Drought Response Based on Modeling of Leaf Photosynthetic Parameters in Two Gossypium Species

Investigators: Daryl Chastain, Bhupinder Singh, and John L. Snider

Date: 2022

Project Summary

Introduction

Cotton is well adapted to dry areas, but progressive water deficits can lead to decline in net photosynthesis (AN), ultimately reducing yield and resulting in excessive aquifer depletion. However, the exact mechanism responsible for this decline in net photosynthesis (stomatal or non-stomatal) is not fully understood under field conditions, partially due to limitations in the ability to collect these critical field-scale data. To this end, a field study was conducted to quantify the impact of drought, as measured by midday stomatal conductance to water vapor (gsw), on cotton leaf metabolism in pima (Gossypium barbadense) and upland (Gossypium hirsutum) cotton. Survey gas exchange and rapid photosynthetic CO2 response (RACiR) were conducted during flowering. We hypothesized that light saturated midday stomatal conductance could define a pattern of photosynthetic response to moderate drought stress in cotton and could be used as basis for rapid comparisons in photosynthetic limitation under drought. Additionally, we hypothesize that chlorophyll fluorescence may be a target for indicating plant stress and may be used to fine-tune the need for irrigation using remote and proximal sensing techniques.

Materials and Methods

Two cotton species PHY490 WRF (Gossypium hirsutum) and PHY881 RF (Gossypium barbadense) were sewn in the 2017 and 2018 growing-seasons to evaluate the effects of mild and moderate drought on leaf-level metabolism and to determine if fluorescence signals are a potential indicator of stress. Plant water status was monitored using midday leaf water potential and gsw as a reference parameter. Survey gas exchange was conducted on uppermost fully expanded leaf after allowing for stability at 60% relative humidity and 400 ppm CO2 using a LiCOR 6800F at flowering stage. Rapid Photosynthetic CO2 response (RACiR) experiments were conducted on the same leaf following survey measurements. Briefly, leaves were acclimated at 400 ppm CO2 before rapidly decreasing to approx. 10 ppm. CO2 was then increased to 1,000 ppm over 10 minutes. Mesophyll conductance (gm), CO2 concentration at the carboxylation site (Cc ), and ETR were also calculated by standard methods.

Results and Discussion

In this study, we observed no relationship between midday leaf water potential (MD) and net photosynthesis (AN), however we did observe a decline in AN as gsw declined for both Gossypium species (Figure 1).

Correlation analysis indicated typical relationships with AN and parameters associated with stomatal limitations (Ci, Cc , gsw, E); however, it was found that while pima exhibited a strong relationship between maximum electron transport rate and instantaneous electron transport rate (ETR), upland cotton did not. Furthermore, when ETR is broken down into proportions contributing to net photosynthesis and photorespiration (ETRA, ETRP , respectively), we found that a greater proportion of ETR is being shuttled to the photorespiratory pathway in upland, relative to pima, as gsw decreases (Figure 2).

Interestingly, the increase in ETRP observed in upland under drought (Figure 2A) suggests it acquires an alternative pathway to consumes excess energy for normal plant metabolism and to prevent oxidative damage of Photosystem I (PSI) by limiting free electrons beyond acceptor side of PSI. Similarly, Figure 2B indicates lowered steady state chlorophyll florescence (Fs), an indication of non-photochemical quenching, in upland cotton under mild drought stress than pima cotton. The relationship between ETRP and basal fluorescence (Fs) to gsw give insights into biochemical limitations leading to interspecific differences in photosynthetic rates under drought stress. The information could be useful to fill gaps associated with stomatal and non-stomatal limitations under drought in cotton. In addition, Fs could be used as an indicator of drought stress in upland cotton and could potentially be measured using remote or proximal sensing to indicate the need for irrigation prior to a yield-limiting stress event.

• Crop Type:
• Cotton

• Topic:
• Irrigation Scheduling
• Irrigation