BIOMETRICS CHANGES IN SUGARCANE
The use of sugarcane single-bud propagules offers advantages over the conventional method of planting, which uses longer sections of the stalk, with propagules from different physiological ages. These advantages involve economic issues, by reducing the costs associated with harvesting and transporting the stalks; logistical issues, by reducing machine traffic in the sugarcane fields (Fluminhan & Fluminhan 2020); phytosanitary issues, by treating the stalks with pest and disease control products (Landell et al. 2012). Homogeneous and vigorous early growth is desirable for sugarcane since biometric gains at this point can have repercussions on the best development of the crop and the yield per hectare (Otto et al. 2022). In this scenario, the use of plant growth regulators, such as biofertilizers, has significant importance as they have been described as capable of improving the early performance of sugarcane (Gazola et al. 2017; Oliveira et al. 2018; Mógor et al. 2022; Almeida et al. 2024).
In this work, when comparing the control treatment with propagules immersed in AQ250, it was seen that the microalga biomass promoted higher sprouting percentages in all sections of the stalk (Fig. 1a). This contrast in the apical propagules shows that even in younger propagules - with a more accelerated metabolism (Baracat et al. 2017; Figueiredo et al. 2020) - the product improved the sprouting rate by 13.6%, and by 32.4% in medial propagules, equaling them statistically, and being superior to the control. In this variable, which is of fundamental importance for the establishment and development of sugarcane (Nalawade et al. 2018), this index gains have repercussions on the greater number of plants in the stand. This shows the great benefit of using this biofertilizer, which provided homogenization of the sprouting of apical and medial propagules - which naturally have different sprouting rates.
AQ250 also promoted gains in the length of the culms (Fig. 1b), especially in the apical propagules, of 19.1% compared to the control. In the same section, a similar result was shown for culm diameter (Fig. 1c), showing biometric gains of 32.5% compared to the control. For fresh culm mass (Fig. 1d), the superior result also occurred for the apical propagules applied with AQ250 (by 64.3% compared to the control), and a positive distinction was also seen in the basal propagules when compared to the control. This suggests that sugarcane cell multiplication may be enhanced in the tissues of propagules immersed in AQ250, due to its composition being rich in L-AAs (Cordeiro et al. 2022a).
The aerial part of sugarcane was also influenced by AQ250, both in terms of leaf area (Fig. 1e) and leaf fresh mass (Fig. 1f), in which there was an inversely proportional increase with the lower physiological age of the propagule. This treatment was also superior to the control in the leaf area from apical and basal propagules (by 45.3% and 206.9%, respectively), and in leaf mass from apical propagules (by 63.4%). This shows that the aerial part - related to the photosynthesizing capacity of this crop - can also be regulated by the biomass of AQ250.
In general, as described in other crops (Cordeiro et al. 2022a; Cordeiro et al. 2022b; Lara et al. 2022; Palma et al. 2022; Marques et al. 2023), the biomass of AQ also triggers a series of metabolic changes in sugarcane, which can regulate plant growth. Furthermore, AQ biomass application was associated with changes in carbon and nitrogen metabolism in sugarcane, linking the stimulation of vegetal biometric and biochemical changes with their interaction with the physiological age of the propagules (Mógor et al. 2022).
BIOCHEMICAL CHANGES IN SUGARCANE
Changes in the metabolism of plants can be started by signaling processes originating from the biochemical composition of the biomasses applied to them (Yakhin et al. 2017). Microalgae biomasses have bioactive compounds such as L-AAs (Mógor et al. 2018) and PAs (Incharoensakdi et al. 2010; Mógor et al. 2017) that have been described as plant growth promoters. Research using AQ biomass has already proven its efficiency as a biofertilizer in other crops such as potatoes (Cordeiro et al. 2022a), onions (Cordeiro et al. 2022b), tomatoes (Lara et al. 2022), soybeans (Palma et al. 2022), beans (Marques et al. 2023) and sugarcane (Mógor et al. 2022).
For sugars results, especially Ts and Rs, it was shown that the product promoted improvements in sugarcane propagules (Table 1) and in the leaf (Table 3) (from basal propagules for Rs; and basal and apical propagules for Ts), but not in the culm (Table 2). This suggests that the remobilization of sugars from the culm to the leaves may be stimulated by the application of AQ250.
In addition, the Try, Tre, and Ser balances (Table 1–3) partly follow other metabolic changes induced by immersion in AQ250. Although, at general levels, total free AAs were not influenced by the application of AQ250, Try, which is an AA and precursor of Ser (Kang et al. 2008), and Tre, which is a metabolite of the indolamine pathway derived from Try (Negri et al. 2021), had their levels modified by the interaction of the biofertilizer with the physiological age of the propagules, with results varying for propagule, culm or leaf tissues.
AQ250 promoted a higher Try content in culms (Table 2) and leaves (Table 3) from basal propagules, while in the propagule tissue (Table 2) AQ250 treatments were superior to the average of the control. Tre, on the other hand, only increased in the apical and basal propagules. For Ser, it only occurred in the medial propagule (Table 1), and for the leaves of the propagules immersed in AQ250 (Table 3), the Try content average was higher than the control. This suggests that immersion in AQ250 may influence the level of the AA Try, which as a metabolic precursor causes changes in the level of Ser in propagules and leaves.
The bioactive amines (Put, Spd, Spm) were altered by the application of AQ250. The tissue least positively influenced by this product was the leaf (Table 3), wherein the basal propagules AQ250 promoted an increase in Put, however in the medial propagules the application decreased Put levels. Something similar occurred for this same amine in the propagule (Table 1), in which the microalga increased Put levels in the medial and basal propagules but reduced them in the apical propagules. Thus, AQ biomass also influences this metabolism in sugarcane, and Put, as the first molecule in this chain of transformation of Put into Spd, then into Spm (Chen et al. 2019), promotes changes in the balance of other polyamines, which have also been related to regulating plant growth (Nandy et al. 2023). This fact helps to understand how this biofertilizer may have promoted biometric gains in sugarcane (Fig. 1a-1f).
For Spd in the propagule (Table 2), on average, the treatment with AQ was superior to the control. In the culm (Table 3), AQ250 increased Spd in the medial propagules, while it reduced it in the apical propagules when compared to the control. Meanwhile, the Spm in the culms (Table 2) from propagules applied by AQ250 was higher, in contrast to the result in the own propagule tissue (Table 1). Changes can occur in the PAs profile with modifications in glucose metabolism and carbon/nitrogen (C/N) signaling pathways (Anwar et al. 2015), and Spd is also capable of improving the flow from the source to the drain and the transport of sugars (Luo et al. 2019).
Based on these results, in which the averages of the propagules sections with AQ were higher than the control, the variables that show the same trend in propagules (Table 1) are Spd, Ts, Rs, and Try. This suggests a metabolic crosstalk between bioactive amines and sugars in sugarcane, enhanced by immersion in AQ250. It should be added that the propagules sections can also be influenced independently by the application of AQ250. This situation occurred several times in the leaf analyses (Table 3), in which Put, Ts, Rs, and Try had their levels raised by AQ250, specifically in the basal propagules, in contrast to the medial propagules. This supports the hypothesis that AQ biomass can promote metabolic signaling in plants and the results may vary depending on the physiological age of the vegetal tissues (Mógor et al. 2022).
Although the pigments did not have their content per gram changed by the application of this biofertilizer, it did promote greater leaf area and fresh mass, which helps the sugarcane's photosynthesizing capacity.
The alterations of PA levels in the plant contents (propagule, culm, and leaves) of sugarcane suggest the influence of AQ250 on metabolic signaling that triggers the synthesis or catabolism of bioactive amines. An example is the oxidation process of Put in Spd and Spm through the catabolism process of PAs (Chen et al. 2019).
PAs also take part in many metabolic processes, such as cell proliferation and differentiation, modulating plant growth (Srivastava et al. 2013). Spm and Spd are associated with plant ontogeny and growth promotion, while Put is associated with plant senescence (Ahmed et al. 2017; Anwar et al. 2015). This change in the PAs profile using microalga biomass has also been reported (Mógor et al. 2017) and related to the AAs content in the biomass of this microalga (Table 1) (Cordeiro et al. 2022a).
Given that the balance in PA levels is controlled by intricate regulatory feedback mechanisms, it has been described that PAs can be regulated directly or indirectly through interaction with signaling metabolites and nitrogen metabolism - in amines and AAs - and carbon metabolism - in sugars (Kamiab et al. 2020; Nandy et al. 2023). This fact has often been seen in this work as the protagonist of biochemical changes associated with better sugarcane sprouting when applied with AQ250.