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He, Jie
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The fundamental aim of this project is to improve the efficiency of growing temperate vegetables in this hot tropical climate. Using 113 recombinant inbred lines (RTL) of lettuce, this research attempts to understand the physiology of temperate plants grown in the tropics. As shoot fresh weight (FW) is a main determinant of the marketability of crop plants, they were used to calculate thermotolerant indices and the RTLs were identified. With only a moderately large root system, relative to the RTL population, heat resistant (HR)-RTL#220 was able to develop a very large shoot. Heat-sensitive (HS)-RTL#162, however, was a smaller plant on the whole. The more extensive root system of 14-day old HR-RTL#220, than HS-RTL#162, conferred seedling stand establishment advantage. When treated with high light and temperature, there was no significant difference in its Chlorophyll (Chl) a/b ratio and Carotenoid (Car) content, whereas post-treatment Chl a/b ratio significantly increased for HS-RTL#162. Tn the reciprocal rootzone temperature (RZT) transfer treatments, no clear relationship between N metabolism and productivity could be identified. Since ethylene is a stress indicator, accumulated RZ ethylene concentrations were measured. Highest concentrations were measured in HS-RTL#162 growing in obviated cooling (ambient-RZT; A-RZT) conditions with reduced frequency of RZ misting, while the lowest concentrations were measured in HR-RTL#220 under cool-RZT (C-RZT) with higher spray frequency - corresponding to the lowest and highest shoot FWs, respectively. HR-RTL#220 recovered by Day 4 of reciprocal RZT transfer as reflected by its Chl fluorescence Fv/Fm ratios, unlike HS-RTL#162 after 6 days of transfer. When ACC was added to the RZ, shoot FW were significantly higher in control than ACC-treated plants after 5 days of exposure. However, there was no significant difference in the shoot FW of HR-RTL#220, after another fortnight of exposure, though it was significantly lower in ACC-treated HS-RTL#162 plants. With the increased occurrence of unpredictably hazy weather, supplementary LED lighting treatments were provided to alleviate reduced productivity. Supplementing with photosynthetic photon flux density (PPFD) 100 ❍mol m⁻² s⁻¹ and four different red-blue (RB) ratios of LED lighting, higher shoot growth was obtained in HS-RTL#162 plants than under natural sunlight. Despite highest leaf expansion rates and stomatal density (SD) at 8B for both genotypes, light- and CO2-saturated photosynthetic oxygen evolution rate (Pmax), photochemical quenching (qP) and electron transport rate (ETR) were lowest - implying that red-blue LED lighting increased growth via leaf numbers and SD rather than photosynthetic efficiency. With supplementary PPFD of 188 ❍mol m⁻² s⁻¹ supplemented during sunset and sunrise, higher productivity was obtained - demonstrating the effectiveness in circumventing decreased productivity under lower light availability. These results would benefit the numerous urban farming projects that are in progress of starting out in Singapore. With more LED research, both indoor and outdoor vertical farms can be developed to produce more vegetables, which could significantly contribute to Singapore's food security.
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SB351.L6 Cho
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Appears in Collections:Doctor of Philosophy (Ph.D.)

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