In coconut, in vitro development and acclimatization of plantlets still constitute the major bottlenecks for the implementation of tissue culture protocols. In the present paper, the photosynthetic status of in vitro grown plantlets resulting from zygotic embryo culture is investigated, combining various complementary approaches, both in vitro and in planta. Patterns of chlorophyll fluorescence were similar in in vitro grown coconut plantlets (aspiré: phi MAX= 0.72 and aspiré: phi = 0.45) and in autotrophic adult palms (aspiré: phi MAX= 0.76 and aspiré: phi = 0.50), whereas the chlorophyll content was lower in in vitro-cultured plantlets(0.92 mg.g-1 FW) than in autotrophic plants (2.43 mg.g-1 FW). The photosynthetic rate (1.14 micro-mol CO2 m-2s-1) was two times lower in in vitro grown plantlets, while transpiration rates were similar in bath in vitro-cultured plantlets and in autotrophic palms. Changes in the PEPC: Rubisco carboxylase ratio during the development of in vitro grown zygotic embryos (from 89.17 to 0.04 micro-mol CO2 h1mg1rsp) reflected an early transition towards a RubisCO-mediated mode of CO2 fixation. The RubisCO content (172.8 mg.g-1) measured in in vitro-cultured plantlets was lower than in autotrophic palms (217.6 mg.g-1). During the acclimatisation phase, the in vitro grown coconut plantlets showed a faster decrease in their PEPC: RubisCO ratio than the seedlings, suggesting that an earlier transition from a heterotrophic to an autotrophic mode of carbon fixation takes place in the in vitro-derived material. Just before acclimatization, the RubisCO activity in in vitro derived plantlets was lower than that in seedlings of the same age. After acclimatisation, RubisCO activities were comparable in both in vitro and in planta germinated material. In vitro grown plantlets displayed several photosynthetic characteristics (aspire: phiMAX, aspiré: phi, PEPC: RubisCO ratio and transpiration rates) similar to those of accl