Resource availabilty and intra-specific competition in maize: A per-plant and canopy-level morpho- and eco-physiological analysis

Christopher R Boomsma, Purdue University

Abstract

Although maize (Zea mays L.) routinely experiences both intra- and inter-specific competition for limited resources, most plant-plant interaction studies in this species have principally focused on maize-weed interactions. Thus very few investigations have considered the impacts of plant crowding and nitrogen (N) availability on maize intra-specific competition. As N fertilizer prices fluctuate upwards, environmental concerns over excessive N application increase, and recommended maize plant densities move progressively higher, it is crucial that the crowding tolerance, N stress tolerance, and N use efficiency (NUE) of current maize germplasm continue to be investigated and improved. Fundamental to these efforts is an understanding of the per-plant and canopy-level morpho- and eco-physiological responses of maize to above- and below-ground intra-specific competition. Achieving such an understanding requires an intense season-long analysis of maize plant hierarchy dynamics that takes both temporal and spatial aspects of intra-specific competition for above- and below-ground resources into consideration. The primary objectives of this three-year field study near West Lafayette, Indiana were to (i) evaluate the N responsiveness, N stress tolerance, and NUE of modern maize genotypes using sub-optimal, optimal, and supra-optimal plant densities (54,000, 79,000, and 104,000 plants ha-1, respectively) with varying levels of side-dress N application (0, 165, and 330 kg N ha -1), (ii) identify key per-plant and canopy-level morpho- and eco-physiological responses to the simultaneous stresses of intense crowding and low N availability, and (iii) investigate plant hierarchy responses to varied resource availability through the intensive, season-long, non-destructive measurement of ≈ 4,000 plants and the employment of theories and analysis techniques frequently used in plant ecology. In addition to multiple canopy-level measurements (e.g., green leaf area index, grain yield), per-plant measurements included, but were not limited to, the number of growing degree days from planting to seedling emergence; anthesis-silking interval; V5, V14, and R1 plant height; R1 and R6 aboveground total biomass; two-dimensional available space theoretically available at planting; R1 total green leaf area and green leaf area ratio; V14, R1, R3, and R5 SPAD; grain yield; kernel number; individual kernel weight; harvest index; and relative vegetative biomass remobilization during the grain-filling period. For the examination of plant hierarchy responses, a plant was classified as dominated, intermediate, or dominant when its grain yield was in the lowermost 25%, middle 50%, or uppermost 25% of the plot-level population of plants, respectively. At optimal and supra-optimal plant densities, maize receiving 165 kg ha-1 of side-dress N displayed strong N responsiveness, high NUE, pronounced crowding tolerance, and plant density independence. However, crowding tolerance was strongly contingent upon side-dress N application for this study's genotypes. Relative to less crowded, N-fertilized environments, the 104,000 plants ha-1, 0 kg N ha-1 treatment combination exhibited (among other features) (i) reduced pre-and post-anthesis plant height and total biomass; (ii) greater pre-flowering leaf senescence and reduced R1 per-plant total green leaf area and canopy-level green leaf area index; (iii) enhanced floral protandry; (iv) lower pre- and post-anthesis leaf chlorophyll content; and (v) reduced kernel weight and lower per-plant kernel number, grain yield, and harvest index. Maize exposed to intense crowding and low N availability also exhibited key characteristics of strongly hierarchical populations of plants including (i) large mean ratios of the dominant plant hierarchy group over the dominated plant hierarchy group for key traits, (ii) high plant-to-plant variability for numerous morpho- and eco-physiological parameters, (iii) frequency distributions with slightly positive skewness and high relative interquartile and total ranges for key parameters, (iv) a high percentage of poorly productive and barren plants, and (v) markedly concave Lorenz curves for per-plant grain yield. Overall, a plant's success within this high stress environment was principally contingent upon its ability to (i) effectively compete for solar radiation through pre-silking stem elongation, (ii) maintain relatively high rates of pre-silking biomass accumulation, (iii) sustain ear biomass accumulation during the critical period bracketing silking (i.e., limit silking delays and early kernel abortion), (iv) produce a relatively large leaf area with high leaf N/chlorophyll levels for sustaining plant and ear growth, (v) maintain leaf N/chlorophyll levels during the grain-filling period (i.e., stay-green) to ensure assimilate availability for kernel growth, (vi) limit the premature remobilization of lower stem assimilates to root tissues, and (vii) markedly remobilize vegetative assimilates for ear growth and development. The results of this three-year study suggest that (i) adequate N availability is critical for high grain production in crowded maize stands since (in part) it reduces plant-to-plant variability for key morpho- and eco-physiological traits and limits the formation of pronounced plant hierarchies and (ii) genetic efforts seeking to improve high plant density tolerance should simultaneously focus on enhancing N stress tolerance and NUE. Enhancing these complex traits mandates an improved understanding of maize per-plant morpho-physiological plasticity and associated plant hierarchy dynamics in conjunction with further evaluation of the maize ideotype. Further extensive studies on the season-long morpho- and eco-physiological dynamics of maize above- and below-ground intra-specific competition are therefore requisite for the genetic improvement of maize abiotic stress tolerance and the agronomic advancement of current maize cropping systems. By finding a strongly negative relationship between early-season plant height variability and overall grain yield in a continuous maize, no-till environment, this study's associated investigation of crop rotation and tillage system impacts on plant-to-plant variability already provides a more robust explanation for low productivity in continuous maize, no-till systems than that previously offered by simple observations of canopy-level reductions in vegetative growth.

Degree

Ph.D.

Advisors

Vyn, Purdue University.

Subject Area

Agronomy|Ecology|Physiology

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