Dynamics of soybean leaf hydraulic conductance and diurnal soybean transcriptome analysis

Leaves must remain hydrated with a constant flow of water through the leaf if stomata are to remain open for CO2 acquisition during the day. Thus, leaf hydraulic conductance (Kleaf), a measure of the leaf’s capacity to transport water to the sites of evaporation within the leaf, is closely linked and potentially limiting to photosynthesis. Because transpiration and photosynthesis are affected by environmental conditions on both short and long time scales, the capacity of soybean Kleaf to acclimate to environmental conditions is of interest, particularly with regard to potential hydraulic limitations to photosynthesis. This dissertation research examines the acclimation responses of soybean Kleaf to climate change environmental conditions and the dynamics of Kleaf on diurnal and season-long time scales. The acclimation of soybean Kleaf to growth at elevated [CO2], elevated temperature, and drought were examined in growth chamber studies and with field-grown soybean at the SoyFACE site in Champaign, IL. Because the expected responses of photosynthesis and transpiration to elevated [CO2] and elevated temperature are well documented, we hypothesized that Kleaf would acclimate in line with photosynthesis and transpiration to remain in balance with water loss through the stomata. However, Kleaf did not acclimate to growth at either elevated [CO2] or elevated temperature, despite reductions in stomatal conductance at elevated [CO2] and increased transpiration demand at elevated temperature. Similarly, Kleaf did not acclimate to a soil moisture deficit, despite declines in both stomatal conductance and leaf water potential. This inability of Kleaf to acclimate to long-term environmental conditions could leave the leaf xylem more vulnerable to cavitation under extreme tensions in the water column, resulting from either very high transpiration demands or very low soil moisture that the continuing progression of climate change portend. Soybean Kleaf proved to be more dynamic over both diurnal and seasonal time scales. Diurnal depressions in Kleaf have been reported in several species, and soybean has been previously reported to have anisohydric regulation of leaf water status, meaning that the stomata remain open for CO2 acquisition during high mid-day vapor pressure deficits at the cost of a decline in leaf water potential. This suggests that soybean Kleaf would need to decline during the day, as low leaf water potential leaves the leaf xylem more vulnerable to cavitation. Diurnal fluctuations in Kleaf were measured at four time points in field-grown soybean, grown at ambient and elevated [CO2] to test the hypothesis that elevated [CO2] would ameliorate a Kleaf decrease because of lower stomatal conductance at elevated [CO2]. Reflecting the pattern of leaf water potential over the course of the day, Kleaf declined over the course of the morning, remained low in the early afternoon, and sometimes began to recover by early evening, although elevated [CO2] did not affect Kleaf depression. Using RNA-seq, 22 soybean aquaporin genes were found to be differentially expressed over the course of the day, suggesting that these proteins may play a role both in the diurnal depression in Kleaf and in refilling embolized vessels. Finally, the diurnal soybean transcriptome was analyzed using RNA-seq to determine what genes, or groups of genes, may contribute to diurnal and circadian cycles in plant function. 30% of expressed soybean genes were found to be differentially expressed over the course of the day. The functional groups with the most diurnally responsive genes included tetrapyrrole synthesis, C1 (methyl and formate) metabolism, and major CHO metabolism (starch and sucrose). These changes were linked to typical diurnal cycles in soybean photosynthesis and metabolism, and diurnal expression patterns for individual genes in these categories were visualized with MapMan software. Overall, this dissertation research illustrates that soybean Kleaf is dynamic over time in a predictable manner, but that it does not have the phenotypic plasticity to acclimate to changing environmental conditions in ways that reflect photosynthetic and gas exchange properties of the leaf. It is likely that soybean Kleaf has been indirectly bred to be well in excess of what would limit gas exchange at current climate conditions, but this may result in hydraulic limitations to photosynthesis as climate change causes more frequent, extreme environmental conditions. Because Kleaf is dynamic over diurnal and seasonal time scales, future research may elucidate the mechanisms controlling these Kleaf changes and direct improvements to Kleaf to keep pace with needed photosynthetic increases for agricultural production