Katsuya Yano

Phosphorus acquisition from non-labile sources in peanut and pigeonpea with mycorrhizal interaction

Abstract Both peanuts and pigeonpeas are believed to utilize non-labile phosphate (P) due to the dissolving abilities of their root exudates, in which the mycorrhizal interaction is rarely considered. We examined a hypothesis that such effective P acquisition would not appear without the mycorrhizal function. First, mycorrhizal responses of peanuts and pigeonpeas were compared with soybeans or a non-mycorrhizal species of yellow lupin in a P-limited soil amended with a mixture of three non-labile P sources (Fe-P, Al-P, and Ca-phytate). Inoculation with Gigaspora margarita greatly increased P uptake in pigeonpeas (10-fold) and peanuts (6-fold), more than in soybeans (3-fold), indicating greater mycorrhizal dependency in both these plants. Second, the mycorrhizal responses of peanuts and pigeonpeas were compared in a soil amended with three P sources individually. P uptake responses were significant, and similar for all the P sources in pigeonpeas. In peanuts, the response was significant for both Al-P and Fe-P, but not for Ca-phytate because of greater P uptake in the non-inoculated plants, implying a direct utilization of Ca-phytate by peanuts. Third, interactive effects of the pigeonpeas’ rhizosphere and extraradical hyphae were investigated in a compartment container, where root exudate movement as well as hyphal penetration into the compartment soil, amended with Al-P, was controlled. A significant increase in P uptake was found only when both the exudates and hyphae were permitted access into the compartment soil, suggesting the importance of the interactive effect. We concluded that the mycorrhiza can accelerate P acquisition by the plants from non-labile sources in soil through interaction between the roots and hyphae. It is a novel finding that peanuts can utilize Ca-phytate without mycorrhiza, but the mechanism is still unclear.

Localized alteration in lateral root development in roots colonized by an arbuscular mycorrhizal fungus

Abstract The morphological responses of root systems to localized colonization by endophytes is not well understood. We examined the responses of lateral roots to the arbuscular mycorrhizal (AM) fungus Gigaspora margarita Becker & Hall inoculated locally into the soil. Peanut (Arachis hypogaea L.) and pigeon pea (Cajanus cajan (L.) Millsp.) were examined. Root boxes filled with nutrient-poor soil in were inoculated in one half with the fungus and in the other half with a sterilized inoculum. Responses were apparent after 30 days but not after 20 days. Overall, lateral root development was more advanced in inoculated soil. This was clearly observed for 2nd- and 3rd-order lateral roots, but less clear for 1st-order lateral roots in both species, although percentage of colonized root length was higher in 1st-order lateral roots. Whilst in peanut the responses were clearly evident at the level of lateral roots initiated on more proximal parts of the tap root axis, they occurred on more distal parts in pigeon pea. We conclude that plants under nutrient-poor conditions give priority to mycorrhizal roots when partitioning assimilation products within the root system. Thus, AM formation may induce local morphological alteration of root systems.

Stomatal density of cowpea correlates with carbon isotope discrimination in different phosphorus, water and CO2 environments

* Stomatal formation is affected by a plants external environment, with long-distance signaling from mature to young leaves seemingly involved. However, it is still unclear what is responsible for this signal. To address this question, the relationship between carbon isotope discrimination (Delta) and stomatal density was examined in cowpea (Vigna sinensis). * Plants were grown under various environments that combined different amounts of soil phosphorus (P), soil water, and atmospheric CO(2). At harvest, stomatal density was measured in the youngest fully expanded leaf. The (13)C : (12)C ratio was measured in a young leaf to determine the Delta in mature leaves. * Results indicated that stomatal density is affected by P as well as by amounts of water and CO(2). However, stomatal responses to water and CO(2) were complex because of strong interactions with P. This suggests that the responses are relative, depending on some internal factor being affected by each external variable. Despite such complicated responses, a linear correlation was found between stomatal density and Delta across all environments examined. * It is proposed that the Delta value is a good surrogate for the long-term mean of the intercellular (C(i)) to the atmospheric (C(a)) CO(2) concentration ratio (C(i) : C(a)) and may be useful in understanding stomatal formation beyond complicated interactions.

Applying hydraulic lift in an agroecosystem: forage plants with shoots removed supply water to neighboring vegetable crops

When a plant encounters spatially heterogeneous soil moisture within its root system, usually drier surface and moister subsurface soils, water can move between these layers through the root system, a plant process known as hydraulic lift or redistribution. The water thus transferred is available not only for the plant itself but also for its neighbors. We examined application of this process as a possible biological irrigation tool. As ‘donors’, we used perennial forage plants with their shoots removed to minimize the effect of light-interception by them on the ‘receiver’ plants growing alongside them. In a horizontally split-root experiment, where an upper container was filled with sand and a lower one with water, superior donor species could maintain the upper sand in a fully hydrated condition for several weeks, increasing stomatal conductance in the receivers. The effects were also confirmed in a water-limited agricultural field, as significant differences were found in canopy temperature and yield in neighboring crop plants in the presence or absence of donor root systems. These results suggest that deep-rooting associate plants with their shoots removed function as an irrigation tool and improve crop production in water-scarce environments.

Arbuscular mycorrhizal formation in undisturbed soil counteracts compacted soil stress for pigeon pea

Abstract Plant growth is sometimes restricted with soil compaction under no-tillage conditions, although undisturbed soils are favorable to arbuscular mycorrhizal (AM) fungi of a symbiont. We examined growth responses of the pigeon pea ( Cajanus cajan (L.) Millsp.) to soil disturbance and inoculation with an AM fungus ( Gigaspora margarita Becker & Hall) in a pot experiment. The AM fungal inoculum was added to the soil and wheat was grown. After 6 months the shoot of wheat was removed and the soil was either disturbed or remained undisturbed. Subsequently, pigeon pea was grown and harvested after 3 months. The colonization and spore density of Gigaspora were significantly greater in undisturbed soil than in disturbed soil. Undisturbed soil showed higher penetrometer resistance and resulted in impaired shoot growth of the pigeon pea with lower shoot-to-root ( S / R ) ratio than disturbed soil. However, inoculation with the AM fungus reduced the stress impact of undisturbed soil on the pigeon pea without affecting the soil resistance and S / R ratio. A possible reason for reducing the stress impact was increase in specific root length, rather than P inflow with the AM formation. It is a novel finding that AM formation in undisturbed soil could promote root elongation despite the fact that soil was seriously compacted.

Mycorrhizal phosphorus enhancement of plants in undisturbed soil differs from phosphorus uptake stimulation by arbuscular mycorrhizae over non-mycorrhizal controls

Previous greenhouse and field studies have shown arbuscular mycorrhizal (AM) plants usually have greater P uptake and growth when raised in undisturbed soil compared to soil disturbed between plantings, such as by tillage. We report here for the first time that AM fungi able to stimulate shoot P uptake in experimental comparisons to non-mycorrhizal plants differ in their ability to bring about similar responses in undisturbed soil compared to disturbed soil. This outcome indicates a difference in functional character between the two stimulation processes. Three isolates of AM fungi were tested for growth promotion of maize (Zea mays L.) in pots in a soil disturbance experiment that included non-mycorrhizal controls. All three fungi colonized roots well and promoted shoot P uptake compared to non-inoculated controls, but only Glomus mosseae was able to stimulate growth in undisturbed soil compared to disturbed soil. This effect was seen when Glomus mosseae was alone or in combination with Gigaspora margarita. However, the presence of Glomus aggregatum in combination with Glomus mosseae prevented any stimulation, presumably due to domination by Glomus aggregatum. The ability of AM fungi to be beneficial to plants in comparison to non-mycorrhizal situations likely relates to the spread of mycelium in the soil and the capacity for nutrient transfer to the root. The ability of an AM fungus to promote growth in undisturbed soil appears to be related to these features and, in addition, a capacity for persistence and retention of functional capacity of the extraradical mycelium from one plant generation to the next.

Root Morphological Plasticity for Heterogeneous Phosphorus Supply in Zea mays L.

Abstract The morphological plasticity of roots in nutrient-enriched patches of soil is regarded as an adaptive response in plants, but its functional efficiency is still debatable. We examined whether the efficiency is dependent upon the patch size, or the amount of phosphate (P) supplied in maize (Zea mays L.). Two levels of P-input (high and low) and three patch sizes (large, medium and small) were used in various combinations in containers filled with soil. Irrespective of the P-inputs, P uptake and biomass were greatest in large patches together with root proliferation restricted to the soil inside patches, indicating that the effect of P-patch size was stronger than the amount of P supplied. Due to the fine root proliferation (about 0.5mm-width) of higher specific root length, the root length promoted was not accompanied by more biomass investment inside the patches. For the medium and small patches, such a localized root proliferation disappeared, resulting in impaired plant growth with limited P acquisition. It was concluded that the efficiency of the root plasticity on P acquisition depends on the size of P patches more strongly than the inputs.

Relationship between the Distribution of Na and the Damages Caused by Salinity in the Leaves of Rice Seedlings Grown under a Saline Condition

Abstract The distributions of Na and chlorophyll in the leaves of rice (Oryza sativa L.) seedlings grown under a saline condition were examined in relation to the anatomical changes caused by salinity. In salt-treated plants, the Na content was higher in older leaves and basal part of the leaf. In the 4th leaf of salt-treated plants, Na content was the highest in the middle part of the leaf sheath and decreased toward the tip of the leaf blade. The chlorophyll content in the 4th leaf was decreased by salt treatment at the tip and middle parts of the leaf blade, whereas it was unaffected in the leaf sheath and at the base of the leaf blade. Electron microscopic studies revealed that salt-treatment caused plasmolysis, vesiculation of cellular membranes and degradation of cytoplasm at the tip of the leaf blade, but scarcely caused such alterations at the base of the leaf blade. The present study suggests that the damage by salinity correlates more strongly with the age of the tissue than the Na content of the tissue.

A Simple Method for Quantitative Estimation of Rhizosphere pH along Root Axes through Visualization

Abstract A pH indicator agar gel is widely used in rhizosphere pH studies, but its use was mostly confined to visualization of pH changes. A few complex methods are available to measure pH in agar gels. We improved such methods to enable non-destructive quantification of pH dynamics along a root axis by using an image scanner and image analysis software. A thin agar gel containing Bromocresol Purple was used for 2-dimensional image analysis. A taproot of cowpea was embedded in the agar gel containing 1 mM nitrate, and incubated in the dark at 30°C. Every 2 hours, the agar gel was scanned to capture a full color image that reflected rhizosphere pH. In image analysis, optical properties of the pH indicator showed a linear (R2 = 0.99) relationship between pH and optical density in the pH range of 4.4 to 7.2. This analysis allowed us to map the pH gradient in the rhizosphere at a resolution of 0.2 pH. An apparent proton flux was calculated to integrate rhizosphere pH gradients, which quantifies alkalizing/acidifying abilities of the root at various portions. Using this protocol, we examined the effect of pre-cultural N-levels on alkalizing (proton influx) ability of root at various portions under subsequent uniformly nitrate-fed conditions. Results showed that the estimated cumulative proton production was about two times higher in high-N- than in low-N pre-cultured roots. The ratio of proton flux in high-N- to low-N pre-cultured roots was the greatest at the middle (2.94), followed by the basal (2.08) and the apical (1.23) portions of the root, suggesting that nitrate uptake is partitioned along the root axis.

Rhizosphere pH changes induced by exposure of shoot to light.

Abstract Rhizosphere pH is known to be strongly influenced by nitrogen sources and plant species. We have evaluated whether the exposure of the shoot to light affects the pH changes in the rhizosphere of legumes and cereals in the presence of different forms of nitrogen (ammonium, nitrate or both). The pH changes in the rhizosphere were quantified as apparent proton fluxes by image analysis of agar gel containing a pH indicator, in which the root was embedded. In the presence of ammonium or ammonium nitrate, the rhizosphere was alkalized under dark conditions and acidified under light conditions in both cowpea and sorghum. This implied that, in the presence of ammonium, acidification of the rhizosphere is induced by exposure of shoot to light. In the presence of nitrate, both plants alkalized their rhizospheres in the dark, whereas in the light cowpea acidified its rhizosphere although sorghum alkalized it. Light-induced acidification, in the presence of nitrate, was also found in chickpea and adzuki bean, but not in maize. We found, that a particular part of the root axis strongly acidifies the rhizosphere in response to the exposure of shoot to light, especially in legumes. We conclude that rhizosphere pH is strongly affected by the light conditions encountered by the shoot, and the pH changes in response to the light locally along the root axis.