Phototropins but not cryptochromes mediate the blue light-specific promotion of stomatal conductance, while both enhance photosynthesis and transpiration under full sunlight

Abstract

Leaf epidermal peels of Arabidopsis (Arabidopsis thaliana) mutants lacking either phototropins 1 and 2 (phot1 and phot2) or cryptochromes 1 and 2 (cry1 and cry2) exposed to a background of red light show severely impaired stomatal opening responses to blue light. Since phot and cry are UV-A/blue light photoreceptors, they may be involved in the perception of the blue light-specific signal that induces the aperture of the stomatal pores. In leaf epidermal peels, the blue light-specific effect saturates at low irradiances; therefore, it is considered to operate mainly under the low irradiance of dawn, dusk, or deep canopies. Conversely, we show that both phot1 phot2 and cry1 cry2 have reduced stomatal conductance, transpiration, and photosynthesis, particularly under the high irradiance of full sunlight at midday. These mutants show compromised responses of stomatal conductance to irradiance. However, the effects of phot and cry on photosynthesis were largely nonstomatic. While the stomatal conductance phenotype of phot1 phot2 was blue light specific, cry1 cry2 showed reduced stomatal conductance not only in response to blue light, but also in response to red light. The levels of abscisic acid were elevated in cry1 cry2. We conclude that considering their effects at high irradiances cry and phot are critical for the control of transpiration and photosynthesis rates in the field. The effects of cry on stomatal conductance are largely indirect and involve the control of abscisic acid levels.

Figure 1.

Cryptochromes and phototropins control stomatal conductance and transpiration rates under natural radiation. Diurnal course of stomatal conductance (A and D) and transpiration per unit leaf area (B and E) in adult plants of the wild type and of the cry1 cry2 and phot1 phot2 double mutants recorded under the indicated PPFD and temperatures (C and F) in a glasshouse in winter (A–C) or outdoors in summer (D–F). Data are means and se of at least three plant replicates. An asterisk close to a mutant genotype symbol denotes significant differences (P < 0.05) with the wild type according to ANOVA and Bonferroni post tests.

Figure 2.

Diurnal course of stomatal conductance in plants of the wild type and of the cry1 cry2 and phot1 phot2 double mutants exposed to different levels of constant irradiance. Plants were grown in a growth chamber under a PPFD = 170 μmol m⁻² s⁻¹ (12 h light/12 h dark, relative humidity [HR] approximately 50%, 22°C) for 32 d and either remained at that irradiance (A) or were transferred to 17 μmol m⁻² s⁻¹ (B) before the onset of the photoperiod in which stomatal conductance was recorded. Data are means and se of six plant replicates. An asterisk close to a mutant genotype symbol denotes significant differences (P < 0.05) with the wild type according to ANOVA and Bonferroni post tests.

Figure 3.

Diurnal course of net CO₂ uptake in plants of the wild type and of the cry1 cry2 and phot1 phot2 double mutants grown under natural radiation in a glasshouse (same as the experiment in Fig. 1, A–C). Data are means and se of at least three plant replicates for each time and genotype. An asterisk close to a mutant genotype denotes significant differences (P < 0.05) with the wild type according to ANOVA and Bonferroni post tests.

Figure 4

. Fluence rate response curves of net CO₂ uptake in plants of the wild type and of the cry1 cry2 and phot1 phot2 double mutants. Stomatal conductance (A) and net CO₂ uptake (B) as a function of PPFD. Plants were grown in a growth chamber (12 h light/12 h dark, PPFD = 170 μmol m⁻² s⁻¹, HR approximately 50%, 22°C). Data are means and se of at least four plant replicates. An asterisk close to a mutant genotype denotes significant differences (P < 0.05) with the wild type according to ANOVA and Bonferroni post tests.

Figure 5.

CO₂ response curves of net CO₂ uptake in plants of the wild type and of the cry1 cry2 and phot1 phot2 double mutants. Stomatal conductance (A), net CO₂ uptake (B), and ratios between intercellular (C_i) and ambient (C_a) CO₂ concentrations as a function of ambient CO₂ concentration (C). Plants were grown in a growth chamber (12 h light/12 h dark, PPFD = 170 μmol m⁻² s⁻¹, HR approximately 50%, 22°C). Data are means and se of at least five plant replicates. An asterisk close to a mutant genotype denotes significant differences (P < 0.05) with the wild type according to ANOVA and Bonferroni post tests.

Figure 6

. The cry1 cry2 double mutant has reduced stomatal conductance even in the absence of blue light. Plants of the wild type and of the cry1 cry2 and phot1 phot2 double mutants were grown in a growth chamber (12 h light/12 h dark, PPFD = 170 μmol m⁻² s⁻¹, HR approximately 50%, 22°C). During the photoperiod when stomatal conductance was recorded, the plants were exposed for 6 h to red light or white light (A; both at 110 μmol m⁻² s⁻¹) or to different irradiances of blue or red light (B). Data are means and se of at least five plant replicates. In A, an asterisk close to a mutant genotype denotes significant differences (P < 0.05) with the wild type according to ANOVA and Bonferroni post tests. In B, the slope and se are indicated. [See online article for color version of this figure.]

Figure 7.

Increased ABA levels in the leaves of the cry1 cry2 mutant. Plants of the wild type and of the cry1 cry2 double mutant were grown in a growth chamber (12 h light/12 h dark, PPFD = 170 μmol m⁻² s⁻¹, HR approximately 50%, 22°C) and harvested either at midday of day 33 (A) or the following night, 9 h after the end of the photoperiod (B). Data are means and se of three replicates. An asterisk denotes significant differences (P < 0.05) with the wild type according to t tests.

Figure 8.

Exogenously applied ABA eliminates the differences in stomatal conductance between the cry1 cry2 and the wild type. Plants of the wild type and of the cry1 cry2 double mutant were grown in a growth chamber (12 h light/12 h dark, PPFD = 170 μmol m⁻² s⁻¹, HR approximately 50%, 22°C), and ABA was either sprayed (100 μM) on the leaves (A) or added with watering (35 μM; B) at the indicated times (arrows). Data are means and se of five replicates. An asterisk denotes significant differences (P < 0.05) between the wild type and cry1 cry2 according to t tests.