Light-use properties in two sun-adapted shrubs with contrasting canopy structures.

Area of Science:

• Plant Physiology
• Ecology
• Photosynthesis Research

Background:

• Sun-exposed ridges in subtropical environments present challenges like high solar irradiance and elevated temperatures.
• Dwarf shrub species, Planchonella obovata var. dubia and Hibiscus glaber, coexist in these harsh conditions.
• Leaf morphology differs significantly: Planchonella has inclined, long-lived, sclerophyllous leaves, while Hibiscus has thinner, horizontal, short-lived leaves.

Purpose of the Study:

• To investigate the impact of high solar irradiance and temperature on carbon gain in two co-occurring dwarf shrub species.
• To test the hypothesis that Planchonella exhibits lower physiological tolerance to high light compared to Hibiscus.
• To explore the relationship between leaf display (morphology) and leaf photochemical efficiency under stress.

Main Methods:

• Field measurements of net photosynthetic rate (P(n)) under high irradiances (photosynthetic photon flux density, PPFD > 500 µmol m⁻² s⁻¹).
• Assessment of photosystem II (PSII) quantum yield and non-photochemical quenching (NPQ) at varying PPFD levels.
• Exposing leaf discs to high light (1900 µmol m⁻² s⁻¹) and high temperatures (37-43°C) followed by dark recovery measurements of PSII quantum yield (Fv/Fm).

Main Results:

• Planchonella exhibited lower P(n) and PSII quantum yield but higher NPQ compared to Hibiscus under high PPFD.
• Mature and young leaves of Hibiscus showed significantly higher P(n) than Planchonella.
• Planchonella demonstrated slower recovery of PSII quantum yield after high light and temperature stress, indicating reduced tolerance.

Conclusions:

• Planchonella possesses lower physiological tolerance to combined high light and temperature stress than Hibiscus.
• Leaf structure and orientation appear linked to photochemical efficiency and stress tolerance in these sun-adapted species.
• The findings suggest a trade-off between leaf longevity/sclerophylly and high-light/heat tolerance in subtropical dwarf shrubs.