Effects of Light Quality and Intensity on Diurnal Patterns and Rates of Photo-Assimilate Translocation and Transpiration in Tomato Leaves

Abstract

Translocation of assimilates is a fundamental process involving carbon and water balance affecting source/sink relationships. Diurnal patterns of CO2 exchange, translocation (carbon export), and transpiration of an intact tomato source leaf were determined during 14CO2 steady-state labeling under different wavelengths at three pre-set photosynthetic rates. Daily patterns showed that photosynthesis and export were supported by all wavelengths of light tested including orange and green. Export in the light, under all wavelengths was always higher than that at night. Export in the light varied from 65–83% of the total daily carbon fixed, depending on light intensity. Photosynthesis and export were highly correlated under all wavelengths (r = 0.90–0.96). Export as a percentage of photosynthesis (relative export) decreased as photosynthesis increased by increasing light intensity under all wavelengths. These data indicate an upper limit for export under all spectral conditions. Interestingly, only at the medium photosynthetic rate, relative export under the blue and the orange light-emitting diodes (LEDs) were higher than under white and red-white LEDs. Stomatal conductance, transpiration rates, and water-use-efficiency showed similar daily patterns under all wavelengths. Illuminating tomato leaves with different spectral quality resulted in similar carbon export rates, but stomatal conductance and transpiration rates varied due to wavelength specific control of stomatal function. Thus, we caution that the link between transpiration and C-export may be more complex than previously thought. In summary, these data indicate that orange and green LEDs, not simply the traditionally used red and blue LEDs, should be considered and tested when designing lighting systems for optimizing source leaf strength during plant production in controlled environment systems. In addition, knowledge related to the interplay between water and C-movement within a plant and how they are affected by environmental stimuli, is needed to develop a better understanding of source/sink relationships.

Effect of Supplementary Light Source on Quality of Grafted Tomato Seedlings and Expression of Two Photosynthetic Genes

“…Jang et al found that a mixture of B and R light was the most beneficial for healing and growing grafted pepper seedlings [37]. However, simply considering the traditionally used red and blue LEDs is not sufficient, and orange and green LEDs should be considered and tested when designing lighting systems [38]. In our previous study of grafted tomato seedlings treated with a combination of R and B light, and W 1 R 2 B 1 , it was demonstrated that W 1 R 2 B 1 was the most suitable for developing vascular bundles and stomatal behaviors [39].…”

Morphological Response of Soybean (Glycine max (L.) Merr.) Cultivars to Light Intensity and Red to Far-Red Ratio

“…These results should be examined further with more levels of PPFD between 100 and 400 µmol m −2 s −1 and measurements of photosynthesis for a better understanding. Earlier studies on soybean found changes in the photosynthetic system [17] and stem carbohydrate concentration as well as composition under low PPFD [21], and changes in carbon export from tomato leaves depending on PPFD and light quality [31].…”

Supplemental Light-Emitting Diode Inter-Lighting Increases Tomato Fruit Growth Through Enhanced Photosynthetic Light Use Efficiency and Modulated Root Activity

“…A greater transport of sucrose might be regulated by transpiration in the LED-treated leaves, as both the hydrostatic tension of water in leaf xylem and the osmotic pressure in leaf phloem defines the turgor pressure that drives phloem transport (Smith and Milburn, 1980;Johnson et al, 1992;Windt et al, 2006). However, a comparison of the effects of light of different wavelengths revealed no close relationship between sucrose transport and transpiration, indicating that sucrose export cannot be explained solely by modulation of leaf turgor pressure (Lanoue et al, 2018). The most promising key player candidates for the regulation of sucrose transport are sucrose transporters (SUT) (Griffiths et al, 2016).…”

“…Canopy inter-lighting is expected to change the photosynthetic rate, but it might also modulate the rate of carbohydrate export from source leaves to sinks. Indeed, recent experiments with LEDs of different wavelengths showed that orange and blue light promoted greater carbohydrate export from the leaves when compared with green or white light (Lanoue et al, 2018); therefore, modulation of the light spectrum by inter-lighting might also affect carbohydrate transport from the lower-canopy leaves in high-wire greenhouse tomatoes. The mechanism driving carbohydrate export from the leaves might include interactions involving phloem and xylem osmotic pressure (Smith and Milburn, 1980;Windt et al, 2006;Nikinmaa et al, 2013), where increased carbohydrate export would be expected when higher rates of transpiration increase the turgor potential gradient between the source and sink (Johnson et al, 1992).…”

“…The mechanism driving carbohydrate export from the leaves might include interactions involving phloem and xylem osmotic pressure (Smith and Milburn, 1980;Windt et al, 2006;Nikinmaa et al, 2013), where increased carbohydrate export would be expected when higher rates of transpiration increase the turgor potential gradient between the source and sink (Johnson et al, 1992). However, a recent comparison of the effects of lights of different wavelength on carbohydrate export and transpiration showed no close relationship between these turgor-related traits (Lanoue et al, 2018), indicating an involvement of some other types of regulatory mechanisms. Several enzymes and transporters are considered to act as potential bottlenecks for carbohydrate transport from the leaves (Lemoine et al, 2013), thereby emphasizing the complexity of the regulation of carbon export.…”