Has photosynthetic capacity increased with 80 years of soybean breeding? An examination of historical soybean cultivars

Abstract

Crop biomass production is a function of the efficiencies with which sunlight can be intercepted by the canopy and then converted into biomass. Conversion efficiency has been identified as a target for improvement to enhance crop biomass and yield. Greater conversion efficiency in modern soybean [Glycine max (L.) Merr.] cultivars was documented in recent field trials, and this study explored the physiological basis for this observation. In replicated field trials conducted over three successive years, diurnal leaf gas exchange and photosynthetic CO2 response curves were measured in 24 soybean cultivars with year of release dates (YOR) from 1923 to 2007. Maximum photosynthetic capacity, mesophyll conductance and nighttime respiration have not changed consistently with cultivar release date. However, daily carbon gain was periodically greater in more recently released cultivars compared with older cultivars. Our analysis suggests that this difference in daily carbon gain primarily occurred when stomatal conductance and soil water content were high. There was also evidence for greater chlorophyll content and greater sink capacity late in the growing season in more recently released soybean varieties. Better understanding of the mechanisms that have improved conversion efficiency in the past may help identify new, promising targets for the future.

The Impacts of Fluctuating Light on Crop Performance

“…Upon low to high light transitions in some species, stomata conductance continues to climb, even after maximum rates of photosynthesis have been reached; thus, these species overshoot the rate of stomatal conductance that achieves maximum photosynthesis, which also reduces water use efficiency (McAusland et al, 2016). Some C 3 crops bred for high photosynthesis rates display lower water use efficiency in fluctuating light (McAusland et al, 2016), likely because selection for yield in non-water-limiting conditions does not penalize accessions with inferior water use efficiencies (Fischer et al, 1998;Koester et al, 2016). Given that crop productivity and yield often are water limited and are likely to become more so (Ort and Long, 2014), improving stomatal closure kinetics and regulation in response to fluctuating light regimes could substantially improve canopy water use efficiency and yield.…”

Variable Mesophyll Conductance among Soybean Cultivars Sets a Tradeoff between Photosynthesis and Water-Use-Efficiency

“…The optimization of harvest indices and interception efficiencies in many of the most widely cultivated crops is nearing its upper limit (Zhu et al, 2010). However, we can improve upon the remaining determinant of yield potential: the efficiency of converting absorbed light to biomass (Beadle and Long, 1985;Slattery et al, 2013;Koester et al, 2014Koester et al, , 2016Slattery and Ort, 2015).…”

Photosynthesis: ancient, essential, complex, diverse … and in need of improvement in a changing world

“…This was nicely demonstrated at the meeting by Lisa Ainsworth (USDA ARS, USA) who reported on mechanisms underlying the historical 80‐year improvement in soybean yield showing that soybean yield has been driven largely by a near doubling of harvest index. While the rate of carbon gain per unit leaf area has increased somewhat in modern soybean cultivars, it has been due to increased stomatal conductance and lower water‐use efficiency, rather than via increases in photosynthetic capacity (Koester et al ., ). Yet, photosynthesis is the only yield determinant that is not close to its biological limits (Zhu et al ., ; Ort et al ., ), suggesting that increases in photosynthesis might indeed lead to increases in yield.…”