Physiological adaptations in Peperomia obtusifolia plants during low light acclimatization

Abstract

Carbon dioxide exchange rates (CER) of Peperomia obtusifolia plants (tops and roots) were measured, using infrared gas analyzer for 12 hours light and dark cycles, at 250 (mu)E . m('-2). s('-1) photosynthetic photon flux density (PPFD) before acclimatization. Plants were transferred to acclimatization PPFD (30 (mu)E . m('-2).s('-1)). Integrated CER measurements, made weekly at acclimatization PPFD were used to calculate carbon balance parameters. Growth conversion efficiency (Y(,g)) remained constant before and after acclimatization (0.73 (+OR-) 0.1 and 0.77 (+OR-) 0.04), while maintenance coefficient (m) decreased (13.0 (+OR-) 0.9 to 7.4 (+OR-) 0.4 mg . g('-1). 24 hr('-1)). Rate of substrate production from photosynthesis ((DELTA)S/(DELTA)t) was reduced after transfer to acclimatization PPFD, but remained constant during acclimatization (18 mgC . plant('-1). 24 hr('-1)). Rate of synthesis of carbon into new material ((DELTA)W/(DELTA)t) increased by 55% (-33 to -15 mgC . plant('-1) . 24 hr('-1)) the first week, and subsequently at a slower rate to - 5 mgC . plant('-1). 24 hr('-1) after 5 weeks. Light compensation point (LCP), light saturation irradiance (LSI) and dark CER were reduced during acclimatization. The activities of ribulose bisphosphate carboxylase (RuBPcase) and malate dehydrogenase (MDH) were reduced during acclimatization. Specific leaf weight (SLW), chlorophyll and carotene content were determined. SLW of control plants was constant but reduced in acclimatization plants. Total chlorophyll (a+b) was increased. Chlorophyll b accounted for increased total chlorophyll, since chlorophyll a content was constant. Carotene content did not change appreciably. This study showed that acclimatization to low PPFD may be occuring faster than previously thought, and could be accomplished at a shorter period. ...