The Response of Baby Leaf Lettuce to Selenium Biofortification under Different Lighting Conditions

Selenium (Se) is an essential microelement for human health and has antioxidant and anticancerous properties. One of the ways to increase its concentration in plants is biofortification through various agronomic practices including artificial lighting. The aim of this study was to determine the responses of baby leaf lettuce to various Se content in hydroponic solution at different ratios of blue (B) and red (R) light of light-emitting diodes (LED) lighting. Lettuce (Lactuca sativa, ‘Little Gem’) was grown hydroponically under B:R light ratios–10%B:90%R, 75%B:25%R. The photon flux density (PFD), photoperiod, temperature, and relative humidity in the growth chamber were maintained at 220 μmol m−2 s−1, 18 h, 21/17 ± 2 °C, and 60 ± 5%, respectively. Two experiments with Se were performed using natrium selenate (Na2SeO4). Results of the first experiment (Se–0, 1, 3 ppm) showed that the content of Se in lettuce was about 15 times higher at 3 ppm compared to 1 ppm. Similar trends were observed for both B and R ratios. However, even the lowest dose of Se in hydroponic solution inhibited lettuce growth and reduced photosynthesis and chlorophyll content. Therefore, a second experiment was performed with lower Se doses (0, 0.5, 1 ppm) at different growth stages (11th and 17th days after sowing (DAS)). It was found that, when Se exposure was initiated at the 17th DAS, the lettuce accumulated a lower content of Se compared to the 11th DAS, but this did not have a negative effect on their growth. The B:R ratio of 10B:90R% resulted in a higher content of Se in the leaves. Overall, these results suggest that properly composed doses of Se, LED lighting and application time could be a suitable way for cultivation of selenium-biofortified baby leaf lettuces without any adverse effects on growth.

Soybean Nutrition in a Novel Single-Nutrient Source Hydroponic Solution

Hydroponic systems are efficient for studying plant nutrition. It is often desirable to adjust individual nutrients for unique species’ needs and/or to create multiple nutrient deficiencies within the same study. However, this is challenging to do with traditional solutions as nutrients are generally added as dual nutrient salts, such as when varying phosphorus (P) concentration also affects nitrogen concentration; potentially, the chemical form of the nutrient taken up when ammonium phosphate is the source for P. This can create unintended consequences with nutrients other than those intended for adjustment. A new hydroponic system has been created to allow for nutrient deficiencies using single-nutrient sources, including ammonium nitrate; phosphoric, sulfuric, hydrochloric, and boric acids; potassium, calcium, magnesium, zinc, and copper carbonates; manganese acetate; sodium molybdate; iron EDDHA; with HEDTA as an additional chelate. This nutrient solution was compared to a traditional “Hoagland” hydroponic solution to grow soybean (Glycine max (L.) Merr). Additional treatments included alteration of pH in the new solution as well as evaluating varying levels of calcium, magnesium, and manganese. This new solution proved effective, as soybean was grown to maturity and performed as well as the traditional Hoagland solution. Adjusting pH downward with hydrochloric acid resulted in healthy plants, but solution pH was not adequately buffered. Adjusting pH with acetic acid resulted in toxicity. Further work is required to provide better pH buffering and approximately align tissue nutrient concentrations with field-grown soybean.

Nutrient Uptake in Huanglongbing-affected Sweet Orange: Transcriptomic and Physiological Analysis

Since the advent of Huanglongbing [HLB (Candidatus Liberibacter asiaticus)] in Florida, several preliminary reports have emerged about the positive effects of mineral nutrition on the performance of HLB-affected citrus (Citrus sp.) trees. HLB-affected trees are known to undergo significant feeder root loss. Therefore, studies have focused on foliar nutrient application instead of soil-applied nutrients speculating that the HLB-affected trees root systems may not be competent in nutrient uptake. Some studies also suggest that HLB-affected trees benefit from micronutrients at higher than the recommended rates; however, the results are often inconclusive and inconsistent. To address this, the goal of the present study was to evaluate the nutrient uptake efficiency and the quantitative and qualitative differences in nutrient uptake of HLB-affected trees. HLB-affected and healthy sweet orange (Citrus sinensis) trees were grown in a 100% hydroponic system with Hoagland solution for 8 weeks. The trees were deprived of any fertilization for 6 months before the transfer of trees to the hydroponic solution. Altogether, the four treatments studied in the hydroponic system were healthy trees fertilized (HLY-F) and not fertilized (HLY-NF), and HLB-affected trees fertilized (HLB-F) and not fertilized (HLB-NF). HLY-F and HLY-NF trees were found to have similar levels of leaf nutrients except for N, which was found to be low in nonfertilized trees (HLY and HLB). Both HLB-F and HLB-NF trees had lower levels of Ca, Mg, and S compared with HLY trees. In addition, HLB-NF trees had significantly lower levels of micronutrients Mn, Zn, and Fe, compared with HLY-NF trees. The hydroponic solution analysis showed that HLB-F and HLY-F trees had similar uptake of all the nutrients. Considering that HLB-affected trees have a lower root-to-shoot ratio than healthy trees, nutrient uptake efficiency per kilogram of root tissue was significantly higher in HLB trees compared with HLY trees. Under nutrient-deficient conditions (day 0) only nine genes were differentially expressed in HLB roots compared with HLY roots. On the other hand, when fertilizer was supplied for ≈1 week, ≈2300 genes were differentially expressed in HLB-F roots compared with HLY-F roots. A large number of differentially expressed genes in HLB-F were related to ion transport, root growth and development, anatomic changes, cell death, and apoptosis compared with HLY-F trees. Overall, anatomic and transcriptomic analyses revealed that HLB-affected roots undergo remarkable changes on transitioning from no nutrients to a nutrient solution, possibly facilitating a high uptake of nutrients. Our results suggest the roots of HLB-affected trees are highly efficient in nutrient uptake; however, a small root mass is a major limitation in nutrient uptake. Certain micronutrients and secondary macronutrients are also metabolized (possibly involved in tree defense or oxidative stress response) at a higher rate in HLB-affected trees than healthy trees. Therefore, a constant supply of fertilizer at a slightly higher rate than what is recommended for micronutrients and secondary macronutrients would be beneficial for managing HLB-affected trees.

Rapid method of screening for drought stress tolerance in maize (Zea mays L.)

Drought stress is the major production constraint in rainfed maize. Screening for drought tolerance is severely affected by the lack of a simple and reliable phenotyping technique. The objective of this study was to standardize a simple hydroponic based drought screening technique in maize. In this context, one week old uniform seedlings of 55 inbreds and 5 hybrids were transferred to hydroponic solution in the glass house. The seedlings were allowed to acclimatize for next one week in hydroponic solution. The drought stress was imposed by removing seedlings from nutrient solution and exposed to air for 6 and 4 hours daily for a period of 5 and 4 consecutive days in hybrids and inbreds, respectively. Data were recorded on all shoot and root parameters, and based on stress symptoms, a drought tolerance score was given to each genotype. The percent deductions in shoot and root fresh weight from non-stress to stress ranged from 11.7 to 84.4 and 2.1 to 77.5, respectively. Six inbred lines, namely, DQL790-4, CML334, CM140, CML422, CM125 and HKI488 and three hybrids namely DMRH1306, DMRH1410 and PMH4 were found drought tolerant. The effectiveness of this screening technique was compared and confirmed using pots screening as well as by expression profiling of key antioxidant genes (Sod2, Sod4, Sod9 and Apx1) playing role in drought stress tolerance. This phenotyping technique is very short, low cost and simple which can be utilized in preliminary drought screening for large set of maize germplasm and mapping populations.

Levels of induced free proline in chelated and unchelated hydroponically grown lettuce (Lactuca sativa L.)

It is well described that under stress conditions many plant species accumulates proline as adaptive response to adverse conditions. In this study, eight weeks old Lactuca sativa L. seedlings were transplanted in nutrient solution containing various doses (0.000, 0.0025, 0.005, 0.0075, and 0.025mg/L) of Pb2+ and Cd2+ and constant concentration of EDDS (0.0025M). The effects of these treatments of Pb2+, Cd2+, combination of Pb2+ + Cd2+and with or without EDDS (Ethylene diamine disuccinate) were compared based on the level of stress induced to the seedlings. Free proline contents induced due to stress by uptake of Pb2+and Cd2+ were investigated in both unchelated and chelated treatment. The results obtained showed that the effect of increasing concentration of Pb2+ in hydroponic solution leads to decrease in the level of stress induced to the seedlings by EDDS while the lower the Cd2+ concentration in the hydroponic solution the more the level of stress induced by EDDS to the plant. The highest level of stress caused to the seedlings by unchelated and chelated Pb2+ solution are 68.294±5.642; 72.681±14.740 µmolg-1 respectively while that of Cd2+ are 28.962±8.703; 48.263±2.455 µmolg-1 respectively. From this, it is evident that Pb2+induced more stress to the plants than Cd2+ in the presence and absence of EDDS (p<0.05). Keywords: Proline, Ethylenediaminedisuccinic Acid (EDDS), Lead, Cadmium, Lettuce.