Why is vascular dysfunction an important target for reducing alternate bearing in Coffea arabica?
Year: 2026
Type: article
Abstract: Coffee is one of the most valuable crops in the world (FAO 2025), and it provides a livelihood for more than 125 million people worldwide (Sachs et al. 2019). The genus Coffea belongs to the Rubiaceae family and comprises 131 described species (Ramalho et al. 2025). The most cultivated coffee varieties descend from two wild Coffea species:Coffea arabica L. (Arabica coffee) and C. canephora Pierre ex A. Froehner. (Robusta coffee) (Vanden Abeele et al. 2021). Coffea arabica is an allotetraploid hybrid (4n = 4x = 44) of C. eugenioides S. Moore and C. canephora and contributes to approximately 60% of world coffee production (Scalabrin et al. 2024). Coffea arabica and C. canephora have similar management requirements; however, C. arabica is more sensitive to biotic and abiotic stress and has a higher alternate bearing intensity (DaMatta, Martins, and Ramalho 2025).Alternate (biennial) bearing (AB) is a reproductive phenomenon in which episodic flowering triggers the reallocation of carbon, nutrients, and hormonal signals, manifesting as cycles of high-yield ("on") years followed by low-yield ("off") years. AB occurs across many crops of global socioeconomic importance, including apple, avocado, coffee, mango, olive, and pistachio (Goldschmidt & Sadka 2021;Monselise & Goldschmidt 1982). Despite its clear implications for production stability, AB has received limited attention in discussions of socio-ecological resilience (Garcia et al. 2024). In Coffea arabica, AB constitutes a major constraint on farmer livelihoods because income and market access depend on consistent harvests in both quantity and quality (Garcia, Kuhl, and Orians 2025). Most research efforts have focused on agronomic measures to increase productivity and climate resilience, but few studies have systematically evaluated how these interventions directly influence AB (Garcia and Orians 2022).In fruit trees, AB is a complex, multi-scale phenomenon framed by different theoretical models, such as the Resource Budget Model (RBM) as applied by Garcia and Orians (2022) (Jan et al. 2022).From a physiological and molecular perspective, AB arises from cross-regulatory pathways that modulate a core set of floral integrators, restricting flowering to favorable environmental and developmental conditions (Khan et al. 2025). Four major genetic pathways control flowering in Arabidopsis thaliana by integrating external signals and the plant's internal state to trigger the switch from vegetative growth to flowering (Blázquez, Koornneef, and Putterill 2001).Although models describing cross-regulated pathways and resource reallocation in alternate bearing exist, an important question persists: Why do current agricultural management strategies not consistently mitigate alternate bearing in Coffea arabica?The probable answer is that agricultural management strategies do not target the plant vascular system, which conducts all the plant's resources and signals. A holistic understanding of vascular function and its regulation is indispensable for unraveling and managing alternate bearing in C. arabica and other perennial crops.Damage to vascular tissues-xylem and phloem-through cavitation, embolism, blockage, or pathogen attack disrupts the integrated transport of water, nutrients, carbohydrates, and signaling molecules and thereby alters the physiological processes that determine flowering, fruit set, and bud development (Zhang and Brodribb 2017;Konrad et al. 2018;Qaderi, Martel, and Dixon 2019). In C. arabica, xylem cavitation occurs when water stress or high evaporative demand increases xylem sap tension, drawing air into the conduits and forming emboli that interrupt the continuous water column and reduce hydraulic conductivity (Nardini, Õunapuu-Pikas, and Savi 2014).Xylem anatomy strongly determines the likelihood and severity of cavitation: wider vessels increase hydraulic efficiency but raise vulnerability to embolism, whereas narrower, more reinforced conduits trade transport capacity for safety (Max et al. 2023).Cavitation limits water supply to leaves and reproductive organs, provoking stomatal closure, reducing photosynthetic carbon gain, and causing leaf wilting, abscission and ultimately lower growth and yield; recovery after rewatering is often incomplete or delayed because embolized vessels may require active refilling processes that depend on carbohydrate availability and intact phloem function (Tausend, Goldstein, and Meinzer 2000;Carréra et al. 2023). Empirical work on coffee documents substantial genotypic variation in hydraulic traits and cavitation vulnerability, so that cultivar choice, rooting depth, and water-use strategy materially influence susceptibility and recovery under intermittent drought (Nardini, Õunapuu-Pikas, and Savi 2014;dos Santos et al. 2025;Tausend, Goldstein, and Meinzer 2000).Phloem dysfunction compounds these effects by impairing the long-distance transport of photosynthates, hormones, and signaling molecules from source leaves to sink organs such as developing fruits, buds, and roots (De Schepper et al. 2013). Sieve tubes and companion cells mediate sucrose movement and the distribution of regulatory compounds that coordinate growth, storage, and developmental transitions; phloem blockage can arise from pathogen invasion, phloem-feeding insects, mechanical injury, or stress-induced callose deposition and collapse of sieve elements (Konrad et al. 2018).When phloem transport is compromised, sinks receive insufficient carbohydrates and hormonal cues, resulting in reduced fruit size and quality, inhibited bud development, leaf chlorosis, and, in severe cases, necrosis and dieback (Nardini, Lo Gullo, and Salleo 2011).Because the phloem actively contributes to refilling embolized xylem conduits, phloem impairment can both limit reserve remobilization and slow hydraulic recovery, creating a coupled vascular failure that undermines the plant's capacity to restore source strength after a heavy crop year (De Schepper et al. 2013).We propose that coupled xylem-phloem dysfunction provides a mechanistic link between genotype, agronomic management, climatic variability, biotic agents, and alternate bearing (AB) in Coffea arabica (Figure 1). Direct evidence tying AB to vascular failure remains limited, but studies of AB species offer indirect support and reveal physiological and ecological patterns consistent with this model.In our framework, genotype strongly mediates the relationship between xylemphloem dysfunction and AB. Under resource-rich conditions (for example, high nitrogen and strong irradiance), high-yielding genotypes produce large fruit loads in "on" years. To mitigate vascular dysfunction, breeding should prioritize genotypes that combine anatomical and physiological traits conferring vascular resilience. Traits of interest include narrower xylem vessels, thicker phloem, larger hydraulic safety margins, greater nonstructural carbohydrate (NSC) reserves, and faster phloem recovery after stress. Additional desirable traits include small leaves with high vein density (Nardini, Õunapuu-Pikas, and Savi 2014) and resistance to vascular-targeting pests and pathogens (for example, phloem-feeding insects and stem cankers) that directly damage conductive tissues or trigger defensive occlusion responses, thereby reducing transport capacity (Venzon 2021). Establishing standardized, high-throughput protocols for measuring these traits will enable selection of genotypes with a lower propensity for AB and provide a mechanistic basis for genotype-specific management.Because new genotypes take many years to reach growers and wholesale replanting is often cost-prohibitive, near-term emphasis should be on affordable agronomic practices that preserve vascular integrity and reduce AB incidence. Priority measures include irrigation, selective pruning, and targeted application of plant growth regulators.Irrigation is likely the most important management tool for coffee because targeted water supply stabilizes flowering and fruit set (Massarirambi et al. 2009), maximizes yield, preserves bean quality (Barros et al. 1997), and maintains production under variable climatic conditions (DaMatta and Ramalho 2006). Large-scale climatic oscillations (e.g., El Niño-Southern Oscillation and the Madden-Julian Oscillation) modulate drought and heat stress, amplifying hydraulic strain and increasing the probability of vascular failure during critical phenological windows (Sarvina et al. 2021); such synchronous stress events can intensify AB at landscape scales.Precision irrigation stabilizes soil-plant water status, preventing recurrent water stress and xylem embolism by avoiding extreme negative stem water potentials that drive cavitation (Anjum et al. 2023). By matching water timing and volume to crop demand, it reduces water-potential excursions, maintains safer midday stem and leaf water status, and promotes root development and soil moisture stability, thereby lowering stomatal extremes, dampening xylem tension swings, and enhancing hydraulic resilience (Bracken, Burgess, and Girkin 2023). Field studies show that irrigation can increase C. arabica yields even in years of otherwise low production (Sakai et al. 2015), supporting an indirect link among water deficit, vascular dysfunction, and AB.Selective pruning performs two complementary functions in managing vascularrelated productivity decline in Coffea arabica. First, when coupled xylem-phloem dysfunction, canopy function is already compromised, and irrigation cannot restore productivity; selective pruning acts as a reinvigoration treatment that promotes recovery of productive capacity (Fernandes et al. 2012). Second, when applied prophylactically, selective pruning reduces the risk of vascular failure by altering canopy architecture and lowering whole-plant hydraulic demand (Pinkard and Beadle 2000;Gokavi et al. 2021).Targeted pruning of orthotropic (vertical) branches further contributes to hydraulic resilience by stimulating the development of vigorous plagiotropic (horizontal) branches.Orthotropic branches typically possess larger vessel diameters, whereas plagiotropic branches have narrower, more numerous vessels (Carréra et al. 2023); reducing the proportion of orthotropic growth therefore decreases susceptibility to hydraulic conductivity loss and may reduce alternate bearing.Exogenous application of plant growth regulators, such as triazole retardants, produces compact canopies with reduced transpiration demand and an increased root: shoot ratio (Desta and Amare 2021). In practice, paclobutrazol (PBZ) currently offers the most consistent, crop-specific evidence for shifting assimilate partitioning and improving drought resilience in C. arabica, but its use requires careful dose and timing control, as well as integration with irrigation and nutrient management (Ribeiro et al. 2017). PBZ enhances drought resilience, thereby reducing susceptibility to hydraulic conductivity loss and potentially mitigating alternate bearing.In conclusion, a mechanistic understanding of xylem-phloem structure and function, coupled with targeted breeding and agronomic practices that preserve vascular integrity, provides a practical strategy to mitigate alternate bearing in Coffea arabica.
Source: Frontiers in Plant Science
Institution: Universidade Federal do Rio de Janeiro
Language: English
Subfield: Plant Science
Domain: Life Sciences
SDG: Zero hunger
Open Access status: gold
Estimated APC paid: $3,801
Awards: G1840770272, G933372841
OpenAlex ID: W7155690499