Analyzing variation in plant canopy conversion efficiency and assessing canopy and leaf photosynthetic efficiency in soybean with reduced chlorophyll content
The conversion efficiency of absorbed radiation into biomass (εc) is a component of yield potential. Unlike other efficiency components of yield potential, εc in C3 and C4 plant groups is estimated to be much lower than the theoretical maxima, signifying that εc limits yield potential but has room for improvement. This makes it the ideal candidate to increase yields to meet the food and fuel demand of the world population. Understanding the causes of variability in εc are important when considering new approaches to improving εc. Therefore, the first part of this research focused on using past literature to quantify environmental and managerial effects on εc and assess the variability in εc between and within major food and biofuel crops. The second part of this research tested the hypothesis that reduced chlorophyll content would increase εc at both the canopy and leaf level using field and microscopy techniques. In chapter one, a meta-analysis was used to statistically quantify the effects of greenhouse gases, weather-related stresses projected to intensify due to climate change, and management practices on εc from 140 published studies. Significant increases in εc were caused by elevated [CO2], shade, and intercropping, whereas εc was reduced by elevated [O3], water stress, temperature stress, and foliar damage. εc curvilinearly increased with nitrogen and phosphorus applications. These findings suggest that extensive variability is present in εc with external factors, and improved management, breeding for greater stress tolerance, and selecting for enhanced responses to positively contributing factors will increase εc and therefore yields and yield potential. In chapter two, past literature was analyzed to determine current statuses and trends in εc across unstressed food and biofuel crops. Data was mined from 153 studies that measured εc in six important food crops (maize, sorghum, rice, wheat, barley, peanut, soybean, chickpea, pigeonpea) that spanned major functional groups and several energy crop species. Determination of εc in all crop and sub-crop groups demonstrated that in general, εc was greatest in C4 energy crops, followed by C4 food crops, then C3 non-legumes, and finally C3 legumes. Changes in food crop εc over the past few decades were mostly attributed to environmental variability with temperature and solar radiation as the most influential factors. Past improvements in εc due to breeding were very low and suggest that additional breeding for increasing εc is needed, especially as crops are faced with enhanced environmental shifts due to climate change. The impacts of reduced chlorophyll (chl) content on leaf and canopy photosynthetic efficiency in soybean were studied across two field seasons in chapter three. Reducing leaf chl content was hypothesized to improve canopy light distribution compared to the wildtype (WT) by creating a more even balance of light availability between leaf layers. Gas exchange measurements at the leaf level demonstrated greater light use efficiency in chl-deficient mutants when chl content was approximately 30% of the WT. Leaves absorbed less light while demonstrating a similar or greater level of photosynthetic performance, which may have been caused by a more even light distribution within the leaf. Despite similar or greater leaf level efficiency in the chl-deficient mutants, canopy level measures of εc and yield were generally lower and suggest that pleiotropic effects of the mutations causing chl-deficiency, such as reduced water use efficiency, were limiting to canopy processes. Based on the greater leaf level photosynthesis and photosynthetic efficiency apparent in chl-deficient soybean field studies, chapter four examined the light environment within leaves of WT and chl-deficient soybean using a novel technique. Light sheet microscopy effectively measured chl fluorescence profiles within leaves to estimate relative absorption profiles. The chl-deficient mutant had a more gradual gradient in light availability in the leaf as predicted with the greatest differences occurring with blue light illumination from the adaxial surface. Predicted photosynthetic profiles based on chl and light profiles demonstrated a more even distribution of photosynthesis among leaf layers as compared to the WT. However, chl content reductions were greater in chamber-grown plants compared to field-grown plants and led to decreased photosynthetic efficiency at the leaf level, suggesting that chl content was below the threshold for normal photosynthetic capacity. Overall, the research from this dissertation illustrates that εc is highly variable with the greatest proportion of variability in important food and biofuel crops due to the environment and management practices. However, high variability and a large capacity for improvement indicate that εc is a prime target to increase yields. While reducing chl content shows promise for improving εc in dense crop canopies such as soybean, a more fine-tuned approach of lowering chl to an optimal concentration while avoiding pleiotropic effects is needed so that the benefits on leaf level processes are translated to canopy productivity and ultimately yields.
photosynthesis, photosynthetic efficiency, energy conversion efficiency, chlorophyll, food security
University of Illinois - Urbana-Champaign
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University of Illinois at Urbana-Champaign