Abstract
Structural–physiological factors affect the accurate estimation of vegetation gross primary production (GPP) under various types of environmental stress. Solar-induced chlorophyll fluorescence (SIF), which is directly linked to photosynthesis, has been effectively used to estimate and monitor GPP. However, understanding of the physiological mechanism linking SIF to GPP under stress remains limited. In this article, the link between SIF and GPP at diurnal and seasonal timescales was explored for a maize field and its response to drought stress (as defined by the crop water stress index, CWSI) using three-years of continuous tower-based measurements was investigated. The results show that the ratio of GPP to SIF decreased with increasing drought stress levels, and the canopy stomata conductance (Gs) declined synchronously. Compared to two canopy structural factors (NDVI and NIRv), both the Pearson and partial correlation coefficients for the relationship between Gs and the ratio of GPP to the total SIF was higher (0.38, p ${{\bf \Phi }_F}$ tracked the changes in LUE well under drought conditions (CWSI > 0.6), which demonstrated that SIF can be a powerful parameter for estimating GPP under drought stress. However, there was a smaller drop in ${{\bf \Phi }_F}$ under drought stress (slope = –0.002) compared to the slope for the relationship between LUE and CWSI (–0.08). The response of light reactions to drought stress may be muted compared to the stomatal response. These findings confirm that the Gs is sensitive to drought and is important for the SIF-based GPP estimation model. It also provides reliable evidence that SIF data include a large amount of physiological information and can serve as a potential indicator for detecting drought and estimating GPP.
Highlights
Photosynthesis is associated with one of the largest carbon dioxide fluxes and is the most important biochemical process in terrestrial ecosystems; the measurement of photosynthetic carbon uptake plays an important role in understanding the global carbon cycle [1,2,3,4,5]
It was observed that fluctuations in the diurnal gross primary productivity (GPP) and Solar-induced chlorophyll fluorescence (SIF) were mainly driven by changes in photosynthetically active radiation (PAR), whereas the diurnal SIF– GPP relationship varied between slight drought and drought days (Fig. 4)
We observed that the GPP decreased at the same SIF level or the SIF increased at the same GPP level on drought days; this effect could be demonstrated from the SCOPE simulation results (Fig. S5a)
Summary
Photosynthesis is associated with one of the largest carbon dioxide fluxes and is the most important biochemical process in terrestrial ecosystems; the measurement of photosynthetic carbon uptake plays an important role in understanding the global carbon cycle [1,2,3,4,5]. The response of gross primary productivity (GPP) to climate has been shown to vary across space and time. This effect has been hypothesized to be the result of the interaction between multiple bioclimatic factors, including environmental energy (i.e., temperature and radiation) and water availability. Drought stress is a significant issue that affects plant photosynthesis [6]. Conventional remote sensing techniques, such as optical, thermal infrared and microwave remote sensing, have been extensively used to estimate soil moisture status by exploiting the surface reflection, surface temperature and backscatter coefficient. Uncertainties remain in estimates of instantaneous vegetation stresses because of a lack of information about plant physiological status
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