Photosynthetic and transpiration responses of in vitro-regenerated Solanum nigrum L. plants to ex vitro adaptation

In vitro regeneration of black nightshade (Solanum nigrum L.) plants was achieved through callus-mediated shoot organogenesis followed by 30 d indoor ex vitro adaptation to nutritional stress under environmental ambience and thereafter 6-d outdoor acclimatization in pots prior to field establishment. Relevant physiological parameters including pigment content, chlorophyll a fluorescence, net photosynthetic rate (P N), transpiration rate (E), and stomatal conductance (g s) of in vitro-regenerated plants were investigated during the course of ex vitro adaptation. During the first 4 d of indoor transplantation to potting substrate, there was a marginal reduction in the leaf chlorophyll and carotenoid contents but P N and E were strongly reduced. The stomatal conductance and E/P N ratio were significantly higher in plants up to 20 d of indoor adaptation than those of comparable age grown naturally from seeds. The shape of the OJIP fluorescence transient varied significantly with acclimatization, and the maximum change was observed at 2.0 ms. The 2.0 ms variable fluorescence (V j), 30 ms relative fluorescence (M ₀), photon trapping probability (TR₀/Abs), and photosystem II (PSII) trapping rate (TR₀/RC) showed initial disturbance and subsequent stabilization during 30 d of indoor acclimatization. Energy dissipation (DI₀/RC) and electron transport probability (ET₀/TR₀) showed an initial phase of increase during the 4 d after plants were transplanted outdoors. During the 6-d outdoor acclimatization after transfer of plants to soil, no significant change in total chlorophylls and carotenoids, E, and g s were observed, but P N improved after reduction on the first d. The OJIP-derived parameters experienced change on the first d but were stabilized quickly thereafter. There was no significant difference between outdoor acclimatized plants and those of the seed-grown plants of comparable age with respect to photosynthetic and fluorescence parameters. Direct transfer of plants without indoor acclimatization, however, showed a completely different trend with respect to P N, E, and OJIP fluorescence transients. The bearing of this study on optimizing micropropagation is discussed.