Opposite effects of exogenous sucrose on growth, photosynthesis and carbon metabolism of in vitro plantlets of tomato (L. esculentum Mill.) grown under two levels of irradiances and CO2 concentration

The long-term effects of exogenous sucrose (3 percnt;) on growth, photosynthesis and carbon metabolism ofin vitro tomato plantlets were investigated under two sets of growth conditions that respectively favor source- or sink-limitations of photosynthesis: 1) low photosynthetic photon flux (PPF) (50 μmol m−2 · s−1) and low CO2 concentration (400 μmol mol−1) and 2) high PPF (500 μmol m−2 · s−1 and high CO2 concentration (4000 μmol mol−1). The supply of sucrose under source-limitation conditions increased the growth, the maximal photosynthetic rate, the chl content, the maximal quantum yield of Photosystem II estimated by the Fv/Fm chl fluorescence ratio as well as the soluble sugars (hexoses, sucrose) and starch contents in roots, young and mature leaves when compared to those of photo-autotrophic plantlets. Also, sucrose feeding under these conditions strongly increased the activity of sucrose synthase (SS) (EC 2.4.1.13) in roots and young leaves whereas the activities of sucrose phosphate synthase (SPS) (EC 2.4.1.14), acid invertase (INV) (EC 3.2.1.26) and ADP-glucose pyrophosphorylase (ADPGppase) (EC 2.7.7.27) were highly stimulated in roots and mature leaves. Contrary to these observations, the supply of sucrose to plantlets developed under high PPF and CO2 concentration decreased growth and led to a somewhat lower maximal photosynthetic rate relative to photo-autotrophic plantlets. These negative responses to exogenous sucrose were accompanied by stronger accumulations of hexose and starch, larger stimulation of INV in mature leaves developed under conditions of sink limitation than those from source limitation conditions. Moreover, under high PPF and high CO2 concentration, exogenous sucrose led to a marked repression of the SPS activity and caused much lower stimulations of ADPGppase in mature leaves than those observed at low PPF and low CO2 concentration. We therefore conclude that under our experimental conditions, the interactive effects of exogenous sucrose and environmental conditions on growth and photosynthesis could be rationalized by the source-sink equilibrium of thein vitro tomato plantlets.