Growth and photosynthesis of virus‐infected and virus‐eradicated orchid plants exposed to different growth irradiances under natural tropical conditions

Abstract

Responses of virus‐infected (VI) and virus‐eradicated (VE) Oncidium Gower Ramsey orchid plants grown under 30% of prevailing solar radiation and those transferred from 30 to 60% and 100% of prevailing solar radiation were studied under natural tropical conditions. Plants grown under 30% of prevailing solar radiation suffered lower leaf and floral production and reduced photosynthesis. When the irradiance was increased to 60% of prevailing solar radiation, enhancement of leaf and floral production and photosynthetic capacities were achieved. However, when the plants were transferred from 30 to 100% of prevailing solar radiation, the growth and photosynthetic capacities of the plants were significantly reduced. All plants exhibited a midday depression in photosynthetic CO2 assimilation (A), stomatal conductance (gs) and Fv/Fm ratio. The degree of midday depression of these parameters was not only associated with high temperatures and high irradiances but also with virus infection. Midday Fv/Fm ratio depression indicated that dynamic photo‐inhibition occurred in all plants grown under all three light conditions. However, chronic photo‐inhibition, measured by pre‐dawn Fv/Fm ratio and chlorophyll content, occurred only in those plants transferred from 30 to 100% of prevailing solar radiation. Hence, it is concluded that the VI Oncidium Gower Ramsey was more susceptible to high irradiance than the VE plants.

Source-to-sink relationship between green leaves and green pseudobulbs of C₃ orchid in regulation of photosynthesis

“…These orchids are intermediate shade plants (He et al 2004) and were acclimatized for two weeks in the greenhouse under a similar light condition with maximal photosynthetic photon flux density (PPFD) of 700-800 µmol m -2 s -1 . The daily ambient temperature ranged from 24 to 33°C.…”

Susceptibility of green leaves and green flower petals of CAM orchid Dendrobium cv. Burana Jade to high irradiance under natural tropical conditions

“…Providing higher growth irradiance under well-watered and well-supplied mineral nutrients environment is one way of enhancing the harvestable yield in the tropical orchid Yong 1994, 1997). Increasing the photosynthetic rate of source leaves ---by increasing irradiance is, however, not feasible because many tropical orchids are shade plants (Hew and Yong 1994) and are virus-infected (He et al 2004). Photosystem 2 (PS2), an intricate thylakoid membrane complex with a light-driven water-plastoquinone-oxidoreductase activity, is the primary target of this phenomenon, which is known as photoinhibition (Osmond 1994).…”

“…First, supra-optimal leaf temperatures that accompany high irradiance (HI) cause reduction in photosynthesis (He et al 1998). Second, virus infection enhances decrease of photosynthesis (He et al 2004). Providing higher growth irradiance under well-watered and well-supplied mineral nutrients environment is one way of enhancing the harvestable yield in the tropical orchid Yong 1994, 1997).…”

“…Another useful indicator of photoinhibition is Chl fluorescence (Björkman and Demmig 1987). Photoinhibited leaves exhibiting a decline in quantum yield of O 2 evolution and maximal photosynthetic rate per leaf area also have a corresponding decrease in F v /F m in plants grown under natural tropical conditions (He et al 1996(He et al , 2004. Differentiation of the allocation of excitation energy to photochemistry or radiation-less dissipation processes can be determined via measurements of photochemical (q P ) and non-photochemical fluorescence quenching (q N ) together with determination of O 2 evolution (Weis et al 1987, Demmig andWinter 1988).…”

“…Photosystem 2 (PS2), an intricate thylakoid membrane complex with a light-driven water-plastoquinone-oxidoreductase activity, is the primary target of this phenomenon, which is known as photoinhibition (Osmond 1994). HI may lead to photoinhibition of the leaves in plants grown under tropical natural conditions (He et al 1996) including orchid plants (He et al 1998(He et al , 2004. When plants are exposed to excess photons, different mechanisms operate to protect the photosynthetic apparatus from 'over-energization'.…”