Introduction
Alginate oligosaccharide (AOS) is a natural biostimulant with low molecular weight, good solubility, high bioactivity, and easy absorption. It consists of 2-25 monosaccharides and is the primary active substance in seaweed extracts. In agriculture, AOS has positive effects on plant growth. These effects include promoting root growth in lettuce, mustard, barley, and soybean seedlings, activating seed germination and metabolite formation, and increasing proline and total soluble solids in tomatoes.
In addition, previous studies have demonstrated the efficacy of AOS in enhancing crop drought tolerance. Specifically, AOS was demonstrated to promote root growth in wheat seedlings, restore normal growth parameters in cucumber plants under drought stress, and increase tomato biomass.
Under drought stress, AOS becomes a signaling molecule or exciton in plants to activate the antioxidant system and up-regulate the expression of relevant genes in the abscisic acid (ABA) pathway. In soil, the network structure of AOS can reduce soil water loss and bind to more nutrients to form chelates and achieve in-situ nutrient fixation.
Citrus is one of the most widely grown cash crops, with an annual production of 158 million tonnes, accounting for about 25% of the world’s total fruit production. It has enormous industrial value in areas such as agriculture, food, pharmaceuticals, cosmetics, and nutraceuticals. However, consumer demand has shifted from quantity to quality with rising living standards. Additionally, the cost of agriculture has increased with the development of society. Therefore, reducing costs while improving yield, quality, and profitability has become a challenge for the citrus industry in modern agriculture.
Improving citrus growth and quality by applying excess water and fertilizers contradicts the optimal use of resources and a sustainable soil ecosystem. Agronomic measures such as improved irrigation techniques (drip, micro-irrigation, partial root-zone irrigation) and precision fertilization could achieve green production without compromising quality.
Soil moisture is the necessary environmental factor for crop growth and development. By regulating the amount of irrigation or water conditions, the nutritional and reproductive development of fruits can be balanced, and their overgrowth can be inhibited, thus achieving water and fertilizer conservation and quality improvement.
However, crop agronomic traits respond differently to the duration and extent of water deficit. Studies have shown that water deficit can promote citrus flowering, but this can lead to compromised pollination and physiological fruit drop in citrus. Deficit irrigation before the physiological fruit drop stage significantly affected the physical qualities of citrus fruit (such as firmness and size) and the formation of chemical qualities (soluble solids and sugars) during fruit expansion.
In addition, citrus fruits are subjected to certain water stress during non-critical water demand periods, and the fruit can enter a compensatory growth state after rehydration, accelerating the growth rate and improving quality. However, some studies have also pointed out that deficit irrigation may lead to drought stress and yield loss in different crop varieties. The side effects of stress on yield and quality can be effectively prevented by monitoring the water and nutrient status of crops in real-time, but this is extremely costly in practice. Therefore, the negative effects of water regulation on crop growth and yield can be overcome by strategies combining appropriate measures, thus achieving their applicability and sustainability.
Quality
The use of AOS under different moisture conditions improved the sugar quality of the citrus fruit (Fig. 1). Total soluble sugar, sugar-acid ratio, sucrose and glucose contents of fruits treated with the same level of AOS at W70 were significantly higher than those treated with AOS at W80 and W60 (p<0.05). For the overall fruit quality, AOS application under W70 conditions outperformed that under the other two moisture conditions.
Plant growth
Leaf photosynthesis and water use efficiency
Effects of AOS application on photosynthesis and water use efficiency of citrus leaves under different moisture conditions are shown in Fig. 2. Pn and Tr were significantly higher in the same levels of AOS treatments at late fruit expansion and ripening under W70 and W80 than AOS treatments under W60. At the late fruit expansion stage, Gs significantly increased with increasing water with the same level of AOS treatment, and at the maturity stage it showed W80, W70 > W60.
The WUEi was significantly higher at maturity in W70 than in W60 with the same level of AOS treatment (p<0.05). In conclusion, the photosynthesis of citrus fruit was significantly improved after several root applications of AOS. However, this effect varied under different moisture conditions, mainly presented as W70 > W80 > W60.
Root system configuration
The effects of AOS application on citrus root growth under different moisture conditions are shown in Fig. 3. Under all moisture conditions, root application of AOS mainly affected root structure in the 0-20 cm soil layer. AOS treatment did not significantly affect total root length, root surface area and root volume of the root system in each soil layer under W60 conditions (p>0.05). In addition, total root length and root surface area were significantly higher in W70 than in W80 and W60 for the same level of AOS treatments, whereas root volume was significantly lower in W60 than in W80 and W70 for the same level of AOS treatments in the 0-20 cm soil layer (p<0.05).
In addition, to directly reflect the effect of AOS on the citrus root system, the root system of citrus seedlings was observed through hydroponic experiments, as shown in Fig. 4. In addition, citrus root activity was increased by 33.4% and 23.5% in AOS2.5 and AOS5.0 treatments than in CK, respectively (Fig. 6B, p<0.05).
Soil properties
Soil water content
Effects of AOS application under different moisture conditions on the soil water content at three stages are shown in Fig. 5. The decreasing time from the upper limit to the lower limit differed for the three moisture conditions. With a higher upper limit of moisture content, the decreasing time to the lower irrigation limit is faster, ranking as W80 > W70 > W60. The multiple root applications of AOS may have improved the water-holding capacity of the soil, particularly under W70 and W60.
Soil nutrients, pH and CEC
Application of AOS mainly affected soil available K and organic C contents under all moisture conditions (Fig. 6).
Soil bacterial community structure
Effects of each treatment under control, conventional fertilizer, and W70 moisture conditions on soil bacterial community structure were analyzed, and the results are shown in Fig. 7. PC1 and PC2 explained 17.4% and 14.3% of the variability, respectively, with insignificant bacterial community differentiation between treatments (Fig. 9C). The bacterial community was analyzed at the phylum and genus level by identifying the taxonomic assignments of OTUs.
A total of 47 bacterial phyla were detected at the phylum level, of which the major taxa were Actinobacteriota (27.4-33.6%), Proteobacteria (20.8-25.1), Firmicutes (9.9-15.3%), Chloroflexi (8.7-10.6%), and Acidobacteriota (7.3-12.6%), with five phyla accounting for more than 80% of the total taxa in each treatment (Fig. 9D). After applying AOS, the number of unique OTUs increased in each treatment (695-939), and both fertilizer and water conditions affected the number of unique OTUs between treatments (Fig. 9F).
The relationship between soil chemical properties and microbial community characteristics was assessed using Pearson’s correlation coefficient. In contrast, there was a strong correlation between soil fertility and bacterial communities at the genus level. Among them, Arthrobacter showed a significant positive correlation with SOC, Sphingomonas and Bradyrhizobium exhibited a significant positive correlation with SN, and Pseudolabrys displayed the most significant positive correlation with SOC, SN, and SK.
Correlation analysis of citrus growth and soil indexes
Spearman correlation analysis was conducted on citrus growth and soil indexes under three water management conditions, as shown in Fig. 8. In general, under W70 water management, the application of AOS enhanced the correlation between soil indexes and citrus tree growth indexes, indicating that the application of AOS had a more positive effect under this water condition.
Effect of water management combined with AOS application on citrus yield, quality, and resource use efficiency
Appropriate fertilizer application and water regulation can not only improve water and fertilizer use efficiency, yield, and quality of crops, but also effectively reduce soil nutrient loss. For example, regulating water supply below crop transpiration levels during certain growing seasons can save water and improve crop quality, and water regulation coupled with fertilizer management can significantly increase sugar accumulation in sugar beets.
In citrus studies, moderate water stress during fruit expansion and ripening can promote fruit physicochemical properties, but the fruit size is severely affected by higher stress. This phenomenon may be because AOS acts as a signaling molecule or exciton, effectively regulating physiological processes related to plant antioxidant system, photosynthesis, and nutrient uptake.
AOS can effectively restore normal growth parameters of cucumbers under drought stress and AOS of DP2-5 could induce the ABA pathway and increase the activity of antioxidant enzymes in plants under drought stress. Thus, AOS may have mitigated the negative effects of moisture conditions on yield and created a better synergy with water stress on fruit sugar and quality.
The K uptake and the water use efficiency of citrus were increased by AOS, thus improving the fruit quality. The increase in P and K can promote total sugar accumulation in citrus. Finally, PCA and the affiliation function method indicate that applying AOS at 60-70% of the soil water holding capacity is optimal for citrus growth.
Effect of water management combined with AOS application on citrus growth conditions
Photosynthesis is the process by which crops accumulate dry matter and transport assimilates. High photosynthetic performance improves the transport of photosynthetic assimilates from citrus leaves to roots, facilitating the accumulation of assimilates. However, when crops are stressed, leaf chloroplasts become swollen and disorganized, and the structure of photosynthetic organs is damaged.
As a result, stomata are closed, and the regeneration of ribulose diphosphate carboxylase is diminished, reducing the leaf photosynthetic rate and assimilate formation. Pn, Tr, and Gs of citrus leaves were significantly reduced at the end of fruit expansion and at harvest, while WUEi showed no significant difference due to the simultaneous reduction of Pn and Tr. Application of AOS significantly increased leaf Pn under all three moisture conditions, with the greatest increase in Pn and a significant increase in leaf WUEi.
However, leaf Tr and Gs did not change with the AOS application. It indicated that only under a certain degree of water stress, AOS can increase leaf photosynthesis and effectively improve leaf transpiration rate and water use efficiency. Similar effects of AOS have been observed in several plant experiments, including the induction of growth hormone-related enzyme activities and gene expression in crops under stress conditions. These effects promote growth and enhance photosynthesis and physiological and biochemical metabolic processes.
Furthermore, the AOS application optimized the root conformation of citrus. The total root length, surface area, and volume of citrus were significantly increased in the 0-20 cm soil layer. The hydroponic experiment more intuitively showed that AOS stimulated citrus root growth and improved root vigor, indicating that AOS favors citrus root development and explaining the increase in Pn. Fruit tree root architecture affects water and mineral uptake, and citrus tree growth and development are directly affected by root vigor.
The positive effects of AOS on the root system have been confirmed in most previous studies, with the AOS of both DP 3 and DP 4 significantly affecting growth-promoting activity in the roots of barley seedlings. In the research on carrots and rice, AOS of DP 5 showed the greatest effect on root growth-promoting activity.
Combined with previous studies on resource use efficiency, it can be concluded that AOS may improve photosynthesis and citrus growth by optimizing the conformation of the citrus root system and increasing the efficiency of K uptake and use. Several studies have also shown that increasing K under drought conditions can improve osmoregulation, activate photosynthesis, and increase the load of photosynthates on the plant phloem, leading to higher growth rate and water use efficiency.
Effect of water management combined with AOS application on soil properties in citrus orchards
In this study, the application of AOS delayed the time to reach the lower moisture limit under all three moisture conditions, suggesting that AOS improved the water-holding capacity of the soil. This effect was enhanced with decreasing moisture, which may be related to the structure of AOS.
Sodium alginate and polysaccharides could increase soil water holding capacity and improve nutrient uptake and utilization of roots by optimizing soil aggregate structure, aeration, and capillary action. The cementation of seaweed polysaccharides with soil aggregates can increase water-stable macroaggregates and soil stability.
The analysis of soil nutrients showed that AOS application significantly increased the available potassium content of the soil in the 0-20 cm soil layer under all three moisture conditions. In the 20-40 cm soil layer, the available potassium content was significantly decreased. In addition, AOS significantly increased organic carbon content in the 0-40 cm soil layer and cation exchange capacity in the 0-20 cm layer at all moisture conditions, and these effects were optimal.
The cation exchange capacity of soil represents the total amount of cations that can be adsorbed by soil colloids, which directly reflects the soil’s ability to conserve, supply and buffer cationic nutrients, and also affects other soil physical and chemical properties. The molecular chain of AOS contains many hydroxyl and carboxyl groups. These groups bind to the metal ions to form a reticulated intercalation complex, further absorbing water and reducing the mobility of the metal ions with a more significant effect as the soil moisture decreases.
This effect contributes to increased nutrient vitality of the soil, and nutrient uptake and utilization by shallow roots, which explains why AOS improves the resource use efficiency of potash and water. In addition, the increase in soil organic carbon content may be related to the microbial community. The increase in organic carbon may be associated with changes in taxa during decomposition and mineralization of soil organic matter. Previous studies have reported that Pseudolabrys are chemoenergetic heterotrophic aerobic bacteria primarily associated with C and N. Aerobic bacteria are also known for their nitrate-reducing and carbon-reducing abilities.
This genus is nitrate-reducing, and the carbon source can be derived directly from organic carbon compounds. Lower moisture conditions reduce soil microbial activity and alter microbial community structure, which may affect soil fertility or mineralization of organic matter. In contrast, AOS can provide nutrients for soil microbial metabolism, influencing the abundance and activity of soil microbes and communities. Soil microorganisms secrete enzymes for catabolism and metabolism of complex compounds that can be stabilized in extracellular humus and mineral particles.
Conclusion
In conclusion, it can be concluded that the integration of moderate water regulation with the application of alginate oligosaccharides (AOS) represents a highly effective and sustainable strategy for citrus cultivation. This combined approach not only significantly enhances fruit quality, photosynthetic efficiency, and root development but also optimizes resource use efficiency by improving soil water-holding capacity, nutrient availability, and soil aggregate stability.
While the positive effects of AOS were observed across different moisture levels, its synergistic benefits were most pronounced under mild water deficit conditions, effectively mitigating the negative impacts of stress while promoting overall plant growth and soil health. Thus, this management practice offers a promising solution to balance high-quality fruit production with resource conservation, contributing to the sustainability and profitability of the citrus industry in modern agriculture.
Dora Arouse is a new elicitor product based on AOS/ADO (Alginate-derived Oligosaccharide). This product is produced in full compliance with organic standards and obtained Ecocert in May 2023. See more details of Dora Arouse (AOS).
Dora Alginate Oligosaccharide(EcoCert)
Dora Arouse is a new elicitor product based on ADO(alginate-derived oligosaccharide). It will activate the plant’s SA & JA immune system and awaken the plant’s vitality from root to the top in rooting, disease defending and adapting to climate changes.