Abstract
Two experiments were carried out to determine the optimum combination of CO2 concentration and photosynthetic photon flux density (PPFD) under low light irradiation controlled atmosphere (LLI-CA) storage conditions at atmospheric O2 concentration for the quality preservation of grafted tomato (Lycopersicon esculentum Mill., scion : cv. Momotaro ; rootstock : cv. Helper M) plug seedlings. In the first experiment, the seedlings were placed in a glass vessel at 10°C, and CO2 exchange rate was measured under different CO2 concentrations (0.05, 0.5 or 1.0%) and PPFDs (0, 1.0, 2.0 or 3.0pmol m-2 s-1) of red light from light-emitting diodes. A 3-dimensional (CO2 concentration, PPFD and CO2 exchange rate) graph was drawn to find a curve expressing the relationship between CO2 concentration and PPFD where the CO2 exchange rate becomes 0. All combinations of CO2 concentration and PPFD on the curve are theoretically the most essential conditions for keeping dry weight of the seedlings nearly constant during storage. In the second experiment, grafted tomato plug seedlings were stored at 10°C for 28 d under the following combinations of CO2 concentration and PPFD selected from the curve : 0.05% + 2.5μmol m-2 s-1 (C0.05P2.5), 0.25% + 1.9 μmol m-2 s-1 (C0.25P1.9), 0.50% + 1.3 μmol m-2 s-1 (CO.SOPL3), 0.75% + 0.9μmol m-2 s-1 (C0.75P0.9) and 1.0% + 0.5 μmol m-2 s-1 (C1.0P0.5) . Storage conditions of 0.05% CO2+0μmol m-2 s-1 PPFD were used as the control (font) . Five-rank subjective visual quality scores of the seedlings and color difference values of the leaves (ΔEab) on the last day of 28 d storage were in the following order: C0.25P1.9>C0.50Pl.3>C0.05P2.5=C0.75P0.9>C1.0P0.5>Cont and Cont = C1.0P0.5 = C0.75P0.9 = C0.05P2.5>C0.50P1.3>C0.25P1.9 (Cont> C0.05P2.5), respectively (where‘>’ and ‘=’ denote significantly greater and not significantly different at the 5% level by LSD, respectively) . Survival percentages in C0.25P 1.9 and C0.50P1.3 were 100% (10 plants out of 10) on the last day of 28 d cultivation following 28 d storage whereas survival percentages in C0.05P2.5, C0.75P0.9, C 1.0P0.5 and Cont were 90, 70, 20 and 0%, respectively. These results showed that the optimum conditions of LL-CA storage for quality preservation of grafted tomato plug seedlings at l0°C under atmospheric O2 concentration were 0.25% CO2 combined with 1.9ymol m-2 s-1 PPFD. These results also indicate that there exists a minimum level of PPFD for successful LL-CA storage, and the minimum was considered to be between 0.9 and 1.3, umol m-2 s-1 for the seedlings used.
