Discussion

Summary of key findings and environmental threshold model
In this study, we demonstrated that Adenium dhofarense seedlings require the concurrent attainment of high temperature (≥35 °C maximum) and high daily irradiance (estimated DLI ≥23 mol m⁻² day⁻¹) to initiate rapid caudex growth. Under partial shade or moderate temperature alone, growth remained minimal, and many individuals experienced leaf abscission. The data reveal a dual‑threshold environmental window governing seedling growth dynamics: outside this window, A. dhofarense appears to revert to a low‑energy, slow‑growth mode.
These results contrast sharply with the behavior of the congeneric A. somalense, which maintained continuous growth across a broader and less extreme range of temperature and light conditions. Thus, A. dhofarense exhibits constrained photosynthetic and growth flexibility relative to congeners, a pattern that cannot be attributed solely to cultivation artifacts but appears to reflect inherent physiological constraints.

Ecological interpretation in light of native habitat seasonality
The strict dual‑threshold requirement aligns closely with the native climatic regime of the Dhofar escarpment. This region is characterized by the khareef monsoon (June–September), during which persistent orographic cloud cover produces cool temperatures (22–28 °C) and severely reduced direct solar irradiance, followed by a brief post‑monsoon interval of intense heat (40–45 °C maxima) and cloudless skies (Horisk et al., 2023, 2024). In its natural habitat, A. dhofarense likely experiences a narrow but intense favorable season, during which rapid biomass accumulation must occur before the onset of the prolonged dry season.

The observed physiological strategy—slow or near‑dormant growth outside of the optimal window, rapid growth only under combined environmental maxima—may represent an adaptive trade‑off, allowing survival through extended unfavorable periods while maximizing carbon gain during the brief favorable interval. This interpretation supports the view that the dual‑threshold growth requirement is not a cultivation artifact but an evolved adaptation to the sharply pulsed resource availability characteristic of its endemic environment (Horisk et al., 2024).

Implications for metabolic strategy: hypothesis of constrained CAM–C₃ flexibility
Although we do not present direct physiological data (e.g., gas‑exchange measurements, diel titratable acidity, or carbon isotope discrimination) to confirm photosynthetic mode, the growth behavior observed in A. dhofarense is consistent with a metabolic strategy in which the plant defaults to a low‑energy, CAM‑like state under suboptimal conditions and transitions to a high‑activity, C₃‑like mode only under optimal external cues. Because no peer‑reviewed studies have conclusively demonstrated the predominant photosynthetic pathway or CAM reversibility in Adenium species, this interpretation does not rely on an assumed “baseline C₃ with facultative CAM” framework for the genus. Instead, our hypothesis is strictly comparative: relative to the typical growth responses reported for congeners under similar conditions, A. dhofarense appears to show an unusually strong dependency on simultaneous high irradiance and high temperature to achieve sustained daytime carbon gain.

This hypothesis is consistent with the observed growth patterns: the extremely slow early growth under moderate conditions (suggestive of limited CO₂ uptake via nocturnal stomatal opening), the abrupt acceleration when dual thresholds are met (consistent with daytime stomatal opening and higher photosynthetic rates; cf. Yamori et al., 2014), and the rapid reversion to dormancy when conditions decline (suggestive of re‑engagement of water‑conserving CAM mode). Such a strategy would represent a novel adaptation within Adenium, potentially convergent with obligate CAM taxa in other succulent lineages.

Intraspecific variation in dormancy onset: ecological implications
The observed variation in dormancy timing among individuals under identical environmental conditions suggests that intrinsic factors—possibly genetic diversity or ontogenetic asynchrony—influence the threshold for metabolic down‑regulation and leaf abscission. While focal individual D‑1 and several cohort members had fully defoliated by mid‑November, a substantial subset retained green, turgid leaves with no apparent dormancy entry. This intraspecific variability may reflect a bet‑hedging strategy, allowing at least some seedlings to exploit variable or extended favorable windows if environmental conditions fluctuate unpredictably—a pattern commonly observed in species endemic to hyper‑seasonal environments (Horisk et al., 2023).

From a conservation perspective, this variability indicates that A. dhofarense populations may harbor considerable physiological diversity, which could be critical for persistence under climate variability. Moreover, the apparent reliance on narrowly defined high‑temperature and high‑irradiance conditions for sustained growth suggests that populations occupying microhabitats with differing thermal or light regimes may experience divergent selective pressures, potentially shaping local physiological differentiation relevant to long‑term species resilience.

Limitations, physiological validation priorities, and broader implications
This study is limited by its reliance on morphological proxies (caudex diameter, leaf phenology) rather than direct measurements of photosynthetic physiology. The central claim of this work—that A. dhofarense exhibits reversible shifts between slow, CAM‑like metabolism and rapid, C₃‑like metabolism in response to environmental conditions—remains a hypothesis requiring physiological validation. The dual‑threshold environmental window (≥35 °C maximum temperature and ≥23 mol m⁻² day⁻¹ DLI) represents the conditions under which this metabolic transition appears to occur, but the precise numerical values should be interpreted as approximate indicators rather than strict physiological constants.

The estimation of DLI from horizontal global irradiance obtained from a meteorological station on the order of 20 km distant introduces uncertainty in absolute values; however, the core evidence lies not in the exact threshold magnitudes but in the demonstrable reversibility of the growth response across distinct environmental regimes. The transition from minimal growth under moderate conditions, to rapid growth when both temperature and irradiance maxima are reached, and back to dormancy when conditions decline, constitutes a reproducible phenological pattern that is inconsistent with constitutive C₃ metabolism but consistent with environmentally triggered metabolic switching.

Gas‑exchange measurements and diel titratable acidity assays planned for 2026 will directly test whether the observed growth transitions correspond to shifts in stomatal behavior and CO₂ uptake timing. Controlled‑environment experiments comparing plants maintained at 25 °C versus 35 °C under standardized irradiance will allow quantification of net photosynthetic rates, stomatal conductance, and nocturnal acid accumulation, following established protocols for CAM assessment (Winter & Holtum, 2014; Yamori et al., 2014). Complementary stable‑carbon isotope analysis (δ¹³C) of leaf tissue produced under contrasting environmental conditions will provide time‑integrated evidence of photosynthetic pathway utilization. Until these data are obtained, the metabolic interpretation presented here should be regarded as a working hypothesis grounded in phenological observation rather than confirmed physiology.

Nonetheless, the phenological evidence for reversible, threshold‑dependent growth mode switching in A. dhofarense is robust and represents a novel pattern within the genus. If confirmed physiologically, this species would exemplify a strategy in which constitutive or near‑constitutive CAM serves as the default state, with facultative engagement of C₃‑like metabolism restricted to brief windows of extreme environmental favorability—a departure from the physiological scenarios discussed by Winter & Holtum (2014), in which CAM expression is typically framed in relation to environmental stress responses, rather than as a default metabolic state. Such a strategy would be consistent with adaptation to the hyper‑seasonal climate of the Dhofar escarpment, where extended periods of marginal conditions (cool, clouded khareef monsoon) are interrupted by short, intense favorable intervals of extreme heat and high irradiance (Horisk et al., 2024). Whether similar threshold‑dependent metabolic flexibility exists in other Adenium species, and whether it represents a shared ancestral trait or independent adaptation to extreme environments, remains to be determined through comparative physiological studies.