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In the shade of vegetation, the R:FR ratio decreases, and this change in spectral quality is a key signal of light competition between neighboring plants. FR-rich shadow light converts active Pfr to Pr and is able to express PIF-activated genes.
Elongation of hypocotyls, stems, and/or petioles, enhancement of apical dominance, reduction of branching, and hypocotyls are typical morphological changes induced by FR light, especially in species adapted to strong light environments.
In addition to photoconversion, Pfr can also relax back to the inactivated form of Pr in a temperature-dependent manner. This process is known as "thermal reversal". Both photoconversion and thermal regression determine the steady state of PHY, i.e., the ratio of active Pfr to the total PHY pool.
Recent studies have shown that PHY dynamics related to light and temperature enable PHYs to act as dual sensors of light and temperature signals and mediate temperature-induced morphological and developmental changes, in addition to their more well-known role in regulating light morphogenesis responses.
Warm temperatures often result in increased leaf/stem growth, low and apical dominance due to the increased rate of thermal regression from Pfr to Pr in PHY and subsequent changes in hormonal signaling. These morphological changes at warm temperatures resemble FR light-induced shading responses.
1. Materials and methods
1.1. Temperature and light treatment
In this experiment, we studied plant responses to two types of temperature cues: 1) cold to warm temperatures, with constant diurnal temperatures at each temperature level, and 2) diurnal temperature fluctuations, with three growth chambers set at temperatures of 20, 24, and 28 °C, respectively.
For both DIF treatments, plants are moved between the 28 °C and 30 °C chambers twice a day for 20 min before the start of the light period, and then at the beginning of the dark period. Each chamber was divided into three sections using reflective cardboard, and the three spectral treatments were randomly assigned to each section by 0% FR, 10% FR, or 20% FR photons.
1.2. Plant growth and morphometry
Morphology and growth parameters were measured at 10 and 20 days post-treatment and then at harvest. On each measurement date, lettuce plant height, width at the widest part of the plant canopy, total leaf count, and length and width of recently matured leaves are measured.
In basil, plant height, total leaf count, petiole length, leaf length, and width of recently mature leaves were measured. Calculate the plant height and width ratio of lettuce and the leaf length to width ratio of lettuce and basil. Leaf area meter at harvest time. In addition, in the 3rd replicate, top-down photographs of the lettuce at harvest were taken using a digital camera placed 120 cm away from the plant.
2. Experimental design and statistical analysis
This experiment was repeated three times. The treatment was designed with a partition area, with temperature as the main graph factor and far-red light percentage as the subplot factor. Each replicate study is treated as a block. In each replicate, at 0/10 °C, 12 plants were used for each spectral treatment and 4 plants were used in all other 3 experimental units.
For basil under 0/2 °C x 3% FR treatment, only the data from replicates 1 and 9 were used for statistical analysis, as the plants in replicate 4 were exposed to the water vapor stream released from the humidifier nozzle and had to be discarded.
3. Results
3.1. Plant morphology
FR light and temperature had significant interactions on leaf elongation, stem and petiole elongation, plant height, leaf number and total leaf area of lettuce and basil. However, the tendency and magnitude of interactions vary depending on specific morphological parameters and species.
3.1.1. Lettuce morphology
At constant diurnal temperatures, higher temperatures accelerate plant growth rates, such as increasing leaf unfolding rates and total leaf counts, and promoting leaf elongation/expansion. The effect of flame retardant light on plant morphological traits is temperature-related.
In general, flame retardant light promotes the rate of leaf extension/expansion at lower temperatures, but as the temperature increases, the stimulation weakens and even inhibits leaf expansion. At 0/20 °C, the FR percentage increased from 48% to 20%, resulting in a 20% increase in lettuce leaf length, but only a 28% increase at 7/28 °C.
Similarly, the leaf length-to-width ratio, plant height, and plant height:width ratio increased with the FR percentage from 0% to 20%, but at 20/20 °C, all 28 parameters increased more than at 28/2 °C. On the other hand, the number of blades decreases as the percentage of FR increases.
3.1.2. Basil pattern
The increase in temperature results in basil plants being taller, larger, with a higher number of leaves, a larger total leaf area, and more slender leaves. At 0% FR, plant height increased by 28% with temperature from 278/20 °C to 20/28 °C. Similarly, the increase in temperature from 20/20°C to 28/28°C resulted in a 401% increase in leaf number and a 433% increase in total leaf area.
Similar to the response observed in lettuce, the effect of FR light on basil morphological traits depends on temperature. In general, a stronger flame retardant effect is observed at lower temperatures, while the amplitude of the flame retardant effect decreases with increasing temperature.
More specifically, increasing the flame retardancy rate from 0% to 20% resulted in a 20% increase in basil plant height at 77/20°C, but a much smaller increase of 24% at 23/24°C, and had no effect on plant height at 28/28°C.
4. Discussion
4.1. Temperature regulates the morphological response of plants to the shadows of far-red light signals
The results showed that flame retardant light and temperature interacted with each other to regulate plant morphology, including stem/petiole/leaf elongation and leaf expansion. A particularly interesting observation is that at lower temperatures, FR light strongly promotes leaf expansion and photon capture in lettuce, but inhibits leaf expansion and photon capture at warm temperatures.
The hypocotyl elongation of lettuce exhibited an inverse response to the flame retardant light x temperature combination compared to leaf expansion, which was consistent with previous studies. Specifically, the stimulating effect of FR light on hypocotyl elongation, a typical shade avoidance response, tends to be stronger at higher temperatures. In addition, we observed that lettuce stems grown at 20% flame-retardant light and high temperatures of 28/28°C were significantly elongated at the expense of leaf expansion.
The temperature difference between day and night also interacts with FR light to regulate plant morphology. FR light combined with +DIF conditions results in larger and taller plants for lettuce and basil. The interaction between DIF treatment and flame retardant light is particularly evident in basil.
Under the +DIF condition, the shade response to FR light was enhanced, but decreased under the –DIF condition. The stimulation effect of low R:FR ratio on cucumber stem, petiole and hypocotyl elongation was stronger under the +DIF condition than under the –DIF condition. The authors also identified the interaction of PHYB involved in the use of PHYB-deficient lh mutants on plant morphology between FR light and DIF treatment.
A significant interaction between FR light and temperature was observed during hypocotyl elongation of lettuce seedlings, suggesting that the interaction began early in growth.
4.2 Interaction of FR light and temperature on plant biomass
In addition to leaf expansion and photon capture, replacing R light with FR light can affect leaf photosynthesis and plant growth. FR light is generally considered ineffective in photosynthesis and is therefore excluded from the definition of photosynthetically effective radiation.
However, recent studies have found that FR light synergizes with shorter wavelength photons to improve the photochemical efficiency of leaves, and short-term photosynthetic measurements further suggest that partial substitution of PAR photons with FR light results in comparable canopy photosynthetic rates for different species.
Therefore, in this study, we replaced a portion of the R photons with FR photons to maintain the same total photon flux under different spectral treatments.
In a long-term crop cultivation study, the photosynthetic rate of leaves of lettuce was significantly reduced after 14-17 days of adaptation to flame-retardant light. This reduction may be due to physiological and morphological adaptations, such as reduced leaf thickness and reduced chlorophyll content per unit leaf area.
In our study, there was a similar reduction in photosynthetic rate per unit leaf area in lettuce plants adapted to flame-retardant light for 20 days, regardless of temperature conditions. At the plant canopy level, replacing shorter wavelength photons with FR light can improve plant growth by enhancing photon capture. These results suggest that when FR light is applied, plant growth may be more dependent on photon capture than photosynthetic efficiency per unit leaf area.
4.3. Chlorophyll content decreases in response to FR light
The chlorophyll content of lettuce and basil was greatly reduced under flame retardant light. The decrease in chlorophyll content is a typical shade avoidance reaction. One possible reason for the decrease in chlorophyll content under FR is the "dilution effect" caused by leaf swelling.
As leaf area increases, leaf expansion and competition for carbohydrates by pigment synthesis may also lead to reduced pigment levels. However, the decrease in leaf chlorophyll content observed in this study cannot be simply attributed to the "dilution effect", as the chlorophyll content on the biomass also decreases with an increase in the percentage of FR.
Previous studies have shown that chlorophyll biosynthesis can be regulated directly by PHY signaling in response to environmental conditions. For example, in rice, active PHYB under R light increases chlorophyll biosynthesis by promoting the expression of prochlorophyll oxidoreductase A, one of the genes involved in chlorophyll biosynthesis.
5. Conclusion
Overall, our findings suggest that FR light and temperature interact to regulate plant morphology. We also found that replacing R light with FR light increased plant biomass yield in lettuce and basil at lower temperatures and +DIF.
However, at higher temperatures and –DIF, FR light had a negative or no effect on plant biomass. These results suggest that the effect of flame-retardant light on plant growth is temperature-dependent, and highlight the importance of considering temperature when using flame-retardant photons in controlled environment production systems to improve crop yields by initiating ideal morphological modifications.
In addition, caution should be exercised when using PPE calculated from a two-state model to predict plant morphology under different temperature conditions, as the predictive power of PPE calculated using this simplified method depends on temperature.