He, Jie

Brassica alboglabra Bailey plants were first grown under full nutrient solution for 2, 3 and 4 weeks and then were transferred to phosphorus (P)-deprivation solution for 3 weeks (–P3), 2 weeks (–P2) and 1 week (–P1), respectively. The total growth duration was 5 weeks for all plants and the full P plants were grown under complete nutrient solution for 5 weeks. Full P and –P1 plants had similar productivity which was significantly higher than –P2 and –P3 plants. P-deprivation treatments caused a reduction of the P concentration in all treated plants compared to full P plants. The total P content per plant was significantly higher in full P than any other P-deprivation plants. P-deprivation did not cause significant changes in chlorophyll (Chl) fluorescence Fv/Fm ratio and Chl content. There were no differences in light saturated photosynthetic CO2 assimilation (Asat) and stomatal conductance (gssat) in young leaves (YL) among all plants. However, P-deprivation resulted in reduction of gssat for old leaves (OL).Full P plants had the lowest soluble sugarsaccumulated inboth YL and OL compared to P-deprivation plants. There were no significant differences in the concentration of insoluble sugar of YL among all plants. The concentration of insoluble sugar in OL of full P and –P3 plants were significantly lower than in –P1 and –P2 plants. This study concludes that B. alboglabraBaileyplants are able to accumulate adequate P in its early growth stages and reserves of P are sufficient one week before harvest for quality crop yield. Relationships among productivity, photosynthesis and carbohydrate levels under P deprivation at different growth stage were discussed.

The facultative halophyte Mesembryanthemum crystallinum (ice plant) has high nutritional values. In Singapore, we have successfully grown M. crystallinum indoors under different LED-lightings using freshwater. Since halophytes are capable of tolerating a wide range of salinities, it provides a basis for cultivating them as vegetables using saline water. This study aimed to investigate the impacts of different NaCl concentrations on productivity and photosynthesis of M. crystallinum. All plants were grown under different salinities by adding 0, 100, 250 and 500 mM NaCl, respectively, to a full strength Netherlands Standard Composition. Photosynthetic photo flux density at 156 μmol m^‑2 s^‑1 (12 h photoperiod) was provided to all plants by LED with red:blue ratio of 2.2. Twenty days after transplanting, plants grown under 100 mM NaCl had the highest shoot fresh weight and largest total leaf area followed by plants grown at 0, 250 and 500 mM NaCl. Plants grown under 500 mM NaCl had the lowest specific leaf area due to its highest leaf dry matter content and lowest water content. However, all plants had similar values of leaf succulence (g FW cm^‑2). Although all plants had chlorophyll fluorescence Fv/Fm ratios >0.8, photochemical quenching and electron transport rate were significantly higher in plants grown under 0 mM NaCl than under 100, 250 and 500 mM NaCl measured under the growth irradiance. CAM acidity of plants grown at 500 mM NaCl was induced while CAM acidity of plants grown with freshwater was undetectable. In conclusion, M. crystallinum requires some salt (100 mM NaCl) to perform healthy growth compared to those grown with freshwater. Low productivity of M. crystallinum under high salinity could mainly be due to its slow leaf growth resulting from the switch of C₃ to CAM photosynthesis.

In this study, butterhead lettuce (Lactuca sativa L. cv. Baby Butter) plants were grown at three root-zone temperatures (RZTs): 25°C, 30°C and ambient-RZT (A-RZT) ranging from 26°C-42°C while their shoots were maintained at hot ambient temperature ranging from 26°C-42°C. Three phosphorus (P) concentrations: -25% P (minus P, 23.25ppm), control (31.00ppm) and +25% P (plus P, 38.75ppm) were supplied to the plants at each RZT using Netherlands Standard Nutrient Solution. Interactions between RZT and P concentrations on productivity, root morphology, maximum photosynthetic O2 evolution (Pmax), P uptake and its partitioning between shoot and root were studied. Lettuce plants grown with the plus P concentration at 25°C-RZT had the highest productivity, highest Pmax, highest shoot and root P concentrations. However, all the root morphological parameters studied were the greatest in plants grown with the minus P at 25°C-RZT. Higher P concentration in the solution and 25°C-RZT resulted in higher portion of the absorbed P partitioned to the shoots. Integrated effects of RZT and P concentration on the productivity, root morphology, photosynthesis and P uptake were discussed.

We have identified certain heat-resistant (HR) and heat-sensitive (HS) lettuce (Lactuca sativa) recombinant inbred lines (RILs) from 113 lines under hot ambient temperature by studying the root morphology, shoot and root productivity. Except for temperature, one of the other major determinants of root morphology is nitrate (NO3–) availability. In this study, total productivity, root morphology, photosynthesis and nitrogen (N) metabolism of two RILs, 168 HS and 200 HR were studied under full N (100% NO3–), +N (125% NO3–) and –N (50% NO3–). The shoot and root productivity of both RILs under +N and –N treatments declined compared to those of full N plants. Reductions in root length, root surface area and total number of root tips were observed in 168 HS plants under both +N and –N treatments. For 200 HR plants, they all had similar values of root parameters regardless of N treatments. There were no significant differences in the light saturated CO2 assimilation (Asat) and stomatal conductance (gs sat) between two RIL plants For each lettuce RIL, no differences in total chlorophyll (Chl ) content and Chl a/b ratio were observed among the different N treatment. For both lettuce RILs, shoot NO3– concentration was highest in +N followed by full N plants and –N plants had the lowest values. There were no differences in root NO3– concentration between +N and full N plants but root NO3– concentration was significantly lower in –N plants than in +N and full N plants. For shoot total reduced N, +N plants had significantly higher concentration in both RILs compared to those of full N and –N plants. All plants had similar root total reduced N concentrations except for 168 HS under –N condition, which had significantly lower total reduced N concentrations. Differences in shoot maximal nitrate reductase (NR) activity among the different N treated plants were similar to those of total reduced N concentration. The relationships among NO3– availability, root morphology, productivity, photosynthesis and N metabolism were discussed.

Under low growth-irradiance and/or excessive nitrogen (N) fertilization, the roots of leafy vegetables could take up nitrate (NO3-) faster than the plant can convert it to organic nitrogen compounds. NO3- is suspected to have carcinogenic effect in human when eaten in high quantity. In this study, lettuce plants were first grown in an aeroponics system with full nutrients (full NO3–) under full sunlight. Six weeks after transplanting, plants were subjected to 7 days of full sunlight and shade with full, 1/2 and 0 NO3– respectively. Shoot NO3– concentration was higher under shade thanunder full sunlight regardless of NO3– availability after 7 days of treatments. The higher shoot NO3– concentration of shade plants was derived from the high NO3– accumulated in their roots during the 6 weeks of growth prior to treatments. There were no significant differences in NO3– concentrations of shoot and root after re-exposing all plants to full sunlight and full NO3– for another 7 days. Total shoot reduced N concentrations were similar among all plants regardless of treatments. These results indicate that reduction or withdrawing NO3– from nutrient solution did not affect N metabolism. Low productivity and photosynthesis under shade condition did not result from NO3– availability but they were directly caused by low growth irradiance. Thus, to prevent high accumulation of NO3– in the shoot, it may be a good practice to withdraw NO3– from nutrient solution during cloudy days or to extend the plant growth period a few more days under full sunlight before harvest.

There is very little study on the effects of LED quality on Mesembryanthemum crystallinum grown under different salinities. In this study, M. crystallinum was grown under red/blue (R/B) LED ratios of 0.9, 1.6. 2.0 and 2.8 with 100 and 500 mM NaCl and exposed to the same level of photosynthetic photon flux density, PPFD (290 µmol m‑2 s‑1, 12 h photoperiod). Fourteen days after transplanting, plants with 100 mM NaCl had significantly higher values of growth parameters including shoot and root fresh weight, total leaf area and specific leaf area than with 500 mM NaCl. Grown with 100 mM NaCl under R/B 0.9, M. crystallinum had significantly lower shoot FW and total leaf area than under other R/B ratios. However, LED quality did not seem to affect the growth parameters of M. crystallinum grown with 500 mM NaCl. All plants had similar higher leaf succulence and water content but lower leaf dry matter content with 100 mM NaCl than with 500 mM NaCl. Crassulacean acid metabolism (CAM) acidity of plants grown with 500 mM NaCl was about 4-fold higher than with 100 mM NaCl, indicating induction and stimulation of CAM photosynthesis. Chlorophyll fluorescence Fv/Fm ratios were greater than 0.8 for all plants, suggesting that maximum efficiency of PSII was unaffected by LED quality and salinity. M. crystallinum grown with 500 mM NaCl had lower electron transport rate, ETR but higher non-photochemical quenching, NPQ than with 100 mM NaCl, implying that photosynthetic light use efficiency was affected when switching C3 to CAM photosynthesis under high salt stress. M. crystallinum grown with100 mM NaCl had higher photochemical quenching, qP and ETR under R/B 0.9 than under R/B 2.8 and all plants with 500 mM NaCl. In conclusion, LED quality had different impacts on photosynthetic light-use efficiency of M. crystallinum grown under different salinities and thus resulting in different growth and productivity.

The edible halophyte Mesembryanthemum crystallinum L. was grown at different NaCl salinities under different combined red and blue light-emitting diode (LED) light treatments. High salinity (500 mM NaCl) decreased biomass, leaf growth, and leaf water content. Interactions between LED ratio and salinity were detected for shoot biomass and leaf growth. All plants had Fv/Fm ratios close to 0.8 in dark-adapted leaves, suggesting that they were all healthy with similar maximal efficiency of PSII photochemistry. However, measured under the actinic light near or above the growth light, the electron transport rate (ETR) and photochemical quenching (qP) of M. crystallinum grown at 100 and 250 mM NaCl were higher than at 500 mM NaCl. Grown under red/blue LED ratios of 0.9, M. crystallinum had higher ETR and qP across all salinities indicating higher light energy utilisation. Crassulacean acid metabolism (CAM) was induced in M. crystallinum grown at 500 mM NaCl. CAM-induced leaves had much higher non-photochemical quenching (NPQ), suggesting that NPQ can be used to estimate CAM induction. M. crystallinum grown at 250 and 500 mM NaCl had higher total chlorophyll and carotenoids contents than at 100 mM NaCl. Proline, total soluble sugar, ascorbic acid, and total phenolic compounds were higher in plants at 250 and 500 mM NaCl compared with those at 100 mM NaCl. An interaction between LED ratio and salinity was detected for proline content. Findings of this study suggest that both salinity and light quality affect productivity, photosynthetic light use efficiency, and proline accumulation of M. crystallinum.

Dendrobium officinale has high medicinal value but grows slowly in natural environment due to its special CAM photosynthetic pathway. In this study, D. officinale were grown aeroponically with light/dark cycles of 12 h/12 h, 4 h/4 h, and 2 h/2 h for 150 d. The photosynthetic electron transfer characteristics, photosynthetic CO₂ fixation pathways, and accumulations of biomass and soluble polysaccharides in D. officinale leaves were studied. The results showed that the photosynthetic apparatus states of D. officinale in aeroponic cultivation under short light/dark cycles of 4 h/4 h and 2 h/2 h were better than that under 12 h/12 h. The dark net CO₂ exchange percentages of D. officinale were negative in short light/dark cycles of 4 h/4 h and 2 h/2 h, and the daily net CO₂ exchange amount and dry/fresh weight increases were doubled compared with those in 12 h/12 h light/dark cycle. High soluble polysaccharides content and the soluble polysaccharides yield of D. officinale were obtained in the shorter light/dark cycle of 2 h/2 h. Therefore, the photosynthetic pathway of D. officinale could be switched from CAM to C3 by short light/dark cycles of 4 h/4 h and 2 h/2 h, and its higher biomass accumulation and soluble polysaccharides yield could be obtained by the shorter light/dark cycle of 2 h/2 h in aeroponic cultivation.

Food security is one of the key global challenges in this century. In Singapore, our research team has been using novel aeroponic technology to produce fresh vegetables since 1997. Aeroponic systems allow for year-round production of not only tropical, but also sub-tropical and temperate fresh vegetables, by simply cooling the roots suspended in aeroponic systems while the aerial parts grow under tropical ambient environments. It has also been used to investigate the impacts of root-zone CO2 on vegetables by enriching root-zone CO2 while their aerial portions were subjected to constant atmospheric CO2. To compensate for the lack of available land, Singapore also needs to develop a farming system that can increase productivity per unit land area by many-fold. Over the past 10 years, my research team has established a commercially viable LED integrated vertical aeroponic farming system to grow different leafy vegetables under different LED spectra, intensities, and durations in the tropical greenhouse. The results demonstrate that it is possible to increase shoot production and rate of shoot production of leafy vegetables by increasing light intensity and extending the photoperiod under effective LED lighting. Furthermore, temperate vegetable crops such as lettuce were able to acclimate to high light intensity under supplementary LED lights to natural sunlight in the greenhouse. Supplementary LED lightings promote both leaf initiation and expansion with increased photosynthetic pigments, higher Cyt b6f and Rubisco protein contents on a per area basis and thus improve photosynthetic capacity and enhance productivity. Plants sense and respond to changes in their immediate environments (microclimate), manipulating the root zone temperature (RZT) and water supply will impact not only their growth and development but also their nutritional quality. Our on-going research aims to investigate if the nutritional quality of leafy vegetables could be improved under suboptimal RZT and mild water deficit through deficit irrigation. If substantial energy and water savings in urban farming can be achieved without substantial yield penalty but with higher nutritional quality, the amount of water and energy saved can bring substantial benefits to society.