Gene Expression and Activities of SOD in Cucumber Seedlings Were Related with Concentrations of Mn^(2+),Cu^(2+),or Zn^(2+) Under Low Temperature Stress

Effects of increasing Mn^2＋, Cu^2＋, or Zn^2＋ on SOD expressions were studied in cucumber seedlings under low temperature stress. Both gene expressions and activities of Cu/Zn-SOD and Mn-SOD in cucumber seedling leaves were induced by increasing Mn^2＋, Cu^2＋, or Zn^2＋ under low temperature stress, especially 48 h afterwards. The activities of Cu/Zn-SOD and Mn-SOD at 0 and 48 h after treatment were in accordance with their gene expression levels, which implied that the transcriptional regulation plays key roles in regulating their activities at the early stage of low temperature stress. Gene expressions of Cu/Zn-SOD and Mn-SOD declined at 96 h, but Cu/Zn-SOD and Mn-SOD activities still remain high, which suggested that Cu/Zn-SOD and Mn-SOD activities might be regulated by other factors after transcription at the later stage of low temperature stress. Therefore, we concluded that the increasing Mn^2＋, Cu^2＋, or Zn^2＋ could increase the capacity of scavenging ROS in cucumber seedlings under low temperature stress by inducing gene expressions of Cu/ Zn-SOD and Mn-SOD, elevating activities of Cu/Zn-SOD, Mn-SOD, or regulating other factors after transcription.

Grafting Increases the Copper Tolerance of Cucumber Seedlings by Improvement of Polyamine Contents and Enhancement of Antioxidant Enzymes Activity

The aim of the study is to determine whether grafting could improve antioxidant enzyme activities and polyamine contents in leaves of cucumber plants（Cucumis sativus L.cv.Xintaimici） under copper stress.Grafted（using Cucurbita ficifolia as rootstock） and ungrafted cucumber seedlings were cultured in deep flow technique（DFT） with the Cu2＋ concentration of 40 μmol L-1.The results showed that on the 9th day of copper stress treatment,the contents of malondialdehyde（MDA） and hydrogen peroxide（H2O2）,superoxide radical（） producing rate,and electrolyte leakage percentage were significantly lower in grafted seedlings in comparison to those of the ungrafted seedlings,whereas the activities of antioxidants such as superoxide dismutase（SOD,EC 1.15.1.1）,peroxidase（POD,EC 1.11.1.7）,ascorbate peroxidase（APX,EC 1.11.1.11）,catalase（CAT,EC 1.11.1.6）,glutathione reductase（GR,EC 1.6.4.2）,and monodehydroascorbate reductase（MDHAR,EC 1.6.5.4） of grafted seedlings were also significantly higher than that of ungrafted seedlings,and the contents of free proline and soluble protein of grafted seedlings were significantly higher than that of ungrafted seedlings.Cu2＋ treatment increased the putrescine（Put） level and lowered the spermidine（Spd） and spermine（Spm） levels,thereby reducing the Put/（Spd ＋ Spm） ratio in leaves of grafted and ungrafted seedlings.Grafting markedly reversed these Cu-induced effects for all three PAs and partially restored the Put/（Spd ＋ Spm） ratio in leaves.These results suggest that grafting can enhance the tolerance of cucumber seedlings to Cu2＋ stress by increasing the activities of antioxidants and the levels of endogenous Spd and Spm,decreasing the Put/（Spd ＋ Spm） ratio and the levels of ROS,promoting free proline and soluble protein synthesis in cucumber seedling leaves.

Interactive Effects of Drought Stresses and Elevated CO_2 Concentration on Photochemistry Efficiency of Cucumber Seedlings

To reveal and quantify the interactive effects of drought stresses and elevated CO2 concentration [CO2] on photochemistry efficiency of cucumber seedlings, the portable chlorophyll meter was used to measure the chlorophyll content, and the Imaging-PAM was used to image the chlorophyll fluorescence parameters and rapid light response curves （RLC） of leaves in two adjacent greenhouses. The results showed that chlorophyll content of leaves was reduced significantly with drought stress aggravated. Minimal fluorescence （Fo） was increased while maximal quantum yield of PSII （Fv/Fm） decreased significantly by severe drought stress. The significant decrease of effective quantum yield of PSII （Y（Ⅱ）） accompanied by the significant increase of quantum yield of regulated energy dissipation （Y（NPQ）） was observed under severe drought stress condition, but there was no change of quantum yield of nonregulated energy dissipation （Y（NO））. We detected that the coefficient of photochemical quenching （qP） decreased, and non-photochemical quenching （NPQ） increased significantly under severe drought stress. Furthermore, we found that maximum apparent electron transport rate （ETRmax） and saturating photosynthetically active radiation （PPFDsat） decreased significantly with drought stress aggravated. However, elevated [CO2] significantly increased FvlFm, qP and PPFDsat, and decreased NPQ under all water conditions, although there were no significant effects on chlorophyll content, Fo, Y（Ⅱ）, Y（NPQ）, Y（NO） and ETRmax. Therefore, it is concluded that CO2-fertilized greenhouses or elevated atmospheric [CO2] in the future could be favorable for cucumber growth and development, and beneficial to alleviate the negative effects of drought stresses to a certain extent.

Effects of light quality on growth and development of cucumber seedlings in controlled environment

Conventional light sources have been successfully used to cultivate a wide variety of horticultural crops.However,they are of limited use due to uncontrollability of spectra and energy inefficiency.Light-emitting diodes(LEDs)emerged with tremendous potential in controlled environment agriculture due to their energy efficiency,longevity,and spectral specificity,but the effects of different types of LEDs on plant growth and development must be examined.In this study,cucumber(Cucumis sativus L.cv.Zhongnong 26)seedlings were grown under four different lighting treatments that each delivered a photosynthetic photon flux density of 200μmol/m2·s at plant canopy including triphosphate fluorescent lamps(TF),high-frequency fluorescent lamps(HF),white LEDs(WL),and red and blue LEDs(RBL).Cucumber seedlings were grown in a growth chamber at(25.0±1.5)℃with 12-hour light and 12-hour dark for 30 days after sowing,and data were subsequently collected.Seedlings grown under the WL were 45%,12%,and 40%taller than those grown under the TF,HF and RBL,respectively.The leaf area was 23%smaller under the TF than under the HF.The shoot dry weight was 16%-22%lower under the TF than under the other lighting treatments.The transplants grown under the RBL had the lowest root dry weight and root to shoot ratio.The seedling quality index was similar among all the lighting treatments.The LEDs treatment yielded more total dry weight with unit electric power compared to the fluorescent lamps.The chlorophyll content was 13%-15%higher in plants grown under the HF and WL than that under the TF and RBL.Plants grown under the WL and RBL had greater photosynthetic rate,transpiration rate,and stomatal conductance than those grown under the TF and HF.It was concluded that high quality cucumber seedlings can be efficiently produced under the broad-spectrum WL that emit a reasonable amount of blue,green and red light,and the lack of green light and/or high ratio of red to blue light under the RBL may cause undesired plant attributes.

Growth of cucumber seedlings in different varieties as affected by light environment

High-quality cucumber seedlings are a prerequisite for ensuring high yield of cucumbers.With the continuous increase of cucumber planting area in China,the demand for high-quality cucumber seedlings is also increasing.One of the important ways to improve the quality of cucumber seedlings is to improve the light environment by using artificial light source.In this study,three cucumber seedlings(cv.Jintong,cv.Yunv and cv.Xiazhiguang)were grown for 23 d under eight levels of daily light integral(DLI)at 4.3 mol/(m^(2)·d),8.6 mol/(m^(2)·d),10.1 mol/(m^(2)·d),13.0 mol/(m^(2)·d),5.8 mol/(m^(2)·d),11.5 mol/(m^(2)·d),14.4 mol/(m^(2)·d)and 17.3 mol/(m^(2)·d),respectively.The results showed that when DLI was 14.4 mol/(m^(2)·d),the seedling height,stem diameter,total leaf area and shoot dry/fresh weight of all three cucumber cultivars reached the maximum,while hypocotyl length decreased with the increase of light intensity.When DLI was 14.4 mol/(m^(2)·d),Jintong and Xiazhiguang had the highest health index,which were 49.29 and 28.56,respectively,while that of Yunv was 81.59(DLI=14.4 mol/(m^(2)·d)).With the increase of DLI,the photosynthetic capacity of cucumber increases gradually.The highest net photosynthetic rate was shown at DLI of 14.4 mol/(m^(2)·d),while the chlorophyll content of cucumber seedlings of all three cultivars were less affected by DLI.Jintong and Yunv had the highest chlorophyll content when DLI was 8.6 mol/(m^(2)·d)because they were adapted to low-light environment.In conclusion,the DLI of 14.4 mol/(m^(2)·d)for Jintong and Xiazhiguang,while that of 17.3 mol/(m^(2)·d)for Yunv are suggested for the light environment design of factory-cultivated seedling for cucumber.

Effects of phosphate solubilization and phytohormone production of Trichoderma asperellum Q1 on promoting cucumber growth under salt stress

Salinity is one of the major abiotic stresses limiting crop growth and yield.This study investigated the underlying mechanisms of Trichoderma asperellum Q1 in promoting cucumber growth under salt stress, including the abilities of the strain to solubilize phosphate and to produce phytohormone.The results showed that T.asperellum Q1 could solubilize inorganic or organic phosphate and the activities of phosphatases and phytase could be detected in the culture supernatant.In hydroponic experiments, the growth of cucumber seedlings was increased in the hydroponic system treated by culture filtrate of strain Q1 with tricalcium phosphate or calcium phytate under salt stress.This strain also exhibited the ability to produce indole acetic acid（IAA）, gibberellic acid（GA） and abscisic acid（ABA） in liquid medium without any inducers.The levels of those three phytohormones in cucumber seedling leaves also increased after inoculated with this strain, along with increased root growth and root activities of the plant.These results demonstrated the mechanisms of T.asperellum Q1 in alleviating the suppression effect of salt stress involving the change of phytohormone levels in cucumber plant and its ability of phosphate solubilization.

Grafting Enhances Copper Tolerance of Cucumber Through Regulating Nutrient Uptake and Antioxidative System

An experiment was carried out to determine plant growth, mineral uptake, lipid peroxidation, antioxidative enzymes, and antioxidant of cucumber plants （Cucumis sativus L. cv. Xintaimici） under copper stress, either ungrafted or grafted onto the rootstock （Cucurbitaficifolia）. Excess Cu inhibited growth, photosynthesis, and pigment synthesis of grafted and ungrafted cucumber seedlings and significantly increased accumulation of Cu in roots besides reducing mineral uptake. Cu concentration in roots of grafted cucumber plants was significantly higher than that of ungrafted plants and obviously lower in leaves. The accumulation of reactive oxygen species （ROS） significantly increased in cucumber leaves under Cu stress and resulted in lipid peroxidation, and the levels of ROS and lipid peroxidation were greatly decreased by grafting. Activities of protective enzymes （superoxide dismutase, SOD; peroxidase, POD; catalase, CAT; ascorbate peroxidase, APX; dehydroascorbate reductase, DHAR; glutathione reductase, GR） and the contents of ascorbate and glutathione in leaves of grafted plants were significantly higher than those of ungrafted plants under Cu stress. Better performance of grafted cucumber plants were attributed to the higher ability of Cu accumulation in their roots, better nutrient status, and the effective scavenging system of ROS.

Grafting Raises the Cu Tolerance of Cucumber Through Protecting Roots Against Oxidative Stress Induced by Cu Stress

A greenhouse experiment was carried out to determine plant growth, reactive oxygen species （ROS） metabolism in roots and functions of plasma membrane （PM） and tonoplast in cucumber seedlings （Cucumis sativus L. ev. Xintaimici） treated with 40 μmol L^-1 CuSO4·5H2O, which were either ungrafted or grafted onto the rootstock （Cucurbitaficifolia）. Cu treatment inhibited growth, induced significant accumulation of H2O2 and led to serious lipid peroxidation in cucumber roots, and the ROS-scavenging enzymes activities in grafted seedlings roots were significantly higher than that of ungrafted plants, thus less accumulation in grafted cucumber roots induced by Cu. As a result, lipid peroxidation in roots decreased. Furthermore, the activities of H^-ATPase, H＋-PPase and Ca^2＋-ATPase in PM and/or tonoplast in grafted cucumber seedlings under Cu stress were obviously higher than that in ungrafted plants, resulting into higher ability in grafted plants to expulse the excess H＋, promote the cytoplasm alkalinization, regulate the intracellular Ca^2＋ concentration and brought the cytoplasma concentration of free Ca2＋ to extremely low level under Cu stress.

Effects of PQQ on Protective System in Cucumber Cotyledons under Low Temperature Stress

The authors have studied the effects of Pyrroioguinloine quinone (PQQ) on superoxide dismutase (SOD), Ascorbid acid(AsA) peroxidase (AsAPOD), glutathion (GSH) and electrolytic leakage of cotyledon in cucumber seedling under low temperature stress, meanwhile, 8-hydroquinone (8-HQ) and AsA (activeoxiygen scavengers) have been made use of in comparison with PQQ. The results indicate that the activities of SOD, AsAPOD and content of GSH can be increased by PQQ. The relative conductivity of cotyledon in cucumber seedling is decreased for PQQ possesses the ability of cleaning up free redical of oxygen. We came to the conclusion that PQQ can act as a kind of active oxygen scavenger and adjust the metabolism on free radical of oxygen to balance in plants and enhance resistance finally in plants.