Analysis of endogenous hormones in different organs reveals the critical role of phthalanilic acid in the yield and quality of pepper (Capsicum annuum) fruits
Introduction
Pepper belongs to the Solanaceae family and is an annual herbaceous plant cultivated all over the world. Its fruit is consumed as a vegetable, spice, or colorant in the form of raw materials, sauces, or powders (Baenas et al., 2019), as well as being an important source of bioactive compounds, including vitamins C and E, provitamin A, carotenoids, and phenolic compounds (Deng et al., 2019). Capsaicin, also derived from pepper fruits, has strong medicinal attributes in animals and humans, such as anti-oxidation, blood pressure lowering, anti-cancer, irritative, and analgesic properties, affecting thermoregulation and adipose tissue metabolism, etc. (Adaszek et al., 2019). In recent years, consumers have dramatically increased interest in or demand for pepper fruits (Gholipoor and Nadali, 2019), also placing higher requirements on food quality (Zaccardelli et al., 2018). Therefore, efforts should be made to maintain or improve quality and yield.
Plant growth regulators (PGRs), defined as synthetic compounds or bioregulators, can promote or inhibit plant physiological processes or morphological traits (Rademacher, 2015; Engin and Gökbayrak, 2019). Its application to improving crop yield and quality is also well known. For example, salicylic acid could improve the fruit yield and quality of pepper and cucumber (Cucumis sativus) (Elwan and El-Hamahmy, 2009; Preciado-Rangel et al., 2019). Spraying exogenous 6-benzyladenine or brassinolide increased the maize yield (Gao et al., 2017). Therefore, PGR plays an important role in promoting crop growth and development.
In general, plant growth and development are highly complex physiological processes controlled by a series of factors, including exogenous and endogenous factors (Montoya et al., 2005). Among the endogenous factors, hormones, mainly ethylene, indole-3-acetic acid (IAA), zeatin riboside (ZR), gibberellins (GA3), and abscisic acid (ABA) exert crucial roles in regulating plant growth, development, nutrient allocation, and source/sink transition (Vanstraelen and Benková, 2012). These hormones interact in a synergistic or antagonistic manner rather than acting alone (Peleg and Blumwald, 2011). By interfering with endogenous hormone balance, PGRs affect the partition of photoassimilates, resulting in an improvement in yield and quality (Azzi et al., 2015). For example, exogenous glycinebetaine enlarged tomato (Solanum lycopersicum) fruits by affecting the balance of endogenous auxin, brassinolide, gibberellin, and cytokinin (Zhang et al., 2019). Exogenous 2, 4-epibrassinolide and jasmonic acid can regulate the proportion of endogenous hormones to improve the quality of grapevine (Vitis vinifera) (Li et al., 2022a). Moreover, plant hormone analogs can also be applied directly to change the endogenous hormone content in crops for the enhancement of yield and quality. For example, the grain yield of wheat (Triticum aestivum) plants treated with exogenous IAA, GA3, or ABA can be improved by regulating the endogenous IAA and ZT (Zeatin) contents (Cai et al., 2014). These fully indicated that the improvement of crop traits by PGRs is closely related to the changes in endogenous hormones mediated by PGRs.
Phthalanilic acid [2-(phenyl carbamoyl) benzoic acid], also known as N-phenyl-phthalamic acid (PPA), is a plant growth regulator developed by the Neviki Research Institute of Hungary in 1982 (Zhao et al., 2014). PPA was commercially produced in 1993 and is mainly used to increase stigma life, pollination, fruit setting, and yield on fruit trees such as apple (Malus pumila), sour cherry (Prunus cerasus), sweet cherry (Prunus avium), and eggplant (Solanum melongena) (Khadivi-Khub and Nosrati, 2013). Although PPA has long been known, its application to vegetables has been poorly understood. Our research group has previously found that PPA was beneficial to the yield and quality of peppers and cowpeas (Vigna unguiculata) (Zhang et al., 2017; Wu et al., 2018; Ma et al., 2021). But its mode of action and regulation of endogenous hormones remain to be further explored.
Therefore, the goal of this study is to describe the internal and external impacts of PPA on pepper fruits and to further evaluate the association among different phenotypic indicators in order to clarify the mechanism by which PPA enhances yield in terms of PPA-induced endogenous hormonal alterations.
Section snippets
Plant and chemicals
The pepper (Capsicum annuum L., Shijihong) seedlings were obtained from the Horticulture College of Northwest A & F University. The PPA 20% soluble concentrate was provided by Shaanxi Sunger Road Bio-science Co., Ltd.
Experimental design
Field experiments were conducted from June to August 2018 at the farm of Northwest A & F University (34°29′ N, 108°07′ E) in Shaanxi Province, China. The soil type of the site was Lou loam, which contains 11.50 g·kg−1 organic matter, 60.30 mg·kg−1 available N, 6.20 mg·kg−1
Effect of PPA on the flowering and fruiting of pepper plants
The results of all five investigations showed that PPA could increase the number of flowers in pepper plants compared with the control (Fig. 1). In particular, PPA (133.3 mg·L−1) had the most significant effect (P < 0.05) on the number of flowers among the three-dose treatments. Compared with the control, the flowering numbers in the first, second, third, fourth, and fifth surveys increased by 54.82%, 44.61%, 41.70%, 34.16%, and 25.13%, respectively. In addition, PPA increased the number of
Discussion
In general, the quality of pepper fruits consists of their visual appearance, nutrition, flavor, and safety. The fruit has a high nutritional value because of its high concentrations of soluble protein, soluble sugar, and ascorbic acid. Sugar and organic acid both have a significant influence on flavor, and a high sugar-acid ratio results in a superior taste (Yao et al., 2011). Capsaicin is a crucial factor in flavor determination (Baenas et al., 2019). Vegetables contain nitrate as a safety
Conclusion
PPA can increase the number of flowers and fruits and change the level of endogenous hormones in peppers. It has also benefited the quality and yield of pepper fruits. The increase in yield may be linked to changes in endogenous hormones induced by PPA. PPA should be generalized in pepper production in the future, with an optimal concentration of 133.3 and 200.0 mg·L−1 (Table 2).
CRediT authorship contribution statement
Xiaopeng Lu: Writing – original draft, Formal analysis, Writing – review & editing. Linlin Jiang: Formal analysis, Investigation, Resources. Zihao Li: Investigation. Hua Wu: Investigation. Zhiqing Ma: Conceptualization, Methodology, Project administration, Validation, Supervision, Writing – review & editing.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
The authors are very grateful to Professor Baomin Wang from China Agricultural University for his assistance in determining pepper plant hormones. This research was supported by Shaanxi Sunger Road Bio-science Co., Ltd and the Shaanxi Provincial Key Research and Development Program (Grant Number: 2019ZDLNY03-04).
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2023, Frontiers in Plant Science