Green light reversal of blue light-stimulated stomatal opening was discovered in isolated stomata. The present study shows that the response also occurs in stomata from intact leaves. Arabidopsis thaliana plants were grown in a growth chamber under blue, red and green light. Removal of the green light opened the stomata and restoration of green light closed them to baseline values under experimental conditions that rule out a mesophyll-mediated effect. Assessment of the response to green light over a daily time course showed that the stomatal sensitivity to green light was observed only in the morning, which coincided with the use of potassium as a guard cell osmoticum. Sensitivity to green light was absent during the afternoon phase of stomatal movement, which was previously shown to be dominated by sucrose osmoregulation in Vicia faba. Hence, the shift away from potassium-based osmoregulation in guard cells is further postulated to entail a shift from blue light to photosynthesis as the primary component of the stomatal response to light. Stomata from intact leaves of the zeaxanthin-less, npq1 mutant of Arabidopsis failed to respond to the removal or restoration of green light in the growth chamber, or to short, high fluence pulses of blue or green light. These data confirm previous studies showing that npq1 stomata are devoid of a specific blue light response. In contrast, stomata from intact leaves of phot1 phot2 double mutant plants had a reduced but readily detectable response to the removal of green light and to blue and green pulses.

Stomata in epidermal strips from growth chamber-grown Vicia faba leaves opened less in response to white light than did stomata from greenhouse-grown leaves. Chlorophyll-mediated, red light-stimulated opening was similar in stomata from the two growth conditions, but stomata from the growth chamber environment had a severely reduced response to blue light. Transfer of plants between the two growth conditions resulted in an acclimation of the stomatal blue light response. Stomata lost blue light sensitivity within 1 d of transfer to growth chamber conditions and gained sensitivity to blue light over an 8 d period after transfer to a greenhouse. Short-term transfer experiments confirmed that the rapid loss of blue light sensitivity was an acclimation response, requiring between 12 and 20 h exposure to growth chamber conditions. The acclimation of the stomatal response to blue light was inversely related to a previously reported acclimation response in which stomata change between high CO2 sensitivity under growth chamber conditions and low CO2 sensitivity under greenhouse conditions. The time courses of the blue light and CO2 acclimation responses were virtually identical, suggesting the possibility of a common acclimation mechanism.

Recent studies have shown that blue light-specific stomatal opening is reversed by green light and that far-red light can be used to probe phytochrome-dependent stomatal movements. Here, blue-green reversibility and far-red light were used to probe the stomatal responses of the npq1 mutant and the phot1 phot2 double mutant of Arabidopsis. In plants grown at 50 μmol m-2 s-1, red light (photosynthetic)-mediated opening in isolated stomata from wild type (WT) and both mutants saturated at 100 μmol m-2 s-1. Higher fluence rates caused stomatal closing, most likely due to photo-inhibition. Blue light-specific opening, probed by adding blue light (10 μmol m-2 s-1) to a 100 μmol m-2 s-1 red background, was found in WT, but not in npq1 or phot1 phot2 double mutant stomata. Under 50 μmol m-2 s-1 red light, 10 μmol m-2 s-1 blue light opened stomata in both WT and npq1 mutant stomata but not in the phot1 phot2 double mutant. In npq1, blue light-stimulated opening was reversed by far-red but not green light, indicating that npq1 has a phytochrome-mediated response and lacks a blue light-specific response. Stomata of the phot1 phot2 double mutant opened in response to 20 to 50 μmol m-2 s-1 blue light. This opening was green light reversible and far-red light insensitive, indicating that stomata of the phot1 phot2 double mutant have a detectable blue light-specific response.

Recent studies have shown that blue light-specific stomatal opening is reversed by green light and that far-red light can be used to probe phytochrome-dependent stomatal movements. Here, blue-green reversibility and far-red light were used to probe the stomatal responses of the npq1 mutant and the phot1 phot2 double mutant of Arabidopsis. In plants grown at 50 mumol m(-2) s(-1), red light (photosynthetic)-mediated opening in isolated stomata from wild type (WT) and both mutants saturated at 100 mumol m(-2) s(-1). Higher fluence rates caused stomatal closing, most likely due to photo-inhibition. Blue light-specific opening, probed by adding blue light (10 mumol m(-2) s(-1)) to a 100 mumol m(-2) s(-1) red background, was found in WT, but not in npq1 or phot1 phot2 double mutant stomata. Under 50 mumol m(-2) s(-1) red light, 10 mumol m(-2) s(-1) blue light opened stomata in both WT and npql mutant stomata but not in the phot1 phot2 double mutant. In npql, blue light-stimulated opening was reversed by far-red but not green light, indicating that npql has a phytochrome-mediated response and lacks a blue light-specific response. Stomata of the phot1 phot2 double mutant opened in response to 20 to 50 mumol m(-2) s(-1) blue light. This opening was green light reversible and far-red light insensitive, indicating that stomata of the phot1 phot2 double mutant have a detectable blue light-specific response.

Previous work has shown that stomata of growth chamber-grown Vicia faba leaves have an enhanced CO2 response when compared with stomata of greenhouse-grown plants. This guard cell response to CO2 acclimatizes to the environmental conditions on the transfer of plants between the two environments. In the present study, air relative humidity is identified as a key environmental factor mediating the changes in stomatal sensitivity to CO2. In the greenhouse environment, elevation of relative humidity to growth chamber levels resulted in an enhanced CO2 response, whereas a reduction in the light level to that comparable to growth chamber conditions had no effect on stomatal CO2 sensitivity. The transfer of plants between humidified and normal greenhouse conditions resulted in an acclimation response with a time-course matching that previously obtained in transfers of plants between greenhouse and growth chamber environments. The high stomatal sensitivity to CO2 of growth chamber-grown plants could be reduced by lowering growth chamber relative humidity and then restored with its characteristic acclimation time-course by an elevation of relative humidity. Leaf temperature was unchanged during this restoration, eliminating it as a primary factor in the acclimation response. Humidity regulation of stomatal CO2 sensitivity could function as a signal for leaves inside dense foliage canopies, promoting stomatal opening under low light, low CO2 conditions.

Reversal by green light of blue-light-stimulated stomatal opening was found across a number of plant species, including leguminous and nonleguminous dicots and grass and nongrass monocots. Simultaneous exposure to equal fluence rates of blue and green light resulted in ∼50% reversal of normal blue light opening. Complete reversal occurred when the fluence rate of green light was approximately twice that of blue light. These results suggest that blue-green reversibility of stomatal opening is a basic photobiological property of guard cells. The blue-green reversibility of stomatal opening has been hypothesized to ensue from the cycling of two interconvertible, isomeric forms of the blue-light photoreceptor, zeaxanthin. Testing of blue-green reversibility could provide a valuable diagnostic tool for zeaxanthin-mediated blue-light photoperception.

Stomata of growth chamber-grown Vicia faba leaves have an enhanced CO2 response, measured as change in stomatal aperture, compared to stomata of greenhouse-grown leaves. Reciprocal transfer experiments showed that the stomatal response to CO2 acclimated to the growing environment. Stomata of growth chamber-grown leaves transferred to a greenhouse lost their high CO2 sensitivity within 2-3 d while stomata of greenhouse-grown leaves transferred to a growth chamber acquired a high CO2 sensitivity within 5-7 d. Experiments measuring the CO2 responses of stomata in detached epidermis showed that growth chamber and greenhouse-grown stomata have the same contrasting CO2 sensitivity observed in the intact leaf, indicating that the responses reflect intrinsic guard cell properties. The acclimation properties of the CO2 response of guard cells have implications for the understanding of stomatal function under the predicted increases in atmospheric CO2.

Guard cells of the orchid genus, Paphiopedilum have been reported to lack developed chloroplasts and detectable chlorophyll a autofluorescence. Paphiopedilum stomata lack a photosynthesis-dependent opening response but have a blue light-specific opening. The present study found that low fluence rate green and red light elicited stomatal opening in Paphiopedilum and this opening was reversed by far red light, indicating the presence of a phytochrome-mediated opening response. Phytochrome-dependent, red light-stimulated opening was largest under low fluence rates and decreased to near zero as fluence rate increased. A recently discovered green light reversibility of blue light-specific stomatal opening was used to probe the properties of the blue light response in Paphiopedilum stomata. Blue light-stimulated opening was completely reversed by green light in the presence of far red light. Red light enhanced the blue light response of Paphiopedilum guard cells when given as a pretreatment or together with blue light. Analysis of guard cell pigments showed that guard cells have small amounts of chlorophyll a and b, zeaxanthin, violaxanthin, antheraxanthin and lutein. Zeaxanthin content increased in response to blue light or ascorbate and declined in the dark or under illumination in the presence of dithiothreitol, indicating the presence of an active xanthophyll cycle. Thus Paphiopedilum stomata possess both a blue light-mediated opening response with characteristics similar to species with normal chloroplast development and a novel phytochrome-mediated opening response.

Summary The guard cell chloroplast is the site of perception of blue light and of photosynthetically active radiation, and of at least one of the mechanisms sensing CO 2 in the guard cell. The guard cell chloroplast has been the focus of intense controversy over its capacity for light sensing and photosynthetic carbon fixation, and the osmoregulatory mechanisms mediating stomatal movements. It is argued here that a primary reason behind these long‐lived controversies is the remarkable plasticity of the guard cell, which has resulted in responses being generalized as basic properties when opposite responses appear to be the norm under different environmental or experimental conditions. Examples of guard cell plasticity are described, including variation of chlorophyll fluorescence transients over a daily course, acclimation of the guard cell responses to blue light and CO 2 , the shift from potassium to sucrose in daily courses of osmoregulation and the transduction of red light into different osmoregulatory pathways. Recent findings on the properties of the guard cell chloroplast are also presented, including the role of the chloroplastic carotenoid, zeaxanthin, in blue light photoreception, the blue‐green reversibility of stomatal movements, and the involvement of phytochrome in the stomatal response to light in the orchid, Paphiopedilum .

Chloroplasts of guard cells and coleoptiles have been implicated in the sensory transduction of blue light. The present study was aimed at establishing whether the chloroplast of the hypocotyl from Arabidopsis, another blue light-responding organ, has similar characteristics to that of sensory-transducing guard cell and coleoptile chloroplasts. Results showed that the phototropic curvature and arch length induced by blue light in Arabidopsis seedlings matched the distribution of mature chloroplasts in the bending hypocotyl. The bending arch consistently included the region of the hypocotyl containing mature chloroplasts, and never extended beyond that region. Manipulation of the extent of greening of dark-grown hypocotyls by varying red light pretreatments elicited blue light-stimulated curvatures and arch lengths that depended on the duration of the red light pretreatment and on the distribution of mature chloroplasts in the hypocotyl. Albino psd2 mutants of Arabidopsis, which lack mature chloroplasts, are devoid of phototropic sensitivity under conditions in which wild-type seedlings show large curvatures. The star mutant of Arabidopsis has a delayed greening and a delayed phototropic response as compared with wild type. Measurements of photosynthetic oxygen evolution and carbon fixation, dark respiration, and light-dependent zeaxanthin formation in the hypocotyl showed features similar to those of guard cells and coleoptiles, and distinctly different from those of mesophyll tissue. These results indicate that the hypocotyl chloroplast has characteristics similar to those associated with guard cell and coleoptile chloroplasts, and that phototropic bending of Arabidopsis hypocotyls appears to require mature chloroplasts.

Extended periods of high temperature can reduce cotton (Gossypium hirsutum L. and G. barbadense L.) lint yield, even under adequate irrigation. High stomatal conductance may confer some adaptive advantage to genotypes that experience supra-optimum temperatures. The primary objective of this research was to practice divergent selection for stomatal conductance in a segregating population (n = 118 F2.3 progenies) derived from the cross NM24016/TM1. Divergent selection for high and low stomatal conductance was practiced in Maricopa, AZ, in 1996. DNA was isolated from all 118 F2 plants in 1995 and a linkage map produced with 199 random amplified polymorphic (RAPD) and simple sequence repeat (SSR) DNA markers. Genetic analysis of the replicated F3 families in 1996 at Maricopa M. Ulloa, USDA-ARS Crop Genet. & Prod. Res. Unit, Stoneville, MS 38776; R.G. Cantrell, Agron. & Hort. Dep., New Mexico State Univ., Las Cruces, NM 88003; R.G. Percy, USDA-ARS Pima Cotton Breeding & Genet. Unit, Maricopa Agric. Ctr., Maricopa, AZ 85239; E. Zeiger, Dep. of Biol., Univ. of California, Los Angeles, CA 90024; and Zhenmin Lu, 1 Huntsman Dr., Boothwyn, PA 19061. Received 3 Sept. 1999. *Corresponding author (rcantrel@nmsu.edu). Abbreviations: QTL, quantitative trait loci; RAPD, random amplified polymorphic DNA; SSR, simple sequence repeat. 11 CANTRELL ET AL.: STOMATAL CONDUCTANCE IN COTTON permitted identification of quantitative trait loci (QTL) influencing stomatal conductance. Replicated experiments of 20 selected F2.4 progeny (10 with high, 10 with low stomatal conductance) were grown in Maricopa and Las Cruces in 1997. The 10 families selected for high stomatal conductance in 1996 averaged 542.6 mmol H2O m s at Maricopa in 1997 and were significantly (P = 0.0001) different from the mean of the low families (472 mmol H2O m $2 s). The two selected groups were not significantly different for stomatal conductance at Las Cruces (P = 0.0631). Lint yield was significantly (P = 0.0027) affected by selection for stomatal conductance in Maricopa. The F3.4 family group with high stomatal conductance produced the highest cotton lint yield averaging 1842 g plot while the family with low stomatal conductance averaged 1655 g plot. Two putative QTLs for stomatal conductance were identified on two cotton linkage groups. Lint yield of cotton can be dramatically reduced by supra-optimal temperatures during peak reproductive development in irrigated conditions in the U.S. Southwest. Recent studies with pima have shown that increases in stomatal conductance have accompanied increases in cotton lint yields (Lu et al., 1994, 1998; Lu and Zeiger, 1994). The level of stomatal conductance at high temperature was positively correlated with stomatal sensitivity to temperature and independent of photosynthesis (Lu et al., 1996). Our working hypothesis was that selection for high yields has imposed indirect selection pressure for elevated stomatal conductance at supra-optimal temperature under irrigated environments. This increased conductance may reduce leaf temperatures and confer tolerance to high temperatures, especially during critical fruiting periods. Genetic modifications of the sensory transduction pathway(s) are required in guard cells for genetically stable changes in stomatal conductance (Zeiger, 1983). This higher level of conductance provides an avoidance mechanism for heat tolerance in hot environments that would not prevail or contribute to higher lint yields under cooler environments, such as the San Joaquin Valley in California (Lu et al., 1998). The adaptive advantage of greater stomatal conductance would be absent at lower leaf temperatures. Genetic studies of stomatal conductance have been restricted to each cultivated species (upland and pima cotton) and not interspecific hybrid populations. Roark and Quisenberry (1977) found stomatal conductance to be under genetic control in upland cotton. Percy et al. (1996) estimated relatively low heritabilities for the trait in pima populations and reported existence of dominance and epistatic interactions controlling the trait. Divergent selection for stomatal conductance in a pima segregating population revealed that high yielding F4 progeny were derived predominantly from F2 plants with high levels of stomatal conductance (Radin et al., 1994). As a result of selection for high lint yields under hot environments in Arizona, the stomatal conductance of some recent pima cultivars has approached that of upland cotton (Radin, 1992). Genes for high levels of stomatal conductance from upland cotton may have contributed to the increases in the trait in advanced pima lines selected for high lint yields in hot Arizona environments (Lu et al., 1994). Introgression of genes between upland and pima cotton has been a long-standing goal of cotton breeders. Traits that have been the target of introgression include heat tolerance from upland sources and fiber quality from pima germplasm. The degree of introgression is hindered by genetic breakdown in segregating interspecific breeding populations (Stephens, 1949). Genetically stable lines that exhibit significant combinations of upland and pima chromatin have been developed (Tatineni et al., 1996; Cantrell and Davis, 1993). These genotypes display a continuous spectrum of morphological traits between the two parental species. The genetic distance of these lines from typical upland TM1 has been determined using molecular and morphological markers. Evaluation of these lines for stomatal conductance under heatstress conditions reveals that lines more genetically similar to TM1 have higher stomatal conductance values than those more similar to pima 3-79 (Cantrell and Zeiger, 1995, personal communication). One of these introgressed lines, denoted as NM24016, exhibits significant introgression from 12 JOURNAL OF COTTON SCIENCE, Volume 4, Issue 1, 2000 Fig. 1. Flowchart of derivation and evaluation of progeny from the population, NM24016 × TM1 cotton. upland and pima (Cantrell et al., 2000). The variability derived from hybridization of NM24016 with upland TM1 would provide a valuable opportunity for associating introgressed chromosome regions with physiological traits, such as stomatal conductance. Most important characteristics of agricultural crops are quantitatively inherited, and stomatal regulation appears to be one such trait. The location and effects of the genes controlling quantitative traits can be determined by DNA marker-based genetic analysis. A region of the genome linked to or associated with DNA markers that affects a quantitative trait is defined as a quantitative trait locus (QTL) (Geldermann, 1975). Shappley (1996) provided the first linkage map of QTLs in an upland cotton cross. Recently, other cotton QTLs have been identified for fiber quality (Shappley et al., 1998; Jiang et al., 1998). To date, no QTLs have been reported for physiological traits in cotton. The objectives of this study were: (i) to determine the effect of divergent selection for stomatal conductance on lint yield in a diverse segregating population derived from the cross NM24016/TM1, (ii) to determine the stomatal conductance and lint yield of selected progenies in two environments representing drastically different temperatures during peak fruiting periods, and (iii) to utilize DNA markers and a linkage map to identify QTLs controlling stomatal conductance. MATERIALS AND METHODS

In Pima cotton (Gossypium barbadense L.), stomatal conductance shows a strong response to temperature. At high temperature (40˚C), the stomatal conductance of greenhouse- and growth chamber-grown leaves is three and four times higher than that measured at lower temperature (25ºC), respectively. The segregation of stom-atal conductance observed in an F2 population obtained from a cross between a primitive cotton (B368) and a modern Pima line (Pima S-6) increased substantially with temperature in both light and darkness. Furthermore, F2 segregants with high stomatal conductance at high temperature were more sensitive to temperature, showing larger changes in conductance in response to an increase in temperature when compared to F2 segregants having low stomatal conductance. Rates of guard cell respiration measured in enzymatically-cleaned epidermal peels, mechanically isolated from the same F2 plants, showed the same temperature dependence. The temperature-induced respiration enhancement was higher in guard cells with high respiration rates. There were positive correlations between stomatal conductance and guard cell respiration rates, and between stomatal conductance and the sensitivity of respiration to changes in temperature. These results imply that guard cell respiration and stomatal conductance co-segregate in Pima cotton plants, suggesting that guard cell respiration is a component of the sensory transduction pathway controlling stomatal responses to temperature.

Blue light-stimulated stomatal opening in detached epidermis of Vicia faba is reversed by green light. A 30 s green light pulse eliminated the transient opening stimulated by an immediately preceding blue light pulse. Opening was restored by a subsequent blue light pulse. An initial green light pulse did not alter the response to a subsequent blue light pulse. Reversal also occurred under continuous illumination, with or without a saturating red light background. The magnitude of the green light reversal depended on fluence rate, with full reversal observed at a green light fluence rate twice that of the blue light. Continuous green light given alone stimulated a slight stomatal opening, and had no effect on red light-stimulated opening. An action spectrum for the green light effect showed a maximum at 540 nm and minor peaks at 490 and 580 nm. This spectrum is similar to the action spectrum for blue light-stimulated stomatal opening, red-shifted by about 90 nm. The carotenoid zeaxanthin has been implicated as a photoreceptor for the stomatal blue light response. Blue/green reversibility might be explained by a pair of interconvertible zeaxanthin isomers, one absorbing in the blue and the other in the green, with the green absorbing form being the physiologically active one.

Guard cells regulate the dimension of stoma-tal pores in the leaf epidermis, and pore size regulates gas exchange between leaves and the atmosphere. Stomatal function couples the rate of water loss by transpiration with the rate of carbon dioxide uptake by photosynthesis. Guard cells sense multiple environmental and hormonal signals and transduce those signals to modulate the pore apertures to conserve water and ensure a supply of CO2 sufficient for photosynthesis.

The Arabidopsis mutant npq1, which cannot accumulate zeaxanthin because of a defective violaxanthin deepoxidase, was used to investigate the role of zeaxanthin in the stomatal response to blue light. Neither dark-adapted nor light-treated guard cells or mesophyll cells of the npq1 mutant contained detectable zeaxanthin. In contrast, wild-type guard cells had a significant zeaxanthin content in the dark and accumulated large amounts of zeaxanthin when illuminated. The well-documented red light enhancement of blue light-stimulated stomatal opening, in which increasing fluence rates of background red light result in increased response to blue light, was used to probe the specific blue light response of Arabidopsis stomata. Stomata from the npq1 mutant did not have a specific blue light response under all fluence rates of background red light tested. On the other hand, stomata from leaves of hy4 (cry 1), an Arabidopsis mutant lacking blue light-dependent inhibition of hypocotyl elongation, had a typical enhancement of the blue light response by background red light. The lack of a specific blue light response in the zeaxanthinless npq1 mutant provides genetic evidence for the role of zeaxanthin as a blue light photoreceptor in guard cells.

The stomatal response to blue light is an intrinsic component of the sensory transducing processes mediating light-stimulated stomatal movements. Guard cell chloroplasts have a specific blue light response with an action spectrum that resembles the action spectrum for blue light-stimulated stomatal opening, suggesting a role of guard cell chloroplasts in the sensory transduction of blue light. The xanthophyll, zeaxanthin has recently been identified as a blue light photoreceptor in guard cells. The inhibitor of zeaxanthin formation, dithiothreitol, inhibits zeaxanthin formation and the stomatal response to blue light in a concentration-dependent fashion. In greenhousegrown leaves, guard cell zeaxanthin content closely tracks incident radiation and it is positively correlated with stomatal apertures. The sensitivity of guard cells to blue light co-varies with guard cell zeaxanthin content. A zeaxanthin-less mutant of Arabidopsis is devoid of a typical stomatal response to blue light. At constant light and temperature, changes in ambient [CO2] in a growth chamber caused large changes in stomata aperture and in guard cell zeaxanthin. The aperturezeaxanthin changes were linearly related over a wide range of [CO2]. Experiments with detached epidermis showed a similar relation among [CO2], stomatal apertures and guard cell zeaxanthin, and DTT inhibited the CO2 response in the light without altering the CO2 response in the dark. These results indicate that blue light sensing by guard cell zeaxanthin has a regulatory role in the light response of stomata. Zeaxanthin also appears to mediate light-CO2 interactions in guard cells.

The mechanisms mediating CO2 sensing and light–CO2 interactions in guard cells are unknown. In growth chamber‐grown Vicia faba leaves kept under constant light (500 μmol m–2 s–1) and temperature, guard cell zeaxanthin content tracked ambient [CO2] and stomatal apertures. Increases in [CO2] from 400 to 1200 cm3 m–3 decreased zeaxanthin content from 180 to 80 mmol mol–1 Chl and decreased stomatal apertures by 7·0 μm. Changes in zeaxanthin and aperture were reversed when [CO2] was lowered. Guard cell zeaxanthin content was linearly correlated with stomatal apertures. In the dark, the CO2‐induced changes in stomatal aperture were much smaller, and guard cell zeaxanthin content did not change with chamber [CO2]. Guard cell zeaxanthin also tracked [CO2] and stomatal aperture in illuminated stomata from epidermal peels. Dithiothreitol (DTT), an inhibitor of zeaxanthin formation, eliminated CO2‐induced zeaxanthin changes in guard cells from illuminated epidermal peels and reduced the stomatal CO2 response to the level observed in the dark. These data suggest that CO2‐dependent changes in the zeaxanthin content of guard cells could modulate CO2‐dependent changes of stomatal apertures in the light while a zeaxanthin‐independent CO2 sensing mechanism would modulate the CO2 response in the dark.

Pima S‐6 (Gossypium barbadense L.) is a modern line with high stomatal conductance, while B368 is a primitive cotton with low conductance. The blue light sensitivity of adaxial guard cells, probed as the blue light‐dependent enhancement of the red light‐induced chlorophyll a fluorescence quenching, was investigated in these two cotton lines with contrasting stomatal conductance. Adaxial guard cells isolated from Pima S‐6 cotton plants had a significantly higher carotenoid content and a higher blue light sensitivity than those isolated from B368 plants. In a growth chamber‐grown F2 population of a cross between these two lines, adaxial stomatal conductances of individual plants segregated over a range exceeding the average conductances of the parents. Carotenoid content and the blue light sensitivity of adaxial guard cells also segregated. The concentrations of xanthophylls and β‐carotene in the adaxial guard cells were poorly correlated with the blue light response, except for zeaxanthin. The co‐segregation of stomatal conductance and blue light sensitivity suggested that the stomatal response to blue light may play a role in the regulation of stomatal conductance in the intact leaf. Zeaxanthin content and blue light sensitivity also co‐segregated, suggesting that both parameters are under genetic control. The co‐segregation of zeaxanthin content, blue light sensitivity and stomatal conductance provides further evidence for a role of zeaxanthin in the blue light photoreception of guard cells.