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 (epsilon c) is a component of yield potential. Unlike other efficiency components of yield potential, epsilonc in C3 and C4 plant groups is estimated to be much lower than the theoretical maxima, signifying that epsilonc 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 epsilonc are important when considering new approaches to improving epsilon c.;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 epsilonc from 140 published studies. Significant increases in epsilonc were caused by elevated [CO2], shade, and intercropping, whereas epsilon c was reduced by elevated [O3], water stress, temperature stress, and foliar damage. epsilonc curvilinearly increased with nitrogen and phosphorus applications. These findings suggest that extensive variability is present in epsilonc with external factors, and improved management, breeding for greater stress tolerance, and selecting for enhanced responses to positively contributing factors will increase epsilonc and therefore yields and yield potential.;In chapter two, past literature was analyzed to determine current statuses and trends in epsilonc across unstressed food and biofuel crops. Data was mined from 153 studies that measured epsilonc 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 epsilonc in all crop and sub-crop groups demonstrated that in general, epsilonc was greatest in C4 energy crops, followed by C4 food crops, then C3 non-legumes, and finally C3 legumes. Changes in food crop epsilonc over the past few decades were mostly attributed to environmental variability with temperature and solar radiation as the most influential factors. Past improvements in epsilonc due to breeding were very low and suggest that additional breeding for increasing epsilonc 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 epsilonc 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. (Abstract shortened by UMI.).