Far-red light-enhanced apical dominance stimulates flower and fruit abortion in sweet pepper

Abstract

Far-red radiation affects many plant processes, including reproductive organ abortion. Our research aimed to determine the role of apical dominance in far-red light-induced flower and fruit abortion in sweet pepper (Capsicum annuum L.). We conducted several climate room experiments where plants were grown under white- or red-rich LED light, with or without additional far-red light. Additional far-red light enhanced apical dominance: it increased auxin levels in the apices of dominant shoots, and caused a greater difference in internode length and apical auxin levels between dominant and subordinate shoots. Additional far-red light stimulated fruit abortion in intact plants but not in decapitated plants, suggesting a crucial role of shoot apices in this effect. However, reducing basipetal auxin transport in the stems with N-1-naphthylphthalamic acid did not influence far-red light-stimulated fruit abortion, although auxin levels in the stem were largely reduced. Applying the synthetic auxin 1-naphthaleneacetic acid on decapitated apices did not influence fruit abortion. However, applying the auxin biosynthesis inhibitor yucasin to shoot apices reduced fruit abortion regardless of the light conditions, accompanied by slight shoot growth retardation. These findings suggest that the basipetal auxin stream does not mediate far-red light-stimulated fruit abortion. Far-red light-stimulated fruit abortion was associated with reduced sucrose accumulation and lower invertase activities in flowers. We suggest that under additional far-red light conditions, increased auxin levels in shoot apices promote fruit abortion probably through enhanced competition for assimilates between apices and flowers, which limits assimilate import into flowers.

Figure 1.

FR stimulates dominance from apical shoots in pepper plants. A) Plant shoot architecture during the vegetative growth experiment. All flowers were removed. Leaves are not shown. BS = bottom stem, the stem before splitting; I1 to I3 = internode 1 to 3; I4 = internode 4, and the apex above. Plants were cultivated with or without FR. B, C) Length ratio and dry weight ratio of the two internodes connected to the same node below, which was calculated by using the value of the larger shoot divided by the smaller one. D) Levels of IAA in 1 cm of apices from larger and smaller shoots, respectively. FW stands for fresh weight. E) Number of lateral shoots with a stem longer than 0.2 cm and a leaf longer than 1.5 cm. “No” indicates no lateral shoots in +FR treatment. Mean values were derived from two statistical replicates, each based on six plants. Dots indicate individual data of each statistical replicate. One-way ANOVA was performed for each internode in (B) and (C). Two-way ANOVA was performed for (D). Error bars in (B) to (E) indicate ± standard error of mean based on the common variance. Different lowercase letters indicate significant differences between treatments according to Fisher's unprotected LSD test at P = 0.10.

Figure 2.

FR has no influence on fruit abortion in decapitated pepper plants. A) Plant shoot architecture during decapitation experiment. Plants were cultivated with or without FR. Plants were pruned to have four main shoots carrying eight flowers. Leaves are not shown. Decapitation was performed when the upper layer of flowers reached anthesis. B) Number of fruits per plant on day 44 since light treatments. Mean values were derived from three statistical replicates, each based on eight plants. Dots indicate individual data of each statistical replicate. ANOVA based on split-plot design was performed. Error bars indicate ±standard error of means based on the common variance. Different lowercase letters indicate significant differences between treatment means according to Fisher's unprotected LSD test at P = 0.10.

Figure 3.

NPA application reduces auxin basipetal transport but does not influence fruit abortion. A) Plant shoot architecture in the NPA experiment. Plants were pruned to have four main shoots carrying 12 flowers. Leaves are not shown. Plants were cultivated with or without FR. NPA (5 mg/g) was applied in lanolin paste as a ring around the stems when the upper layer of flowers reached anthesis, while sole lanolin was applied to the control group. 1 cm of apices; 1 cm of stem above the NPA application site (stem A); 1 cm of stem below the NPA application site (stem B) were collected for auxin determination. B) Number of fruits per plant on day 52 since light treatments. C) Levels of IAA in apex, stem A, and stem B. FW stands for fresh weight. Mean values were derived from two statistical replicates, each based on six plants. Dots indicate individual data of each statistical replicate. ANOVA based on split-plot design was performed for (B) and (C). Error bars indicate ±standard error of means based on the common variance. Different letters for the same tissue indicate significant differences between treatment means according to Fisher's unprotected LSD test at P = 0.10. In (C), small letters are for apex, capital letters are for stem A, and small letters in bold and Italics are for stem B.

Figure 4.

NAA application to decapitated shoots does not promote fruit abortion. A) Plant shoot architecture in NAA experiment. Plants were pruned to have four main shoots carrying eight flowers. Plants were cultivated with or without FR. NAA (1% w/w) was applied in lanolin paste on the cutting surface after decapitation, while sole lanolin was applied to the control group. Decapitation and NAA application were performed on the same day when the upper layer of flowers reached anthesis. Afterwards, NAA was renewed every week for two more weeks. Topmost tissues of each plant (either 1 cm of apices in intact plants, or 1 cm of topmost stem segment in decapitated plants); 1 cm of nodes where the higher and the lower layer flowers were attached to (higher node and lower node); leaf discs from the topmost mature leaves were collected for auxin determination. B) Number of fruits per plant on day 53 since light treatments. C) Levels of IAA in and plant apex/topmost stem, lower node, higher node, and leaf. FW stands for fresh weight. Mean values were derived from two statistical replicates, each based on six plants. Dots indicate individual data of each statistical replicate. One-way ANOVA was performed for (B) and (C) for each tissue type. Error bars indicate ±standard error of means based on the common variance. Different letters for the same tissue indicate significant differences between treatment means according to Fisher's unprotected LSD test at P = 0.10. In (C), small letters are for apex/topmost stem, capital letters are for higher node, small letters in bold and italics are for lower node, and capital letters in bold and italics are for topmost mature leaf.

Figure 5.

Inhibition of auxin biosynthesis in shoot apices reduces fruit abortion independent of FR. A) Plant shoot architecture upon treatment with the auxin biosynthesis inhibitor yucasin. Plants were cultivated with or without FR. Plants were pruned to have four main shoots carrying eight flowers. Yucasin solution (50 mM for first application, and 25 mM for second to fourth application) was sprayed on the apical shoots every week since the anthesis of lower layer of flowers. The control group was sprayed with the solvent. One week after the last application, 1 cm of apices; 1 cm of nodes where the higher and the lower layer flowers were attached to (higher node and lower node); leaf discs from the topmost mature leaves were collected for auxin determination. B) Number of fruits per plant on day 60 since light treatments. C) Plant height on day 60 since light treatments. D) Levels of IAA in apex, higher node, lower node, and leaf. FW stands for fresh weight. Mean values were derived from four statistical replicates, each based on three plants. Dots indicate individual data of each statistical replicate. ANOVA based on split-plot design was performed for (B) to (D). Error bars indicate ±standard error of means based on the common variance. Different letters for the same tissue indicate significant differences between treatment means according to Fisher's unprotected LSD test at P = 0.10. In (D), small letters are for apex, capital letters are for higher node, small letters in bold, and italics are for lower node, and capital letters in bold and italics are for topmost mature leaf.

Figure 6.

Additional FR leads to higher sugar content except for sucrose content in flowers. A) Plant shoot structures when sampling tissues for sugar and hormone measurements. Plants were pruned to have four main shoots, each carrying one flower. Plants were cultivated with or without FR. Sampled tissues are flowers (including pedicels); 1 cm of apex; 1 cm of node where the flowers are attached to, and leaf discs from the topmost mature leaf and the leaf adjacent to the flowers. Samples were collected within the last hour of light period. B) The glucose, fructose, sucrose, and starch levels in the indicated tissues sampled on day 7 before anthesis, at anthesis, and on day 7 after anthesis. DW stands for dry weight. Means values were derived from four statistical replicates, each based on two plants. Split-plot ANOVA was performed on all parameters. Error bars indicate ±standard error of means based on the common variance. Different lowercase letters indicate significant differences between treatment means according to Fisher's unprotected LSD test at P = 0.10.

Figure 7.

Effect of additional FR on the activity of sucrose cleavage enzymes. Plants were cultivated with or without FR. The activity of soluble AI, cell wall invertase (CWI), NI, and SuSy in flowers and apex on day 7 before anthesis, at anthesis, and on day 7 after anthesis. FW stands for fresh weight. Samples were collected within the last hour of light period. Mean values were derived from four statistical replicates, each based on two plants. Split-plot ANOVA was performed on all parameters. Error bars indicate ±standard error of means based on the common variance. Different lowercase letters indicate significant differences between treatment means according to Fisher's unprotected LSD test at P = 0.10.