He, Jie

Brassica alboglabra plants were first grown aeroponically with full nutrients under full sunlight with average midday photosynthetic photon flux density (PPFD) of 1200 μmol m−2 s−1. Thirty days after transplanting, plants were respectively, subjected to 10 days of average midday PPFD of 1200 (control, L1), 600 (L2) and 300 μmol m−2 s−1 (L3). Productivity, photosynthetic CO2 assimilation and stomatal conductance were significantly lower in low-light (L2 and L3) plants than in high-light (L1) plants. Low light plants had the highest nitrate (NO3−) accumulation in the petioles. Low light also had an inverse effect total reduced N content. After different light treatments, all plants were re-exposed to another 10 days of full sunlight. Low-light plants demonstrated their ability to recover their photosynthetic rate, enhance productivity and reduce the NO3− concentration. These results have led to the recommendation of not harvesting this popular vegetable during or immediately after cloudy weather conditions.

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.

Mesembryanthemum crystallinum is a succulent, facultative crassulacean acid metabolism (CAM) plant. Plant growth and photosynthetic characteristics were studied when M. crystallinum plants were grown indoor under light emitting diodes (LED)-lighting with adequate water supply. Plants were cultured aeroponically for a 16-h photoperiod at an equal photosynthetic photon flux density of 350 mmol m-2 s-1 under different red:blue LED ratios: (1) 100:0 (0B); (2) 90:10 (10B); (3) 80:20 (20B); (4) 70:30 (30B); (5) 50:50 (50B); and (6)100:0 (100B). M. crystallinum grown under 10B condition had the highest shoot and root biomass and shoot/root ratio while those grown under 0B condition exhibited the lowest values. Compared to plants grown under 0B condition, all other plants had similar but higher total chlorophyll (Chl) and carotenoids (Car) contents and higher Chl a/b ratios. However, there were no significant differences in Chl/Car ratio among all plants grown under different red- and blue-LEDs. Photosynthetic light use efficiency measured by photochemical quenching, non-photochemical quenching, and electron transport rate, demonstrated that plants grown under high blue-LED utilized more light energy and had more effective heat dissipation mechanism compared to plants grown under 0B or lower blue-LED. Statistically, there were no differences in photosynthetic O2 evolution rate, light-saturated CO2 assimilation rate (Asat), and lightsaturated stomatal conductance (gssat) among plants grown under different combined red- and blue-LEDs but they were significantly higher than those of 0B plants. No statistically differences in total reduced nitrogen content were found among all plants. For the total soluble protein, all plants grown under different combined red- and blue-LEDs had similar values but they were significantly higher than that of plants grown under 0B condition. However, plants grown under higher blue-LEDs had significant higher ribulose-1,5-bisphosphate carboxylase oxygenase (Rubisco) protein than those plants grown under lower blue- LED. High Asat and gssat but very low CAM acidity of all M. crystallinum plants during light period, imply that this facultative CAM plant performed C3 photosynthesis when supplied with adequate water. Results of this study suggest that compared to red- or blue-LED alone, appropriate combination of red- and blue-LED lighting enhanced plant growth and photosynthetic capacities of M. crystallinum.

Effects of root-zone (RZ) CO₂ on crisphead-type lettuce (Lactuca sativa L. cv. ‘Wintergreen’ were measured in an aeroponic system under photosynthetic photon flux of 650 μmol m^-2 s^-1, 12 h photoperiod at 36^oC/30^oC and 28^oC/22^oC (day/night), with three enriched RZ CO₂ levels (2000 ppm, 10,000 ppm and 50,000 ppm). Leaf growth was monitored after elevated RZ CO₂ had been supplied for one week. Leaf areas with elevated RZ CO₂ were greater than ambient controls at both temperatures, while shoot and root weights were also higher. Increasing temperature reduced biomass overall, but the relative response to RZ CO₂ was greater. Elevated RZ CO₂ stimulated photosynthetic CO₂ assimilation, with greater increase at higher temperatures. Elevated RZ CO₂ decreased stomatal conductance at both temperatures, reducing transpiration water loss.

Mesembryanthemum crystallinum L. is a nutritious edible facultative halophyte. This study aimed to investigate the physiology and quality of M. crystallinum L. grown under different salinities and salt-priming conditions. All plants were first grown in 10% artificial seawater (ASW) for 10 days. After that, some plants remained in 10% ASW while the others were transferred to 20%, 30%, 40%, or 50% ASW for another 10 days. Some plants also underwent a salt priming by transferring them gradually from 10% to 100% ASW over a span of 10 days (defined as salt primed). All plants were green and healthy. However, there were reductions in shoot and root productivity, leaf growth, and water content, but also an increase in leaf succulence after transferring plants to higher salinities. The salt-primed plants showed higher photosynthetic light use efficiency with higher chlorophyll concentration compared to other plants. The concentrations of proline, ascorbic acid (ASC), and total phenolic compounds (TPC) increased as percentages of ASW increased. The salt-primed plants switched from C3 to crassulacean acid metabolism photosynthesis and accumulated the greatest amounts of proline, ASC, and TPC. In conclusion, higher salinities and salt priming enhance the nutritional quality of M. crystallinum L. but compromises productivity.

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.

Portulaca oleracea L. (known as purslane) is one of the most nutritious leafy vegetables owing to its high content of antioxidants. In this study, all plants were grown indoors hydroponically with different NaCl salinities. Photosynthetic photo flux density (PPFD) at 200 μmol m−2 s−1 (12 h) was provided to all plants by LED with red:blue ratio of 2.2. Thirty days after transplanting, plants grown with100 mM NaCl had the highest productivity and the fastest leaf growth followed by those with 0, 200 and 300 mM NaCl. Grown with 300 mM NaCl, purslane had the lowest specific leaf area due to its highest leaf dry matter content and its lowest water content. All plants had similar values of leaf succulence except for those with 300 mM NaCl. Total chlorophyll and carotenoids contents were significantly higher in plants grown with 0 and 100 mM NaCl than with 200, and 300 mM NaCl. All plants had Fv/Fm ratios close to 0.8. However, electron transport rate and ΔF/Fm′ were significantly higher in plants grown with 0 and 100 mM NaCl than with 200 and 300 mM NaCl. CAM-induced purslane with 300 mM NaCl had higher non-photochemical quenching. Maximum net photosynthetic O2 evolution rate and Cyt b6f concentration were significantly lower with 300 mM NaCl compared to all other plants while all plants had similar PS II concentration. Proline concentration increased with increasing salinities. All plants had similar levels of total soluble sugars. Plants grown with 0 and 100 mM NaCl had significantly higher concentrations of NO3−, total reduced nitrogen, total leaf soluble protein, Rubisco protein, total ascorbic acid, and total phenolic compounds than with 200 and 300 mM NaCl. The highest concentrations of K, Ca, and Mg were found in purslane grown under 0 mM NaCl. Statistically, no significant differences in Fe concentrations were observed among all plants. However, salinity seems to increase Fe concentration. In conclusion, it is feasible to grow purslane under 100 mM NaCl as it is the most optimal condition to achieve higher productivity and better quality. However, the production of antioxidants may depend on not only salinity but also other growth conditions.

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.