Please use this identifier to cite or link to this item: http://hdl.handle.net/10497/22435
Title: 
Authors: 
Supervisor: 
He, Jie
Issue Date: 
2016
Abstract: 
In Singapore, aeroponic technology has been used to produce all types of vegetable year round. For example, temperate lettuce can be now grown in tropical countries such as Singapore by simply cooling the roots of plants suspended in aeroponic troughs while the aerial parts grow under tropical ambient environments. However, for all aeroponically grown vegetables, NO3- is the only source of nitrogen (N). NO3- is considered to be of low toxicity but when converted to NO2, it interacts with haemoglobin and affects oxygen transport, leading to a condition known as methaemoglobin. NO3- accumulation in plants varies according to growth temperature and irradiance as the NO3- concentration in the plant tissues is closely related to photosynthesis and N metabolism. Under suboptimal growth conditions such as high temperature and low growth NO3- to protein and other organic N compounds. Furthermore, N metabolism is closely related to nutritional quality. This project investigated the effects of different root-zone temperatures (RZTs), NO3- availability and LED lighting during growth and just before harvest on productivity, NO3- accumulation and nutritional quality of lettuce (Lactuca sativa cv. Canasta) grown in a tropical greenhouse.

To achieve the above stated objective, in this project, there were two set of experiments. In the first set, lettuce (L.sativa cv. Canasta) plants were grown in an aeroponic system under different RZTs. After 3 weeks of transplanting, the shoot and root fresh weight (FW) and dry weight (DW), shoot /root FW ratio, shoot /root DW ratio, root morphology, photosynthetic pigments, Fv/Fm ratio, A sat, gs sat, NO3- concentration, total reduced nitrogen (TRN), total soluble protein (TSP), phenol, ascorbic acid, and dietary minerals such as calcium (Ca), magnesium(Mg), potassium (K) and iron (Fe) were analysed.

Three weeks after transplanting, both 20 oC– and 25 oC–RZT plants had higher shoot FW, higher shoot/root FW and DW ratios than those of 30 oC–RZT plants. Except for lower number of root tips observed in plants grown at 30 oC–RZT, there were no significant differences in total root length, root surface area and average root diameter. These results indicate that lower shoot productivity in plants grown under higher RZT (30 oC–RZT) was not due to root growth and development. There were no significant differences in total chlorophyll (Chl) and carotenoid (Car) contents, Chl a/b ratio and Chl/Car ratio. All plants had Chl fluorescence Fv/Fm ratio greater than 0.8, indicating that all plants were healthy.

Furthermore, all plants had reasonable high light-saturated photosynthetic CO2 assimilation (A sat) and stomatal conductance (gs sat), although these two parameters were slightly lower in 30 oC–RZT plants than in 20 oC– and 25-RZT plants. There were no differences in shoot concentrations of NO3- TRN and dietary minerals such as Ca, Mg, K and Fe among the plants grown under the three different RZTs. All plants had similar levels of TSP and phenol concentrations. There results suggest that RZTs used in the study did not affect the nutritional values of lettuce plants in terms of NO3- accumulation, the concentrations of TRN and dietary minerals. However, plants grown under 30 oC–RZT had significantly higher ascorbic acid than the plants grown under 20 oC– and 25-RZT.

To investigate if NO3- availability before harvest affects productivity, photosynthesis, NO3- accumulation and nutritional quality those plants grown at 25oC–RZT for 5 weeks were subjected to three different NO3- treatments as 25oC–RZT plants had the highest shoot productivity. The 3 NO3- treatments were 0 N, ½ N and full N. All plants continued to grow after being subjected to different treatments. Eight days after different N treatments, NO3- availability did not result in significant differences in shoot FW although root FW was significantly higher in plants grown under 0N compared to those grown under ½ N and full N. These results indicate that all plants accumulated sufficient NO3- to support their growth before the treatments. All plants had similar levels of photosynthetic pigments. Compared to plants grown under full N, plants grown under ½ N and 0N had significantly reduced NO3- accumulation in shoots, lower shoot TRN but there were no changes in TSP and phenol concentrations. It was interesting to see that 0N plants had higher ascorbic acid than that of ½ N and full N plants. These results imply that reduction of NO3̶ supply to lettuce plants before harvest is a good horticultural practice.

In the second set of experiments, lettuce plants were grown at 25oC–RZT under natural sunlight with different supplementary LED lightings to study the effects of light on productivity, photosynthesis, NO3- accumulation and nutritional quality. Lettuce plants were grown under natural sunlight (L1), and natural sunlight with supplementary LED lighting (L2 - L5) of various combinations of red, white and blue light. The LED lamps were set at PPFD of 200 ±20 μmol photon m-2s-1, while L1, the natural sunlight only had a maximal PPFD of about 120 μmol photon m-2s-1. Three weeks after transplanting, parameters such as shoot and root FW and DW, shoot /root FW ratio, shoot /root DW ratio, root morphology, pigment content, Fv/Fm ratio, A sat, gs sat, NO3- concentration, concentration of TRN, TSP, phenol, ascorbic acid and dietary minerals such as Ca, Mg, K and Fe were evaluated. Five weeks after transplanting, the L1 plants were subjected to growth under L2 without changing the RZT and nutrient solution for the next 12 days. Parameters such as shoot and root FW and DW, shoot /root FW ratio, shoot /root DW ratio, pigment content, NO3- concentration, TRN, TSP, phenol, ascorbic acid and dietary minerals such as Ca, Mg, K and Fe were examined. Higher shoot FW and DW and root FW and DW were obtained in all plants grown under natural sunlight with supplementary LED lighting than in plants grown under natural sunlight only. Plants grown under L2 conditions (65R:0B:35W) showed highest shoot FW and DW and root FW and DW. However, L1 (natural sunlight only) plants showed highest shoot/root FW ratio and L3 plants (65R:20B:15W) had highest shoot/root DW ratio. L4 (65R:25B:10W) and L5 plants (65R:30B:5W) had highest total root length and number of root tips respectively while highest root surface area and average diameter was in L2 plants. No significant differences were observed in Chl, Car, Chl a/b ratio, and Chl/ Car ratio. Fv/Fm ratio in all the treatments, indicating that all plants were healthy. Asat and gs sat were highest in L4 and L1 plants respectively. L4 plants showed the highest shoot NO3- while root NO3- was highest in L2 plants. TRN, TSP, Ca, Mg and K showed no significant differences. Phenol concentration was the highest in L3 plants while ascorbic acid concentration was the highest in L2 plants. The above results suggest that different combinations of LED lighting affected different physiological processes and resulted in different qualities of lettuce plants. Overall, all supplementary LED lighting enhances shoot and root productivity compared to natural sunlight.

In the last stage of this project, lettuce plants that were grown under natural sunlight at 25oC-RZT for five weeks were supplemented with L2 LED (65R:0B:35W) lighting for 12 days. After supplementing with LED, lettuce plants showed a steep increase in shoot FW and DW and root FW and DW from day 0 to day 12. Even though there was a small dip observed on day 4, the shoot/ root FW ratio and shoot/root DW ratio increased from day 4 to day 12. The concentrations of Chl and Car decreased on day 4 but an increase was observed on day 8 and then remained constant. The Chl a/b ratio increased from day 4 onwards while Chl/ Car ratio increased from day 0 to day 8 and then remained constant. The shoot NO3- decreased on day 4 but increased from day 4 to day 12. No such variation was obtained in root NO3- from day 0 to day 12. There were no significant differences in TRN and ascorbic acid in plants between n day 0 and day 12 of LED treatments while phenol content was higher on day 12 compared to that of day 0. TSP was higher on day 0. There were no significant differences in concentration of dietary minerals such Ca, Mg, K and Fe in plants between day 0 and days 12 of treatment. These results indicate that supplementary LED lighting enhanced productivity of lettuce plants tremendously in the later stage of its growth cycle. However, there was no clear conclusion on the effects of LED lighting on quality of lettuce vegetable.
URI: 
Issued Date: 
2016
Call Number: 
QK753.N54 She
File Permission: 
Restricted
File Availability: 
With file
Appears in Collections:Master of Science (Life Sciences)

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