Ecophysiology of soursop seedlings fertilized with calcium silicate under different irrigation frequencies

Technical Summary: Ecophysiology of Soursop Seedlings Fertilized with Calcium Silicate Under Different Irrigation Frequencies

Problem Statement
The soursop tree (Annona muricata L.) is a tropical fruit crop of significant economic and nutritional value, particularly in the semi-arid regions of Brazil. However, its cultivation is severely constrained by water scarcity and irregular rainfall patterns, which induce water deficit stress. This stress negatively impacts morphological development, physiological processes (such as photosynthesis and gas exchange), and biochemical stability, ultimately reducing biomass and seedling quality. While deficit irrigation is a necessary management strategy in water-scarce regions, it often compromises plant productivity. There is a need to identify complementary mitigation strategies, such as the application of calcium silicate (CaSiO₃), which contains both silicon (Si) and calcium (Ca)—elements known to enhance stress tolerance in various species but whose specific interaction in soursop seedlings under varying irrigation regimes remains underexplored.

Methodology
The study was conducted in a greenhouse at the Federal Rural University of the Semi-Arid (UFERSA) in Mossoró, Brazil, from September to December 2023. The experiment utilized a randomized block design with a 4 × 2 factorial scheme and four replications.

• Factors:
  1. Irrigation Frequencies: Four schedules were tested: TR1 (daily), TR2 (every two days), TR3 (every four days), and TR4 (every six days).
  2. Fertilization: Presence or absence of calcium silicate (CaSiO₃) applied to the soil at a dose of 3.5 g per plant.
• Experimental Conditions: Soursop seedlings (cultivar Morada) were transplanted into 5 L pots containing a sand and commercial substrate mixture. Irrigation was managed gravimetrically to maintain specific water retention capacities. CaSiO₃ was applied 30 days after transplanting.
• Measurements: At 120 days after treatment application, the study evaluated growth parameters (height, stem diameter, leaf area, root volume), biomass accumulation, leaf gas exchange (transpiration, stomatal conductance, CO₂ assimilation), chlorophyll a fluorescence, photosynthetic pigments, osmotic adjustment markers (proline and total soluble sugars), and leaf nutrient content (K, Ca, Si). Seedling quality was assessed using the Dickson Quality Index (DQI), and drought tolerance was calculated via Drought Tolerance Efficiency (DTE) and Drought Sensitivity Index (DSI).

Key Results

• Impact of Irrigation Frequency: Severe water deficit (TR4, irrigation every six days) significantly reduced plant height, leaf number, leaf area, root volume, and total dry mass compared to daily irrigation (TR1). TR4 also led to a reduction in chlorophyll content and an increase in initial fluorescence (F₀), indicating damage to Photosystem II. Conversely, irrigation intervals of up to four days (TR3) maintained growth and physiological performance comparable to daily irrigation (TR1) in the absence of CaSiO₃.
• Effect of Calcium Silicate (CaSiO₃):
  • Gas Exchange: A significant interaction was observed. Under moderate stress (TR3), the application of CaSiO₃ significantly increased transpiration (E), stomatal conductance (gs), and CO₂ assimilation rate (AN) compared to unfertilized seedlings. Specifically, TR3 with CaSiO₃ exhibited the highest gas exchange values among all treatments, representing increases of 142.25%, 122.31%, and 68.44%, respectively, compared to the lowest performance observed (TR1 without CaSiO₃).
  • Biomass and Nutrients: CaSiO₃ application increased leaf dry mass by approximately 15% across irrigation frequencies. It significantly increased silicon and calcium content in leaf tissues, particularly under TR3 and TR4.
  • Osmotic Adjustment: The application of CaSiO₃ regulated proline synthesis. Under moderate stress (TR3), seedlings without CaSiO₃ exhibited a massive accumulation of proline (192.30% higher than fertilized seedlings), whereas fertilized seedlings maintained significantly lower proline levels. This suggests that CaSiO₃ alleviated stress intensity in TR3, reducing the plant's need for excessive osmotic adjustment via proline synthesis.
  • Stress Tolerance: The presence of CaSiO₃ improved Drought Tolerance Efficiency (DTE) and reduced the Drought Sensitivity Index (DSI), particularly under TR3 and TR4.

Key Contributions and Significance
The paper demonstrates that calcium silicate acts as an effective mitigator of water deficit stress in soursop seedlings, specifically under moderate irrigation intervals.

• Physiological Mechanism: The study posits that CaSiO₃ enhances drought tolerance by maintaining stomatal function and osmotic balance. The deposition of silicon in cell walls likely reinforces structural rigidity and reduces excessive water loss, while calcium acts as a signaling molecule to regulate stomatal opening and antioxidant mechanisms.
• Agronomic Viability: The results indicate that irrigation every four days (TR3) combined with CaSiO₃ fertilization allows for significant water savings without compromising seedling development, gas exchange, or biomass accumulation. This combination outperformed severe deficit irrigation (TR4) and showed superior physiological efficiency compared to unfertilized seedlings under the same stress.
• Sustainable Management: The findings suggest that for soursop production in semi-arid regions, the integration of calcium silicate fertilization with moderate deficit irrigation (up to four-day intervals) is a viable strategy to optimize water use efficiency and maintain seedling quality.

Conclusion
The application of CaSiO₃ increases the tolerance of soursop seedlings to deficit irrigation regimes, primarily under moderate stress conditions (four-day irrigation intervals). This treatment maintains stomatal regulation, enhances photosynthetic efficiency, and improves nutrient uptake (Si and Ca), thereby reducing the physiological impact of water scarcity. While irrigation every six days severely suppresses growth regardless of fertilization, the combination of a four-day irrigation schedule and CaSiO₃ fertilization offers a sustainable approach for seedling production in water-limited environments. The authors conclude that further research is needed to evaluate different dosages of CaSiO₃ to fully expand its agronomic potential for this species.