The Magnitude of the Stomatal Response to Blue Light

Abstract

The effect of leaf-air vapor pressure difference (VPD) on the magnitude of the stomatal response to blue light was investigated in soybean (Glycine max) by administering blue light pulses (22 seconds by 120 micromoles per square meter per second) at different levels of VPD and temperature. At 20 OC and 25 OC, the magnitude of the integrated conductance response decreased with increasing VPD (0.4 to 2.6 kiloPascals), due to an earlier onset of stomatal closure that terminated the pulse response. In contrast, at 30 OC this magnitude increased with rising VPD (0.9 to 3.5 kiloPascals), due to an increasing maximum excursion of the conductance response despite the accelerated onset of stomatal closure. When the feedforward response of stomata to humidity caused steady state transpiration to decrease with increasing VPD, the magnitude of the pulse-induced conductance response correlated with VPD rather than with transpiration. This suggests that water relations or metabolite movements within epidermal rather than bulk leaf tissue interacted with guard cell photobiological properties in regulating the magnitude of the blue light response. VPD Experimental ProtocolThe leaf was placed in the cuvette and allowed to reach steady-state levels of assimilation and stomatal conductance at a low VPD (ca. 0.4 kPa at 20°C, 0.9 kPA at 30 C). A preliminary blue pulse was administered so that all test pulses, including the first, would be preceded by blue light stimulation. Steady-state was defined as near stability over ten minutes and was quantified as the average over that time of conductance, assimilation, transpiration, VPD, and RH. Blue light pulses were then administered and gas exchange parameters monitored without adjusting any system controls until conductance regained a steady state value. The magnitude of the blue light response (Fig. 1) was quantified as the integrated conductance above the baseline following stimulation by blue light (11,27). Under conditions of slowly changing baseline conductance (increasing or decreasing) the drifting baseline was incorporated into estimates of pulse magnitude as shown diagramatically in Figure 1. The components that determined this magnitude, including duration of response, maximum excursion of stomatal conductance above baseline, and time to reversal from stomatal opening to stomatal closure (T7&v) were also determined (Fig. 1). Kinetic parameters associated with rates of conductance increase and recovery were determined from the slopes of tangents to the ascending and descending portions of the pulse response curves as described previously (2). The water cost of a response to blue light was defined as the integrated transpiration above the baseline after a blue light pulse. Several replicate experiments were performed at each of 20 C, 25 C, and 30 'C. Figures depict data from representative leaves, while tables contain statistical evaluations of the pooled data. RESULTSThe Blue Light Response A transient increase in stomatal conductance was observed following a brief p...

Stomatal responses to VPD utilize guard cell intracellular signaling components

“…In particular, the mechanism underlying stomatal closure following short-term exposure to high VPD, believed to stem from metabolically-controlled solute release from guard cells ( Grantz and Zeiger, 1986 ; Buckley, 2005 ), lacks substantial experimental evidence. Early observations that high VPD accelerates the rate of transient stomatal conductance changes in response to pulses of blue light in both the dicot, soybean, and the graminaceous monocot, sugarcane ( Assmann and Grantz, 1990a ; Assmann and Grantz, 1990b ), are consistent with the hypothesis that VPD modulates intracellular signaling cascades, such as those well-documented ( Shimazaki et al., 2007 ; Assmann and Jegla, 2016 ; Matthews et al., 2019 ) for the phototropin-mediated ( Kinoshita et al., 2001 ; Hosotani et al., 2021 ) rapid blue light response.…”

“…Previous work has suggested possible involvement of localized “water loss sensors” in the guard cells, responding more to transpiration than to RH itself ( Mott and Parkhurst, 1991 ). However, as VPD increases, transpiration can actually decline because of decreased stomatal conductance, obviating a monotonic relationship between stomatal conductance and transpiration that would be expected if transpiration were the parameter being sensed ( Assmann and Grantz, 1990a ; Assmann and Grantz, 1990b ). Due to its important role in stomatal regulation, ABA has been considered as a possible key player in the process of stomatal closure after VPD perturbation ( McAdam and Brodribb, 2016 ).…”

Stomatal response to blue light: water use efficiency in sugarcane and soybean*

“…Stomatal response was assayed as a transient inerease in stomatal conductance following the blue light pulse. Stomata did not respond to equivalent pulses of red light (Assmann & Grantz, 1990a), indicating that the responses reported here were not mediated by guard cell or mesophyll photosynthesis. Attainment of steadystate conditions was defined as relative stability of gas exchange parameters over a lO-min period prior to each blue light stimulus, and was quantified as average values over this period of conductance (g), assimilation (A), transpiration (E), and leaf-air vapour pressure difference (V).…”