Suboptimal Root Zone Temperature Research Articles

Microclimate control to increase productivity and nutritional quality of leafy vegetables in a cost-effective manner

Food security is one of the key global challenges in this century. In Singapore, our research team has been using novel aeroponic technology to produce fresh vegetables since 1997. Aeroponic systems allow for year-round production of not only tropical, but also sub-tropical and temperate fresh vegetables, by simply cooling the roots suspended in aeroponic systems while the aerial parts grow under tropical ambient environments. It has also been used to investigate the impacts of root-zone CO2 on vegetables by enriching root-zone CO2 while their aerial portions were subjected to constant atmospheric CO2. To compensate for the lack of available land, Singapore also needs to develop a farming system that can increase productivity per unit land area by many-fold. Over the past 10 years, my research team has established a commercially viable LED integrated vertical aeroponic farming system to grow different leafy vegetables under different LED spectra, intensities, and durations in the tropical greenhouse. The results demonstrate that it is possible to increase shoot production and rate of shoot production of leafy vegetables by increasing light intensity and extending the photoperiod under effective LED lighting. Furthermore, temperate vegetable crops such as lettuce were able to acclimate to high light intensity under supplementary LED lights to natural sunlight in the greenhouse. Supplementary LED lightings promote both leaf initiation and expansion with increased photosynthetic pigments, higher Cyt b6f and Rubisco protein contents on a per area basis and thus improve photosynthetic capacity and enhance productivity. Plants sense and respond to changes in their immediate environments (microclimate), manipulating the root zone temperature (RZT) and water supply will impact not only their growth and development but also their nutritional quality. Our on-going research aims to investigate if the nutritional quality of leafy vegetables could be improved under suboptimal RZT and mild water deficit through deficit irrigation. If substantial energy and water savings in urban farming can be achieved without substantial yield penalty but with higher nutritional quality, the amount of water and energy saved can bring substantial benefits to society. Keywords: leaf vegetables, LED lighting, microenvironment, nutrient spraying intervals, nutritional quality, photosynthetic performance, productivity, root-zone temperature DOI: 10.25165/j.ijabe.20221503.7367 Citation: He J, Qin L. Microclimate control to increase productivity and nutritional quality of leafy vegetables in a cost-effective manner. Int J Agric & Biol Eng, 2022; 15(3): 55–61.

24-Epibrassinolide promotes NO3\u2212 and NH4+ ion flux rate and NRT1 gene expression in cucumber under suboptimal root zone temperature

BackgroundSuboptimal root zone temperature (RZT) causes a remarkable reduction in growth of horticultural crops during winter cultivation under greenhouse production. However, limited information is available on the effects of suboptimal RZT on nitrogen (N) metabolism in cucumber seedlings. The aim of this study is to investigate the effects of 24-Epibrassinolide (EBR) on nitrate and ammonium flux rate, N metabolism, and transcript levels of NRT1 family genes under suboptimal RZT in cucumber seedlings.ResultsSuboptimal RZT (LT) negatively affected on cucumber growth and proportionately decreased EBR contents, bleeding rate, root activity, enzyme activities of nitrate reductase (NR), nitrite reductase (NiR), glutamine synthetase (GS), and glutamate synthase (GOGAT), nitrate (NO3−) influx rate, ammonium (NH4+) efflux rate, and transcript levels of nitrate transporter (NRT1) encoding genes. However, exogenous EBR reduced the harmful effects of suboptimal RZT and increased endogenous EBR contents, bleeding rate, root activity, enzyme activities of NR, NiR, GS, and GOGAT, NH4+ and NO3− flux rates and contents, and N accumulation. EBR-treated seedlings also upregulated the transcript levels of nitrate transporters CsNRT1.1, CsNRT1.2A, CsNRT1.2B, CsNRT1.2C, CsNRT1.3, CsNRT1.4A, CsNRT1.5B, CsNRT1.5C, CsNRT1.9, and CsNRT1.10, and downregulated CsNRT1.5A and CsNRT1.8. LT treatment upregulated the expression level of CsNRT1.5A, while exogenous BZR application downregulated the expression level of NRT1 genes.ConclusionThese results indicate that exogenous application of EBR alleviated the harmful effects of suboptimal RZT through changes in N metabolism, NH4+ and NO3− flux rates, and NRT1 gene expression, leading to improved cucumber seedlings growth. Our study provides the first evidence of the role of EBR in the response to suboptimal RZT in cucumber, and can be used to improve vegetable production.

Exogenous 24-epibrassinolide alleviates the detrimental effects of suboptimal root zone temperature in cucumber seedlings

ABSTRACTSuboptimal root zone temperature (RZT) is major hurdle for vegetables production that causes a remarkable reduction in growth and yield during winter and early spring. The aim of this study is to assess the effects of 24-epibrassinolide (EBR) on physiological and biochemical mechanism on cucumber seedling under suboptimal RZT. The treatments were NT (normal RZT [22 ± 2°C]), LT (suboptimal RZT [14 ± 2°C]), EBR (0.1 µM EBR + suboptimal RZT [14 ± 2°C]) and BZR (brassinazole, a specific EBR biosynthesis inhibitor) (4 µM BZR + suboptimal RZT [14 ± 2°C]). The results showed that LT and BZR significantly decline cucumber seedlings growth, chlorophyll (Chl) contents, photosynthetic capacity, antioxidant enzyme activities and nutrients accumulation. Exogenous EBR application significantly enhanced cucumber seedling growth, Chl contents, photosynthetic capacity, nutrient contents, activities of antioxidant enzymes and reduced the contents of H2O2 (hydrogen peroxide), (superoxide radical) and malondialdehyde. The transcript levels of EBR-biosynthesis genes are reduced by LT, while reported significantly higher in EBR treated seedling. Our results suggest that exogenous EBR could alleviate the harmful effects of suboptimal RZT by modulating EBR-biosynthesis genes expression and antioxidant, nutrient, Chl and photosynthetic machinery, thus leading to improved plant growth.

Effect of Gibberellin on Growth and Accumulation of Nitrogen of Cucumber Under Suboptimal Root-zone Temperature

Effect of Gibberellin on Growth and Accumulation of Nitrogen of Cucumber Under Suboptimal Root-zone Temperature

Improving Soybean (Glycine max L.) N2 Fixation under Stress

Soybean is a major leguminous plant that has the ability to establish a symbiotic association with the N-fixing bacteria, Bradyrhizobiumjaponicum. Soils are usually subjected to stress including salinity, drought, acidity, and suboptimal root zone temperature, adversely affecting the symbiotic process between soybean and the bacteria. One of the important processes affecting the performance of soybean under stress is the inhibited exchange of symbiosis-related signaling molecules, specifically genistein, between the host legume and B. japonicum during the initiation of symbiosis. Interestingly, inoculation of B. japonicum with the signal molecule genistein can partially or completely alleviate the stress. Understanding the techniques and the precise molecular pathways, which may be influenced by the signaling molecules during the stress, can be useful to determine parameters that enhance the plant’s ability to cope with stress. For example, the use of proteomic techniques to identify proteins expressed under stress can help characterize those proteins and their involvement in stress. Biotechnological-genetic techniques, either breeding or transformation, are also among the effective methods of improving soybean’s ability to fix N2 under stress. This can be achieved by identifying the genes, which may be expressed under stress in tolerant bacterial and plant species, and inserting them into the non-tolerant species. This article highlights some important advances in soybean N2 fixation under different stress conditions, and reviews some of the techniques used to improve the ability of plants and bacteria to efficiently fix N2 under stress.

Temperature effects on the reactive oxygen species formation and antioxidant defence in roots of two cucurbit species with contrasting root zone temperature optima

Suboptimal root zone temperature (14°C) was imposed on chilling-sensitive cucumber (Cucumis sativus L.) and chilling-tolerant figleaf gourd (Cucurbita ficifolia Bouche) plants. Exposure of roots to low temperature for up to 10 days caused a strong growth inhibition in cucumber compared with figleaf gourd. Physiological analysis showed that generation of reactive oxygen species (ROS) such as hydrogen peroxide and superoxide anion was significantly induced in cucumber plants as fast as 1 day after low root zone temperature treatment. In addition to the significant induction of antioxidant superoxide dismutase activity, low root zone temperature also increased the mitochondrial electron transport allocated to alternative pathway while decreased cytochrome pathway salicylhydroxamic acid-resistant respiration. However, these defense responses could not compensate for the ROS production, resulting in membrane lipid peroxidation and loss of root cell viability in the low root zone temperature treated cucumber roots. In contrast, 14°C root zone temperature had no significant effects on figleaf gourd plant growth, antioxidant enzymes, ROS levels and alternative respiratory pathway. Hence, difference in ROS metabolism would be associated with the remarkable difference in adaptability of cucumber and figleaf gourd plants in response to suboptimal root zone temperature condition.

Ionomic response of Lotus japonicus to different root-zone temperatures

The effects of root-zone temperature (RZT) on the ion uptake, distribution and interactions of 21 (11 essential and 10 non-essential) elements using Lotus japonicus were studied. After acclimation in hydroponics at ambient (25°C) RZT for three weeks, seedlings were exposed to sub-optimal (15°C and 20°C) and ambient RZTs. After one week, plants were harvested and then growth and mineral elements were analyzed. The highest shoot and root biomass was at 25°C and lowest at 15°C RZT. In shoots, the highest concentrations of all elements, except for nickel (Ni), were at 25°C RZT. At lower RZTs, the reductions in concentrations were more evident especially for trace elements and heavy metals. Except for magnesium (Mg), the highest root concentrations were at 15°C RZT for all essential elements. Unlike other non-essential elements, the highest cadmium (Cd) and cobalt (Co) concentrations in roots were at 25°C RZT. The shoot/root ratios of almost all elements were affected by sub-optimal RZTs. Only manganese (Mn) and Co showed increased shoot/root ratio at sub-optimal RZTs; and this ratio decreased for other elements. Nickel in shoots and Mn, Co and Cd in roots tended to be negatively correlated with most other elements (either, essential or non-essential) in each respective organ. The present study showed that RZT significantly changed the ion profile of L. japonicus and suggests possible connection between global warming and phyto-accumulation of heavy metals, especially in temperate regions, since most metals tended to accumulate in shoots at higher RZT.

Inhibitory effects of sub-optimal root zone temperature on leaf bioactive components, photosystem II (PS II) and minerals uptake in Trichosanthes cucumerina L. Cucurbitaceae

We hypothesized that sub-optimal root zone temperature (RZT) will cause a reduction in the bioactive components contents, adversely affect PS II and hinder uptake/partitioning of mineral elements in the “Light Green Variant” of the African snake tomato (Trichosanthes cucumerina L.). Three RZT temperatures (20, 25 and 30°C) were evaluated in a digitally controlled growth chamber. Results showed that for all the mineral nutrients analyzed (Ca, Mg, P, K, Fe and Mn), the amounts absorbed by the plant increase as RZT increases with each nutrient displaying different characteristics with respect to the quantity partitioned into root, stem and leaf at the different RZT. At sub-optimal RZT (20°C), significantly higher amounts of Ca and K were found in the root, whereas at normal RZT (25 and 30°C) higher amounts of Ca were recorded in the stem, and about 50% of the amounts in the stem were found in the leaves. For all the RZT, the amounts of Mg in the leaves were significantly higher than in the root and stem, while the amounts in the stems were also significantly higher than the amounts in the roots. At normal RZT (25 and 30°C) almost equal amounts of P were present in the root, stem and leaf. The amounts of phenolics, ascorbic acid, chlorophyll a and b and total chlorophyll increase as the RZT increases. Photosystem analyses showed that at 30°C the Fv/Fm (relative photochemical efficiency) was 0.76, while at 20 and 25°C the values were 0.35 and 0.60, respectively. The Fv/Fm value (0.35) obtained at 20°C confirmed the adverse effects of sub-optimal RZT on the photosystem II (PS II). Photosynthetic measurements showed that as the RZT increased, A (net photosynthetic efficiency), E (transpiration rate), Ci (intercellular carbon dioxide concentration) and g1 (stomata conductance) also increased. We postulate that the higher E and g1 at high RZT have a great physiological implication on plant performance, because transpiration cooling would be improved, especially during the summer; but the lost water must be complemented by adequate irrigation. The totality of the results confirm our hypothesis that sub-optimal RZT will cause a reduction in the bioactive components contents, adversely affect PS II and hinder uptake/partitioning of mineral elements in T. cucumerina.

Using signal molecule genistein to alleviate the stress of suboptimal root zone temperature on soybean-Bradyrhizobium symbiosis under different soil textures

Suboptimal root zone temperatures (RZTs) (below 25°C) in Canada until July may adversely affect the secretion of interorganismal signal molecules such as genistein by soybean and hence, the soybean-Bradyrhizobium symbiosis. We also proposed for the first time that soil texture might play a role in these biochemical communications. Soybean plants, planted in undisturbed soil samples (with sandy, loamy and clay textures), collected from the field, were subjected to three different soil temperatures (14, 19 and 24°C). Bradyrhizobium japonicum (strain 532C) inocula, preincubated with four levels of genistein (0, 5, 10 and 20 µM), were used to inoculate the plants. The experiment was conducted at the research greenhouse of Macdonald Campus, McGill University, Canada. The effects of genistein 5 and 20 µM on soybean nodulation and growth were significant at 14°C. As genistein was more effective in loamy and clay soils, soil texture may also be a determining factor in the biochemical communications between B. japonicum and soybean.

Root zone temperature influences growth, partitioning, leaf morphology and physiology of the peach rootstock, Green Leaf Nemaguard

This paper examines the hypothesis that root zone temperature (RZT) affects the growth of stone fruit plants. This hypothesis was tested by growing plants of the peach rootstock, Green Leaf Nemaguard (Prunus persica L. Batsch), at a diurnally variable (26/15°C) or at constant (5, 15, 20°C) RZTs; the plants were either actively growing or emerging from dormancy when the treatments were applied. These trials demonstrated that RZT, independently of air temperature and light intensity, influences growth, dry matter partitioning, leaf morphology and physiological processes. The growth of plants emerging from dormancy was more sensitive to RZT than that of actively growing plants, therefore, phenology can influence sensitivity to RZT. The area, numbers and daily rates of expansion of leaves, correlated positively with RZT for both sets of plants. However, plants exposed to a diurnally variable 26/15°C RZT were smaller with respect to overall growth and aspects of leaf morphology than plants exposed to a constant 20°C RZT, despite the daily mean RZTs for both treatments being similar. This could be due to supraoptimal RZTs during the day and/or suboptimal RZTs at night. Root mass ratio, in both actively growing and plants exiting dormancy was highest at 20° RZT. In contrast, the stem mass ratio of actively growing plants was maximised at 5°C, and for plants exiting dormancy, the stem mass ratio was minimised at this RZT. RZTs influence the rate of leaf expansion, which in turn affects the total number of leaves and leaf area and, along with its effect on CO2 assimilation rates, results in reductions in DM production. This research illustrates the importance of RZTs, particularly in the spring, on growth and leaf development and suggests the need to incorporate RZT into development models for peaches.

Pre‐incubation of Bradyrhizobium japonicum with jasmonates accelerates nodulation and nitrogen fixation in soybean (Glycine max) at optimal and suboptimal root zone temperatures

Jasmonic acid (JA) and methyl jasmonate, collectively known as jasmonates, are naturally occurring in plants; they are important signal molecules involved in induced disease resistance and mediate many physiological activities in plants. We studied the effect of JA and its methyl ester, methyl jasmonate (MeJA), on the induction of nod genes in Bradyrhizobium japonicum GG4 (USDA3) carrying a plasmid with a translational fusion between B. japonicum nodY and lacZ of Escherichia coli, and the expression activity was measured by β‐galactosidase activity. Both JA and MeJA strongly induced the expression of nod genes. They have little or no deleterious effects on the growth of B. japonicum cells, while genistein (Gen) showed inhibitory effects. We further studied the effect of JA‐ and MeJA‐induced B. japonicum on soybean nodulation and nitrogen fixation under optimal (25°C) and suboptimal (17°C) root zone temperature (RZT) conditions. B. japonicum cells were grown in liquid yeast extract mannitol media and induced with a range of Gen, JA, and MeJA concentrations, including a treatment control with no inducer added. Soybean seedlings were grown at 25 or 17°C RZT with a constant air temperature (25°C) and inoculated, at the vegetative cotyledonary stage, with various B. japonicum induction treatments. Addition of Gen or jasmonates to B. japonicum, prior to inoculation, enhanced nodulation, nitrogen fixation, and plant growth at suboptimal RZT conditions. A higher concentration of Gen was inhibitory at 25°C, while this same concentration was stimulatory at 17°C. Interestingly, pre‐incubation of B. japonicum with JA and MeJA enhanced soybean nodulation and nitrogen fixation under both optimal and suboptimal RZTs. We show that jasmonates are thus a new class of signaling molecules in the B. japonicum‐soybean symbiosis and that pre‐induction of B. japonicum with jasmonates can be used to enhance soybean nodulation, nitrogen fixation, and early plant growth.

Arbuscular mycorrhiza colonization and development at suboptimal root zone temperature.

Temperature has a strong influence on the activity of living organisms. This study, involving two indoor experiments, evaluated the effects of root zone temperature (10, 15 and 23 degrees C) on the formation and development of arbuscular mycorrhizae (AM). In the first trial, greenhouse-grown sorghum [ Sorghum bicolor (L.) Moench] was either colonized by Glomus intraradices Schenck & Smith or left non-mycorrhizal. Root length, root and shoot weight and root colonization were measured after 5, 10 and 15 weeks of plant growth. Although suboptimal root zone temperatures reduced growth in both mycorrhizal and non-mycorrhizal plants, mycorrhizal plants were larger than non-mycorrhizal plants after 15 weeks at 15 and 23 degrees C. At suboptimal root zone temperatures, mycorrhizal inoculation sometimes slightly reduced root development. AM colonization was more affected than root growth at suboptimal root zone temperatures. Colonization was markedly reduced at 15 degrees C compared with 23 degrees C, and almost completely inhibited at 10 degrees C. The second experiment was conducted in vitro using transformed carrot ( Daucus carota L.) roots supporting G. intraradices. Mycelium length and spore number were measured weekly for 15 weeks. Spore metabolic activity (iodonitrotetrazolium reduction), root length and percentage root colonization were measured after 15 weeks. G. intraradices sporulation was reduced at temperatures below 23 degrees C, while spore metabolic activity was significantly reduced only at 10 degrees C. Root length and in particular percentage colonization were decreased at suboptimal temperatures. A negative interaction between AM hyphal growth and root growth resulting in reduced probability of contact at suboptimal root zone temperatures is proposed to explain the greater reduction observed in root colonization than in root and hyphal growth.

An inducible activator produced by a Serratia proteamaculans strain and its soybean growth-promoting activity under greenhouse conditions

Serratia proteamaculans 1-102 (1-102) promotes soybean-bradyrhizobia nodulation and growth, but the mechanism is unknown. After adding isoflavonoid inducers to 1-102 culture, an active peak with a retention time of about 105 min in the HPLC fractionation was isolated using a bioassay based on the stimulation of soybean seed germination. The plant growth-promoting activity of this material was compared with 1-102 culture (cells) and supernatant under greenhouse conditions. The activator was applied to roots in 83, 830 and 8300 HPLC microvolts (microV) per seedling when plants were inoculated with bradyrhizobia or sprayed onto the leaves in same concentrations at 20 d after inoculation. The root-applied activator, especially at 1 ml of 830 microV per seedling, enhanced soybean nodulation and growth at the same level as 1-102 culture under both optimal and sub-optimal root zone temperatures. Thus, this activator stimulating soybean seed germination is also responsible for the plant growth-promoting activity of 1-102 culture. However, when sprayed onto the leaves, the activator did not increase growth and in higher concentrations decreased average single leaf area. The results suggest that this inducible activator might be a lipo-chitooligosaccharide (LCO) analogue. LCOs act as rhizobia-to-legume signals stimulating root nodule formation. The activator could provide additional 'signal', increasing in the signal quality (the signal-to-noise ratio, SNR) of the plant-rhizobia signal exchange process.

Sensing Iron—A Whole Plant Approach

Regulatory mechanisms leading to cellular Fe homeostasis were investigated inPlantago (Plantago lanceolata L.) plants grown hydroponically at different temperature regimes either in the presence or absence of iron. During the experimental period of 6d, growth was not affected by Fe availability, but was decreased by lowering the root zone temperature (RZT) from 24 to 12°C. Cultivating plants at low RZT decreased the reduction activity for ferric chelates in Fe-deficient plants. In the presence of iron, the temperature regime did not affect Fe accumulation by root cells, but decreased translocation of Fe to the shoot, and chlorosis of young leaves was observed at suboptimal RZT. Under these conditions root-mediated reduction of ferric chelates was increased. In cold-treated plants this effect was specific to Fe and could not be evoked by Mn2+and Zn+2additions. Supplementing the medium with the ferrous scavenger ferrozine caused a further enhancement in reduction rates, probably due to mobilization of apoplastic Fe. These results can be explained plausibly if different sites of Fe sensing are postulated and if it is assumed that both the absence and presence of iron could be a signal increasing root reduction activity.

Root and Rhizosphere Colonization of Soybean [Glycine max (L.) Merr.] by Plant‐Growth‐Promoting Rhizobacteria at Low Root Zone Temperatures and under Short‐Season Conditions

Co‐inoculation of plant‐growth‐promoting rhizobacteria (PGPR) with B. japonicum has been shown to increase soybean [Glycine max (L.) Merr.] nodulation, nitrogen fixation, growth and physiological activity at suboptimal root zone temperatures (RZTs). We studied the survival and growth of seven PGPR inoculated on soybean in a sterile rooting medium at three RZTs (25, 17.5 and 15 °C) on a growth bench. The survival of the two most promising strains (Serratia liquefaciens 2‐68 and S. proteamaculans1‐102) was studied under field conditions in methyl bromide fumigated and non‐fumigated soils. In general, population densities varied with temperature. PGPR strains generally colonized the rhizosphere and root surface effeciently at higher RZTs; however, S. proteamaculans 1‐102 colonized best at a low RZT (15 °C). The population of PGPR applied to the rhizosphere either with or without addition of B. japonicum increased over time in fumigated soil as compared to non‐fumigated soil, indicating that the PGPR survive and proliferate better under fumigated conditions. S. liquefaciens 2‐68 had higher population densities both on the root and in the rhizosphere, demonstrating, their ability to colonize under short‐season conditions. The possible interactions between the two cultivars (Maple Glen and AC Bravor) and the PGPR were generally not significant, despite observations that growth and yield of AC Bravor respond more strongly to PGPR inoculation.

Combined effects of plant growth‐promoting rhizobacteria and genistein on nitrogen fixation in soybean at suboptimal root zone temperatures

Application of plant growth‐promoting rhizobacteria (PGPR) or the plant to bacteria signal molecule genistein has been shown to increase nodulation and nitrogen (N) fixation by soybean [Glycine max (L.) Merr.] over a range of root zone temperatures (RZTs) and, specifically, off‐sets at least some of the ill‐effects of low RZTs. Two sets of controlled‐environment experiments, one on a growth bench and the other in a greenhouse, were conducted to examine the combined ability of both PGPR and genistein to reduce the negative effects of low RZT on soybean nodulation and N fixation. Each of two the PGPR strains, Serratia proteamaculans 1–102 and Serratia liquefaciens 2–68 were co‐inoculated with Bradyrhizobium japonicum USDA 110 preincubated with 17.5 (somewhat inhibitory), and 15°C (very inhibitory). At RZTs of 25 and 17.5°C PGPR strains and genistein in combination increased the number of nodules and the amount of Nn fixed. The most stimulatory effect was observed at 17.5°C for the combination: S. proteamaculans 1–102 plus B. japonicum USDA 110 pre‐incubated in 15 μM genistein under greenhouse conditions. For most treatment combinations the stimulatory effects of PGPR and genistein were additive at RZTs of 17.5 and 25°C. Surprisingly, the combination of these two factors resulted in antagonism at the very inhibitory RZT of 15°C. The results suggest that the negative effects of certain low RZTs could be more effectively off‐set by combined treatments of PGPR plus geneistin pre‐incubation of rhizobial cultures than by their individual treatment.

Root Zone Temperature and Calcium Effects on Phosphorus, Sulfur, and Micronutrients in Winter Wheat Forage

AbstractAn understanding of how environmental factors can alter accumulation of nutrients by plants is needed to anticipate and prevent mineral deficiencies in both plants and grazing animals. To determine the effects of root zone temperature (RZT) and Ca level on mineral concentrations of winter wheat (Triticum aestivum L.), ‘Centurk’ seedlings were grown under three RZT regimes (constant 8°C, constant 16°C, and transferral from 8°C after 23 d to 16°C) and four Ca levels (0.2, 0.6, 2.0, and 5.0 mM) in nutrient solution. Plants grown at 8°C RZT had significantly lower shoot and root concentrations and unit absorption rates of P, S, Cu, Zn, and Mn than did those grown at 16°C. Within 2 wk after transfer from 8°C to 16°C RZT, concentrations of P, S, Cu, Zn, and Mn in shoots and roots increased significantly. Increasing Ca levels in solution significantly increased shoot and root concentrations and unit absorption rates of P and Cu. In contrast, increasing Ca levels significantly decreased concentrations and unit absorption rates of Mn and Zn. Thus, a suboptimal RZT could depress accumulation of P, S, Cu, Zn, and Mn by winter wheat forage, and possibly induce mineral deficiencies in both plants and grazing animals. A high solution Ca level could partially ameliorate this adverse effect on P and Cu accumulation by wheat, but it also could exacerbate the problem of low Mn and Zn concentrations at a cool RZT.

Plant Growth-promoting Rhizobacteria and Soybean [Glycine max(L.) Merr.] Growth and Physiology at Suboptimal Root Zone Temperatures

Application of plant growth-promoting rhizobacteria (PGPR) has been shown to increase legume growth and development under optimal temperature conditions, and specifically to increase nodulation and nitrogen fixation of soybean [ Glycine max (L.) Merr.] over a range of root zone temperatures (RZTs). Nine rhizobacteria applied into soybean rooting media were tested for their ability to reduce the negative effects of low RZT on soybean growth and development by improving the physiological status of the plant. Three RZTs were tested: 25, 17.5, and 15 °C. At each temperature some PGPR strains increased plant growth and development, but the stimulatory strains varied with temperature. The strains that were most stimulatory at each temperatures were as follows: 15 °C— Serratia proteamaculans 1–102; 17.5 °C— Aeromonas hydrophila P73, and 25 °C— Serratia liquefaciens 2–68. Because enhancement of plant physiological activities were detected before the onset of nitrogen fixation, these stimulatory effects can be attributed to direct stimulation of the plant by the PGPR rather than stimulation of plant growth via improvement of the nitrogen fixation symbiosis.

Application of genistein to inocula and soil to overcome low spring soil temperature inhibition of soybean nodulation and nitrogen fixation

In the soybean (Glycine max. (L.) Merr)– Bradyrhizobium japonicum symbiosis, suboptimal root zone temperatures (RZTs) slow nodule development by disruption of the interorganismal signal exchange between the host plant and bradyrhizobia. Two field experiments were conducted on two adjacent sites in 1994 to determine whether the incubation of B. japonicum with genistein prior to application as an inoculant, or genistein, without B. japonicum, applied onto seeds in the furrow at the time of planting, increased soybean nodulation, N fixation, and total N yield. The results of these experiments indicated that genistein application increased nodule number and nodule dry matter per plant and hastened the onset of N fixation during the early portion of the soybean growing season, when the soils were still cool. Because these variables were improved, total fixed. N, fixed N as a percentage of total plant N, and N yield increased due to genistein application. The interaction between genistein application and soybean cultivars indicated that genistein application was more effective on N-stressed plants.

Effects of suboptimal root zone temperatures and shoot demand on net translocation of micronutrients from the roots to the shoot of maize

The effects of suboptimal root zone temperatures (RZTs) on net translocation rates from the roots to the shoots and the concentrations of Fe, Mn, Zn, and Cu were examined in maize grown in nutrient solution or soil. Plants were grown at 12 °C, 18 °C and 24 °C RZT. At each RZT, the growth-related shoot demand for nutrients was varied by independently modifying the temperature of the shoot base (SBT) including the apical shoot meristem. The net translocation rates of Mn and Zn from the roots to the shoots were reduced at low RZTs, irrespective of the SBT and of the substrate (soil or nutrient solution). Obviously, the net translocation rates of Mn and Zn at low RZT were mainly regulated by temperature effects on the roots and not by the chemical nutrient availability in the rhizosphere or by shoot growth rate as controlled by SBTs. When both RZT and SBT were reduced, the decrease in net translocation rates of Mn and Zn was similar to the decline in the shoot growth rate and concentrations of Mn and Zn in the shoot fresh matter were not greatly affected or were even increased by low RZT. However, at high SBT and low RZT in nutrient solution, the depressed net translocation rates of Mn and Zn combined with the increased shoot growth resulted in significantly decreased concentrations of Mn and Zn in the shoot, indicating that Mn and Zn may become deficient even at high chemical availability. By contrast to Mn and Zn, the net translocation rates of Fe and Cu at all RZTs were markedly enhanced by increased SBTs. Accordingly, the concentrations of Fe and Cu in the shoot fresh matter were not greatly affected by RZTs, irrespective of the SBTs. These results indicate that the ability of roots to supply Fe and Cu to the shoot was internally regulated by the growth related shoot demand per unit of roots.