**4. Roles of melatonin in regulating growth and development of horticultural crops**

## **4.1 Effects of melatonin on growth and yield formation of horticultural crops**

Hernández-Ruiz et al. [18] were the first to propose that melatonin was a hormone-like growth regulator in plants. Because melatonin promotes the hypocotyl growth of albino lupine (*Lupinus micranthus* Guss.) in vitro, its action is related to the concentration gradient in plant tissues. The conclusion was consistent within barley (*Hordeum vulgare* L.), wheat (*Triticum aestivum* L.) and sunflower (*Helianthus annuus* L.) [29]. In terms of chemical structure, the side chain of melatonin has no carboxyl group of IAA, and its growth-promoting activity is about 10–50% of the effect of IAA. Moreover, melatonin does not bind to the IAA receptor, indicating that although melatonin and IAA are co-indoleamine compounds, their signal regulation mechanism is quite different. Since no specific binding receptor of melatonin has been found in plants, melatonin cannot be defined as a plant hormone. Nevertheless, more and more studies are focusing on exploring and identifying the functional and signaling networks that melatonin performs in plants.

Byeon and Back [30] obtained transgenic rice with excessive melatonin accumulation via overexpression of sheep *SNAcT* gene in rice. The growth potential of transgenic rice was significantly higher than that of wild rice and showed the characteristics of delayed flowering and reduced yield. This is consistent with the viewpoint mentioned in Section 2 of this report that "melatonin is a kind of antiaging substance," indicating that melatonin can inhibit reproductive growth and promote nutritional growth. According to this characteristic, we predicted that through the modification of melatonin-related genes or the exogenous spraying or watering melatonin, the horticultural crops with vegetative organs, as products, could effectively obtain higher economic yield, but the specific effect needs further test to verify.

Other studies have shown that 50 μmol L<sup>−</sup><sup>1</sup> of melatonin solution can significantly promote the growth of soybean seedlings and increase the yield of soybean [31]. Liu et al. [32] found that in the late stage of pear fruit development, spraying of 100 μmol L<sup>−</sup><sup>1</sup> of melatonin solution into the pear tree could promote the accumulation of endogenous melatonin in pear fruit. Melatonin increased the size of pear fruit by increasing the net photosynthetic rate and maximum quantum efficiency of photosystem II. During the ripening period, melatonin increased the concentrations of soluble sugar, especially sucrose and sorbitol, which may be the result of improving the accumulation of starch and promoting the expansion of pear fruit, which had a significant effect on increasing yield. In a study on grapes, Meng et al. [33] found that spraying of 100 mol L<sup>−</sup><sup>1</sup> of melatonin solution on young grape fruits could promote the accumulation of endogenous melatonin in grape fruits and promote the expansion of grape fruits, which had a significant effect on increasing production.

From these effects of melatonin on crop growth and yield, it becomes apparent that the promotion effect of melatonin on crop growth is a common feature. The difference was that the increase of endogenous melatonin concentrations reduced the rice yield, while the exogenous melatonin treatment increased the soybean and grape yield. The effect of melatonin on crop yield seems contradictory; however, we think that first, the soybean experiment adopted melatonin in treatment at seedling stage, but no melatonin was added at the yield formation stage. However, the melatonin level of transgenic rice was high in the whole growth period, so soybean showed an increase in yield, and rice showed a decreased yield. Second, according

**113**

*Review of Melatonin in Horticultural Crops DOI: http://dx.doi.org/10.5772/intechopen.90935*

concentrations of melatonin (10 μmol L<sup>−</sup><sup>1</sup>

research hotspot in the future.

**of horticultural crops**

**4.2 Melatonin regulates the ripening, aging and preservation** 

found that pretreatment of tomato with 50 μmol L<sup>−</sup><sup>1</sup>

Ripening, aging and preservation are contradictory processes in the production of horticultural products. Interestingly, more and more studies have shown that melatonin can be used to flexibly regulate the ripening, aging and storage of horticultural crops under different concentrations and conditions. Sun et al. [36]

could promote tomato ripening. Its regulatory mechanism can be summarized as follows: melatonin activates the expression of *ACS4* gene and enhances the synthesis of ethylene. Ethylene signal promoted the synthesis of lycopene by expression of *PSY1* gene through the signal transduction pathways such as *NR*, *ETR4*, *EILs* and *ERF2*, which made the tomato turn color, regulated the expression of *TBG4*, *PG2A*, *Exp1*, *XTH5* and *PE1* genes to degrade the cell wall, softened the fruit, regulated the expression of *PIPs* gene to control fruit water loss, and regulated the expression of *ADH2* and *AAT* genes to promote sugar conversion. Thus, phenotypic characteristics promoting tomato ripening were formed. Similar results have been found in grapes [37]. Lei et al. [38] showed that the synthesis of melatonin

(1 mmol L<sup>−</sup><sup>1</sup>

to the phenotypic difference, melatonin promoted plant growth in soybean more than it did in rice. The full growth of vegetative body provides more abundant photosynthetic products for reproductive growth. Third, the essence of melatonin inhibiting yield formation lies in inhibiting reproductive growth, that is, seed formation. The berry of grape is developed from ovary, which is similar to tomato and other berry crops. The expansion of ovary in the early stage of fruit development needs the stimulation of IAA to form a strong storage for nutrition. The effects of melatonin and IAA are similar, which may be the specific mechanism of melatonin in promoting the expansion of grape fruit. Fourth, Zhao et al. [34] showed that low

photosynthesis, loading and transportation of sucrose in the maize phloem, thus promoting the growth of maize plants. However, high concentrations of melatonin

of excessive sucrose, hexose and starch in leaves. As a result, leaf photosynthesis and the growth of maize plants were inhibited. Zhang et al. [35] also obtained similar results on the effects of melatonin concentrations on flowering in apple tree. Overexpression of *SNAcT* gene in rice is similar to the continuous supplementation of melatonin, while in the soybean and grape trials, the short-term supplementation of melatonin resulted in the difference in melatonin supply. Combined with the results of Zhao et al. [34], we analyzed that the concentrations of melatonin in rice was too high, inhibiting the loading of sugars in the phloem, which leads to reduced transport of sugars to the endosperm to form a low-yielding phenotype. The concentration of melatonin in soybean and grape is suitable, which is conducive to the transport of sugar to endosperm and ovary, thus forming a high-yield phenotype. In conclusion, these four factors may be the main reasons behind the differences in the characterization of melatonin on yield. Based on previous studies, we predicted that the right amount of melatonin could increase the yield of root vegetables, leaves, stems, fruits and fruit vegetables as well as the pomology with flesh as edible organs, while the effect on the yield of cauliflower vegetables (such as cauliflower) and pomology with seeds as edible organs (such as walnuts) remains to be verified. However, different crops have different demands on melatonin in different periods. In view of the importance of yield characters in agricultural production, the influence of melatonin on the yield of horticultural crops should become a scientific

) could inhibit sucrose loading in phloem, resulting in the accumulation

) could promote the metabolism of sugars,

and 100 μmol L<sup>−</sup><sup>1</sup>

melatonin

### *Review of Melatonin in Horticultural Crops DOI: http://dx.doi.org/10.5772/intechopen.90935*

*Melatonin - The Hormone of Darkness and Its Therapeutic Potential and Perspectives*

**4. Roles of melatonin in regulating growth and development of** 

**4.1 Effects of melatonin on growth and yield formation of horticultural crops**

Hernández-Ruiz et al. [18] were the first to propose that melatonin was a hormone-like growth regulator in plants. Because melatonin promotes the hypocotyl growth of albino lupine (*Lupinus micranthus* Guss.) in vitro, its action is related to the concentration gradient in plant tissues. The conclusion was consistent within barley (*Hordeum vulgare* L.), wheat (*Triticum aestivum* L.) and sunflower (*Helianthus annuus* L.) [29]. In terms of chemical structure, the side chain of melatonin has no carboxyl group of IAA, and its growth-promoting activity is about 10–50% of the effect of IAA. Moreover, melatonin does not bind to the IAA receptor, indicating that although melatonin and IAA are co-indoleamine compounds, their signal regulation mechanism is quite different. Since no specific binding receptor of melatonin has been found in plants, melatonin cannot be defined as a plant hormone. Nevertheless, more and more studies are focusing on exploring and identifying the functional and signaling networks that melatonin performs in

Byeon and Back [30] obtained transgenic rice with excessive melatonin accumulation via overexpression of sheep *SNAcT* gene in rice. The growth potential of transgenic rice was significantly higher than that of wild rice and showed the characteristics of delayed flowering and reduced yield. This is consistent with the viewpoint mentioned in Section 2 of this report that "melatonin is a kind of antiaging substance," indicating that melatonin can inhibit reproductive growth and promote nutritional growth. According to this characteristic, we predicted that through the modification of melatonin-related genes or the exogenous spraying or watering melatonin, the horticultural crops with vegetative organs, as products, could effectively obtain higher economic yield, but the specific effect needs further

cantly promote the growth of soybean seedlings and increase the yield of soybean [31]. Liu et al. [32] found that in the late stage of pear fruit development, spraying

lation of endogenous melatonin in pear fruit. Melatonin increased the size of pear fruit by increasing the net photosynthetic rate and maximum quantum efficiency of photosystem II. During the ripening period, melatonin increased the concentrations of soluble sugar, especially sucrose and sorbitol, which may be the result of improving the accumulation of starch and promoting the expansion of pear fruit, which had a significant effect on increasing yield. In a study on grapes, Meng et al.

fruits could promote the accumulation of endogenous melatonin in grape fruits and promote the expansion of grape fruits, which had a significant effect on increasing

From these effects of melatonin on crop growth and yield, it becomes apparent that the promotion effect of melatonin on crop growth is a common feature. The difference was that the increase of endogenous melatonin concentrations reduced the rice yield, while the exogenous melatonin treatment increased the soybean and grape yield. The effect of melatonin on crop yield seems contradictory; however, we think that first, the soybean experiment adopted melatonin in treatment at seedling stage, but no melatonin was added at the yield formation stage. However, the melatonin level of transgenic rice was high in the whole growth period, so soybean showed an increase in yield, and rice showed a decreased yield. Second, according

of melatonin solution into the pear tree could promote the accumu-

of melatonin solution can signifi-

of melatonin solution on young grape

**horticultural crops**

plants.

test to verify.

of 100 μmol L<sup>−</sup><sup>1</sup>

production.

Other studies have shown that 50 μmol L<sup>−</sup><sup>1</sup>

[33] found that spraying of 100 mol L<sup>−</sup><sup>1</sup>

**112**

to the phenotypic difference, melatonin promoted plant growth in soybean more than it did in rice. The full growth of vegetative body provides more abundant photosynthetic products for reproductive growth. Third, the essence of melatonin inhibiting yield formation lies in inhibiting reproductive growth, that is, seed formation. The berry of grape is developed from ovary, which is similar to tomato and other berry crops. The expansion of ovary in the early stage of fruit development needs the stimulation of IAA to form a strong storage for nutrition. The effects of melatonin and IAA are similar, which may be the specific mechanism of melatonin in promoting the expansion of grape fruit. Fourth, Zhao et al. [34] showed that low concentrations of melatonin (10 μmol L<sup>−</sup><sup>1</sup> ) could promote the metabolism of sugars, photosynthesis, loading and transportation of sucrose in the maize phloem, thus promoting the growth of maize plants. However, high concentrations of melatonin (1 mmol L<sup>−</sup><sup>1</sup> ) could inhibit sucrose loading in phloem, resulting in the accumulation of excessive sucrose, hexose and starch in leaves. As a result, leaf photosynthesis and the growth of maize plants were inhibited. Zhang et al. [35] also obtained similar results on the effects of melatonin concentrations on flowering in apple tree. Overexpression of *SNAcT* gene in rice is similar to the continuous supplementation of melatonin, while in the soybean and grape trials, the short-term supplementation of melatonin resulted in the difference in melatonin supply. Combined with the results of Zhao et al. [34], we analyzed that the concentrations of melatonin in rice was too high, inhibiting the loading of sugars in the phloem, which leads to reduced transport of sugars to the endosperm to form a low-yielding phenotype. The concentration of melatonin in soybean and grape is suitable, which is conducive to the transport of sugar to endosperm and ovary, thus forming a high-yield phenotype. In conclusion, these four factors may be the main reasons behind the differences in the characterization of melatonin on yield. Based on previous studies, we predicted that the right amount of melatonin could increase the yield of root vegetables, leaves, stems, fruits and fruit vegetables as well as the pomology with flesh as edible organs, while the effect on the yield of cauliflower vegetables (such as cauliflower) and pomology with seeds as edible organs (such as walnuts) remains to be verified. However, different crops have different demands on melatonin in different periods. In view of the importance of yield characters in agricultural production, the influence of melatonin on the yield of horticultural crops should become a scientific research hotspot in the future.
