Flowering and Runnering of Seasonal Strawberry under Different Photoperiods Are Affected by Intensity of Supplemental or Night-Interrupting Blue Light

Abstract

The strawberry (Fragaria × ananassa Duch.) "Sulhyang" is a typical seasonal flowering (SF) strawberry that produces flower buds in day lengths shorter than a critical limit (variable, but often defined as <12 h). There is a trade-off between photoperiod-controlled flowering and gibberellin (GA) signaling pathway-mediated runnering. Some related genes (such as CO, FT1, SOC1, and TFL1) participating in light signaling and circadian rhythm in plants are altered under blue light (BL). Sugars for flowering and runnering are mainly produced by photosynthetic carbon assimilation. The intensity of light could affect photosynthesis, thereby regulating flowering and runnering. Here, we investigated the effect of the intensity of supplemental blue light (S-BL) or night-interrupting blue light (NI-BL) in photoperiodic flowering and runnering regulation by applying 4 h of S-BL or NI-BL with either 0, 10, 20, 30, or 40 μmol·m^-2·s^-1 photosynthetic photon flux density (PPFD) in a 10 h short-day (SD10) (SD10 + S-BL4 or + NI-BL4 (0, 10, 20, 30, or 40)) or 14 h long-day (LD14) conditions (LD14 + S-BL4 or + NI-BL4 (0, 10, 20, 30, or 40)). Approximately 45 days after the photoperiodic light treatment, generally, whether S-BL or NI-BL, BL (20) was the most promotive in runnering, leading to more runners in both the LD and SD conditions. For flowering, except the treatment LD14 + S-BL, BL (20) was still the key light, either from BL (20) or BL (40), promoting flowering, especially when BL acted as the night-interrupting light, regardless of the photoperiod. At the harvest stage, larger numbers of inflorescences and runners were observed in the LD14 + NI-BL4 treatment, and the most were observed in the LD14 + NI-BL (20). Moreover, the SD10 + NI-BL4 was slightly inferior to the LD14 + NI-BL4 in increasing the numbers of inflorescences and runners, but it caused earlier flowering. Additionally, the circadian rhythm expression of flowering-related genes was affected differently by the S-BL and NI-BL. After the application of BL in LD conditions, the expression of an LD-specific floral activator FaFT1 was stimulated, while that of a flowering suppressor FaTFL1 was inhibited, resetting the balance of expression between these two opposite flowering regulators. The SD runnering was caused by BL in non-runnering SD conditions associated with the stimulation of two key genes that regulate runner formation in the GA pathway, FaGRAS32 and FaGA20ox4. In addition, the positive effects of BL on enhancing photosynthesis and carbohydrate production also provided an abundant energy supply for the flowering and runnering processes.

Keywords: GA pathway; carbohydrate accumulation; circadian rhythm; light intensity; photoperiodic response; photosynthetic efficiency; seasonal flowering; supplemental or night-interrupting blue light.

Figure 8

The temporal expression patterns of blue light photoreceptor or flowering-related genes in leaf or shoot apex of strawberry “Sulhyang” under different intensities of supplemental or night-interrupting blue light for 10 days of exposure to the photoperiodic light treatments. For RNA extraction and RT-PCR, the youngest mature compound leaves or new shoot apices were harvested at 0, 2, 4, 6, 8, 10, 12, 14, 16, 18, 20, 22, and 24h after lights-on (from 8:00 a.m.), respectively (ZT 0, 2, 4, 6, 8, 10, 12, 14, 16, 18, 20, 22, and 24). (A) The horizontal white and black bars represent the period of day and night, respectively; the blue bars represent the periods with different intensities of supplemental or night-interrupting blue light. (B) The Fragaria ananassa Duch. homologs of CONSTANS (FaCO), (C) Cryptochrome2 (FaCry2), and (D) FLOWERING LOCUS T1 (FaFT1) have been measured in the youngest ternately compound leaves. The Fragaria ananassa Duch. homologs of (E) SUPPRESSOR OF THE OVER-EXPRESSION OF CONSTANS1 (FaSOC1), (F) APETALA1 (FaAP1), and (G) FRUITFULL1 (FaFUL1) have been measured in the shoot apex samples. Data were averagely normalized against the expression of FaEF1 (Acttin gene-1) and FaMSI1 (Acttin gene-2). The maximum value in each experiment was set to “1”. Vertical bars indicate the means ± standard error (n = 12), using RNA from separate plants. See Figure 11 for details of photoperiodic light treatments with blue light. See Table 1 for the detailed primer information.

Figure 1

The morphology of strawberry “Sulhyang” under different intensities of supplemental or night-interrupting blue light for 45 days of exposure to the photoperiodic light treatments. Strawberry plants grown under short day 10 h conditions with 4 h of supplemental or night-interrupting blue light treatments (A,B); strawberry plants grown under long day 14 h conditions with 4 h of supplemental or night-interrupting blue light treatments (C,D). Numbers 0, 10, 20, 30, and 40 refer to the blue light intensity. See Figure 11 for details of photoperiodic light treatments with blue light.

Figure 2

Runnering and growth parameters of strawberry “Sulhyang” under different intensities of supplemental or night-interrupting blue light for 45 days of exposure to the photoperiodic light treatments. Shoot height (A), fresh weight (B), dry weight (C), and average number of runners (D) and daughter plants (G) of mother plants; runner mean length (≥2 cm) (E) and days to the first visible runner (F). Strawberries in SD10 treatment were not included in the runner-related statistics. Vertical bars indicate the means ± standard error (n = 12). Different lowercase letters indicate significant separation within treatments by Duncan’s multiple range test at p ≤ 0.05. See Figure 11 for details of photoperiodic light treatments with blue light.

Figure 3

Flowering of strawberry “Sulhyang” under different intensities of supplemental or night-interrupting blue light for 45 days of exposure to the photoperiodic light treatments. Average number of inflorescences per mother plant (A) and days to the first visible inflorescences (B). Strawberries in LD14 treatment were not included in the statistics of inflorescences. Vertical bars indicate the means ± standard error (n = 12). Different lowercase letters indicate significant separation within treatments by Duncan’s multiple range test at p ≤ 0.05. See Figure 11 for details of photoperiodic light treatments with blue light.

Figure 4

Photosynthetic pigment contents of strawberry “Sulhyang” under different intensities of supplemental or night-interrupting blue light for 45 days of exposure to the photoperiodic light treatments. Content of chlorophyll a (A), chlorophyll b (B), chlorophyll a + b (C), and carotenoids (D), and ratio of chlorophyll a to b (E). Vertical bars indicate the means ± standard error (n = 12). Different lowercase letters indicate significant separation within treatments by Duncan’s multiple range test at p ≤ 0.05. See Figure 11 for details of photoperiodic light treatments with blue light.

Figure 5

Photosynthetic characteristics of strawberry “Sulhyang” under different intensities of supplemental or night-interrupting blue light for 45 days of exposure to the photoperiodic light treatments. (A) Net photosynthetic rate (Pn). (B) Transpiration rate (Tr). (C) Stomatal conductance (Gs). (D) Intercellular CO₂ concentration (Ci). Vertical bars indicate the means ± standard error (n = 12). Different lowercase letters indicate significant separation within treatments by Duncan’s multiple range test at p ≤ 0.05. See Figure 11 for details of photoperiodic light treatments with blue light.

Figure 6

Carbohydrate contents of strawberry “Sulhyang” under different intensities of supplemental or night-interrupting blue light for 45 days of exposure to the photoperiodic light treatments. Soluble sugar content (A) and starch content (B). Vertical bars indicate the means ± standard error (n = 12). Different lowercase letters indicate significant separation within treatments by Duncan’s multiple range test at p ≤ 0.05. See Figure 11 for details of photoperiodic light treatments with blue light.

Figure 7

Enzymatic activities of strawberry “Sulhyang” under different intensities of supplemental or night-interrupting blue light for 45 days of exposure to the photoperiodic light treatments. Sucrose synthesis enzymes: (A) sucrose synthase (SS) and sucrose phosphate synthase (SPS), (B) phosphoenolpyruvate carboxykinase (PEPC) and phosphoenolpyruvate phosphatase (PEPP). Starch synthesis enzymes: (C) adenosine diphosphate glucose pyro-phosphorylase (ADPGPPase), uridine diphosphate glucose pyrophosphorylase (UDGPPase) and (D) soluble starch synthase (SSS). Vertical bars indicate the means ± standard error (n = 12). Different lowercase letters indicate significant separation within treatments by Duncan’s multiple range test at p ≤ 0.05. See Figure 11 for details of photoperiodic light treatments with blue light.

Figure 9

Expression patterns of flowering-related genes in the youngest ternately compound leaf or new shoot apex of strawberry “Sulhyang” under different intensities of supplemental or night-interrupting blue light for 45 days of exposure to the photoperiodic light treatments. The youngest ternately compound leaves or new shoot apices were harvested at ZT4 (4 h after lights-on, from 8:00 a.m.) for RNA extraction and RT-PCR. The Fragaria ananassa Duch. homologs of CONSTANS (FaCO), Cryptochrome2 (FaCry2), and FLOWERING LOCUS T1 (FaFT1) were measured in the youngest mature compound leaves. The Fragaria ananassa Duch. homologs of SUPPRESSOR OF THE OVER-EXPRESSION OF CONSTANS1 (FaSOC1), APETALA1 (FaAP1), FRUITFULL1 (FaFUL1), and TERMINAL FLOWER1 (FaTFL1) were measured in the shoot apex samples. Data were averagely normalized against the expression of FaEF1 (Acttin gene-1) and FaMSI1 (Acttin gene-2). The maximum value in each experiment was set to “1”. Vertical bars indicate the means ± standard error (n = 12) using RNA from separate plants. See Figure 11 for details of photoperiodic light treatments with blue light. See Table 1 for the detailed primer information.

Figure 10

Expression patterns of runnering-related genes in the youngest ternately compound leaf or new shoot apex of strawberry “Sulhyang” under different intensities of supplemental or night-interrupting blue light for 45 days of exposure to the photoperiodic light treatments. The youngest ternately compound leaves or new shoot apices were harvested at ZT4 (4 h after lights-on, from 8:00 a.m.) for RNA extraction and RT-PCR. The Fragaria ananassa Duch. homologs of (A) the REPRESSOR OF GA1 (FaRGA1) and (B) GRAS transcription factor family (FaGRAS32) were measured in the shoot apex samples. The Fragaria ananassa Duch. homolog of (C) the gibberellic acid (GA) biosynthesis gene Gibberellin 20-oxidase 4 (FaGA20ox4) was measured in the youngest ternately compound leaves. Data were averagely normalized against the expression of FaEF1 (Acttin gene-1) and FaMSI1 (Acttin gene-2). The maximum value in each experiment was set to “1”. Vertical bars indicate the means ± standard error (n = 12) using RNA from separate plants. Different lowercase letters indicate significant separation within treatments by Duncan’s multiple range test at p ≤ 0.05. See Figure 11 for details of photoperiodic light treatments with blue light. See Table 1 for the detailed primer information.

Figure 11

The light spectral distribution of experimental light treatments (A): the white light (~400–720 nm and peaked at 450 nm) provided by white LEDs and blue light (peaked at 450 nm) from blue LEDs used as the supplemental or night-interrupting light. The experimental light schemes employed in this study (B): the light period (the white bar) started and the dark period (the black bar) ended at everyday 8:00 a.m.; plants in the control groups were in a 10 h short-day (SD10) or 14 h long-day (LD14) condition, without any blue light; the 4 h blue light (the blue bar) with either 10, 20, 30, or 40 μmol·m⁻²·s⁻¹ PPFD of intensities was used to (1) supplement the white light at the end of the SD10 (SD10 + S-BL4) and LD14 (LD14 + S-BL4) or (2) provide night interruption (NI) in the SD10 (SD10 + NI-BL4) and LD14 (LD14 + NI-BL4). (B) Blue light. The experimental layout in the plant factory (C): for each treatment, three replications (ten plants/replication) were located alone in an opaque compartment; the 0, 10, 20, 30, and 40 refer to the blue light with intensities of either 0, 10, 20, 30, or 40 μmol·m⁻²·s⁻¹ PPFD.