He, Jie

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.

Sweet basil (Ocimum basilicum) is valued for its culinary and medicinal properties. It thrives in full sunlight and long daylight hours under natural conditions. This study examined the effects of extended photoperiod on sweet basil grown in a hot and humid tropical greenhouse. Some plants received only natural sunlight (SL), while others had SL supplemented with LED light for 6 h (6 h) before sunrise and/or after sunset. Plants grown under only natural SL (L1) had a smaller leaf number, smaller leaf area per plant, lower shoot, and root productivity than those grown under other light conditions. The shoot fresh weight of basil grown under supplemented LED light for 3 h before sunrise and 3 h after sunset (L2), 6 h after sunset (L3), and 6 h before sunrise (L4) was 2.68, 2.33, and 1.94 times higher than L1 conditions, respectively. The maximum quantum efficiency of PSII, electron transport rate, effective quantum yield of PSII, and Chl a/b ratio were also higher in L2, L3, and L4. The total leaf soluble protein, ascorbic acid, total phenolic compounds, and dietary minerals followed the same trend. Among all treatments, L2 consistently showed significantly higher values, making it the optimal lighting strategy for extended photoperiod.

Leafy vegetables have a huge demand in subtropical global markets for its high nutritional value and low cost. Accurate estimation of traits like leaf nitrogen concentration and shoot biomass is critical for optimal fertilizer dosing, nutrient content assessment as well as phenotyping studies. Hand-held smartphone with high-density Light Detection and Ranging (LiDAR) systems capture huge amounts of three-dimensional (3D) plant structural and intensity information that can be used to estimate plant traits. Thus, we propose a semiautomatic proximal sensing approach to model plant nitrogen content and shoot biomass using the structural and intensity information acquired by a smartphone based LiDAR sensor. The performance of the models in estimating leaf nitrogen concentration and shoot dry-weight biomass was quantified on Chinese broccoli (Brassica oleracea) plants with prior knowledge of nitrogen dossing concentrations, and produced a minimal root mean squared error of 8.75 mg nitrogen per gram of dry weight, and 2.38 g, respectively. The potential of the proposed modelling approach to accurately estimate leaf nitrogen concentration and shoot dry-weight biomass, from smartphone LiDAR data is proven.

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.

Perennials improve soil strength and stabilize the slope. However, they are very prone to drought stress (DS). To identify plant health indicators, this study investigated the responses of five tropical perennials commonly grown in Singapore’s slope to DS and re-watering (RW) in the greenhouse. The durations for mild, intermediate, and severe DS defined as T1, T2, and T3, respectively, before RW were based on the extents of reduced Fv/Fm ratio (maximal quantum efficiency of PSII) and the levels of wilting. After RW, soil water content (SWC) increased until field capacity in all DS soil, although they were significantly lower than in well-watered (WW) soil. Overall, the Fv/Fm ratios and leaf water content (LWC) decreased significantly in all DS plants compared to those of WW plants, but all increased to the similar level as WW plants after RW. Nitrogen deficiency did not occur in any plants during DS. There were clear positive correlations of SWC with Fv/Fm ratios, LWC, effective quantum yield of PSII (∆F/Fm’), electron transport rate (ETR), and photochemical quenching (qP) for all species. To monitor plant health, it would recommend using both non-destructive measurements such as SWC and Fv/Fm ratios and destructive parameters like LWC, ∆F/Fm’, ETR, and qP.

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.

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.

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.

Tuscan kale was grown aeroponically with 5, 30 and 60 min nutrient spraying intervals (defined as 5 minNSIs, 30 minNSIs and 60 minNSIs). Four weeks after transplanting, some 5 minNSI plants were transferred to a 60 minNSI (5 minNSI → 60 minNSI) and 90 minNSI (5 minNSI → 90 minNSI) for one more week. Significantly lower light-saturated rates of photosynthesis and stomatal conductance were observed for plants grown with a 60 minNSI than with a 5 minNSI. However, all plants had similar internal CO₂ concentrations and transpiration rates. Reduced light use efficiency but increased energy dissipation was observed in plants grown in a 60 minNSI. A higher nitrate concentration was observed in 60 minNSI plants compared to 5 minNSI and 30 minNSI plants, while all plants had similar concentrations of total reduced nitrogen, leaf soluble protein and Rubisco protein. Plants grown with prolonged NSIs (deficit irrigation) had lower biomass accumulation due to the inhibition of leaf initiation and expansion compared to 5 minNSIs. However, there was no substantial yield penalty in 5 minNSI → 60 minNSI plants. Enhancements in nutritional quality through deficit irrigation at pre-harvest were measured by proline and total soluble sugar. In conclusion, it is better to grow Tuscan kale with a 5 minNSI for four weeks followed by one week with a 60 minNSI before harvest to reduce water usage, yield penalty and enhance nutritional quality.