The effects of lead,nickel, and strontium nitrates on cell division and elongation in maize roots

The effects of Pb, Sr, and Ni nitrates on the root growth, its cell division and elongation were studied. Two-day-old maize seedlings were incubated on the 35 μM Ni(NO₃)₂, 10 μM Pb(NO₃)₂, or 3 mM Sr(NO₃)₂ in the presence or absence of 3 mM Ca(NO₃)₂. Metal toxicity was evaluated after the inhibition of root growth for the first and second days of incubation in comparison with the roots kept on water or Ca(NO₃)₂ solution. The contents of metals were determined in the apical (the first centimeter from the tip) and basal (the third centimeter from the kernel) root parts by voltamperometry and atomic-absorption spectrophotometry. We measured the length of the meristem, the length of the fully elongated cells, counted the mitotic index (MI) in the meristem and the number of meristematic cells in the cortex row; we also calculated duration the cell cycle. In the absence of Ca(NO₃)₂, the metal content in the apical root region was higher than in basal one. In the presence of Ca(NO₃)₂, we observed reverse ratio most pronounced in the case of Pb and Sr. All metals tested markedly reduced MI in the cortex, which was determined by the increase in the cell cycle duration and accompanied by the meristem shortening. These metals affected differently cell division and elongation: Ni inhibited mainly cell division and to a lesser degree their elongation, whereas Sr and Pb affected both cell division and elongation; only Sr treatment resulted in the increased length of the fully elongated cells. In the presence of Ca, all studied growth indices changed less than in the absence of Ca, which was manifested in the less severe suppression of the root growth and was in agreement with the lower accumulation of the metals in the root tips. Possible causes for the heavy metal action on growth are discussed in connection with the specificity of their transport and accumulation.     

Survival and growth of eastern white pine shoot apical meristemsin vitro

Shoot apical meristems of seedling and mature eastern white pine trees were excised and grownin vitro. Placing the meristems on filters instead of directly on agarose-solidified nutrient medium enhanced survival of both juvenile and mature meristems. Applying forcing treatments to mature branches improved survival and growth of dissected meristems compared with meristems from non-forced branches in experiments conducted over two years. No consistent differences were observed among 2-, 4-, and 6-week forcing treatments. Including 5.37 nM (0.001 mg l^-1) l-naphthaleneacetic acid in the culture medium did not affect meristem survival or growth. Some meristems from seedlings grew rapidly, produced primary leaves, underwent internode elongation, and in three cases, produced adventitious roots. Meristems from mature trees did not grow as rapidly as seedling meristems. The leaves produced by mature meristems appeared to be scale leaves and a few of these had brachyblast primordia in the axils. The shoots derived from mature meristems did not produce adventitious roots.     

The involvement of wheat and common bean lectins in the control of cell division in the root apical meristems of various plant species

The effects of wheat germ agglutinin (WGA) and phytohemagglutinin (PHA) at the concentration of 1 mg/l on the rate of cell division in the root apical meristem of wheat (Triticum aestivum L.), barley (Hordeum vulgare L.), rice (Oryza sativa L.), and common bean (Phaseolus vulgaris L.) seedlings were compared. WGA enhanced cell division in the roots of barley and rice approximately similarly as in wheat roots but did not affect division of meristematic cells in the roots of common bean seedlings. In contrast PGA enhanced mitotic activity in the root apical meristem of common bean seedlings but did not affect division in the wheat and barley roots. Seedling treatment with lectins shifted the hormonal balance in them toward accumulation of growth activators (IAA and cytokinins). The relationship between lectin and hormonal systems in the control of cell division is discussed.     

Reactive oxygen species localization in roots of <Emphasis Type="Italic">Arabidopsis thaliana</Emphasis> seedlings grown under phosphate deficiency

Arabidopsis plants responding to phosphorus (P) deficiency increase lateral root formation and reduce primary root elongation. In addition the number and length of root hairs increases in response to P deficiency. Here we studied the patterns of radical oxygen species (ROS) in the roots of Arabidopsis seedlings cultured on media supplemented with high or low P concentration. We found that P availability affected ROS distribution in the apical part of roots. If plants were grown on high P medium, ROS were located in the root elongation zone and quiescent centre. At low P ROS were absent in the elongation zone, however, their synthesis was detected in the primary root meristem. The proximal part of roots was characterized by ROS production in the lateral root primordia and in elongation zones of young lateral roots irrespective of P concentration in the medium. On the other hand, plants grown at high or low P differed in the pattern of ROS distribution in older lateral roots. At high P, the elongation zone was the primary site of ROS production. At low P, ROS were not detected in the elongation zone. However, they were present in the proximal part of the lateral root meristem. These results suggest that P deficiency affects ROS distribution in distal parts of Arabidopsis roots. Under P-sufficiency ROS maximum was observed in the elongation zone, under low P, ROS were not synthesized in this segment of the root, however, they were detected in the apical root meristem.     

ALKALINE EARTH ELEMENTS AND ZOOSPORE RELEASE AND DEVELOPMENT IN PROTOSIPHON BOTRYOIDES

O'Kelley , Joseph C., and Walter R. Herndon . (U. Alabama, University.) Alkaline earth elements and zoospore release and development in Protosiphon botryoides . Amer. Jour. Bot. 48(9): 796–802. Illus. 1961.—Cells of Protosiphon botryoides Klebs from depleted nutrient medium containing Ca were washed and resuspended in fresh complete medium with Ca; or in media with a Sr, Ba or Na replacement, respectively, for Ca; or in an equivalent CaCl₂ solution or deionized water. Zoospore release was observed in these media upon illumination following a 12-hr dark period. Free zoospores were less abundant in Sr-, Ba- and Na-replacement media than in the Ca medium. Zoospore production and release also were depressed in solutions of only CaCl₂ and in deionized water. In the Sr and Ba media, zoospores were formed but not released from the parent cell, as a rule; some zoospores were released in mass within a gelatinous vesicle which did not liquefy and set the zoospores free; these zoospores lost motility and continued development in Sr, producing characteristic, spheroidal clusters of aplanospores. In the Na medium, protoplasmic cleavage preceding zoospore formation was severely inhibited. A study of the reversibility of Sr inhibition of the zoospore-release mechanism revealed evidence of reversion 12 hr after replacement of Sr by Ca. Walls of cells produced in Ca are rich in ruthenium red-positive materials, whereas cells produced under conditions of Sr replacement lack these materials. The significance of these findings in relation to the Ca requirement of other algal species is discussed.     

Effects of Cytokinins on the Growth of Primary Roots of Zea mays

Intact primary roots of Zea mays seedlings, apical 6-mm segmentsisolated from the intact primary roots and 5-mm detipped segments,prepared from the 6-mm apical segments by removal of the apical1-mm meristematic region, were incubated in potassium-phosphatebuffer that contained various concentrations of kinetin, 6-benzylaminopurine(BAP) or zeatin. These cytokinins inhibited the growth of intactprimary roots but they promoted the growth of both tipped anddetipped apical segments. In other words, they promoted thegrowth of root segments irrespective of the presence or absenceof apical meristematic regions. Detipped segments were stoodvertically, with their apical or basal cutends in contact withan agar plate that contained the abovementioned buffer and variousconcentrations of kinetin, BAP or zeatin so that cytokininswere supplied either from the apical or basal cut-ends. Cytokininssupplied from the top promoted the growth of the segments, whilethose supplied from the base did not. These results indicate that the response of roots to the exogenouslyapplied cytokinins is not influenced by the presence of theroot meristem but is significantly affected by the way in whichcytokinins are supplied. (Received October 17, 1995; Accepted August 28, 1995)     

Studies on induced changes in growth patterns and polarity in roots of Vicia faba L.

Summary Roots of Vicia faba were treated with colchicine (0.025%), or IAA (4.7×10^-6 M), or both, for 3 hours and fixed at various intervals over the following 11 days. The axis of spindle orientation and the distribution of mitotic figures, lateral root primordia and xylem vessel elements was examined in the apical 10 mm of median longitudinal sections of these roots.No effect of IAA was found on the orientation of the spindle. However, evidence was obtained indicating that the systems controlling the polarity of cell division and cell expansion differ in some way.The number of lateral root primordia formed was greater in roots treated with IAA or colchicine than in control roots. These primordia were always initiated adjacent to a xylem vessel. Thus, no primordium was closer to the apex than the most apical xylem vessel, suggesting that an endogenous factor involved in primordia initiation is transported in the xylem. The primordia which develop after colchicine treatment grow out as lateral roots; this is in contrast with those which form after IAA treatment and which do not undergo elongation. These results, which it must be emphasized apply only to the apical 1 cm of treated roots, indicate that lateral root primordia become sensitive to IAA at a certain stage in their development. Exogenous IAA acts as an inhibitor.The new meristem, which forms in the primary root apex after colchicine treatment, contains both diploid and polyploid cells, i.e. it was formed from cells that were unaffected and from cells that were affected by colchicine. Following colchicine treatment the size of the meristem shrinks and this can be prevented by treatment with IAA. This and other evidence presented here, suggests that IAA is a factor involved in the control of the size of the apical meristem in normal roots.     

Effects of heavy metals and strontium on division of root cap cells and meristem structural organization

In order to define relations between the behavior of quiescent center cells and the condition of root cap cells, effects of various metal salts on the root meristem structure, root growth, and division of root cap cells were investigated. Two-day-old maize (Zea mays L., cv. Diamant) seedlings were incubated on solutions containing 35 μM Ni(NO₃)₂), 10 μM Pb(NO₃)₂, or 3 mM Sr(NO₃)₂ in the absence or in the presence of 3 mM Ca(NO₃)₂. Toxic effects of metals were assessed from inhibition of the primary root length increment following 24-h and 48-h incubations as compared to the roots grown on water or on 3 mM Ca(NO₃)₂ solution. Metal localization in the root apex tissues following 24-h and 48-h incubations was determined using histochemical techniques. Cell lengths in three upper layers of root cap columella were determined, and the mitotic index in these cells was calculated. In the absence of Ca(NO₃)₂, the metals were found both in the meristem and in the root cap. Pb and Sr were revealed primarily in the cell walls, and Ni, in the cell protoplasts. In the presence of Ca(NO₃)₂, metal content in all root tissues was decreased, and their toxic effect on root growth was ameliorated. Pb and Ni inhibited cell division in the root cap. Pb caused an increase in the root cap cell length as early as following 24-h incubation, and Ni, only following 48-h incubation. Pb activated division of quiescent center cells in the direction of root cap. These effects, as well as possible involvement of dermatogen and cortex cells, resulted in a regrowth of a new root cap already after a 24-h incubation period. In this case, the meristem was transformed from a closed structure into the open one. Following 48-h incubation, Ni brought about only few divisions of quiescent center cells in the direction of root cap. It was suggested that inhibition of divisions of the root cap upper layer cells and a decrease in the sloughing off its cells can stimulate the quiescent center cell divisions. A similarity of the quiescent center and animal stem cells is discussed.     

Effect of sub-optimal root zone temperatures at varied nutrient supply and shoot meristem temperature on growth and nutrient concentrations in maize seedlings (Zea mays L.)

Maize seedlings were grown for 10 to 20 days in either nutrient solution or in soils with or without fertilizer supply. Air temperature was kept uniform for all treatments, while root zone temperature (RZT) was varied between 12 and 24°C. In some treatments the basal part of the shoot (with apical shoot meristem and zone of leaf elongation) was lifted up to separate the indirect effects of root zone temperature on shoot growth from the direct effects of temperature on the shoot meristem.Shoot and root growth were decreased by low RZT to a similar extent irrespective of the growth medium (i.e. nutrient solution, fertilized or unfertilized soil). In all culture media Ca concentration was similar or even higher in plants grown at 12 as compared to 24°. At lower RZT concentrations of N, P and K in the shoot dry matter decreased in unfertilized soil, whereas in nutrient solution and fertilized soil only the K concentration decreased.When direct temperature effects on the shoot meristem were reduced by lifting the basal part of the shoot above the temperature-controlled root zone, shoot growth at low RZT was significantly increased in nutrient solution and fertilized soil, but not in unfertilized soil. In fertilized soil and nutrient solution at low RZT the uptake of K increased to a similar extent as plant growth, and thus shoot K concentration was not reduced by increasing shoot growth rates. In contrast, uptake of N and P was not increased, resulting in significantly decreased shoot concentrations.It is concluded that shoot growth at suboptimal RZT was limited both by a direct temperature effect on shoot activity and by a reduced nutrient supply through the roots. Nutrient concentrations in the shoot tissue at low RZT were not only influenced by availability in the substrate and dilution by growth, but also by the internal demand for growth.     

Étude de la discrimination strontium-calcium chez le Haricot en fonction du rapport et des teneurs respectives en Sr et Ca de la solution nutritive

The discrimination between strontium and calcium is not only conditioned by the Sr/Ca ratio of the nutrient solution but also by the total dose (Ca + Sr) of the medium. The variation of the observed ratio in relation to the total dose is closely related with the Sr/Ca ratio of the solution. Sr is preferentially fixed by the roots when the Sr/Ca ratio of the solution is low; such a result is observed in the shoots for higher ratios. For all the treatments, the OR of the leaves is lower than unity (discrimination against strontium). When the stable strontium concentration of the solution is not detectable, an increase in the calcium concentration reduces the radiostrontium retention by the roots.     

Element localization in ultrathin cryosections of high-pressure frozen ectomycorrhizal spruce roots

We present, for the first time, elemental mapping of ultra-thin cryosections from high-pressure frozen ectomycorrhizal roots of Picea abies–Hebeloma crustuliniforme. The maps provide interpretable information on the relationship between elements and the structure of inhomogeneous objects. Cryoultramicrotomy together with energy dispersive X-ray microanalysis (EDX) offers the potential to study the subcellular localization of specific ions and ecologically important tracers (Cs and Sr) in ectomycorrhizal roots under conditions resembling the natural slate as closely as possible. Structural changes of the ectomycorrhizal roots, in particular the absence of a Hartig net at high NH₄⁺ levels in the nutrient solution, were accompanied by elemental modification of Ca in cortical cell walls, where markedly higher concentrations of Ca were found. Cs and Sr applied to the nutrient solution were localized in root and fungal cells of the Hartig net. Cs accumulated mainly in the vacuoles of the Hartig net hyphae and its distribution was very similar to the distribution of K. In contrast to Cs, Sr was found to occur mainly in electron-opaque and P-rich granules. From this study, (here is no indication that Ca is the only ion accompanying P in the P-rich granules. Several elements including Ca, K, Cl, S, Cs and Sr, with highest concentrations for S, can occur together with P in these granules. The occurrence of the P-rich electron-opaque deposits in fungal cells might be the first evidence of polyphosphate granules in the native state, since our specimen preparation technique did not include chemical fixation.     

Improving of oilseed rape lateral root formation by physiological analogues of auxin

The aim of the present work was to study the effect of auxin physiological analogue TA-12 [1-(2-chloroethoksicarbonylmethyl)-4-naphthalenesulfonic acid calcium salt] on the formation of oilseed rape lateral root and on the mitotic activity of apical meristem cells. Spring oilseed rape (Brassica napus L. ssp. oleifera annua Metzg.) cultivar ‘Mascot’ was chosen as a test object. Anatomical, cytological and histological studies on root development suggest that compound TA-12 induces the activity of parent root pericycle cells, stimulates the formation of lateral roots and enhances the division of apical meristem cells. The auxin transport inhibitor 2,3,5-triiodobenzoic acid suppresses the division of apical meristem cells, while this process is restored by the auxin physiological analogue TA-12 and naphthaleneacetic acid. The compound TA-12, by stimulating primary root growth and lateral root induction, optimised the formation of the oilseed rape root system.     

Factors affecting the establishment and maintenance of ‘Titan’ red raspberry root organ cultures

Root organ cultures of red raspberry (Rubus ideaus cv. Titan) were established from two cm long terminal segments of adventitious roots induced in vitro on micropropagated shoot cultures. Growth of root organ cultures was almost entirely by initiation and elongation of lateral roots rather than by terminal elongation from the apical meristem of the original root explants. Indolebutyric acid (IBA) was required for lateral root initiation and elongation. The optimal IBA concentration for lateral root initiation decreased from 0.5 to 0.1 mg/L from the first to the fifth passage in culture. Two cm root explants initiated more lateral roots per unit length than four cm explants. Liquid Anderson's medium was superior to other basal nutrient and vitamin formulations tested. Root growth in liquid culture was stimulated by aeration. Root organ cultures also grew on media solidified with gelrite or agar. A rapid transfer technique was developed for subculturing these which involved cutting and transferring 1 cm discs of roots and underlying medium. Adventitious bud formation occurred spontaneously, but sporadically in liquid cultures, and was not influenced by cytokinin, auxin (spat) light, or chilling.     

Regeneration of roots from callus reveals stability of the developmental program for determinate root growth in Sonoran Desert Cactaceae

In some Sonoran Desert Cactaceae the primary root has a determinate root growth: the cells of the root apical meristem undergo only a few cell division cycles and then differentiate. The determinate growth of primary roots in Cactaceae was found in plants cultivated under various growth conditions, and could not be reverted by any treatment tested. The mechanisms involved in root meristem maintenance and determinate root growth in plants remain poorly understood. In this study, we have shown that roots regenerated from the callus of two Cactaceae species, Stenocereus gummosus and Ferocactus peninsulae, have a determinate growth pattern, similar to that of the primary root. To demonstrate this, a protocol for root regeneration from callus was established. The determinate growth pattern of roots regenerated from callus suggests that the program of root development is very stable in these species. These findings will permit future analysis of the role of certain Cactaceae genes in the determinate pattern of root growth via the regeneration of transgenic roots from transformed calli.     

Effect of abscisic acid on the cell cycle in the growing maize root

The mechanism by which the rate of cell proliferation is regulated in different regions of the root apical meristem is unknown. The cell populations comprising the root cap and meristem cycle at different rates, proliferation being particularly slow in the quiescent centre. In an attempt to detect the control points in the cell cycle of the root apical meristem of Zea mays L. (cv. LG 11), quiescent-centre cells were stimulated to synthesise DNA and to enter mitosis either by decapping or by immersing intact roots in an aqueous 3,3-dimethyl-glutaric acid buffer solution. From microdensitometric and flow-cytometric data, we conclude that, upon immersion, the G₂ phase of the cell cycle of intact roots was shortened. However, when 50 M abscisic acid (ABA) was added to the immersion buffer, parameters of the cell cycle were restored to those characteristic of intact roots held in a moist atmosphere. On the other hand, decapping of primary roots preferentially shortened the G₁ phase of the cell cycle in the quiescent centre. When supplied to decapped roots, ABA reversed this effect. Therefore, in our model, applied ABA retarded the completion of the cell cycle and acted upon the exit from either the G₁ or the G₂ phase. Immersion of roots in buffer alone seems to trigger cells to more rapid cycling and may do so by depleting the root of some ABA-like factor.Abbreviations ABA cis-abscisic acid - DGA 3,3-dimethyl-glutaric acid - DAPI 4,6-diamidino-2-phenylindole - LI labelling index We thank Pierre Zaech of the Ludwig Institute, Epalinges, Switzerland, for expert assistance in flow cytometry and Dr. Jean-Marcel Ribaut of our Institute for providing data on exodiffusion and metabolism of ABA.     

MITOTIC ACTIVITY IN THE ROOT APICAL MERISTEM OF AZOLLA FILICULOIDES LAM., WITH SPECIAL REFERENCE TO THE APICAL CELL

The meristematic activity of the apical cell and its derivatives (merophytes) in the unbranched, determinate roots of Azolla filiculoides Lam. was investigated. The plane of division of the apical cell indicates that it is the initial of each merophyte. The division plane of each newly formed merophyte is strictly periclinal to the root surface and provides confirmation that the immediate derivatives of the apical cell cannot be the ultimate root initials. The frequency of cell division as determined by the mitotic index, and by the duration of the cell cycle as determined by the colchicine method, confirmed the meristematic activity of the apical cell. As roots increase in length, the duration of the cell cycle in the total meristem increases, with the apical cell possessing the longest cell cycle, whereas the immediate derivatives maintain approximately the same cycle duration as in shorter roots. In determinate Azolla roots, cell division appears to play a major role up to a certain root length, then increase in length is produced mainly by cell elongation.     

HORMONAL REQUIREMENTS OF EXCISED DIANTHUS CARYOPHYLLUS L. SHOOT APICAL MERISTEM IN VITRO

Whereas a medium containing kinetin alone enabled a few Dianthus caryophyllus L. apical meristem dome explants to develop into rooted plants, the highest frequency of plants was obtained in one containing supplements of both IAA and kinetin. In an unsupplemented medium, continued development required that explants have 2 pairs of primordial and a pair of expanding leaves. Kinetin alone caused production of many new leaves, but the development was significantly less than when it was furnished in combination with IAA. IAA given alone caused meristem explants to develop primarily callus, roots, and a few leaves. Gibberellin and abscisic acid were without promotive effects on leaf and shoot formation. A balance of hormonal substances, synthesized in young leaf structures and relocated to the meristem, is proposed as the fundamental mechanism that regulates new leaf initiation in the shoot apex.     

The symplasmic coupling of L2-cells diminishes in early floral development of Iris

The transition from vegetative to reproductive development involves extensive revisions of cellular collaboration at the apical meristem and results in the production of novel appendages. In Dutch Iris (Iris xiphium) the transition from vegetative apical meristem to inflorescence meristem was morphologically signalled by the appearance of a `spathe leaf '. After enlargement of the inflorescence meristem, a second spathe leaf and a double floral meristem were formed. The then undulated surface corresponded to general topological changes and a beginning of altered cell division patterns. Throughout, all cells produced in the meristem remained in contact via plasmodesmata (Pd), thus maintaining the symplasmic unity of the meristem. Since the symplasm harbours part of the signal network that coordinates the activities of the meristem cells, we investigated if alterations in Pd numbers could underlie meristem transitions. Prior to Pd counting, potentially important borders inside the meristem, representing primary and secondary cell contacts, were identified by the construction of a symplasmetric map. During the transition, significant alterations did take place at some of the borders defined by the map. Within the second tunica layer (L₂), and between the L₂ and adjacent cells, Pd numbers were strongly reduced. Within the first tunica layer (L₁) and within the corpus they remained the same. The reduction was 25% between L₂-cells, and between L₂- and L₁-cells; it was 40% between the L₂ and the outer corpus layer. The reductions appeared to be due to a lowered production of primary and secondary Pd by L₂-cells towards each other and towards adjacent cells. As a result of this the integration of all L₂-cells and, as a consequence, of the L₁ as a whole in the symplasmic network of the meristem was reduced. The implied gain in autonomy of the individual L₂-cells and of the L₁-layer may reflect their new functions in the floral meristem. Received: 29 October 1996 / Accepted: 20 March 1997     

Comparative developmental anatomy of seedlings in nine species of podostemaceae (subfamily Podostemoideae)

The developmental anatomy is described for seedlings of nine Asian and Australian species of Podostemaceae, subfamily Podostemoideae. The hypocotyl is rudimentary (except in Zeylanidium olivaceum) and does not form a primary root in any of the species examined. An adventitious root forms endogenously in the hypocotyl of six species with ribbon-like or flattened subcylindrical roots, and in Z olivaceum with foliose roots. In contrast, it forms exogenously in Hydrobryum griffithii and Synstylis micranthera with foliose roots. The juvenile root becomes flattened and dorsiventral, branches exogenously (in Polypleurum stylosum, P. wallichii and Z. lichenoides) and produces shoots endogenously (in P. stylosum, P. wallichii, S. micranthera and Z. lichenoides). The root meristem is simple, composed of surface and uniform inner cells, and is devoid of root cap initials in all species. The reduced meristem morphology of seedling roots may be primitive in the Asian-Australian Podostemoideae. A root cap or protective tissue did not form during the culture period, even in the seven species with capped adult roots, probably due to its delayed development. It was absent throughout ontogeny in the other two species. No obvious shoot apical meristem forms between the cotyledons. One to several leaves occupy the shoot apical area in species with endogenous adventitious roots, while no leaves are formed in species with exogenous roots. These differences suggest recurrent origins of foliose roots in the Asian clade. Similarities between the unique seedling morphology and mutant Arabidopsis phenotypes are discussed.     

Root growth, developmental changes in the apex, and hydraulic conductivity for Opuntia ficus-indica during drought

Developmental changes in the root apex and accompanying changes in lateral root growth and root hydraulic conductivity were examined for Opuntia ficus-indica (L.) Miller during rapid drying, as occurs for roots near the soil surface, and more gradual drying, as occurs in deeper soil layers. During 7 d of rapid drying (in containers with a 3-cm depth of vermiculite), the rate of root growth decreased sharply and most root apices died; such a determinate pattern of root growth was not due to meristem exhaustion but rather to meristem mortality after 3 d of drying. The length of the meristem, the duration of the cell division cycle, and the length of the elongation zone were unchanged during rapid drying. During 14 d of gradual drying (in containers with a 6-cm depth of vermiculite), root mortality was relatively low; the length of the elongation zone decreased by 70%, the number of meristematic cells decreased 30%, and the duration of the cell cycle increased by 36%. Root hydraulic conductivity ( L _P) decreased to one half during both drying treatments; L _P was restored by 2 d of rewetting owing to the emergence of lateral roots following rapid drying and to renewed apical elongation following gradual drying. Thus, in response to drought, the apical meristems of roots of O. ficus-indica near the surface die, whereas deeper in the substrate cell division and elongation in root apices continue. Water uptake in response to rainfall in the field can be enhanced by lateral root proliferation near the soil surface and additionally by resumption of apical growth for deeper roots.