Plants growing on soil with insufficient moisture need deep and dense roots to avoid water stress. In crop plants, the production of dry matter during ripening of grains is critically important for grain yield. We postulated that shoot growth would be suppressed but root growth would continue under an insufficient soil moisture condition before heading, while shoot growth would be more vigorous than root growth under a sufficient soil moisture condition. We anticipated that the plants growing under an insufficient soil moisture condition before heading would produce more dry matter and grain under an insufficient soil moisture condition during ripening. In order to examine our hypotheses and to determine the fundamental conditions for improving grain yield and efficient use of irrigated water under limited irrigation, we grew wheat plants (Triticum aestivum L., cv. Ayahikari) in pots (30 cm in diameter, 150 cm in height) with insufficient soil moisture (PD-D pots) or sufficient soil moisture (PW-D pots) for six weeks before heading followed by full irrigation, and then insufficient soil moisture condition during ripening. The growth of shoots was suppressed significantly but that of roots was not before heading in PD-D plants, with a higher resultant ratio of root to shoot than in PW-D plants. The former retained a high leaf water potential and, therefore, were able to produce more dry matter and grain during soil moisture depletion during ripening as compared with the latter plants. We also obtained similar results with field-grown plants.
The rate of water uptake by seeds is assumed to be an important factor affecting the susceptibility of seeds to flooding injury, but the traits which could contribute to restricting rapid imbibition by seeds remain undetermined in yellow-seeded soybeans. This study was conducted to determine the possible factors controlling the rate of imbibition in soybeans with yellow seed coats. The imbibition behavior of two yellow-seeded cultivars, Tamahomare and Tachinagaha, with different flooding susceptibilities was examined in relation to initial seed moisture content, the hydration location of the seed surface during imbibition, and the state of the seed coat. Low seed moisture increased the water uptake by Tamahomare intact seeds and even allowed substantially faster imbibition than occurred in its embryos, while in Tachinagaha seed moisture content had little influence on seed imbibition. This rapid imbibition by low-moisture Tamahomare seeds was not due to alterations in the permeability of the seed coat nor in water uptake by the embryo, but rather to increased movement of water along the abaxial sides of the cotyledons. The seed coat of Tamahomare loosened readily upon submergence whilst that of Tachinagaha rather tightly adhered to the embryo, suggesting that the seed coat of Tamahomare adheres very loosely to the embryo and may aid in conducting water laterally between the seed coat and embryo. The degree of adherence of the coat to the embryo and/or the ease with which the coat can be loosened upon hydration appear to play a crucial role in determining the rate of water uptake in yellow-seeded soybeans.
Seeds of five maize inbred lines, including 3681-4, were treated with gibberellin A3 (GA3) and uniconazole (UCZ) under deep seeding and shallow seeding in order to elucidate the physiological mechanism of maize mesocotyl elongation under different seeding depths. GA3 was the most effective at 10-5 M and the mesocotyl elongation of 3681-4, 178, Zong 3, Huang C, Han 21 was promoted at this concentration by 60.4%, 44.6%, 42.5%, 24.2% and 44.2% under 20 cm seeding depth, respectively. Under 2 cm seeding depth, however, there were no significant differences between mesocotyl length of all inbred lines at any concentration. UCZ treatment at concentrations higher than 10-5 M, significantly inhibited mesocotyl elongation in all inbred lines under seeding depths of both 20 cm and 2 cm. Interestingly, mesocotyl elongation of only 3681-4 was significantly inhibited at the concentration of 10-7 M. These results suggested that mesocotyl elongation was more sensitive to gibberellin A (GA) under deep seeding than under shallow seeding, and that 3681-4 was more sensitive to GA than the other inbred lines. Endogenous gibberellin A1 (GA1) contents in the mesocotyls of 3681-4 and 178 treated with GA3 and UCZ were accordant with their morphological responses. GA3 promoted cell elongation rather than cell division, and that UCZ inhibited both cell elongation and cell division in 3681-4 and 178.
Flooding during germination inhibits the sprouting and emergence of soybean [Glycine max (L.) Merr.], reducing subsequent growth and yield. Previous studies revealed that cultivars tolerant to pre-germination flooding have a mechanism of reducing water absorption speed (WAS) during the initial stage of inundation; however, seed structures involved in WAS have not been fully clarified to date. The objectives of this study were to identify possible seed structures responsible for cultivar difference in WAS and pre-germination flooding tolerance. WAS of two tolerant cultivars (Peking and Williams) and two susceptible cultivars (Nakasennari and Enrei), which were identified in our previous study, was compared in relation to seed structures. In Peking, WAS was markedly lower than that of the other cultivars, either in intact seed or seed with the seed coat removed, suggesting that both the seed coat and the embryo have a mechanism of reducing WAS in this cultivar. WAS of the hilum side tended to be higher than that of the back side, and sealing of the micropyle significantly lowered WAS, showing that the micropyle rather than the hilum appeared to be responsible for the higher WAS of the hilum side regardless of cultivar. A comparison of cross section area of the hilum revealed that the tolerant cultivars tended to have a larger area than the susceptible cultivars, suggesting that an inner space of the hilum can act as a reservoir at the initial stage of inundation, thereby reducing WAS in tolerant cultivars.
Plant roots release mucilage and root border cells (RBCs) into rhizosphere, which function as a complex at the root-soil interface. The dynamics of RBCs in rhizosphere soil, however, remains unknown. In this study, the ratio of crushed root cap cells during root penetration into soil and survival of the RBCs after the release from the root cap were estimated in maize seminal root. In addition, the effects of long term soil compaction on RBCs release were investigated. During the root penetration into rhizosphere soil, 78, 56, and 45% of sloughed root cap cells were estimated to be crushed at the first, second, and third day after planting, respectively. The number of surviving RBCs decreased with time, but 6% of the RBCs in the rhizosphere still retained their cell walls at one month after planting. These cells were estimated to remain in the soil for at least 10 d after the release from lateral roots. Furthermore, RBCs release from newly emerged nodal root increased with aging of plants, and the cell release was significantly increased by soil compaction only at the seedling stage. In conclusion, significant number of RBCs were crushed during root penetration into soil, however many RBCs remained in the rhizosphere soil for a relatively longer period. Soil compaction significantly increased cell release only at the seedling stage.
Thirty-nine chromosome segment substitution lines (CSSLs) population derived from a Koshihikari/Kasalath cross was used for quantitative trait locus (QTL) analysis of plant type in rice (Oryza sativa L.). Putative rough QTLs (26.2—60.3cM of Kasalath chromosomal segments) for culm length, plant height, panicle number, chlorophyll content of flag leaf blade at heading and specific leaf weight, were mapped on the several chromosomal segments based on the comparison of CSSLs with Koshihikari in the field experiment for 3 years. In order to verify and narrow QTLs detected in CSSLs, we conducted QTL analyses using F2 populations derived from a cross between Koshihikari and target CSSL holding a putative rough QTL. The qPN-2, QTL for panicle number was mapped on chromosome 2. In traits of flag leaf, the qCHL-4-1 and qCHL-4-2 for chlorophyll content was mapped on chromosome 4, and the qSLW-7 for specific leaf weight on chromosome 7. All QTLs were detected in narrow marker intervals, compared with rough QTLs in CSSLs. The qPN-2, qCHL-4-1 and qCHL-4-2 had only additive effect. On the other hand, the qSLW-7 showed over-dominance. It could be emphasized that QTL analysis in the present study with the combination of CSSLs and backcross progeny F2 population can not only verify the rough QTLs detected in CSSLs but also estimate allelic effects on the QTL.
Drought and waterlogging that occur sequentially under field conditions are important abiotic stresses affecting plant growth and development. The ability to maintain the root system development during the contrasting moisture stresses may be one of the key traits for plant adaptation. This study aimed to identify the key root traits that contributed to the above ability by comparatively examining the effects of the two moisture stresses in succession on root system development. The chromosome segment substitution lines (CSSLs) from the crosses between the japonica rice cultivar Nipponbare and indica rice cultivar Kasalath were used for precise comparison of root system development. The rice seedlings were grown by hydroponics under a continuously well-aerated condition for 14 days (non-stressed), a drought condition for 7 days followed by an oxygen (O2)-deficient (stagnant) condition for 7 days (drought-to-stagnant, D-S), or a stagnant condition for 7 days followed by drought condition for 7 days (stagnant-to-drought, S-D). CSSL43 and 47 did not show any significant differences in growth from Nipponbare under the non-stressed condition, but exhibited greater lateral root production under the stresses. Lateral root production was most closely related to faster seminal root elongation mediated by higher aerenchyma formation in the D-S condition, and to more branching of lateral roots on the seminal root axis in the S-D condition. The D-S condition severely affected lateral root production due to reduced seminal root elongation and aerenchyma formation. These results confirmed the fact that those root traits previously identified using different cultivars greatly contribute to plant adaptation. Oxygen deficiency preceded by drought (D-S) was more stressful to roots than drought preceded by O2 deficiency (S-D), because drought reduced root aerenchyma formation during the subsequent stagnant condition.
This study was conducted to clarify the effect of varietal differences in the endosperm structure on the high-degree polishing properties of Hattan varieties of rice suitable for brewing original Hiroshima sake. Four varieties were used: Hattan No.35, Hattan-nishiki No.1, Hattan-nishiki No.2, and Yamada-nishiki. Ellipsoidal-white-core grains occurred at a higher rate in Hattan-nishiki No.1 and No.2, whereas lined-white-core grains were observed at a higher rate in Hattan No.35 and Yamada-nishiki. Hattan No.35 and Yamada-nishiki showed low void polishing rates, and Hattan-nishiki No.1 and No.2 high void polishing rates after 50% polishing. Hattan-nishiki No.1 and No.2 showed inferior properties; that is, many broken and cracked grains, particularly cracked grains, after 50% polishing. The contents of broken grains and cracked grains in Hattan No.35 were higher than those in Yamada-nishiki and lower than those in Hattan-nishiki No.1 and No.2. In Hattan No.35 and Yamada-nishiki, fewer airspaces were observed between the amyloplasts at the center of the white-core as compared with Hattan-nishiki No.1 and No.2. This study showed that the differences in the endosperm structure of white-cores in the brewers’ rice grain are related to varietal differences in the tolerance to high-degree polishing.
We evaluated the effects of functional variation in three starch synthases in rice (Oryza sativa L.)-granule-bound starch synthase I (GBSSI, wx), starch synthase I (SSI, SSI), and starch synthase IIa (SSIIa, alk)-between indica cultivar Kasalath and japonica cultivar Nipponbare on starch properties and eating quality. We used three near-isogenic lines-NIL(Wxa), NIL(SSIk), and NIL(Alk)-containing chromosomal segments of Kasalath on a Nipponbare genetic background. The Wxa allele explained most of the difference in amylose content between the two cultivars, and decreased the peak viscosity and breakdown to less than half of those of Nipponbare. These changes reduced the quality of cooked rice both just after cooking and after storage at 5°C. The variation in SSIIa also affected the eating quality after storage of cooked rice at 5°C : NIL(Alk) became harder and less sticky than Nipponbare, although the rices were comparable just after cooking. Differential scanning calorimetry revealed faster retrogradation of the once-gelatinized starch in NIL(Alk). The variation in SSI alleles hardly affected these properties.
Root growth into deep soil is an important factor for stable production in wheat under drought conditions. Root penetrating capacity (RP) shown by pot experiments with a paraffin-Vaseline layer (PV layer) may be a useful indicator estimating deep rooting ability of wheat genotypes. Previously, we identified genotypes of durum wheat (Triticum turgidum L. var. durum) and bread wheat (T. aestivum L.) with diverse RP by the pot experiments. In this study, we investigated the root distribution of three Ethiopian landraces of durum wheat with high RP, three recent cultivars of durum wheat with low RP and one Japanese cultivar of bread wheat ‘Haruyutaka’ with low RP using: (1) pots with a PV layer, (2) root boxes, (3) artificial field and (4) a normal field to analyze the relationship between RP estimated by pot experiment and root development in the field. In the pot experiments, RP was evaluated by the number of roots penetrating through the PV layer (NRP). In the root-box and field experiments, the root distribution was evaluated by the number of roots on the vertical surface of soil as the root frequency (RF: root number cm-1 soil surface). Ethiopian landraces had a significantly larger NRP than recent cultivars in the pot experiment. The root box and field experiments showed that Ethiopian landraces tended to have a higher RF than recent cultivars in deep soil layer. We concluded that RP estimated by pot experiments with a PV layer is a useful indicator of deep rooting ability under field conditions.
The previous work revealed that the polarization of light reflected from heading wheat canopies allowed the detection of changes in the canopy structure, i.e., the leaf inclination angle. Accordingly, in order to improve measurement accuracy in this study we examined the effects of the solar zenith angle (=90°-solar elevation) and weather conditions at the time of polarization measurements for the light reflected from wheat canopies that were fertilized by different means. We measured polarization in the 660 nm spectral band from the heading canopies of wheat, which were grown in plots fertilized with a basal dressing and then top-dressed at the jointing and booting stages. The radiometric measurements were carried out at various solar zenith angles: 22°-41° on two proximal days, one overcast and the other clear. An empirical method for the adjustment of view zenith angle, based on the solar position at the time of measurement, was effective for the measurement of the degree of polarization (i.e., ratio of the polarized part of reflected light to the total reflected light energy) to eliminate interference due to the change in solar zenith angle. Although the mean values of polarization degree measured in overcast conditions were significantly lower than those measured under clear conditions, the plots top-dressed at the jointing stage could be detected via the polarized reflected light measured under both conditions of illumination.
Basal dressing is generally considered important in the conventional cultivation of green soybean (edamame). In this experiment, we investigated the influence of the sowing time and nitrogen topdressing at the flowering stage on its yield and pod character. Seeds were sown on April 10 (early) and April 20 (late); the total amount of nitrogenous fertilizer applied was maintained constant, and the ratio of nitrogen applied as basal dressing to that applied as topdressing was changed : 10 : 0, 7 : 3, 3 : 7, and 0 : 10. The plant top growth during the early growth stages and its dry weight during the flowering stage increased with the increase in the amount of nitrogen applied as basal dressing, but the length of the main stem, number of branches, and total number of nodes at the time of harvesting did not. The yield, number of pods, and proportion of high-quality 3-grained pods increased with the amount of nitrogen topdressing. The number of pods set and the green soybean yield tended to decrease with the delay in the sowing time. However, in both early and late sowing, the yield tended to be higher when large quantities of nitrogen topdressing were applied. These results suggest that nitrogen topdressing after the flowering stage is effective in improving the yield of green soybean.