Photosynthetic electron transport rate (ETR) in the littoral herb Launaea sarmentosa known as mole crab in Thailand.

Abstract

Launaea sarmentosa (Willd.) Sch. Bip ex Kunze (Asteracaeae) is a littoral sand dune herb found in the Indian Ocean region, used as a folk medicine and as a savory vegetable in Thailand. It is in the transition stage from a kitchen & cottage industry to a commercial proposition. Rapid light curves to measure the photosynthetic electron transport rate (ETR) were conducted on the plants over the course of daylight from 6:00 to 18:00 using a PAM fluorometer on plants grown under 50% (nominal) green horticultural and 20% black (nominal) shade cloth and in the open. Plants grown in the open were sun plants. Eopt (µmol photon m-2s-1) decreased slightly under shade cloth (open air: 890 > green shade cloth, 778 > green shade cloth, 713). Launaea shows limited shade adaptation. ETR decreased under shade cloth on both a surface area and Chl a basis (µmol e- g Chl a-1s-1) [open air: 388 > (green shade cloth, 209 = black shade cloth, 263)]. Maximum non-photochemical quenching (NPQmax) was significantly decreased under black shade cloth [(open air: 1.02 = green shade cloth, 0.969) > black shade cloth, 0.694]. ETR showed midday inhibition (9:00-15:00) when irradiance exceeded the Eopt of Launaea. Daily total photosynthetic electron transport was (mmol e- m-2 d-1): open air, 1890 ± 157; green shade cloth, 1620 ± 203, black shade cloth, 1217 ± 143. Green shade cloth has no effect on total daily photosynthetic electron transport of Launaea but offers some protection from desiccation and excessive evapotranspiration. Waterlogging decreases ETR by about 40%. Launaea can be grown watered with brackish (½ seawater) water unlike many other vegetables. Launeae is a physiologically undemanding cottage industry/market garden crop suitable for sandy coastline fishing communities.