Within a few minutes after landing on in the approximately eight weeks the trichomes, the pollen grains initiate between pollination and physiological pollen tubes. These pollen tubes always maturity (approximately 35 percent grain seem to grow near the silk vascular moisture). For approximately the first bundle (Figure 3). This may occur three weeks after pollination, embryo because these vascular tissues contain a cells are rapidly differentiating and readily available source of water and dividing to produce the tissues necessary nutrients essential for growth. for the embryonic corn plant contained Depending upon water availability and within the kernel. The remaining weeks environmental conditions, it may take of grain fill are devoted primarily to anywhere from a few hours to starch and storage tissue deposition to approximately one day for pollen tubes support new plant growth when this to grow all the way to the ovules. When generation of seed is planted. All kernels the corn plant is under greater drought are attached to the cob, and all kernels stress, pollen tube growth is slower and compete for available food and water. Only those kernels receiving ample

moisture and nutrients live. Typically, lack thereof) indicates the time stress kernels near the butt of the ear develop a occurred — whether it occurred before little earlier and are closer to the source or during pollination or during grain fill. of nutrients than kernels at the tip of the If a portion of the cob is barren, with no ear. When stress is present, the ear will evidence of viable kernel formation, the often sacrifice tip kernels in favor of stress occurred at or before pollination. kernels at the butt of the ear (Figure 5). If a portion of the cob shows either very Depending on the severity of the stress, small kernels or kernel dieback, the tip kernel dieback will continue until the stress occurred sometime during the point at which the corn plant has the grain-filling process. If tip kernels did ability to supply adequate water and not abort, but their test weight is nutrients to support growth of the decreased, the stress occurred only remaining kernels. Kernel formation (or during the very latter part of grain fill.

Figure 1. Development of a primary ear, attached to the V14 node, harvested from corn at V9 (left) and V12. Photos courtesy of Dr. Antonio Perdomo, Pioneer Hi-Bred.

Figure 2. Pollen attachment to silk trichomes. Photo courtesy of Dr. Don Aylor, University of Connecticut.

Pollen Tubes Grow Along Silk Vascular Tissue

Antonio Perdomo, Pioneer Antonio Perdomo, Pioneer

Figure 3. Pollen tube growth in corn silk. Photos courtesy of Dr. Antonio Perdomo, Pioneer Hi-Bred.

Grain Weight and Kernel Count of Ears Exposed to Pollen for One Day

120 Grain weight Field is 50% 100 Kernel count silk on Day 4 80

0 Percent of Normal Ear Day Day Day Day Day Day Day Day Day Day Day 234567891011 1 2 3 4 5 6 7 8 9 10 11 Normal Day of Pollen Exposure Sample Day

Grain Yield Per Ear as a Function of Kernels Per Ear

(oz/ear) 2

Grain Harvest Wt 0 0 100 200 300 400 500 600 700 800 Kernels per Ear

Figure 4. Relating kernel count to grain yield. Silks of ears 2-11 were exposed to open pollination only on the designated day and were covered during the rest of the pollination window. Silks of the “normal” ear were exposed during the entire pollination period.

Figure 5. Stress during grain fill resulting in tip kernel dieback and kernel abortion.