On the contrasting morphological response to far-red at high and low photon fluxes

Abstract

Plants compete for sunlight and have evolved to perceive shade through both relative increases in the flux of far-red photons (FR; 700 to 750 nm) and decreases in the flux of all photons (intensity). These two signals interact to control stem elongation and leaf expansion. Although the interacting effects on stem elongation are well quantified, responses for leaf expansion are poorly characterized. Here we report a significant interaction between far-red fraction and total photon flux. Extended photosynthetic photon flux density (ePPFD; 400 to 750 nm) was maintained at three levels (50/100, 200 and 500 µmol m^-2 s^-1), each with a range of 2 to 33% FR. Increasing FR increased leaf expansion in three cultivars of lettuce at the highest ePPFD but decreased expansion at the lowest ePPFD. This interaction was attributed to differences in biomass partitioning between leaves and stems. Increased FR favored stem elongation and biomass partitioning to stems at low ePPFD and favored leaf expansion at high ePPFD. In cucumber, leaf expansion was increased with increasing percent FR under all ePPFD levels showing minimal interaction. The interactions (and lack thereof) have important implications for horticulture and warrant further study for plant ecology.

Keywords: far-red; leaf expansion; photosynthetic photon flux density; phytochrome; shade avoidance; shade tolerance.

Figure 1

Experimental design of the study. Treatments included four percentages of far-red at three levels of extended photosynthetic photon flux density (ePPFD). The lowest ePPFD in the cucumber study was 50, instead of 100, µmol m^-2 s^-1.

Figure 2

Representative spectral photon distributions for the four fractions of far-red (FR) from the ePPFD = 200 µmol m^-2 s^-1 intensity treatments. Spectral photon distributions from the other two intensities had the same shape with different scales on the y-axis. Photon fluxes of blue, green and red were maintained at 20, 40 and 40%, respectively, as a percent of photosynthetically active radiation (PAR; 400 to 700 nm) for each treatment of FR. Percent FR (%FR) treatments included 2% (grey), 9% (orange), 17% (purple), and 33% (cyan).

Figure 3

Overhead view of all the treatments in one replicate for (A) lettuce and (B) cucumber. The white arrow on the left indicates increasing extended photosynthetic photon flux density (ePPFD) and the red arrow on the bottom indicates increasing percent far-red.

Figure 4

Side view of all the treatments in one replicate for (A) lettuce and (B) cucumber. The white arrow on the left indicates increasing extended photosynthetic photon flux density (ePPFD) and the red arrow on the bottom indicates increasing percent far-red.

Figure 5

Effects of percent far-red (FR) and extended photosynthetic photon flux density (ePPFD) on shoot dry mass in lettuce (A, C) and cucumber (B, D). (A, B) represent the original values of dry mass and (C, D) are the normalized response, where data from each replicate in time was normalized to its respective 2% FR (no added FR) control treatment for each ePPFD. In (C, D), * indicates that the treatment is statistically different from 1 (using a student’s t-test), which represents the response of the 2% FR control. Trend lines are included to guide the eye, and not meant to be used as a statistical representation. Error bars represent standard error for n = 3 replicates in lettuce and n = 5 replicates in cucumber. Letters are not included in (D) because the treatments were determined to be insignificantly different from each other, although several treatments were significantly different from their respective controls.

Figure 6

Effect of percent far-red (FR) at different levels of extended photosynthetic photon flux density (ePPFD) on specific leaf area (SLA) in lettuce (A, C) and cucumber (B, D). (A, B) represent the original values of SLA, while (C, D) are the normalized response, where data has been normalized to the average response in the 2% FR control treatment for each ePPFD. Increasing the percent FR and decreasing the ePPFD increased SLA in both species. Error bars represent standard error for n = 3 replicates in lettuce and n = 5 replicates in cucumber.

Figure 7

Effect of percent far-red (FR) and extended photosynthetic photon flux density (ePPFD) on leaf area in lettuce (A, C) and cucumber (B, D). (A, B) represent the original values of leaf area and (C, D) are the normalized response, where data from each replicate in time has been normalized to its respective 2% FR control treatment for each level of ePPFD. In (C, D), * indicates that the treatment is statistically different from 1 (using a student’s t-test), which represents the effect of the 2% FR control. Trend lines are included to guide the eye, and not meant to be used as a statistical representation. Error bars represent standard error for n = 3 replicates in lettuce and n = 5 replicates in cucumber.

Figure 8

Effect of percent far-red (FR) at different levels of extended photosynthetic photon flux density (ePPFD) on stems. (A) Percent stem mass in lettuce, representative of biomass partitioning to stems, (B) percent stem mass in cucumber, (C) stem length in lettuce, and (D) stem length in cucumber. A two-way ANOVA analysis and Tukey’s post hoc test indicated no difference in percent stem mass between 2 and 9% FR for all three levels of ePPFD, but it was significantly higher at the lowest ePPFD for both the 2% and 9% FR treatments. The letters indicate this effect showing percent stem mass was significantly higher at 2 and 9% FR at an ePPFD of 50 μmol m^-2 s^-1 than 2 and 9% FR at an ePPFD of either 200 or 500 µmol m^-2 s^-1. Because the 2% data was removed to simplify the model and all remaining data was analysed with linear mixed-effects regression analysis. Error bars represent standard error for n = 3 replicates in lettuce and n = 5 replicates in cucumber. No significant differences