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

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 ( g _s) and F _v/ F _m 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 F _v/ F _m 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 F _v/ F _m 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.