Degradation of autotoxic chemicals and maintainence of electrical conductivity recover growth, yield and nutrient absorption of successive lettuce in recycled hydroponics

In recycled hydroponics, successive crop cultivation by maintaining electrical conductivity (EC) suffers lower growth performance due to accumulating autotoxic root exudates. In this study, the efficiency of alternate current electro degradation (AC-ED) was evaluated for degrading allelochemicals and recovering retarded lettuce yield cultivated in EC-adjusted repeatedly used nutrient solutions. From benzoic acid (BA)-added nutrient solution, BA was completely degraded after 24 h by applying AC-ED at 551 and 940 Hz frequency with 50 and 80% electrical duty. In lettuce bioassay, fresh mass was negatively affected without the AC-ED-treated solution. Finally, lettuce seedlings were hydroponically grown in a plant factory using a half-strength Enshi nutrient solution. Culture solutions were unchanged in non-renewed solutions. Nutrient elements were supplied based on the EC (1.42 dS m−1) of culture solutions. The fresh weight of lettuce was gradually decreased in subsequent cultures. Nutrient absorption rate was reduced in non-renewed solutions though enough of all nutrient elements were available in the solution. In the final culture, the highest shoot fresh weight (SFW) was recorded in the renewed (83.0 g plant−1) solution which was similar to the AC-ED-treated solution (81.0 g plant−1) and the lowest (58.0 g plant−1) was in the non-renewed solution. By applying AC-ED, 40% lettuce yield was recovered in the EC-adjusted solution without renewing. Therefore, it is recommended that the continuous application of AC-ED with the capacity of 551 Hz and 50% duty would be applied for recovering the retarded lettuce yield cultivated with repeatedly used culture solutions in recycled hydroponics.</p

Root-tip-mediated inhibition of hydrotropism is accompanied with the suppression of asymmetric expression of auxin-inducible genes in response to moisture gradients in cucumber roots

<div><p>In cucumber seedlings, gravitropism interferes with hydrotropism, which results in the nearly complete inhibition of hydrotropism under stationary conditions. However, hydrotropic responses are induced when the gravitropic response in the root is nullified by clinorotation. Columella cells in the root cap sense gravity, which induces the gravitropic response. In this study, we found that removing the root tip induced hydrotropism in cucumber roots under stationary conditions. The application of auxin transport inhibitors to cucumber seedlings under stationary conditions suppressed the hydrotropic response induced by the removal of the root tip. To investigate the expression of genes related to hydrotropism in de-tipped cucumber roots, we conducted transcriptome analysis of gene expression by RNA-Seq using seedlings exhibiting hydrotropic and gravitropic responses. Of the 21 and 45 genes asymmetrically expressed during hydrotropic and gravitropic responses, respectively, five genes were identical. Gene ontology (GO) analysis indicated that the category auxin-inducible genes was significantly enriched among genes that were more highly expressed in the concave side of the root than the convex side during hydrotropic or gravitropic responses. Reverse transcription followed by quantitative polymerase chain reaction (RT-qPCR) analysis revealed that root hydrotropism induced under stationary conditions (by removing the root tip) was accompanied by the asymmetric expression of several auxin-inducible genes. However, intact roots did not exhibit the asymmetric expression patterns of auxin-inducible genes under stationary conditions, even in the presence of a moisture gradient. These results suggest that the root tip inhibits hydrotropism by suppressing the induction of asymmetric auxin distribution. Auxin transport and distribution not mediated by the root tip might play a role in hydrotropism in cucumber roots.</p></div

Asymmetrically expressed genes in cucumber roots exposed to moisture gradients, as determined by RNA-Seq.

<p>Asymmetrically expressed genes in cucumber roots exposed to moisture gradients, as determined by RNA-Seq.</p

RT-qPCR to examine the auxin-inducible expression of asymmetrically expressed genes during hydrotropism and/or gravitropism in cucumber roots.

<p>Root segments excised from cucumber seedlings (No treatment), root segments from which auxin was depleted for 2 h (After auxin starvation), and root segments that were continuously treated for 2 h with the indicated concentration of auxin were subjected to gene expression analysis. Each data point represents the mean ± SD of three independent experiments. Different letters indicate statistically significant differences in relative expression levels (<i>P</i><0.05), which were analyzed by one-way ANOVA and Tukey HSD tests.</p

Effects of root tip removal on the expression of asymmetrically expressed genes during hydrotropism in intact cucumber roots.

<p>Roots in which the root tip was either not removed (Intact) or removed (De-tipped) were exposed to a moisture gradient induced by K₂CO₃ under stationary conditions for 4 h and cut in half, RNA was isolated from the wet side and the dry side. Each data point represents the mean ± SD of three independent RT-qPCR experiments. Statistically significant differences in relative expression levels between the wet and dry sides (determined by Student’s <i>t</i> test) are indicated by single (<i>P</i><0.05) and double (<i>P</i><0.01) asterisks.</p

Expression of asymmetrically expressed genes in gravitropically bending cucumber roots.

<p>FPKM (Fragments Per Kilobase of transcript per Million mapped reads) values representing mRNA levels from RNA-Seq (left) and relative expression levels from RT-qPCR (right) of <i>CsIAA1(12)</i>, <i>CsIAA24</i>, <i>CsPID</i>, <i>CsGH3</i>.<i>5</i>, and <i>CsSAUR9</i> are shown. The roots of 24-h-old cucumber seedlings grown in a vertical position (24-h-old), seedlings placed in a vertical position for 1 h, and seedlings placed in a horizontal position for 1 h were cut in half, and RNA was isolated from the left/lower side and right/upper side. For the RNA-Seq results, a single asterisk indicates asymmetric gene expression (<i>Q</i>-value <0.05). For RT-qPCR, each data point represents the mean ± SD of three independent experiments. Statistically significant differences between the left/lower side and the right/upper side (determined by Student’s <i>t</i> test) are indicated by single (<i>P</i><0.05) and double (<i>P</i><0.01) asterisks.</p

High osmolarity vitrification using a Frozebag and mesh sheet.

(a) Mesh sheet and embryos (arrows). (b) Mesh bag. (c) Frozebag before modification. (d) Equilibration with CZB medium in an incubator (5% CO, 37°C). (e) Mesh bag on the scoop. (f) Incubation of the Frozebag (5% CO2, 37°C). (g) Solution/medium exchange using 30 ml and 50-ml syringes.</p

Embryo Thawing and Culturing unit (ETC).

(a) Frozebag with mesh cap. (b) Frozebag with mesh wall. (c) Blastocyst collected from the Frozebag. Although the quality of this embryos was insufficient, this was the first time that an embryo was thawed, washed, and cultured for 4 days without direct contact. (d) The top of cryotube was cut to produce the smallest tube (V-tube). (e) Protocol for thawing, washing, and culturing embryos in the ETC. (f) Blastocysts were harvested from the ETC after culture for 4 days. (g) Healthy offspring developed from the morulae/blastocysts derived from 3 days cultured embryos in ETC.</p