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Ornamental traits such as size of flowers and color of petals are fundamentally determined by the metabolic status on the whole-plant level. Extensively performed agricultural practice and continuously increased concentration of CO2 in the atmosphere have both aggravated N conditions in soil. This study has been designed to specifically investigate the effects of ammonium or nitrate as the sole source of inorganic N on the developmental and chemical features of floral tissues in Petunia.
According to the total N content on the whole-plant level after each treatment (Fig. 1a), we think that the Petunia x hybrida cv 'Mitchell Diploid' plants we used in this study do not have preferences to a specific form of inorganic N. To be noted, given the same amount of inorganic N (2 mM), plants treated with NO3-N alone accumulated more biomass. Previous studies[28] pointed out that responses of Petunia plants to different forms of inorganic N depended on the concentrations of N applied to plants and the medium used for plant growth. Ammonium is more effective than nitrate on the growth promotion in rice[29] and maize[30]. Complex effects of ammonium and nitrate on plant growth, CO2 assimilation and activities of key enzymes involved in N metabolism have also been reported in blueberry, raspberry and strawberry[31]. Similarly, different ratios of nitrate to ammonium applied to sweet pepper at different developmental stages bring various effects on flowering, fruit set and yield[32]. It is then clear to us that effects of different forms of inorganic N on ornamental plants should be discussed case by case[17].
We found that in Petunia plants, NH4-N and NO3-N treatment could both enhance the number of branches and flowers at the cost of leaf and corolla area. Plants can generally benefit from the direct application of NH4-N at an appropriate level since the assimilation of ammonium is far more energetically efficient relative to nitrate assimilation. Meanwhile, nitrate is more mobile which can compensate this limitation. Therefore, applications of NH4-N and NO3-N alone could be superior to NH4NO3 treatment. N can shape the architecture of plants. Nitrate regulates shoot branching on the molecular level to control the outgrowth of axillary buds[33]. Nitrate can also control the transition from the vegetative to reproductive phase and therefore, is involved in flowering time regulation[34]. On the other hand, ammonium application usually leads to accumulation of proteins which is generally beneficial to crop growth. However, there are clear differences between the two treatments. Studies on tobacco have also shown that ammonium nitrogen treatment results in a reduction in leaf area[35]. 15N labeling experiments confirmed that only NH4-N changed the distribution of 15N between various tissues with floral tissues absorbing and utilizing more 15N. Meanwhile, relatively smaller amounts of 15N was distributed into the roots and leaves in NH4-N treated plants. Taken together, when ammonium is used as the sole source of inorganic N, Petunia plants can manage to import more N into floral organs and produce more flowers with smaller size. More importantly, both NH4-N and NO3-N treatment can modify the amino acid profile on the whole-plant level. And as the basic compounds for plant development and signaling molecules[36], amino acids may play direct roles in the formation of plant architecture via regulating the meristem tissues[37], which requires further study.
Here we need to clarify that in most cases, ammonium (NH3 or
) is a stress cue[38,39]. However, ammonium nutrition can also be advantageous as it can improve crop quality[40]. For example, when grown under ammonium nutrition, compared with nitrate, wheat grains can accumulate more proteins leading to an increased wheat bread-making quality[41]. This is somehow consistent with what we have found in Petunia plants. Petal color traits and amino acid profiles measured in this study are highly informative. For Petunia x hybrida 'Mitchell Diploid' with white flowers (Fig. 3), NH4-N and NO3-N treatments could significantly increase petal yellowness/blueness (b*) and chroma (C*) and decrease lightness (L*) and hue angle (h°). We got very similar data with the commercially available white-flower cultivar (Fig. 8c). And data collected for commercially available cultivars with pink-, dark pink- and red-flowers, further indicated that effects of NH4-N and NO3-N on petal pigmentation could be different when petal colors differed. Further studies should be carried out to determine an appropriate ratio of ammonium to nitrate which can be applied to ornamental plants to modify their petal colors.${\text {NH}_4^+} $ For amino acids in petals, there are few studies available. Roles of amino acids in flower development and senescence have been briefly reported in Eustoma grandiflorum[42]. Although most local amino acids in floral tissues are imported from roots and leaves, it has been recently proposed that some amino acids can be synthesized in flowers[18]. For example, de novo synthesis of Asn may exist in flowers due to the fact that asparagine synthetase1 (ASN1) is expressed abundantly in Arabidopsis flowers[43]. The biological roles of Asn in floral tissues can be important. On one hand, Asn carries more N than other amino acids and serves as a good N sink[44]. On the other hand, hydrolysis of Asn can release ammonium which in turn can be used to synthesize Gln[45,46]. More importantly, amino acids provide precursors and energy for petal color and scent. We think the metabolic roles of amino acids in regulating ornamental traits should be much more emphasized in this field. Accumulation of free amino acids is usually considered a sign for ammonium stress[47]. The fact that total content of 14 amino acids in NH4-N petals were not higher than those in NH4NO3 indicates that the NH4-N in our study was not a stress treatment. Given that the assimilation of inorganic N into amino acids primarily occur in roots, the 15N label data and petal amino acid profiles collected in this study indicated that Petunia plants treated with NH4-N alone loaded relatively more amino acids such as Glu, Gln, Asp and Asn to their flowers (Fig. 5a, b). We also found that changes in petal amino acid profiles were correlated with changes in their color traits (Fig. 7), which should be further investigated at the molecular level in ornamental plants.
CONCLUSIONS
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In summary, we have analyzed effects of ammonium and nitrate alone on plant growth, the tissue-level N utilization and distribution, petal color traits and petal amino acid profiles in Petunia plants. We have confirmed that NH4-N can increase 15N distribution to floral tissues, modify the amino acid profiles in petals which in turn bring changes to petal colors. Mechanisms of governing flower development by different forms of N or other environmental cues require further investigation and will definitely lead to a better understanding of the metabolic regulation of ornamental traits.
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About this article
Cite this article
Guo H, Wang G, Zhang P, Wang G, Bao Z, et al. 2022. Ammonium and nitrate impact petal color traits and amino acid profiles differently in Petunia. Ornamental Plant Research 2:14 doi: 10.48130/OPR-2022-0014
Ammonium and nitrate impact petal color traits and amino acid profiles differently in Petunia
- Received: 05 August 2022
- Accepted: 06 September 2022
- Published online: 22 September 2022
Abstract: Nitrogen (N) is vital for ornamental plants to fulfill fundamental cellular functions. Effects of inorganic N, including nitrate and ammonium, on ornamental traits at the biochemical level are less reported. Given that the continuously increased concentration of CO2 in the atmosphere has greater negative effects on nitrate reduction than ammonium utilization in C3 plants, it is vital for us to dissect various effects of different inorganic N forms on flower development, which will lead to a better understanding of the metabolic regulation of ornamental traits. In this study, Petunia × hybrida cv 'Mitchell Diploid' (MD) plants at 6−8-leaf stage were treated hydroponically with the same and regular amount of inorganic N for one month but in three different forms, 1 mM NH4NO3 as the control (NH4NO3), 2 mM NH4Cl alone (NH4-N) and 2 mM NaNO3 alone (NO3-N). Results indicated that relative to the control, effects of NH4-N and NO3-N treatment alone on plant growth, N utilization and distribution, petal color traits and petal amino acid profiles were different by varying degrees. We have confirmed that NH4-N could specifically increase 15N distribution to floral tissues, change the amino acid profiles in petals which in turn bring changes to petal colors. Results with four commercially available Petunia cultivars with different flower colors also validated our findings. This study has been designed to specifically investigate effects of ammonium or nitrate as the sole source of inorganic N on the developmental and chemical features of floral tissues in Petunia.
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Key words:
- ammonium /
- nitrate /
- petal /
- color traits /
- amino caid profiles /
- 15N label