A micropropagation protocol forCinnamomum camphora

Summary A micropropagation protocol was developed forCinnamomum camphora (L.) Sieb., using as initial explants 3–5-mm shoot tips from newly emerged laterals of 2-yr-old trees. Performance of small shoot tips was compared with that of 2.0-cm nodal segments during subculture. Murashige and Skoog medium (MS) supplemented with different concentrations of N₆-benzyladenine (BA) or thidiazuron (TDZ) was used to examine shoot proliferation. In separate experiments, MS was supplemented with 1-naphthaleneacetic acid (NAA) for rooting of shoots, and the commercial preparation EM₂ for prevention of hyperhydricity. BA stimulated shoot formation and callus development, whereas TDZ promoted only callus development. Both cytokinins induced hyperhydricity when small shoot tips were used, with severity being directly related to concentrations. Hyperhydricity was avoided in subcultures by using larger nodal segments. EM₂ did not alter degree of hyperhydricity but suppressed callus development and strongly promoted shoot multiplication. The number of new shoots after a 6-wk subculture was 9 per nodal segment when supplemented solely with 4.4 μM BA and 18 per segment when further supplemented with 1000 mg EM₂ per I. Rooting of shoots occurred best when supplemented solely with 0.54 μM NAA, averaging 7 roots per shoot in 4 wk. Ninety percent of rooted shoots survived transfer to the greenhouse.     

The effect of bacterial contamination on the growth and gas evolution ofIn vitro cultured apricot shoots

Summary Shoots of “San Castrese” and “Portici” apricots (Prunus armeniaca L.) free of cultivable bacteria, shoots of the same origin exhibiting bacterial contamination after repeated subcultures, and contaminated shoots treated with cefotaxime were compared for gas exchange, proliferation rate, and fresh and dry weight. Cultures of San Castrese contaminated byBacillus circulans andSphingomonas paucimobilis, and of Portici contaminated withStaphylococcus hominis andMicrococcus kristinae, including those treated with cefotaxime, showed comparable shoot weights and lower proliferation rates than healthy cultures. Bacteria, even if not visible until the end of subculture, markedly influenced the gaseous composition of the jar headspace. Healthy cultures clearly showed photosynthetic activity at 60 μM·m⁻²·s⁻¹ photosynthetically active radiation; in contrast, oxygen quickly decreased and carbon dioxide increased in contaminated cultures, including those treated with cefotaxime, in which bacteria became visible in the culture medium only after repeated subcultures.     

In vitro plant regeneration of Ilex paraguariensis (aquifoliaceae)

Summary Nodal segments as well as shoot tips and apical meristems of 2-yr-old “maté” plants (Ilex paraguariensis St. Hil.) were cultured in vitro to establish micropropagation systems. Maximum shoot regeneration was achieved when nodal segments were cultured with 1/4 Murashige and Skoog (MS) medium with 3% sucrose. We induced roots to differentiate by transferring the regenerated shoots onto the same medium, solidified with 2.5 g Phytagel per 1 and supplemented with indole-3-butyric acid (7.4 μM) and finally transferring shoots to 1/4 MS medium with 3% sucrose and lacking growth regulators. Plants were successfully established in soil.     

Plant regeneration from Rosa shoot tips cryopreserved by a combined droplet vitrification method

Shoot tips obtained from in vitro Rosa plants (three cultivars) were successfully cryopreserved by a combined droplet vitrification method and subsequently shoots regenerated. The excised shoot tips (1–4 mm long) were incubated in a liquid MS medium supplemented with 2.5 mg l⁻¹ thiamine, 0.2 mg l⁻¹ biotin, 0.2 mg l⁻¹ pyridoxine, 0.25 mg l⁻¹ 6-benzylaminopurine (BAP), 0.5 mg l⁻¹ gibberellic acid (GA₃) and 0.08 M sucrose, for 24 h. Following that incubation shoot tips were pre-cultured in this MS medium containing 0.1 till 1.0 M sucrose for 24 and 48 h, respectively. Pre-cultured shoot tips were dehydrated with concentrated PVS2 cryoprotective solution for 10–30 min at room temperature, prior to a direct plunge in liquid nitrogen. After rapid rewarming in the above mentioned liquid medium shoot tips were plated on a modified MS medium (5 g l⁻¹ agar) supplemented with vitamins and plant growth regulators as mentioned above for regrowth. Cryopreserved shoot tips resumed growth within 10 days and regenerated shoots within 3 weeks. The highest numbers of regrowing shoot tips were 64.44% for cv. Kardinal, 67.73% for cv. Fairy and 57.57% for cv. Maidy.     

In vitro Storage of Strawberry and Raspberry in Calcium-Alginate Beads at 4°C

Three-millimeter-long shoot tips of strawberry 'Senga Sengana' and raspberry 'Norna' encapsulated in calcium alginate were stored in vitro at 4 °C in the dark. The cultures which were donors for the shoot tips were grown before encapsulation on shoot multiplication media (Boxus medium with 2.2 µM BAP and 2.46 µM IBA for strawberry, and MS medium with NH₄NO₃ and KNO₃ reduced by 50%, and with 3.55 µM BAP and 0.49 µM IBA for raspberry) as well as on these media supplemented with 10 g l^–1 mannitol or paclobutrazol (1.7 µM for strawberry and 3.4 µM for raspberry). Sodium alginate was dissolved in water, water with sugar or in a culture medium without growth regulators. Regrowth ability of the stored explants and in vitro multiplication in three successive subcultures were evaluated. The encapsulated shoot tips could be stored for 9 months in beads containing sugar or a culture medium. The pre-conditioning of the donor cultures on a mannitol containing medium was beneficial for regrowth ability. The multiplication rate of strawberry and raspberry shoots in the first subculture after storage was lower than that of non-stored cultures. Particularly low multiplication was obtained for strawberry which had been stored for 9 months and for raspberry stored for 3 and 6 months, in combinations where the beads were prepared by dissolving sodium alginate in water. Multiplication of strawberry in the second subculture was generally higher than in non-stored cultures, but multiplication of raspberry was lower also in the second subculture, with the exception of the combination stored for 9 months and pre-cultured on mannitol. In the third subculture, shoot multiplication in both species was similar to that in non-stored cultures.     

Morphogenesis and tissue cultures of three citrus species

Studies were performed to define tissue culture techniques and culture conditions for morphogenesis, callus culture and plantlet culture of sweet orange (Citrus sinensis (L.) Osb.), citron (C. medica L.) and lime (C. aurantifolia) (Christm. Swing). The optimal concentrations of NAA to induce root formation on stem segments were 10 mg l^-1 for sweet orange and lime, and 3 mg l^-1 for citron. The optimal BA concentration for shoot and bud proliferation was 3 mg l^-1 for sweet orange and citron, and 1 mg l^-1 for lime. Callus initiation was accomplished in a culture medium containing 10 mg l^-1 NAA and 0.25 mg l^-1 BA. Callus was maintained by periodical subculture into the same medium supplemented with 10% (v:v) organge juice. In vitro plantlets of the three species were obtained by rooting of shoots developed from bud cultures, and of citron and lime by development of shoots from root cultures. The plants were successfully established on soil.     

A simple chromosomal marker can reliably distinguishes <Emphasis Type="Italic">Poncirus</Emphasis> from <Emphasis Type="Italic">Citrus</Emphasis> species

Several chromosome types have been recognized in Citrus and related genera by chromomycin A₃ (CMA) banding patterns and fluorescent in situ hybridization (FISH). They can be used to characterize cultivars and species or as markers in hybridization and backcrossing experiments. In the present work, characterization of six cultivars of P. trifoliata (“Barnes”, “Fawcett”, “Flying Dragon”, “Pomeroy”, “Rubidoux”, “USDA”) and one P. trifoliata × C. limonia hybrid was performed by sequential analyses of CMA banding and FISH using 5S and 45S rDNA as probes. All six cultivars showed a similar CMA⁺ banding pattern with the karyotype formula 4B + 8D + 6F. The capital letters indicate chromosomal types: B, a chromosome with one telomeric and one proximal band; D, with only one telomeric band; F, without bands. In situ hybridization labeling was also similar among cultivars. Three chromosome pairs displayed a closely linked set of 5S and 45S rDNA sites, two of them co-located with the proximal band of the B type chromosomes (B/5S-45S) and the third one co-located with the terminal band of a D pair (D/5S-45S). The B/5S-45S chromosome has never been found in any citrus accessions investigated so far. Therefore, this B chromosome can be used as a marker to recognize the intergeneric Poncirus × Citrus hybrids. The intergeneric hybrid analyzed here displayed the karyotype formula 4B + 8D + 6F, with two chromosome types B/5S-45S and two D/5S-45S. The karyotype formula and the presence of two B/5S-45S chromosomes clearly indicate that the plant investigated is a symmetric hybrid. It also demonstrates the suitability of karyotype analyses to differentiate zygotic embryos or somatic cell fusions involving trifoliate orange germplasm. During the submission of this paper, we analyzed 25 other citrus cultivars with the same methodology and we found that the chromosome marker reported here can indeed distinguish Poncirus trifoliata from grapefruits, pummelos, and one variegated access of Citrus, besides the previously reported access of limes, limons, citrons, and sweet-oranges. However, among 14 mandarin cultivars, two of them displayed a single B/5S-45S chromosome, whereas in Citrus hystrix D.C., a far related species belonging to the Papeda subgenus, this chromosome type was found in homozygosis. Since these two mandarin cultivars are probably of hybrid origin, we assume that for almost all commercial cultivars and species of the subgenus Citrus this B type chromosome is a useful genetic marker.     

<Emphasis Type="Italic">In vitro</Emphasis> propagation and cryostorage of <Emphasis Type="Italic">Syzygium francissi (Myrtaceae)</Emphasis> by the encapsulation-dehydration method

Summary An efficient procedure for the in vitro propagation and cryogenic conservation of Syzygium francissi was developed. The maximum number of shoots per explant was obtained on a Murashige and Skoog (MS) medium supplemented with 4.5 μM benzyladenine and 0.5 μM indole-3-butyric acid (IBA). The in vitro-propagated shoots produced roots when transferred to MS medium containing IBA, indold-3-acetic acid, or naphthaleneacetic acid at various concentrations. Rooted microshoots were transferred to a coco-peat, perlite, and vermiculite (1∶1∶1) mixture, and hardened off under greenhouse conditions. Ninety-five percent of rooted shoots successfully acclimatized in the greenhouse. Shoot tips excised from in vitro-grown plants were successfully cryostoraged at −196°C by the encapsulation-dehydration method. A preculture of formed beads on MS medium containing 0.75 M sucrose for 1 d, followed by 6 h dehydration (20% moisture content) led to the highest survival rate after cryostorage for 1h. This method is a promising technique for in vitro propagation and cryopreservation of shoot tips from in vitro-grown plantlets of S. francissi germplasm.     

Probable identity of substances in citrus that repress asexual embryogenesis

Summary The embryogenetic responseof culturedDaucus carota L. ‘Queen Anne's Lace’ callus was employed to attempt fractionation and identification of a repressive factor produced byCitrus medica L. ovules. The factor was evidently synthesized and released into the medium continuously, inasmuch as citron ovules that had been autoclaved with the medium were completely infeffective. The inhibition could be attributed to volatile and nonvolatile components. A substantial part of the inhibition was prevented by continuously refereshing the atmosphere within the cultures with filtered air. Monitoring of the gases produced by citron ovule sections under conditions simulating bioassays disclosed significant evolution of carbon dioxide, ethylene and ethanol. Repression of embryogenesis was not averted by trapping the liberated ethylene. On the other hand, ethanol in concentrations equivalent to those released by citron ovules suppressed asexual embryogenesis dramatically. The adverse effect of ethanol was reversed immeditaley upon transfer to ethanol-free medium. Another investigation had disclosed anti-embryogenetic effects of auxin, abscisic acid and gibberellin. Analysis ofCitrus ovules excised from young fuits disclosed those of monoembryonic citron to contain concentrations of IAA, ABA and GA₃ several times higher than those of polyembryonic Ponkan mandrain. The nonvolatile protion might be identified with these hormonal substances. This paper is part of B. Tisserat's PhD. dissertation in Botany at the University of California, Riverside. The research was supported in part by the Elvenia J. Slosson Fellowship in Ornamental Horticulture awarded to T. Murashige.     

Efficient plant regeneration from embryogenic suspension cultures of sweetpotato

Summary Using 15 Chinese and Japanese cultivars of sweetpotato, Ipomoea batatas (L.) Lam., we succeeded in developing an efficient plant regeneration system from embryogenic suspension cultures. The embryogenic callus derived from shoot apices of the 15 cultivars was used to initiate embryogenic suspension cultures in Murashige and Skoog (MS) medium containing 9.05 μM 2,4-dichlorophenoxyacetic acid (2,4-D). Rapidly proliferating and well-dispersed embryogenic suspension cultures were established. Cell aggregates 0.7–1.1 mm in size from embryogenic suspension cultures were transferred to solid MS medium supplemented with 9.05 μM of 2,4-D and formed embryogenic callus with somatic embryos. The embryogenic callus with somatic embryos was further transferred to MS medium supplemented with 3.78 μM of abscisic acid, resulting in the germination of somatic embryos. Within 20 wk after the initiation, the frequencies of cell aggregates forming plantlets reached approximately 100% for the 15 tested cultivars. These plantlets, when transferred to soil, showed 100% survival. No morphological variations were observed.     

Assessment of Gene Flow from Genetically Modified Anthracnose-Resistant Chili Pepper (Capsicum annuum L.) to a Conventional Crop

We conducted a 2-year field assessment of the gene flow from genetically modified (GM) chili pepper (Capsicum annuum L.), containing the PepEST (pepper esterase) gene, to a non-GM control line “WT512” and two commercial hybrid cultivars, “Manidda” and “Cheongpung Myeongwol (CM).” After seeds were collected from the pollen-recipient non-GM plants, hybrids between them and the GM peppers were screened by a hygromycin assay. PCR with the targeting hpt gene was performed to confirm the presence of transgenes in hygromycin-resistant seedlings. Out of 7,071 “WT512” seeds and 6,854 “Manidda” seeds collected in 2006, eight and 12 hybrids, respectively, were detected. In 2007, 33 hybrids from 3,456 “WT512” seeds and 50 hybrids from 3,457 “CM” seeds were found. The highest frequency of gene flow, 6.19%, was observed in that 2007 trial. These results suggest that a limited isolation distance would be sufficient to prevent gene flow from GM to conventionally bred chili peppers.     

Transformation of sweetgum via microprojectile bombardment of nodule cultures

Summary A sweetgum (Liquidambar styraciflua) nodule culture system was developed and integrated with genetic transformation by microprojectile bombardment. Nodule cultures were established from seedling hypocotyls and proliferated in liquid medium containing 0.1 mg (0.45 μM) thidiazuron (TDZ) per 1 and 0.01 mg (0.045 μM) 2,4-dichlorophenoxyacetic acid (2,4-D) per 1. Shoots differentiated from the nodules in liquid media containing (per 1) 1 mg (4.4 μM) benzyladenine (BA), 0.5 mg (2.2 μM) BA, and 0.01 mg (0.054 μM) naphthaleneacetic acid (NAA), or 0.5 mg BA, 0.01 mg NAA, and 0.05 mg (0.23 μM) TDZ under the light. Differentiating shoots required 4 wk of dark treatment for further development on semisolid medium containing 1 mg BA per 1. Elongated shoots were harvested and the basal ends were soaked in a solution containing 10 mg (49.2 μM) indole-3-butyric acid (IBA) per 1 before being planted in potting mix for ex vitro rooting. Roots formed and leaves expanded in 2 wk. Sweetgum nodules were stably transformed by microprojectile bombardment with a 7.4-kb plasmid, pTRA 140, harboring CaMV 35S-HPH and CaMV 35S-GUS. Evidence that nodules growing in the presence of hygromycin B were stably transformed was provided by polymerase chain reaction analysis and β-glucuronidase activity. Sweetgum shoots differentiated in liquid medium in the presence of hygromycin B. Shoots transferred to solid medium lacking hygromycin B elongated and displayed β-glucuronidase activity in their expanding leaves and stems. Southern analysis confirmed the presence of the GUS gene in nodules and shoots. Transgenic shoots initiated roots and showed leaf expansion 2 wk after being planted in potting mix.     

Phloroglucinol enhances growth and rate of axillary shoot proliferation in potato shoot tip cultures in vitro

Efficacy of phloroglucinol in promoting growth and development of in vitro-derived shoot tips was studied in six potato (Solanum tuberosum L.) genotypes. Different concentrations of phloroglucinol (0, 0.08, 0.4, 0.8, 1.2 and 1.6 mM) were tested in combination with either 0.1 or 0.2 M sucrose in shoot tip proliferation medium based on MS (Murashige and Skoog, 1962) medium supplemented with 5.8 μM GA₃ (gibberellic acid), 1.1 μM BA (N⁶-benzyladenine) and 8.39 μM D-calcium pantothenate. Phloroglucinol fostered multiple shoot formation, promoted axillary shoot proliferation in terms of shoot tip fresh weight and shoot length, and stimulated root formation on the shoot tips. There was significant phloroglucinol × sucrose interaction for number of shoots developed per shoot tip, shoot tip fresh weight and number of roots induced per shoot tip. The beneficial effect of phloroglucinol on shoot tip survival was conspicuous only in genotypes that showed poor survival in the control proliferation medium. There were significant differences in response between the two sucrose levels with regard to shoot tip fresh weight and number of roots per shoot tip. Phloroglucinol in combination with 0.2 M sucrose induced maximum number of roots per shoot tip. Optimum shoot tip growth was fostered in medium containing 0.8 mM phloroglucinol and 0.2 M sucrose. High frequency multiple shoot formation in this medium ensures a faster rate of potato shoot tip multiplication within a limited time and space. This revised version was published online in June 2006 with corrections to the Cover Date.     

Plant regeneration from aged-callus of the woody ornamental species Lonicera japonica cv. “Hall's Prolific”

Summary Callus cultures were initiated from in vitro grown leaf, stem and root segments of Lonicera japonica “Hall's Prolific”, on a medium containing 10.7 μM α-naphthtylacetic acid and 2.7 μM benzyladenine, while media with 2,4-dichlorophenoxyacetic acid led to a rapid necrosis of explants. Shoot regeneration from true-callus (i.e. without any part of the original explant) was achieved for the three different source tissues within 12 weeks. The highest rate of regeneration was obtained by using benzyladenine (4.4 to 44.4 μM) as the sole hormone in the medium. The regenerated shoots were readily elongated and rooted on the same medium as used for multiplication, and plantlets were subsequently transferred to greenhouse conditions, where nearly 100% of them were successfully acclimatized. This is the first example of plant regeneration from aged (≥6 month-old) true-callus of a woody ornamental species.     

Use of acephate, benomyl and alginate encapsulation for eliminating culture mites and fungal contamination from in vitro cultures of hardy hibiscus (Hibiscus moscheutos L.)

Summary Shoot cultures of three Hibiscus moscheutos (L.) cultivars were infested with micro-arthropods (mites). Nodal segments (1 cm long) were excised from these cultures and encapsulated in a sodium alginate gelled Driver and Kuniyuki Walnut DKW medium containing 10, 50, or 100 mg l⁻¹ acephate (insecticide) or 10 mg l⁻¹ acephate plus 0, 50, or 100 mg l⁻¹ benomyl (fungicide), then placed in refrigerated (5°C) darkness for 4 wk. Acephate was tested alone if visible fungus was not touching the shoot masses and benomyl was tested if fungus was in contact with the proliferating shoots. Cold-stored encapsulated nodes were then placed on DKW medium with 0.1 μM thidiazuron for 6 wk for subsequent shoot development. The presence of acephate in the encapsulation medium completely eradicated or killed the mites, with 38–69% of cultures fungus-free; 12% of the fungal-contaminated nodes encapsulated with 100 mg l⁻¹ benomyl were fungus-free.     

High-frequency generation of transgenic tobacco plants after modified leaf disk cocultivation withAgrobacterium tumefaciens

A modified protocol for theAgrobacterium tumefaciens-mediated transformation of tobacco (Nicotina tabacum L.) leaf disks was developed for greater recovery of transgenic plants. Modifications include transformation ofAgrobacterium by a freeze-thaw procedure, initial cocultivation of leaf disks andAgrobacterium under vacuum, subsequent growth with nurse cells for one week, rooting of shoots in medium lacking carbenicillin, longer, growth in rooting medium, and a shortened “hardening” step. By this procedure, an average of 1.3 kanamycin-resistant calli were obtained per leaf disk, and 38% of, the callus cultures used were regenerated to produce 133 independently transformed tobacco plants.     

Liquid culture for efficient micropropagation of Wasabia Japonica (MIQ.) matsumura

Summary An efficient protocol for in vitro propagation of the valuable medicinal plant, Wasabia japonica (Miq.) Matsumura is described through shoot tip proliferation and direct regeneration. Multiple shoots were induced from shoort tips cultured on Murashige and Skoog (MS) semi-solid medium containing various concentrations (0.5–50 μM) of N⁶-benzyladenine (BA), thidiazuron, kinetin, and zeatin. A comparison was made on shoot multiplication between semi-solid and liquid culture media. Well-developed shoots were obtained using full-strength MS semi-solid medium containing 5.0 μM BA. However, the greatest shoot proliferation was achieved on either full- or half-strength MS liquid media supplemented with 5.0 μM BA for 4 wk (15.3±0.9 and 15.0±0.7 shoots per explant, respectively), and on half-strength MS liquid medium for 6 wk (25.8±1.3 shoots per explant) in culture. In contrast, the maximum number of shoots per explant on full-strength MS semi-solid medium was achieved with either 5.0 μM BA (10.4±0.6 shoots per explant) or 10.0 μM kinetin (10.9±0.8 shoots per explant). Fresh weight of explants and length of shoots derived from full-strength MS liquid medium (1055±77 mg and 34.2±1.0 mm, respectively) were significantly higher than those derived from full-strength MS semisolid medium (437.6±17.3 mg and 15.4±0.7 mm, respectively). Quarter-strength MS liquid medium had no significant difference in shoot proliferation when compared to quarter-strength MS semi-solid medium. Elongated shoots were separated and rooted on half-strength MS semi-solid media fortified with 1-naphthaleneacetic acid (NAA), indole-3-butyric acid (IBA), or indole-3-acetic acid (IAA) ranging from 0.1 to 10.0 μM. Root formation was greatest with IBA when compared with IAA and NAA. One hundred percent of shoots were rooted on half-strength MS medium with 5.0 μM IBA, while vigorous roots were obtained with 10.0 μM IBA. Micropropagated plantlets were successfully established in soil with 95% survival rate after heardening.     

Cryopreservation of hairy root cultures of <Emphasis Type="Italic">Maesa lanceolata</Emphasis> and <Emphasis Type="Italic">Medicago truncatula</Emphasis>

To study the production of secondary metabolites of Maesa lanceolata and Medicago truncatula, hairy root cultures of both plant species were established. Because maintenance of large numbers of cultures is laborious and costly, we developed a cryopreservation protocol and stored different isolated lines over time. Using encapsulation-dehydration, high survival rates were observed for both Maesa and Medicago hairy roots. Root tips were isolated and encapsulated in calcium-alginate beads, containing 0.1 M sucrose. The encapsulated hairy roots were precultured for 3 days using basal medium containing high sucrose concentrations. Medicago root tip growth during the preculturing time lead to unwanted outgrowth which could be tempered by addition of plant growth inhibitors. After preculturing, the beads were dehydrated in the air flow of a laminar flow until 35–40% of the initial bead weight was reached. Dehydrated beads were plunged into liquid nitrogen and after different storage times thawed in a water bath at 40°C. The survival rates were 90% for Maesa and 53% for Medicago, which are sufficient to allow implementation in large storage experimental set-ups.     

Hydraulic and Non-hydraulic Root-sourced Signals in Old and Modern Spring Wheat Cultivars in a Semiarid Area

Absract Non-hydraulic root-sourced signal (NRS) is so far affirmed to be a unique “early-warning” response to soil drying in plants, but little is known about the quantitative effect of this early-warning mechanism on crop production. To evaluate the link of NRS to a drought tolerance profile, a pot-culture study was carried out in a plant growth chamber with eight spring-wheat (Triticum aestivum L.) cultivars bred in semiarid China. The NRS was judged to begin when there was a significant lowering of stomatal conductance without any change in leaf relative water content (RWC), and the hydraulic root signal (HRS) was judged to begin when leaf RWC changed significantly. Soil water contents (SWC), at which the NRS and HRS were switched on, differed among the eight cultivars. For “Monkhead” and “Jinby,” representing “old” cultivars, the NRS and HRS were initiated successively at about 60% FWC (field water capacity) and 45% FWC, respectively. Conversely, “Longchun8139-2” and “Plateau 602” (recent cultivars) showed the NRS and HRS occurring between 70% FWC and 35% FWC, a much wider range. The events of the other four non-old cultivars were generally intermediate. This threshold range (TR) of soil FWCs between the onset of NRS and HRS also narrowed over the successive developmental stages from seedling to seed filling. Fewer survival days (SD), lower maintenance rate of grain yield (MRGY), and higher lethal leaf water potentials (LLWP) had been found in old cultivars. Widening TR was significantly correlated with increasing SD and MRGY (r = 0.8713** and 0.7318*, respectively), and with decreased LLWP (r = 0.8591**). This survey of different-decade cultivars suggests that advances in grain yield and drought tolerance would be made by targeted selection for a wider TR of root-sourced signals.