Differential sensitivities of photosynthetic processes and carbon loss mechanisms govern N-induced variation in net carbon assimilation rate for field-grown cotton

Abstract

Nitrogen (N) deficiency limits net carbon assimilation rates (AN), but the relative N sensitivities of photosynthetic component processes and carbon loss mechanisms remain relatively unexplored for field-grown cotton. Therefore, the objective of the current study was to define the relative sensitivity of individual physiological processes driving N deficiency-induced declines in AN for field-grown cotton. Among the potential diffusional limitations evaluated, mesophyll conductance was the only parameter substantially reduced by N deficiency, but this did not affect CO2 availability in the chloroplast. A number of metabolic processes were negatively impacted by N deficiency and these effects were more pronounced at lower leaf positions in the cotton canopy. RuBP regeneration and carboxylation, AN, and gross photosynthesis were the most sensitive metabolic processes to N deficiency, whereas photosynthetic electron transport processes, electron flux to photorespiration, and dark respiration exhibited intermediate sensitivity to N deficiency. Among thylakoid specific processes, the quantum yield of PSI end electron acceptor reduction was the most sensitive process to N deficiency. It was concluded that AN is primarily limited by Rubisco carboxylation and RuBP regeneration under N deficiency in field grown cotton and the differential N-sensitivities of the photosynthetic process and carbon loss mechanisms contributed significantly to photosynthetic declines.

Diffusional and Biochemical Limitations to Photosynthesis Under Water Deficit for Field‐Grown Cotton

“…For generating RACiR curves, the leaf chamber's environmental settings were kept the same as for light‐adapted gas exchange assessments, except for reference CO 2 concentration. For each response curve, reference CO 2 concentration was ramped up from 10 to 1000 μmol mol −1 at a rate of 100 μmol mol −1 min −1 , and A N was recorded every 2 s (Singh et al 2013, Pilon et al 2018, Parkash et al 2023). The reference CO 2 range was chosen based on our previously published observations that traditional curves extending to 1400 μmol mol −1 yielded similar results to curves only extending to 1000 μmol mol −1 (Parkash et al 2023).…”

“…For each response curve, reference CO 2 concentration was ramped up from 10 to 1000 μmol mol −1 at a rate of 100 μmol mol −1 min −1 , and A N was recorded every 2 s (Singh et al 2013, Pilon et al 2018, Parkash et al 2023). The reference CO 2 range was chosen based on our previously published observations that traditional curves extending to 1400 μmol mol −1 yielded similar results to curves only extending to 1000 μmol mol −1 (Parkash et al 2023). Raw RACiR curve data were filtered and corrected using empty chamber CO 2 responses based on previously published recommendations (Coursolle et al 2019).…”

“…The maximum rates of RuBP regeneration ( J max ), Rubisco‐driven carboxylation ( V c,max ), maximum photosynthetic rate at light and CO 2 saturated conditions ( A max ), and CO 2 compensation point ( Γ ; C i value at which A N was equal to zero) were derived from A ‐ C i response curves using an R package ‘plantecophys’ in RStudio software (R version 4.2.2) as explained in previous studies (Duursma 2015, Pilon et al 2018, Parkash et al 2023). Specific kinetic parameters for cotton that were necessary inputs for the derivation of J max and V c,max included the apparent CO 2 compensation point ( ᴦ * = 44 μmol mol −1 ) and the Michaelis–Menten constants for O 2 ( K o = 9.59 kPa) and CO 2 ( K c = 35.79 Pa) documented elsewhere (Harley et al 1992a, Singh et al 2013, Lei et al 2021, Parkash et al 2023). Estimated values of g m from the present study were also used during model fitting (Gu et al 2010, Duursma 2015).…”

“…A number of studies have documented reductions in A N under water deficit conditions in cotton (Loka et al 2011, Deeba et al 2012, Chastain et al 2014, Snider et al 2014, Yi et al 2016b, Hasan et al 2018, Meeks et al 2019). However, several component processes govern A N (Parkash et al 2023), and it is important to identify the most drought‐sensitive processes in field‐grown cotton. This information will deepen our understanding of crop response to water deficit stress and potentially identify targets for photosynthetic improvements.…”

“…In potted‐plant experiments on cotton, ETR was negatively impacted by water deficit stress, depending upon the severity and duration of the stress (Ennahli and Earl 2005, Zou et al 2022). In the event that drought stress limits ETR , chlorophyll a fluorescence induction kinetics (commonly referred to as OJIP fluorescence) can provide detailed information on the structure and function of thylakoid components (Strasser et al 1995, Strasser et al 2000, Strasser et al 2010, Snider et al 2018, Snider et al 2022, Parkash et al 2023). OJIP fluorescence provides information on energy absorption by the antenna complex of the photosystem II (PSII), energy trapped by PSII reaction centers, electron flux to electron acceptors present between PSII and PSI, and finally electron flux to reduce electron acceptors at the PSI end (Strasser et al 2010).…”