Photosynthetic ability of in vitro grown coconut (Cocos nucifera L) plantlets derived from zygotic embryos

Photosynthetic parameters have been investigated using complementary approaches throughout the in vitro development of coconut zygotic embryos into plantlets. Patterns of chlorophyll fluorescence were comparable in in vitro grown coconut plantlets (theta(P)(MAX)=0.72 and theta(P)=0.45) and in autotrophic adult palms (theta(P)(MAX)=0.76 and theta(P)= 0.50). Chlorophyll content was lower in in vitro-cultured plantlets (0.92 mg/g fresh weight (FW)) than in autotrophic plants (2.43 mg/g FW). The photosynthetic rate (1.14 mu mol CO2/m(2) per s) of autotrophic palms was half that of in vitro grown plantlets, while transpiration rates were similar in both. Changes in the PEPC:RubisCO ratio during the development of in vitro grown plantlets (from 89.17 to 0.04 mu mol CO2/h per mg total soluble protein (TSP)) reflected a transition from a heterotrophic towards a RubisCO-mediated mode of CO2 fixation. The RubisCO enzyme capacity (2.83 mu mol CO2/h per mg TSP) and content (172.8 mg/g TSP) measured in in vitro-cultured plantlets were lower than those measured in autotrophic palms (6.60 mu mol CO2/h per mg TSP and 217.6 mg/g TSP respectively). Transmission electronic microscopy (TEM) observations showed a complete ultrastructural organisation of chloroplasts in plantlets at the end of the in vitro culture process (6 weeks under light). All the studied parameters have shown that plantlets at the end of the in vitro culture process exhibit photosynthetic characteristics (theta(P)(MAX), theta(P), PEPC:RubisCO ratio and transpiration rates) similar to those of acclimatized plants. These results suggest an early establishment of a photosynthetic metabolism during the in vitro development of coconut plantlets. Nevertheless, RubisCO content and capacity together with chlorophyll content were found to remain lower in in vitro grown plantlets, which might explain the lower photosynthetic rates recorded, as compared to the autotrophic coconut palm. (C) 1997 Elsevier Science Ireland Ltd.