Enhancement of the Stomatal Response to Blue Light by Red Light, Reduced Intercellular Concentrations of CO₂, and Low Vapor Pressure Differences

Abstract

The effects of environmental parameters on the blue light response of stomata were studied by quantifying transient increases in stomatal conductance in Commelina communis following 15 seconds by 0.100 millimole per square meter per second pulses of blue light. Because conductance increases were not observed following red light pulses of the same or greater (30 seconds by 0.200 millimole per square meter per second) fluences, the responses observed could be reliably attributed to the specific blue light response of the guard cells, rather than to guard cell chlorophyll. In both Paphiopedilum harrisianum, which lacks guard cell chloroplasts, and Commelina, the blue light response was enhanced by 0.263 millimole per square meter per second continuous background red light. Thus, the blue light response and its enhancement do not require energy derived from red-light-driven photophosphorylation by the guard cell chloroplasts. In Commelina, reduction of the intercellular concentration of CO2 by manipulation of ambient CO2 concentrations resulted in an enhanced blue light response. In both Commelina and Paphiopedilum, the blue light response was decreased by an increased vapor pressure difference. The magnitude of blue-light-specific stomatal opening thus appears to be sensitive to environmental conditions that affect the carbon and water status of the plant.Light is one of several environmental parameters which affect rates of water loss and CO2 uptake in the leaves of higher plants (24). Following illumination, uptake of K+ and Cl-and synthesis of malate by the guard cells results in osmotic swelling and an increase in the apertures of the stomatal pores through which gaseous diffusion occurs (15).In the grasses, blue light elicits an initial rapid increase in transpiration followed by a second, slower increase. The initial increase requires blue light (4) and is presumably mediated by a specific blue light response that initiates ion uptake via a chemiosmotic mechanism (27) Various hypotheses have been proposed to explain the enhancement of the blue light response by red light. In the present study, the plausibility of these hypotheses was investigated, using gas exchange techniques in which both stomatal conductance and mesophyll assimilation could be measured. Conductance is a more reliable indicator of stomatal responses and guard cell turgor than is transpiration alone, since transpiration is affected by both stomatal aperture and VPD. The effects of c, and VPD on the blue light response were also investigated. The experiments were performed using Paphiopedilum harrisianum, an orchid species which lacks guard cell chloroplasts (5, 12), and Commelina communis. As with other nongrasses (9), these species lack the rapid blue-light-induced transpiration increase, but exhibit a less rapid stomatal opening response which has been clearly attributed to the specific blue light system (8,17,26).The present results provide information on the mechanistic basis of the enhanced blue light response and are also relev...

Stomatal Blue Light Response Is Present in Early Vascular Plants

“…These observations indicate that the decreased Ci with RL is not sufficient for stomatal opening in these plant species and further implicates the functional roles of BL-dependent stomatal opening. The BL response of stomata depends on Ci in the leaf, and therefore, one role of this response is to meet the demand of CO 2 for photosynthesis by opening stomata (Assmann, 1988). This notion is supported by a recent study showing that a mutant variant lacking a stomatal BL response was less sensitive to decreased CO 2 than wild-type plants (Takemiya et al, 2013a).…”

“…Since such stomatal opening requires BL under the RL or PAR, we call the opening reaction a BLdependent response of stomata. BL-dependent stomatal response takes place and proceeds in natural environments because the sunlight contains both BL and RL and facilitates photosynthetic CO 2 fixation (Assmann, 1988;Takemiya et al, 2013a). In this stomatal response, BL and PAR (BL, RL, and other wavelengths of light) seem to act as a signal and an energy source, respectively.…”

The Magnitude of the Stomatal Response to Blue Light

“…In studying the stomatal response to blue light, the introduction of a gas exchange protocol involving pulse stimulation superimposed on high intensity background red light ( 11) allowed induction of a rapid response to blue light ( Fig. 1) without direct excitation by PAR of photosynthesis in either mesophyll or guard cells (1,11,27). This allowed quantitation of the magnitude of the blue light response, independently of photosynthetic effects.…”

Phosphorylation of BLUS1 kinase by phototropins is a primary step in stomatal opening

“…The greater enhancement of stomatal aperture in response to decreased CO 2 in the WT compared with blus1 is partly due to that the BL-specific response of stomata is stimulated by low intercellular concentration of CO 2 (ref. 28). …”