He, Jie

This study investigated the impact of different photoperiods of red- and blue-LED lighting on the growth, as well as, photosynthetic characteristics of heat-resistant (HR) and heat-sensitive (HS) RILs of Lactuca sativa grown in a vertical farming system. Lettuce plants were grown under three different photoperiods (12, 16 and 18 h) of red- (85%) and blue- (15%) LED lighting, at a mean photosynthetic photon flux density (PPFD) of 300 μmol m-2 s-1. 27 days after transplanting, both HR- and HS-RIL plants had significantly higher shoot and root fresh (FW) and dry (DW) weights, leaf number and total leaf area (TLA) when subjected to extended photoperiods of 16 and 18 h than the normal 12 h photoperiod. Highest growth parameters were observed for the longest photoperiod of 18 h. However, photosynthetic properties were not greatly enhanced under the extended photoperiods of 16 and 18 h - only chlorophyll (Chl) a/b ratio was increased. Light saturated photosynthetic CO2 assimilation (Asat) and stomatal conductance (gs sat) among plants exposed to different durations of LED lighting showed no significant difference among the various photoperiods. Though longer photoperiods resulted in increased total carbon assimilation, this would have been due to the plants having a larger TLA. The decrease in soluble and insoluble sugar concentrations in lettuce leaves, under longer photoperiods, was most probably due to the dilution of carbohydrates as a result of increased leaf number and total leaf area. Thus, the results suggest a lack of feedback inhibition of photosynthetic end products in the leaves of lettuce plants, despite its exposure to longer photoperiods.

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.

The soil–water characteristic curve (SWCC) is one of the most crucial and fundamental soil properties in unsaturated soil mechanics. Many theories and equations have been developed to describe and best fit SWCC with unimodal or bimodal characteristics. In this study, a general best-fitting equation for SWCC with multimodal characteristics (bimodal and trimodal SWCC) is proposed. The parameters in the proposed equation are closely related to the properties of soil and variables in the SWCC. To evaluate the performance of the proposed equation in best-fitting multimodal SWCC that has more than two modalities, a trimodal SWCC soil mixture that consists of sand, kaolin and activated carbon has been prepared and tested in the laboratory in this study. The proposed equation has been evaluated with data from published literature and the newly developed soil mixture of trimodal SWCC. The proposed equation has shown high accuracy for best-fitting bimodal SWCC and trimodal SWCC.

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.

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.

Plant propagation via in vitro culture is a very laborious and time-consuming process. The growth cycle of some of the crop species is slow even in the field and the consistent commercial production is hard to maintain. Enhanced methods of reduced cost, materials and labor significantly impact the research and commercial production of field crops. In our studies, stem-segment explants of Brassica species were found to generate adventitious roots (AR) in aeroponic systems in less than a week. As such, the efficiency of rooting from stem explants of six cultivar varieties of Brassica spp was tested without using any plant hormones. New roots and shoots were developed from Brassica alboglabra (Kai Lan), B. oleracea var. acephala (purple kale), B. rapa L. ssp. chinensis L (Pai Tsai, Nai Bai C, and Nai Bai T) explants after 3 to 5 days of growing under 20 ± 2 C cool root zone temperature (C-RZT) and 4 to 7 days in 30 ± 2 C ambient root zone temperature (A-RZT). At the base of cut end, anticlinal and periclinal divisions of the cambial cells resulted in secondary xylem toward pith and secondary phloem toward cortex. The continuing mitotic activity of phloem parenchyma cells led to a ring of conspicuous white callus. Root initials formed from the callus which in turn developed into ARs. However, B. rapa var. nipposinica (Mizuna) explants were only able to root in C-RZT. All rooted explants were able to develop into whole plants, with higher biomass obtained from plants that grown in C-RZT. Moreover, explants from both RZTs produced higher biomass than plants grown from seeds (control plants). Rooting efficiency was affected by RZTs and explant cuttings of donor plants. Photosynthetic CO2 assimilation rate (Asat) and stomatal conductance (gssat) were significantly differentiated between plants derived from seeds and explants at both RZTs. All plants in A-RZT had highest transpiration rates.

Temperate crops cannot grow well in the tropics without rootzone cooling. As cooling increased production costs, this experiment aimed to study the growth of various Lactuca genotypes and propose possible ways of reducing these costs, without compromising productivity. A recombinant inbred line (RIL) of lettuce and its parental lines (L. serriola and L. sativa “Salinas”) were grown aeroponically in a tropical greenhouse under 24°C cool (C) or warm fluctuating 30–36°C ambient (A) rootzone temperature (RZT). Their roots were misted with Netherlands standard nutrient solution for 1 min, at intervals of either 5 min (A5, C5) or 10 min (A10, C10) in attempting to reduce electricity consumption and production costs. Lower mortality and higher productivity were observed in all genotypes when grown in C-RZT. Higher shoot fresh weight was observed under C5 than C10, for the RIL and L. serriola. Since “Salinas” had similar shoot fresh weight at both C-RZ treatments, this may indicate it is more sensitive to RZT than water availability. Under A-RZ treatments, higher carotenoid content, with correspondingly higher nonphotochemical quenching, was observed in A10 for the RIL and “Salinas.” Further, total chlorophyll content was also highest at this RZ treatment for the RIL though photochemical quenching was contrastingly the lowest. Cumulatively, productivity was compromised at A10 as the RIL seemed to prioritize photoprotection over efficiency in photosynthesis, under conditions of higher RZT and lower water availability. Generally, higher RZ ethylene concentrations accumulated in A10 and C10 than A5 and C5, respectively—probably due to spray frequency exerting a greater effect on RZ ethylene accumulation than RZT. In the C5 RZ treatment, lowest RZ ethylene concentration corresponded with highest shoot fresh weight. As such, further research on ethylene (in)sensitivity and water use efficiency could be conducted to identify Lactuca cultivars that are better suited for growth in the tropics, so as to allay production costs with reduced cooling and spray intervals.

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 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.