Photosynthetic characteristics of the invasive weed Chromolaena odorata and other co-occurring species in KwaZulu-Natal

In this study, we tested the hypothesis that photosynthetic characteristics contributed to the success and spread of the invasive, C-3 perennial weed, Chromolaena odorata (L.) R. M. King & H. Rob. Measurements of gas exchange, chlorophyll fluorescence and water relations parameters were taken in summer and winter, as well as in sun and shade plants. Typically, diurnal CO2 exchange increased from dawn to a mid-morning maximum between 10:00 h and 11:00 h, and thereafter decreased gradually for the rest of the day. Light response curves indicated saturation of CO2 uptake and electron transport rate (ETR) through Photosystem II (PSII) at a photosynthetic photon flux density (PPFD) +/- 1800 mu mol m(-2) s(-1). Maximal CO2 exchange was 15.64 & PLUSMN; 0.87 (+/- SE) in summer and 13.15 & PLUSMN; 0.26 mu mol m(-2) s(-1) in winter. Leaf conductance (g), CO2 uptake (A) and transpiration (E) followed trends similar to those for PPFD. Leaf water potential (psi) and water use efficiency (WUE) declined from the predawn maximum values to a minimum at midday and thereafter recovered at dusk. Diurnal trends in actual quantum yield (phi(PSII)) and maximum quantum yield (F-v/F-m) decreased from dawn to a minimum at midday, followed by complete recovery at dusk. The relationship between A and g, and A and ETR was linear. Trends in A, phi(PSII) and ETR through PSII were tightly coupled to those of incident PPFD. Efficient light utilisation is achieved through modifying plastic traits such as leaf size, specific leaf area and chlorophyll content. Chromolaena is a facultative shade-tolerant weed that exhibits superior light utilisation to maximise carbon gain, while efficient photoprotective mechanisms minimise photoinhibition and photodamage to the photosystems. These adaptive architectural and physiological strategies may probably confer competitive advantage over other species.