Leaf area, photosynthetic potential and crop growth rate are important indexes for representing the photosynthetic capacity of a population. Wu et al. suggested that the leaf area, photosynthetic potential and population growth rate of superhybridized japonica rice were significantly greater than those of Shanyou 63 control rice at the middle and late stages. Huo et al. reported that, with a delayed sowing date, the LAI significantly decreased at the JS, HS and dough stage and slightly decreased at the MS. The photosynthetic potential significantly or extremely significantly declined from sowing until the JS and from the JS to the HS. The cropgrowth rate markedly increased from sowing until the JS, and no significant difference was observed from the JS to the HS, whereas the rate decreased significantly from the HS to the MS. The net assimilation rate noticeably increased from sowing until the JS and from the JS to the HS but noticeably decreased from the HS to the MS. However, few reports exist on the effects of seedling age at transplanting on the photosynthetic production characteristics of mechanically transplanted HLMS.
In this research, with increasing seedling age, the LAI decreased for both cultivars at the HS and MS, the photosynthetic potential also decreased from the TS to the JS and from the JS to the HS, and the crop growth rate decreased from the TS to the JS, flood table except for 6 Liangyou 9368 in 2015. No obvious pattern of differences in crop growth rate was observed from the JS to the HS, but the rate was greater for the young seedlings than for the old seedlings from the HS to the MS. With increasing seedling age, the Pn markedly decreased at the HS, but there was no significant difference in Pn between the 13- and 20-day-old 6 Liangyou 9368 seedlings.Selenium is an essential micro-nutrient for maintaining human health . When enters in metabolism, Se can enhance the anti-carcinogenic capacity of the human body . Vegetables and cereals are important sourcesof Se for humans. However, the low bioavailability of Se in the soil in some areas restricts Se accumulation in vegetables and cereals , which leads to inadequate Se intake levels to possibly prevent cancer . Se deficiency in the diet is a worldwide problem, especially in China, the UK, Eastern Europe and Australia . Therefore, there is an increasing demand for Se-enriched food . Vegetables play important roles in the human diet.
The consumption of Se-rich vegetables could be a safe and effective way to solve the problem of Se deficiency. It is known that plants can uptake Se in the form of selenate, selenite and organic species . The exogenous application of Se has been particularly effective in increasing the Se concentration in plants, but this effect varies among species . An excessive intake of Se can also lead to chronic toxicity for humans, with the recommended daily maximum intake of dietary Se not exceeding 400 mg per day . The target to regulate the Se concentration in vegetables is still therefore unknown. Consequently, there is an urgent need to develop adaptive agricultural strategies to regulate Se uptake and distribution in vegetables. Nitrate is one of the main forms of nitrogen used for plant growth and development and is widely used in vegetable production, especially in leafy vegetables grown in hydroponic systems . Lettuce is the main crop grown in greenhouses and is consumed worldwide due to its flavour and high levels of phytochemicals. However, lettuce is a hyperaccumulator of nitrate and easily accumulates a large amount of nitrate in its leaves . The daily consumption of vegetables with high amounts of nitrate is associated with a higher risk for cancers and methemoglobinemia . In our previous study, we reported that exogenous Se application had a positive effect in restricting nitrate accumulation in hydroponic lettuce . However, little is known about the relationship between Se accumulation and nitrate reduction in plants under different forms of exogenous Se.
Light is not only the driving force for photosynthesis but also serves as the transduction signal to regulate metabolism in plants . Compared with light intensity and light duration, light spectra have more complex roles in regulating plant growth and development . To date, red and blue light emitting diodes have been proven to be the most efficient artificial light source for driving photosynthesis and are widely used in vegetable production . Previous studies reported that the light spectral composition plays an important role in regulating the accumulation of mineral elements, such as N, P and K in plants . However, few studies have focused on the effect of light spectra on Se uptake, translocation and accumulation under different forms of exogenous Se treatment. In this study, we conducted a selenate concentration screening experiment and then comparatively investigated the effects of the light spectral composition and Se forms on the uptake, translocation and accumulation of Se, nitrate metabolism and photosynthetic performance of lettuce grown hydroponically. The main objectives of this study were to: investigate the effect of the combination of light spectra and Se forms on the accumulation and distribution of Se and nitrate reduction and investigate the relationship between the light spectral composition and the nitrate/ Se content in lettuce under different forms of exogenous Se treatment.
The results of this study are crucial to understanding and revealing the mechanisms that are responsible for Se uptake, distribution and toxicity in plants. Furthermore, the information from this study can provide guidance on producing high nutritional quality vegetables with safe Se concentrations.In the main light experiment, the plants and previous growth conditions were the same as above. At the end of the dark period of day 21, similar size seedlings were transplanted into 25-L containers with 0.5 μmol L–1 selenite or 10 μmol L–1 selenate in Hoagland solution. These plants were grown under different light spectra for 25 days. There were five different LED light treatments: monochromatic red LED light , monochromatic blue LED light , combined red and blue LED light with a red to blue ratio at 4 , 8 and 12 , respectively. The plants exposed to FL were used as controls. The details of the light spectra from the light sources used in this study are summarized in Appendix A. The light intensity at the plant canopy was monitored every other day using a light intensity metre . The light intensity was maintained at 200 mmol m–2 s−1 by changing the distances between the light sources and plants. Other environmental factors were the same as those at the seedling stage. There were three replicates with a total of 48 plants per treatment. The nutrient solution with the same Se treatment was replaced every 5 days.The RCF and TF could be used to reflect the capacities for Se uptake, accumulation and translocation in response to the light spectra and exogenous Se. The RCF and TF of Se were significantly affected by the light spectral composition and applied forms of Se.
Regardless of the concentration difference, the RCF for exogenous selenite was approximately 5 times higher and the TF was 6.8–11.9 times lower than that for exogenous selenate.With regard to the selenate treatment, the values of RCF and TF in the plants exposed to red and blue LED were lower than those under FL. The RCF of Se under R/B=8 was markedly higher but the TF of Se was lower than that under the other LED light spectral treatments. Interestingly, these parameters for the plants under R/B=4 changed in an opposite direction compared to those for R/B=8, as shown by the lowest RCF and highest TF under R/B=4. These results indicate that R/B=8 was more efficient in promoting Se absorption, while R/B=4 was more effective in promoting Se transportation in lettuce plants. In contrast, the highest and lowest RCF of Se under exogenous selenite was observed in the plants exposed to R and R/B=4, respectively. However, the RCF under the other light treatments was comparable to that under the FL treatment. Compared with FL,rolling benches red and blue LED light led to significant decreases in the TF of Se in the plants treated with exogenous selenite, and the lowest TF of Se was observed under R.No significant difference was observed for the total nitrogen content under different light spectra and exogenous Se treatments . However, the nitrate contentsand N assimilation enzyme activities in the lettuce leaves were markedly affected by the Se forms and light spectral composition . The nitrate contents in the lettuce treated with selenate were lower than those under selenite, indicating that compared with selenite, exogenous selenate was more efficient in retarding nitrate accumulation in the lettuce plants. Relative to FL, the nitrate contents were significantly lower under red and blue LED light. Interestingly, regardless of the Se form, the lowest nitrate content was observed in the plants exposed to R/B=8. NR activity was significantly affected by the light spectra, applied Se forms and their interaction.
The NR activity of the plants treated with selenate was higher than that under the selenite treatments. The highest and lowest NR activity were observed under R/B=8 and B, respectively, after exogenous selenate and selenite application. Unlike the changes in NR activity, NiR activity was mainly affected by the light spectral composition. The highest NiR activity was observed under R/B=8, while this parameter under the other light treatments was comparable to that under FL. The activities of GS and GOGAT under LED light were higher than those under FL. The highest GS activities were observed under R/B=8, while the GOGAT activities were the highest in the plants exposed to B for both the exogenous selenite and selenate treatments.Under the exogenous selenate and selenite treatments, the Se and nitrate contents in the lettuce leaves were both negatively correlated with the percentage of red light from the light sources . However, there was a significant linear relationship between the Se and nitrate content under exogenous selenate and selenite combined with different light spectral compositions . These results indicate that the accumulation of Se and nitrate were regulated by the light spectral composition and that a higher ratio of red light was not conducive to Se and nitrate accumulation in lettuce.Se is an essential mineral element for both humans and animals and is mainly acquired from plants . Exogenous Se application can increase the concentration of Se in the edible parts of plants. However, Se has not been proven to be an essential mineral element for plants, and excessive Se can be toxic. In this study, the effect of selenate on lettuce growth was concentrate-dependent: low concentration promoting growth and high concentration inhibiting growth.
These results are consistent with those of previous studies showing that a small amount of Se is beneficial for plant growth, while excessive doses could induce a decrease in the photosynthetic capacity, ultimately leading to reduced biomass or even plant death . In the present study, the significant effect of the light spectra and the concomitantly marked interaction between light spectra and Se forms on most of the plant biomass parameters indicate that the light spectrum plays an important role in the process of Se regulation of plant growth. Under mixed red and blue LED light, the higher biomass of selenatetreated plants observed in our study is partly attributed to their high photosynthetic capacity, as shown by higher Pn under selenate than under selenite . Similar results were also reported in wheat by Kaur and Sharma . Compared with other trace elements, Se is arguably one of the most interesting elements because of the very narrow window between deficiency and toxicity . An excessive intake of Se in the diet can also be harmful to human beings and animals . The ability to control Se concentration at safe levels in the edible parts of plants is important, alongside promoting Se accumulation in plants. In plants, mineral element uptake and translocation are regulated by many factors. Light can strongly affect plant cell potentialsor fluxes of ions other than those associated only with energy . Previous studies have found that the uptake capacity of mineral elements of plant tissue was affected by the light spectral composition .