(13) STRESS CONTROL FOR ACHIEVING HIGH YIELS: THE QUEST FOR 300 BUSHEL PER ACRE CORN

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1 (13) STRESS CONTROL FOR ACHIEVING HIGH YIELS: THE QUEST FOR 300 BUSHEL PER ACRE CORN Fred E. Below 1 and Martín Uribelarrea ABTRACT Corn is continually subject to environmental and biological stresses that lower its yield potential, and many of the physiological responses to these stresses are modulated by the natural plant hormone ethylene. We are investigating technologies that alter the level of, or the sensitivity to, ethylene as means of protecting the corn plant from stress, and thereby increasing its productivity. The competitive ethylene inhibitor 1-MCP, which decreases plant sensitivity to ethylene, minimizes the negative impacts of supra-optimal plant populations and decreases yield reductions due to high temperature stress. We also show a late-season retention of leaf chlorophyll from strobilurin-based fungicides, which is associated with a decrease in ethylene biosynthesis, and which we believe acts as a long-term signal to alter the senescence trajectory. Tools to manage plant ethylene are clearly one way to mitigate plant stress resulting in higher corn productivity and we discuss these in relationship to other management factors that impact corn productivity. INTRODUCTION Crop productivity results from the integration of multiple environmental and genetic processes throughout the life of the crop, which differ in intensity and timing of action. To maximize grain yield, plants must establish both optimal source and sink capacity and then maintain a high rate of photosynthesis during the grain filling period in order for the sink to achieve that capacity. These environmental and genetic interactions during the growing season alter maize productivity by determining the level of assimilate production from the vegetative source, as well as the ability of the kernel sink to utilize and accumulate those assimilates (Uribelarrea et al., 2009). For the most part, losses in crop productivity are due to abiotic and, to a lesser extent, biotic stresses (Chaerle et al., 2007). These stresses vary widely in their persistency and severity and come from a range of factors that limit the potential growth. Evaluation of the stress source and level to which plants are subjected is critical to quantify their detrimental effects on productivity and to take the proper corrective actions. Abiotic stresses can be caused by temperatures that depart from the optimum range, by a nonoptimal amount or distribution of precipitation, or by an inadequate supply of mineral nutrients, particularly nitrogen. Diseases and pests can also induce stress by reducing green leaf area or by altering plant metabolism. The symptoms of the different stresses can be similar: (i) lower leaf area (LAI), (ii) lower photosynthetic rate per unit of leaf area, and (iii) lower duration of the photosynthetic leaf area. In most stresses, the plant hormone ethylene plays a major role in both perception of stress by the plant and the plant s response to that stress (Davies, 1987; Morgan and Drew, 1997). Understanding 1 Crop Physiology Laboratory, Department of Crop Sciences, University of Illinois at Urbana-Champaign, Urbana, IL

2 how plant stress decreases yield is an important component in attempts to increase crop productivity. Continuous increases in the cost of production inputs serves as an economic limitation to crop productivity, necessitating a better understanding of the value of individual management practices in achieving high productivity. In this regard, we developed the concept of The Seven Wonders of the Corn Yield World as a tool to teach farmers (and crop consultants) the relative importance of management factors that can impact corn productivity. The Seven Wonders ranks the top seven factors that can positively (and sometimes negatively) impact corn yields, and assigns an average bushel per acre value to each wonder (Table 1). One important characteristic of the seven wonders is that they can interact with each other to either magnify or lesson a wonder s impact on yield. Also, as a rough rule, the higher the ranking of a particular wonder, the more control it can exert over the wonders below it. Summing each of the bushel-values of each wonder gives an idea of the yield that can be achieved when all the wonders are optimized, which for our research is 260 bushels per acre. Although still short of the desired 300 bu/acre goal, managing the yield wonders and their positive interactions, gives growers the opportunity to increase productivity with crop management. In this paper, we review the concept of the seven wonders of corn yield, and we present some of our ongoing research investigating the role of ethylene-mediated crop stress on altering these yield wonders and as a result crop productivity. RESEARCH APPROACH: THE SEVEN WONDERS OF CORN YIELD Unfortunately, the first wonder of the corn yield world is the one that farmers have the least control over, the weather. Weather, in the form of rainfall and temperature is a major determinant of when the crop can be planted, and it has a huge impact on plant growth and yield potential (Table 1). Especially affected by weather is nitrogen fertilization (N), the number two wonder of the corn yield world. The ability to apply fertilizer N, its availability or susceptibility to loss, and its impact on grain yield are all heavily impacted by weather. As a result, weather and the availability of N are usually the two factors exerting the greatest impact on corn yields. They can act independently, or be closely linked, and their effects can be to either increase or decrease crop growth and yields. Table 1. The Seven Wonders of the corn yield world. Rank Factor Value bu acre -1 1 Weather Nitrogen 70 3 Hybrid 50 4 Previous Crop 25 5 Plant Population 20 6 Tillage 15 7 Growth Regulators 10 46

3 Nitrogen use also interacts strongly with the third wonder of the corn yield world, hybrid selection, and there is considerable interest in improving the efficiency of N use with genetics or biotechnology. Hybrid selection is probably the most important decision made by farmers, and most do not realize the large difference in yield potential among elite commercial hybrids. Arrays of commercial hybrids, grown under conditions where the other wonders are presumed to be optimized, typically exhibit a 50 bushel range in grain yield. The forth wonder of the corn yield world, previous crop, is gaining in importance as the acreage of continuous corn has steadily increased. This is despite the 25 bushel/acre yield penalty associated with continuous corn, and the higher input cost, especially fertilizer N. Previous crop clearly interacts with the first and second wonders, and if sufficient N is available in a good growing year, the continuous corn yield penalty can be reduced or eliminated. Wonders five, six, and seven have a smaller individual impact on yield, but a larger possibility for interactions with wonders one through four (weather, N, hybrid, and previous crop). It is well known that the fifth wonder of the corn yield world, plant population, has increased steadily over the last 20 years. But, what probably is not well known is how adapt modern hybrids are at flexing their ear components (i.e. kernel number and weight), which makes them less sensitive to differences in plant stand. Still we find that many farmers would benefit from correcting plant populations that are too low, especially if the other yield wonders are optimized. The difficulty is that plant population interacts heavily with the other yield wonders, with high plant populations being particularly susceptible to unfavorable weather conditions. Similarly, some hybrids are clearly more tolerant to high populations than others, and previous crop and tillage can impact plant population by altering seed germination and seedling emergence. The sixth wonder of the corn yield world, tillage, comes in varying degrees, or in differing times. Both aspects interact heavily with the other yield wonders, with the relative advantages or disadvantages of a particular tillage system or time depending largely on the weather, and often interacting with N availability and hybrid. The degree of tillage and the time of tillage can also make a big difference with previous crop since most of the yield penalty associated with continuous corn is due to the residue. Similarly, the tillage system can have a big impact on plant population. Overall, our research shows a 15 bushel yield range due to the various tillage systems, which farmers should optimize for each or their individual fields. The seventh wonder of the corn yield world is a catch-all that we call growth regulators. This includes plant growth regulators, and compounds that exert growth-regulator like effects, and that as a result lead to a positive change in growth or yield determination. Late-season leaf-greening from certain foliar fungicides, and ethylene sensing technologies that make the plant less sensitive to environmental stresses fit into this category. 47

4 RESULTS AND DISCUSSION: STRESS CONTROL WITH PLANT GROWTH REGULATORS As noted earlier, continuous and varied stresses (both biotic and abiotic) are responsible for most decreases in crop productivity, and there is evidence that the plant hormone ethylene is involved in both the perception of the stress by the plant, as well as in the response of the plant to that stress (Davies, 1987; Morgan and Drew, 1997). While some stresses are weather related and therefore out of the reach of farmers, other type of stresses can be managed. In most cases what can be managed are the different yield wonders, and, most importantly, the interactions that occur between them to minimize the negative effects of stress on crop yield. In this work, we focused on the interactions that ethylene-altering chemicals can have with the other yield wonders to alleviate plant stress and increase crop yield. Some ethylene altering compounds are listed in Table 2. An example of a seventh wonder growth-regulator like effect is the late season leaf greening induced by strobilurin fungicides. Strobilurin-treated plants exhibit a slower rate of decline in leaf chlorophyll during the later stages of grain fill, especially for the leaves below the ear (Figure 1). Although the cause of leaf-greening has not been completely elucidated, one prevailing theory is a decrease in the synthesis of ethylene (Grossmann and Retzlaff (1977). While we nearly always observe late-season leaf greening in corn from a strobilurin fungicide application, yield increases are more variable, and appear to be related to weather conditions, mainly rainfall around flowering (data not shown) Table 2. List of some ethylene altering compounds, their effect, and their commercial names. Product Active ingredient/type Role over ethylene Commercial name Strobilurin Lower synthesis Headline (BASF) Fungicide Lower synthesis Quilt (Syngenta) Lower synthesis Stratego (Bayer CropScience) Plant growth regulators aminoethoxyvinylglycine Lower synthesis Retain (Valent BioSciences) 1-methylcyclopropene Lower sensitivity Invinsa TM (AgroFresh) Ethephon Extra ethylene Florel (Rhone Poulenc) Ethephon Extra ethylene Etherl (Bayer CropScience) Since corn exhibits a determinant growth habit, vegetative and reproductive growth phases do not co-exist and yield potential is determined and adjusted at a few key stages based on the growth conditions at that stage. We believe it is these stages that should be targeted with growth regulator chemicals to improve crop productivity, especially those stages in which ethylene is actively involved. To help determine at which growth stages the corn plant is sensitive to excess ethylene we applied exogenous ethylene (as ethephon see Table 2) at five stages during plant development (Table 3). Excess ethylene decreased plant height compared to the untreated controls when applied during any vegetative growth stage, with the greatest magnitude of decrease at the earliest growth stages. Excess ethylene (via ethephon application) also tended to decrease grain yield, especially when it was applied during the earlier vegetative growth stages. Conversely, when 48

5 applied during early grain development (at the R2 growth stage), excess ethylene did not affect plant height or grain yield. Table 3. Effect of a single application of Ethephon (250 g a.i./ha) on different biomass accumulation and partitioning parameters for a corn hybrid grown in Champaign, Il in Plant population and N rate were in accordance with low recommendations for high yield. (*) Indicates significant difference from untreated control (UTC) (p<0.10). Ethephon Plant Stover Grain treatment height weight yield growth stage cm g plant -1 bu acre -1 UTC V5 201* * V10 205* * V15 214* VT 217* * R Ethylene is a gaseous plant hormone usually associated with fruit ripening, kernel abortion and leaf senescence. More recent evidence, suggests that ethylene is also involved in the perception of neighboring plants (together with light quality, and that ethylene responses are enhanced under stressful environments (Pierik et al., (2007). However, as pointed out by Pierik et al, (2006) ethylene (like all plant growth regulators) exhibits a Janis Face, or a bi-model response, where some growth processes are inhibited and others enhanced by the level of ethylene. We depict this response graphically in Figure 2. According to this model altering the level/sensitivity of ethylene in a plant with an exogenous chemical application will either improve or decrease productivity depending on which part of the response curve the plant is at before the application. Based on the results presented in Table 3, we originally believed that excess ethylene during vegetative development was generally detrimental to plant growth and ultimately yield, and as such this was the time that should be targeted for application of ethylene reducing technologies. Contrary to our expectations, but in accordance with the bi-model growth response depicted in Figure 2, we found that too little ethylene can also be detrimental to crop growth when ear abnormalities exhibiting hollow husk symptoms appeared in our trials designed to decrease the level or the sensitivity of the plant to ethylene (Below et al., 2009). Hollow husk is characterized by normal appearing husks which feel hollow due to the cessation in ear development and a lack of silk emergence. It occurred when the foliage of actively growing corn plants was sprayed at the V10 growth stage with a chemical treatment that either lowered the level of plant ethylene (i.e. a strobilurin fungicide), and to a lesser extent one that decreased the plant s sensitivity to ethylene (i.e. 49

6 1-MCP). The percentage of plants exhibiting hollow husk symptoms depended on the management conditions that promoted rapid plant growth. Hollow husk successively increased with increases in N supply and decreased with increases in plant population (Table 4). Almost three times as many plants exhibited hollow husk symptoms when they were grown at the lowest population and with the highest N, compared to those grown with the highest population and with the lowest N. Similar ear abnormalities were observed with AVG (see Table 2) applications in other experiments, further implicating a role for ethylene in ear development. Nevertheless, plants exhibiting the highest severities of hollow husk symptoms, also had a strong late-season greening effect (data not shown), that suggests the ethylene growth response curve depicted in Figure 2 is tissue specific. We believe that the growth response of a plant to an ethylene altering compound will depend on the growing conditions, and how these conditions affect the plant s ethylene level/sensitivity at the time of application (Figure 2). In this regard we demonstrate with data presented in Table 5 that decreasing plant sensitivity to ethylene with 1-MCP can only overcome the negative effect of supra-optimum plant populations only when sufficient N is supplied. On the other hand, when N was deficient (i.e. 40 lbs/acre), both the lack of N and the application of 1-MCP application decreased grain yield. Table 4. The interactive effects of plant population and fertilizer N rate on the percentage of plants with hollow husk symptoms induced by strobilurin application at the V11 growth stage. Plant Pop. N rate (lb N acre -1 ) pl (x1000) acre Average % of plants a b c Average 32.4 c 42.9 b 48.2 a Letters indicate significant differences (p<0.10) within populations or N rates. CONCLUSIONS In summary, we know that the Seven Yield Wonders and the interactions that occur between them can be managed to relieve stresses, thereby improving crop growth and yield. Ethylene plays an important role in how plants sense and respond to stress and technologies are available to decrease the synthesis and/or the perception of ethylene, therefore modulating the plants response to stress. While we find an average increase of 10 bushels/acre from the seventh wonder category, the success of these compounds is highly dependent on the other yield wonders, especially weather and hybrid, and this category has the widest range. Like all of the other yield wonders, there is the possibility for detrimental effects from the seventh wonder, depending on how the other yield 50

7 wonders are altering plant growth at the time of application. Therefore, a good understanding of crop development and biology is needed to successfully use ethylene altering compounds to increase productivity. Table 5. Effect of a single application of 1-MCP (at V11) on grain yield of corn grown under different levels of cultural stress. (*) Indicates significant difference from untreated control (UTC) (p<0.10). Data averaged across 4 hybrids. N rate Plant population UTC 1-MCP N rate (lb N acre -1 ) pl (x1000) acre -1 bu acre * * * * ACKNOWLEDGEMENTS We thank AgroFresh, Inc., BASF, and Syngenta for supporting this research. LITERATURE CITED Below F.E., Duncan K.A., Uribelarrea M., Ruyle T.J. (2009) Occurance and proposed cause of hollow husk in maize. Agron. J. 101: Chaerle L., Leinonen I., Jones H.G., Van der Straeten D. (2007) Monitoring and screening plant populations with combined thermal and chlorophyll fluorescence imaging. J Exp. Bot 58: Davies P.J. (1987) Plant hormones and their role in plant growth and development. (ed.). Martinus Nijhoff Publ., Dordrecht, the Netherlands Grossmann K., Retzlaff G., (1977) Bioregulatory effects of the fungicide strobilurin kresoxim-methyl in wheat (Triticum aestivum). Pest Sci. 50: Morgan P.W., Drew. M.C. (1997) Ethylene and plant responses to stress. Physiol. Plant. 100: Pierik R., Tholen D., Poorter H., Visser E., Voesenek L. (2006) The Janus face of ethylene: Growth inhibition and stimulation. Trends Plant Sci. 11: Pierik R., Sasidharan R., Voesenek L. (2007) Growth control by ethylene: adjusting phenoptypes to the environment. J. Plant Growth Regul. 26: Uribelarrea M., Crafts-Brandner S.J., Below F.E. (2009) Physiological N response of field-grown maize hybrids (Zea mays L.) with divergent yield potential and grain protein concentration. Plant Soil 316: DOI: /s

8 Figure 1. Graphic depiction of strobilurin-induced late season leaf greening in corn plants grown in IL in Fungicide was applied at VT and picture was taken 60 days later (R5). Plant Response to Growth Regulators Optimum level Relative growth Too little ethylene Too much ethylene Ethylene level/sensitivity Figure 2. Theoretical growth response model of a given plant tissue to varying levels of or sensitivity to ethylene. 52