Chlorophyll fluorescence as an indicator of photosynthetic functioning of in vitro grapevine and chestnut plantlets under ex vitro acclimatization

This study reports the effects of light availability during the acclimatization phase on photosynthetic characteristics of micropropagated plantlets of grapevine (Vitis vinifera L.) and of a chestnut hybrid (Castanea sativa × C. crenata). The plantlets were acclimatized for 4 weeks (grapevine) or 6 weeks (chestnut), under two irradiance treatments, 150 and 300 mol m^–2 s^–1 after in vitro phases at 50 mol m^–2 s^–1. For both treatments and both species, leaves formed during acclimatization (so-called `new leaves') showed higher photosynthetic capacity than the leaves formed in vitro either under heterotrophic or during acclimatization (so-called `persistent leaves'), although lower than leaves of young potted plants (so-called `greenhouse leaves'). In grapevine, unlike chestnut, net photosynthesis and biomass production increased significantly with increased light availability. Several parameters associated with chlorophyll a fluorescence indicated photoinhibition symptoms in chestnut leaves growing at 300 mol m^–2 s^–1. The results taken as a whole suggest that 300 mol m^–2 s^–1 is the upper threshold for acclimatization of chestnut although grapevine showed a better response than chestnut to an increase in light.