Thomas R. Sinclair

Radiation Use Efficiency

Publisher Summary This chapter reviews the various aspects of radiation use efficiency (RUE). Topics that are covered include theoretical analyses, experimental determination and measures, and sources of variability. Crop growth rate (dm/dt) could be readily estimated from successive harvests through the growing season. The studies that examined theoretically the nature of RUE will be reviewed. These theoretical studies help to give a background and framework for evaluating potential sources of variation in experimental measures of RUE. Specifically, theoretical studies help to identify variations in the environment and crops that might influence RUE. The various experimental measures of RUE are also discussed. RUE was calculated based on differing leaf photosynthetic rates of various crop species and varying levels of solar radiation. The chapter compares RUE reported for various species and experiments. The focus is particularly on data collected under optimum, usually control conditions, to compare observations on potential RUE from each study. The collection of a baseline of potential RUE data could serve as a useful reference in developing a realistic perspective on the upper limits of RUE that might be reasonably expected for individual crop species.

Water-Use Efficiency in Crop Production

A prime concern in cultivating crops has always been water availability. The earliest crops may have been seeded about 18,000 years ago on the high dunes area of the Nile floodplain immediately after the flood waters receded (Wendorf et al. 1982). This practice assured adequate moisture for plants to grow and produce grain. Plant water-use efficiency was a topic for early scientific study (Briggs and Shantz 1913, Lawes 1850, Woodward 1699). Knowledge of the factors influencing crop water-use efficiency and a hope to improve the efficiency has continued to be an objective in many modern investigations. Wittwer (1975) identified water as the second-most limiting factor, behind land area, to increasing food production. He argued that a high research priority should be an improvement in the efficiency of water use by plants. Considerable research has been done on crop water-use efficiency during the past century, but much work resulted in empirical conclusions that seemed confusing or contradictory. However, recent developments in the understanding of the physical and physiological processes regulating crop growth and water loss allow crop water-use efficiency to be analyzed in quantitative, mechanistic terms.

Water and nitrogen limitations in soybean grain production I. Model development

A simple, phenomenological model was developed to describe the carbon, nitrogen, and water budgets of a soybean (Glycine max (L.) Merr) crop from emergence to maturity. Daily meteorological data input to the model were minimum temperature, maximum temperature, solar radiation, and precipitation. Leaves were grown to give a linear function of mean daily temperature. From the calculated leaf area index the daily intercepted solar radiation by the crop was determined. Daily carbon accumulation was calculated as a linear function of intercepted radiation. The daily nitrogen accumulation rate was calculated as a linear function of vegetative biomass. Leaf growth, carbon accumulation and symbiotic-nitrogen fixation rates were all made sensitive to the amount of soil water. Experimental data were collected to develop the empirical relationships between these three physiological processes and the amount of transpirable soil water. These data showed nitrogen fixation rates to be more sensitive to soil dehydration than either leaf growth or leaf gas exchange. Preliminary tests showed that model predictions compared favorably with field observations. An analysis of the models behavior showed that crop yield was most sensitive to those variables influencing the interception of solar radiation and its conversion to biomass.

Future contributions of crop modelling—from heuristics and supporting decision making to understanding genetic regulation and aiding crop improvement

Crop modelling has evolved over the last 30 or so years in concert with advances in crop physiology, crop ecology and computing technology. Having reached a respectable degree of acceptance, it is appropriate to review briefly the course of developments in crop modelling and to project what might be major contributions of crop modelling in the future. Two major opportunities are envisioned for increased modelling activity in the future. One opportunity is in a continuing central, heuristic role to support scientific investigation, to facilitate decision making by crop managers, and to aid in education. Heuristic activities will also extend to the broader system-level issues of environmental and ecological aspects of crop production. The second opportunity is projected as a prime contributor in understanding and advancing the genetic regulation of plant performance and plant improvement. Physiological dissection and modelling of traits provides an avenue by which crop modelling could contribute to enhancing integration of molecular genetic technologies in crop improvement.

Potential yield and water-use efficiency benefits in sorghum from limited maximum transpiration rate

Limitations on maximum transpiration rates, which are commonly observed as midday stomatal closure, have been observed even under well-watered conditions. Such limitations may be caused by restricted hydraulic conductance in the plant or by limited supply of water to the plant from uptake by the roots. This behaviour would have the consequences of limiting photosynthetic rate, increasing transpiration efficiency, and conserving soil water. A key question is whether the conservation of water will be rewarded by sustained growth during seed fill and increased grain yield. This simulation analysis was undertaken to examine consequences on sorghum yield over several years when maximum transpiration rate was imposed in a model. Yields were simulated at four locations in the sorghum-growing area of Australia for 115 seasons at each location. Mean yield was increased slightly (5-7%) by setting maximum transpiration rate at 0.4 mm h-1. However, the yield increase was mainly in the dry, low-yielding years in which growers may be more economically vulnerable. In years with yield less than ∼450 g m-2, the maximum transpiration rate trait resulted in yield increases of 9-13%. At higher yield levels, decreased yields were simulated. The yield responses to restricted maximum transpiration rate were associated with an increase in efficiency of water use. This arose because transpiration was reduced at times of the day when atmospheric demand was greatest. Depending on the risk attitude of growers, incorporation of a maximum transpiration rate trait in sorghum cultivars could be desirable to increase yields in dry years and improve water use efficiency and crop yield stability.

Physiological traits for crop yield improvement in low N and P environments

Nitrogen and phosphorus are recognized as essential elements in crop production, but the full extent of the requirement for these elements in the physiological processes leading to crop growth seems not to be always fully appreciated. Virtually all the biochemical compounds in plants that support development and growth contain N and/or P. Deficiencies in either element lead to a lost ability for plant growth such that there is a quantitative relationship between crop yield and accumulation by plants of each of these elements. Few options appear to exist to greatly diminish the requirement for either element in crop growth and the formation of seed yield. Consequently, crop yields cannot be increased without increased acquisition of N and P by plants. If the soil environment does not offer these elements, then crop yield will necessarily be restricted. While little opportunity exists to increase N recovery under low nutrient environments, several options can be investigated for increasing P accumulation by the crop. Ultimately, however, the rigid limitation on yields of inadequate N means that without external supplies of N for the cropping system, biological fixation of N must be enhanced to increase N input. In particular, it appears that considerable research needs to be focused on whole-plant processes in legumes that lead to enhanced symbiotic N fixation. A critical aspect of increased legume production will be improved management of P to allow legumes to achieve high N fixation rates and yields.

Criteria for publishing papers on crop modeling.

Abstract Manuscripts describing crop models are a common feature in crop science journals. Many of these papers offer important conceptual insights and advances in the understanding of crop science but some fail to offer the scientific innovation expected in a scientific publication. Even though manuscripts may describe modeling efforts of practical perspective with localized interest, they may not present an analysis of general, scientific interest. A difficult challenge for journal referees and editors is to make decisions on submitted manuscripts concerning their acceptability for journal publication. The discussion presented in this paper is intended to initiate a consideration of those traits expected of a manuscript describing a crop model. We suggest three criteria that should be met in a crop modeling paper to make it suitable for scientific publication: a clear statement of a scientific objective with a defined domain of relevance, a mechanistic framework, and an evaluation of the scientific innovation offered in the new model. We also discuss the use and abuse of three widely used modeling concepts: calibration, validation, and universality.

Challenges in breeding for yield increase for drought

Crop genetic improvement for environmental stress at the molecular and physiological level is very complex and challenging. Unlike the example of the current major commercial transgenic crops for which biotic stress tolerance is based on chemicals alien to plants, the complex, redundant and homeostatic molecular and physiological systems existing in plants must be altered for drought tolerance improvement. Sophisticated tools must be developed to monitor phenotype expression at the crop level to characterize variation among genotypes across a range of environments. Once stress-tolerant cultivars are developed, regional probability distributions describing yield response across years will be necessary. This information can then aid in identifying environmental conditions for positive and negative responses to genetic modification to guide farmer selection of stress-tolerant cultivars.

On Systems Thinking, Systems Biology, and the in Silico Plant

The recent summary report of a Department of Energy Workshop on Plant Systems Biology (P.V. Minorsky [2003] Plant Physiol 132: 404–409) offered a welcomed advocacy for systems analysis as essential in understanding plant development, growth, and production. The goal of the Workshop was to consider methods for relating the results of molecular research to real-world challenges in plant production for increased food supplies, alternative energy sources, and environmental improvement. The rather surprising feature of this report, however, was that the Workshop largely overlooked the rich history of plant systems analysis extending over nearly 40 years (Sinclair and Seligman, 1996) that has considered exactly those challenges targeted by the Workshop. Past systems research has explored and incorporated biochemical and physiological knowledge into plant simulation models from a number of perspectives. The research has resulted in considerable understanding and insight about how to simulate plant systems and the relative contribution of various factors in influencing plant production. These past activities have contributed directly to research focused on solving the problems of increasing biomass production and crop yields. These modeling approaches are also now providing an avenue to enhance integration of molecular genetic technologies in plant improvement (Hammer et al., 2002).

A model to assess nitrogen limitations on the growth and yield of spring wheat

Under many conditions the availability of soil nitrogen imposes an important constraint on wheat (Triticum aestivum L.) yields. In this paper a simple, mechanistic model of spring-wheat growth and yield was proposed to account for nitrogen uptake and use by the crop. A soil nitrogen balance was developed so that crop nitrogen uptake was restrained when the available mineral nitrogen in the soil was exhausted. The crop nitrogen uptake rate was calculated as a function of cumulative thermal units. Leaf area development and maintenance, radiation-use efficiency, and stem growth were assumed to depend on accumulated nitrogen. Seed growth resulted in nitrogen transfer from leaves and stems to the seeds. The model was compared against data collected in nine years of experimentation at Gilat, Israel. Good agreement between observed and simulated yields was obtained for varying nitrogen fertilizer treatments (r2=0.94) and levels of soil nitrogen following fallow (r2=0.93).