Lethal yellowing (LY), a disease caused by a phytoplasma, is the most devastating disease affecting coconut (Cocos nucifera) in Mexico. Thousands of coconut palm trees have died on the Yucatan peninsula while plantations in Central America and on the Pacific coast of Mexico are severely threatened. Polymerase chain reaction assays enable identification of incubating palm trees (stage 0+, phytoplasma detected but palm asymptomatic). With the development of LY, palm trees exhibit various visual symptoms such as premature nut fall (stage 1), inflorescence necrosis (stages 2 to 3), leaf chlorosis and senescence (stages 4 to 6), and finally palm death. However, physiological changes occur in the leaves and roots prior to onset of visual symptoms. Stomatal conductance, photosynthesis, and root respiration decreased in stages 0+ to 6. The number of active photosystem II (PSII) reaction centers decreased during stage 2, but maximum quantum use efficiency of PSII remained similar until stage 3 before declining. Sugar and starch concentrations in intermediate leaves (leaf 14) and upper leaves (leaf 4) increased from stage 0- (healthy) to stages 2 to 4, while root carbohydrate concentrations decreased rapidly from stage 0- to stage 0+ (incubating phytoplasma). Although photosynthetic rates and root carbohydrate concentrations decreased, leaf carbohydrate concentrations increased, suggesting inhibition of sugar transport in the phloem leading to stress in sink tissues and development of visual symptoms of LY.

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Gradual climate change and discrete extreme climatic events have driven shifts in the structure of populations and the distribution of species in many marine ecosystems. The most profound impacts of recent warming trends have been generally observed at species' warm edges and on large conspicuous species. However, given that different species and populations exhibit different responses to warming, and that responses are highly variable at regional scales, there is a need to broaden the evidence to include less conspicuous species and to focus on both local‐ and regional‐scale processes. We examined the population dynamics of canopy‐forming seaweed populations situated at the core range of their distribution during a regional marine heatwave (MHW) event that occurred in the Mediterranean Sea in 2015, to determine between‐site variability in relation to the intensity of the MHW. We combined field observations with a thermo‐tolerance experiment to elucidate mechanisms underlying observed responses. Despite our study populations are located in the species core range, the MHW was concomitant with a high mortality and structural shifts in only one of the two surveyed populations, most likely due to differences in habitat characteristics between sites (e.g. degree of shelter and seawater transfer). The experiment showed high mortalities at temperatures of 28°C, having the most severe implications for early life stages and fertility, which is consistent with warming being the cause of population changes in the field. Crucially, the regional‐scale quantification of the MHW (as described by satellite‐derived SSTs) did not capture local‐scale variation in MHW conditions at the study sites, which likely explained variation in population‐level responses to warming. Synthesis. Enclosed and semi‐enclosed seas, such as the Mediterranean Sea, often highly impacted by human perturbations, are also global hotspots for ocean warming and are highly susceptible to future MHWs. Our findings highlight that local‐scale variability in the magnitude of extreme climatic events can lead to local extinctions of already fragmented populations of habitat‐forming seaweeds, even towards the species' core range. However, our results highlight the potential for local‐scale climatic refugia, which could be identified and managed to safeguard the persistence of canopy‐forming seaweeds.

SUMMARYNoon stomatal conductance of lethal-yellowing (LY) affected coconut palms (Cocos nucifera L. Atlantic Tall ecotype) decreased progressively as the disease developed. In the early stages of the disease and before leaf yellowing started, stomatal conductance decreased to a minimum in leaves at both the top, the middle and the base of the crown. Since altered stomatal behaviour might affect gas exchange and related processes in the plant, such as water movement and photosynthesis, these results support the hypothesis that LY-induced stomatai closure is central to the development of LY symptoms in coconut palms, and therefore to the mode of pathogenicity of the LY mycoplasma-like organism. Leaf yellowing occurred simultaneously with a decrease in photosynthetic rates and a decrease in protein, chlorophyll and carotenoid contents. Based on these biochemical changes a second hypothesis is proposed, that LY-associated leaf 3'ellowing is part of a leaf senescence process. The concentration of abscisic acid in the leaves and the capacity of leaf tissue to form ethylene increased in diseased palms, suggesting that a hormonal imbalance occurs in LY-afTected palms. Furthermore, treatment of symptomless palms with an ethylene-releasing agent (ethephon) resulted in symptoms that mimicked some of the LY symptoms. A third hypothesis is therefore proposed, that a hormonal imbalance might be related to LY symptom development, at least with respect to nutfall and leaf senescence.

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