Seed coat cell turgor responds rapidly to air humidity in chickpea and faba bean

The turgor pressure in cells of chickpea (Cicer arietinum L.) and faba bean (Vicia faba L.) seed coats was measured with a pressure probe. Measurements were made under in situ conditions by removing a section of wall from a pod, which remained attached to the plant, and exposing the intact seed. If the pod wall was removed and the turgor measurements made under ambient laboratory conditions of 50% to 70% relative humidity (RH), cell turgor pressure declined over time, typically reaching 0 MPa. If the pod wall was removed and the turgor measurements made under conditions of 100% RH, however, cell turgor pressure was stable over time, relatively uniform within the seed coat tissue, and was found to be 0.1-0.3 MPa for chickpea, and 0.1-0.2 MPa for faba bean. In both species there was a marked decline in cell turgor, beginning within about 60 s, when humidification was discontinued. The decline in cell turgor occurred regardless of the depth of the cell within the seed coat tissue, and this decline could be stopped, but not entirely reversed, when humidification was restored. An increase in cell turgor could also be caused by wetting of the seed. These responses indicate that a very rapid water exchange can occur within the seed coat tissue in situ. The rapid and, in some cases, relatively permanent loss of seed coat cell turgor in the absence of humidification raises serious concerns regarding desiccation artefacts which may be involved in the empty seed coat technique, often used to study seed carbon and water relations in grain legumes.     

Internal recycling of respiratory CO2 in pods of chickpea (Cicer arietinum L.): the role of pod wall, seed coat, and embryo

It has previously been proposed that respiratory CO2 released from the embryo in grain legume pods is refixed by a layer of cells on the inner pod wall. In chickpea this refixation process is thought to be of significance to the seed carbon budget, particularly under drought. In this study it is reported that the excised embryo, seed coat, and pod wall in chickpea are all photosynthetically competent, but the pod wall alone is capable of net O2 evolution over and above respiration. The predominant role of the pod wall in refixation is supported by measurements of fixation of isotopically labelled CO2, which show that more than 80% of CO2 is fixed by this tissue when provided to the pod interior. Chlorophyll concentrations are of the same order for embryo, seed coat, and pod wall tissues in younger pods on both an area and a fresh weight basis, but decline differentially with development from 12-30 d after podding. Imaging of chlorophyll distribution in the pod wall suggests that less than 15% of chloroplasts are located in the inner layer of cells thought to refix CO2 in legumes; this would be sufficient to refix less than 40% of respired CO2. It is concluded that while all tissues of the pod are capable of refixing respiratory carbon, the entire pod wall is responsible for the majority of this process, rather than a specialized layer of cells on the inner epidermis. The role of this fixed carbon in the pod for reallocation to the seed is discussed     

Soybean Seed Water Relations during in Situ and in Vitro Growth and Maturation

Water, osmotic, and pressure potentials of soybean (Glycine max [L.] Merrill) embryos and related maternal tissues were measured during periods of seed growth and maturation to test the involvement of embryo water relations in seed maturation. Seeds were matured in situ or in an in vitro liquid culture medium in detached pods or as isolated seeds. Changes in water relations of embryo tissues were independent of maternal tissues. During seed maturation in situ, water and osmotic potentials in both embryo and maternal tissues declined sharply near the time of maximum dry weight. During in vitro seed culture with and without pods, water and osmotic potentials in axis and cotyledon tissues declined continuously during growth. Water and osmotic potentials of the seed coat, which was present only during in vitro seed culture with pods, changed little during the culture period. Positive turgor in the embryo was maintained beyond maximum dry weight and the loss of green color during in vitro culture but declined to zero at maturity in situ. The osmotic potential in embryo tissues declined from −1.1 megapascals at early pod fill to between −1.65 and −2.2 megapascals at maximum seed dry weight across all maturation environments. It is suggested that the decreasing osmotic potential in the growing soybean embryo reaches a threshold level that is associated with cessation of growth and onset of seed maturation.     

Turgor-dependent unloading of assimilates from coats of developing legume seed. Assessment of the significance of the phenomenon in the whole plant

The in vivo significance of turgor-dependent unloading was evaluated by examining assimilate transport to and within intact developing seeds of Phaseolus vulgaris (cv. Redland Pioneer) and Vicia faba (cv. Coles Prolific). The osmotic potentials of the seed apoplast were low. As a result, the osmotic gradients to the seed coat symplast were relatively small (i.e. 0.1 to 0.3 MPa). Sap concentrations of sucrose and potassium in the seed apoplast and coat symplast accounted for some 45 to 60% of the osmotic potentials of these compartments. Estimated turnover times of potassium and sucrose in the seed apoplast of < 1 h were some 5 to 13 times faster than the respective turnover times in the coat symplast pools. The small osmotic gradient between the seed apoplast and coat symplast combined with the relatively rapid turnover of solutes in the apoplast pool, confers the potential for a small change in assimilate uptake by the cotyledons to be rapidly translated into an amplified shift in the cell turgor of the seed coat. Observed adjustments in the osmotic potentials of solutions infused between the coat and cotyledons of intact seed were consistent with the in vivo operation of turgor-dependent unloading of solutes from the coat. Homeostatic regulation of turgor-dependent unloading was indicated by the maintenance of apoplast osmotic potentials of intact seeds when assimilate balance was manipulated by partial defoliation or elevating pod temperature. In contrast, osmotic potentials of the coat symplast adjusted upward to new steady values over a 2 to 4 h period. The resultant downward shift in coat cell turgor could serve to integrate phloem import into the seed coat with the new rates of efflux to the seed apoplast. Circumstantial evidence for this linkage was suggested by the approximate coincidence of the turgor changes with those in stem levels of ³²P used to monitor phloem transport. The results obtained provide qualified support for the in vivo operation of a turgor homeostat mechanism. It is proposed that the homeostat functions to integrate assimilate demand by the cotyledons with efflux from and phloem import into the coats of developing legume seed.     

Metabolism of Phloem-borne Amino Acids in Maternal Tissues2Pisum sativum L.)

The amino acid composition of the EDTA-induced phloem exudatereaching the fruit and the seed, and of the solutes releasedby the seed coat during fruit development were determined inglasshouse-grown pea (Pisum sativum L. cv. Finale) suppliedeither with nitrate-free nutrients (nodulated plants) or withcomplete medium (non-nodulated plants). The EDTA-promoted exudationtechnique was used supposedly to collect phloem sap and theempty seed technique supposedly to collect the solutes secretedby the seed coat to the embryo sac cavity. In young seeds embryosac liquid was sampled directly from the embryo sac. The maincarbohydrate transported and secreted was sucrose. The mainamino acids reaching the fruit were asparagine, glutamine, andhomoserine. Their proportions were steady during a day-nightcycle and throughout fruit development. Amino acid compositionchanges occurred first in the pathway from fruit stalk to seedfunicle, due to the formation of threonine (probably from homoserine)and in the seed coat due to production of glutamine, alanineand valine which, together with threonine were the main secretedamino acids. The temporary nitrogen reserves of the pod walland seed coat were remobilized as asparagine during senescence.Phloem exudate of nodulated plants showed a higher (about twice)proportion of asparagine but lower proportions of homoserineand glutamine than in EDTA-induced phloem exudate of nitrate-fedplants. The two types of nitrogen nutrition also produced somechanges in relative proportions of threonine and homoserinesecreted by the seed coat. Key words: Pisum sativum, phloem, amino acids, pod wall, seed coat     

Formation of the vascular system of developing bean (Phaseolus limensis L.) seeds according to nuclear magnetic resonance microtomography

1H magnetic resonance microtomography imaging was applied to study vascular systems in developing bean (Phaseolus limensis L.) seeds. Using the gradient echo method, we recorded 2D tomographic sections in the sagittal and axial planes of the fruits sampled from a vegetating plant on days 10, 17, 24, and 31 after fertilization. Any vascular connection between the tissues of maternal plant (bean pod and seed coat) and the embryo were undetectable. The embryo has an autonomous branched network of procambial strands in the cotyledons, converging to the embryonic axis. The bean pods are covered with a network of vascular bundles; large vascular strands run along the dorsal and ventral sutures. The seed coat vascular bundles are formed in the process of seed ripening and are represented by a developed vascular system multiply branching in the middle part of the ground parenchyma at the stage of physiological maturity. They are connected with the source of assimilates via the lateral pod veins and a large vascular bundle, entering the seed below the hilum via the placenta. Assimilates enter the external part of the seed coat, which contains no vascular bundles, via the funiculus vascular bundles and hilum tissue.     

西鄂尔多斯地区强旱生小灌木的水分参数

应用PV技术研究了西鄂尔多斯地区绵刺、红沙、四合木和霸王柴4种超旱生灌木的水分关系参数膨压(ψ_P)、细胞弹性模量(ε)、细胞体积比(RCV)及其相互关系.结果表明：在4种荒漠旱生灌木中,红沙保持最大膨压的能力最强(a=2.4593).不同荒漠旱生灌木保持膨压的方式不同：绵刺通过弹性调节保持膨压(ε_max=8.4005 MPa);红沙通过渗透调节来保持膨压(ψ_π100=-3.1302 MPa;ψ₀=-3.5074 MPa);四合木通过渗透调节和弹性调节的协同作用来维持膨压;霸王柴通过渗透调节来保持膨压,而弹性调节能力较弱.绵刺具有柔软而高弹性的细胞壁,是构成其根茎系统快速吸收和传导水分能力的因素之一.四合木具有较柔软而高弹性的细胞壁且ψ_P的变化随RCV减小而趋于缓慢,说明四合木具有较强的持水能力和抗脱水能力.     

Formation of the vascular system of developing bean (<Emphasis Type="Italic">Phaseolus limensis</Emphasis> L.) seeds according to nuclear magnetic resonance microtomography

¹H magnetic resonance microtomography imaging was applied to study vascular systems in developing bean (Phaseolus limensis L.) seeds. Using the gradient echo method, we recorded 2D tomographic sections in the sagittal and axial planes of the fruits sampled from a vegetating plant on days 10, 17, 24, and 31 after fertilization. Any vascular connection between the tissues of maternal plant (bean pod and seed coat) and the embryo were undetectable. The embryo has an autonomous branched network of procambial strands in the cotyledons, converging to the embryonic axis. The bean pods are covered with a network of vascular bundles; large vascular strands run along the dorsal and ventral sutures. The seed coat vascular bundles are formed in the process of seed ripening and are represented by a developed vascular system multiply branching in the middle part of the ground parenchyma at the stage of physiological maturity. They are connected with the source of assimilates via the lateral pod veins and a large vascular bundle, entering the seed below the hilum via the placenta. Assimilates enter the external part of the seed coat, which contains no vascular bundles, via the funiculus vascular bundles and hilum tissue.     

Growth-induced water potentials originate from wall yielding during growth.

Multicellular plants display growth-induced water potentials that generate tensions on water in the apoplast and move water into the growing cells. The potentials are sometimes assumed to arise from wall yielding, keeping the turgor pressure below what otherwise would occur. There has been no direct test of this theory, and therefore whole plants or growing regions of stems (hypocotyls) of dark-grown soybean (Glycine max L. Merr.) seedlings were sealed in a pressure chamber, and wall yielding was decreased by applying external pressure. In whole plants, external pressure had little effect because the plants and water supply were uniformly exposed to the pressure. If pressure was applied to the stem while the roots were outside in water, stem elongation was markedly inhibited because the pressure raised the water potential of the growing region and decreased water entry, reducing wall yielding. Further increasing the pressure prevented water entry completely and measured the tensions in the apoplast in the same growing regions. Tensions were about 0.19 MPa at low external pressure, but diminished as wall yielding was inhibited. At external pressures of about 0.63 MPa, wall yielding was abolished and tensions approached zero. There was a linear relation between wall yielding and tension, supporting the theory that wall yielding lowers the turgor thus causing most of the growth-induced water potential.     

Differential response of carbon and nitrogen metabolism to fluoride application in fruiting structures of chickpea (Cicer arietinum)

The effect of sodium fluoride (10 and 50 mol·m⁻³) on the activities of sucrose metabolizing enzymes, transaminases and glutamine synthetase in relation to the transformation of free sugars to starch and protein in the fruiting structures (pod wall, seed coat, cotyledons) of chickpea was studied by culturing detached reproductive shoots in a liquid medium. Addition of fluoride to the culture medium drastically reduced starch content of the cotyledons and caused a marked build-up of total free sugars comprised mainly of reducing sugars in the pod wall and seed coat, and sucrose in the cotyledons. Concomitantly, the activity of soluble invertase was stimulated in the pod wall but reduced in the cotyledons. However, soluble protein content of both the pod wall and the cotyledons increased in conjunction with an increase in the activities of glutamate-oxaloacetate transaminase, glutamate-pyruvate transaminase and glutamine synthetase. Disruption of starch biosynthesis under the influence of fluoride and the resulting accumulation of free sugars possibly resulted in their favoured utilization in nitrogen metabolism. Labelling studies with [U-¹⁴C]-sucrose showed that the ¹⁴C incorporation into total free sugars was enhanced by fluoride in the pod wall but reduced in the seed coat and cotyledons, possibly due to an inhibitory effect on their translocation to the developing seeds.     

A Cascade of Sequentially Expressed Sucrose Transporters in the Seed Coat and Endosperm Provides Nutrition for the Arabidopsis Embryo

Developing plant embryos depend on nutrition from maternal tissues via the seed coat and endosperm, but the mechanisms that supply nutrients to plant embryos have remained elusive. Sucrose, the major transport form of carbohydrate in plants, is delivered via the phloem to the maternal seed coat and then secreted from the seed coat to feed the embryo. Here, we show that seed filling in Arabidopsis thaliana requires the three sucrose transporters SWEET11, 12, and 15. SWEET11, 12, and 15 exhibit specific spatiotemporal expression patterns in developing seeds, but only a sweet11;12;15 triple mutant showed severe seed defects, which include retarded embryo development, reduced seed weight, and reduced starch and lipid content, causing a “wrinkled” seed phenotype. In sweet11;12;15 triple mutants, starch accumulated in the seed coat but not the embryo, implicating SWEET-mediated sucrose efflux in the transfer of sugars from seed coat to embryo. This cascade of sequentially expressed SWEETs provides the feeding pathway for the plant embryo, an important feature for yield potential.     

Gas exchange by pods and subtending leaves and internal recycling of CO(2) by pods of chickpea (Cicer arietinum L.) subjected to water deficits

Terminal drought markedly reduces leaf photosynthesis of chickpea (Cicer arietinum L.) during seed filling. A study was initiated to determine whether photosynthesis and internal recycling of CO(2) by the pods can compensate for the low rate of photosynthesis in leaves under water deficits. The influence of water deficits on the rates of photosynthesis and transpiration of pods and subtending leaves in chickpea (cv. Sona) was investigated in two naturally-lit, temperature-controlled glasshouses. At values of photosynthetically active radiation (PAR) of 900 micromol m(-2) s(-1) and higher, the rate of net photosynthesis of subtending leaves of 10-d-old pods was 24 and 6 micromol m(-2) s(-1) in the well-watered (WW) and water-stressed (WS) plants when the covered-leaf water potential (Psi) was -0.6 and -1.4 MPa, respectively. Leaf photosynthesis further decreased to 4.5 and 0.5 micromol m(-2) s(-1) as Psi decreased to -2.3 and -3.3 MPa, respectively. At 900--1500 micromol m(-2) s(-1) PAR, the net photosynthetic rate of 10-d-old pods was 0.9-1.0 micromol m(-2) s(-1) in the WW plants and was -0.1 to -0.8 micromol m(-2) s(-1) in the WS plants. The photosynthetic rates of both pods and subtending leaves decreased with age, but the rate of transpiration of the pods increased with age. The rates of respiration and net photosynthesis inside the pods were estimated by measuring the changes in the internal concentration of CO(2) of covered and uncovered pods during the day. Both the WW and WS pods had similar values of internal net photosynthesis, but the WS pods showed significantly higher rates of respiration suggesting that the WS pods had higher gross photosynthetic rates than the WW pods, particularly in the late afternoon. When (13)CO(2) was injected into the gas space inside the pod, nearly 80% of the labelled carbon 24 h after injection was observed in the pod wall in both the WW and WS plants. After 144 h the proportion of (13)C in the seed had increased from 19% to 32% in both treatments. The results suggest that internal recycling of CO(2) inside the pod may assist in maintaining seed filling in water-stressed chickpea.     

Maternal control of seed weight in rapeseed (Brassica napus L.): the causal link between the size of pod (mother,source) and seed (offspring,sink)

Seed size/weight is one of the key traits related to plant domestication and crop improvement. In rapeseed (Brassica napus L.) germplasm, seed weight shows extensive variation, but its regulatory mechanism is poorly understood. To identify the key mechanism of seed weight regulation, a systematic comparative study was performed. Genetic, morphological and cytological evidence showed that seed weight was controlled by maternal genotype, through the regulation of seed size mainly via cell number. The physiological evidence indicated that differences in the pod length might result in differences in pod wall photosynthetic area, carbohydrates and the final seed weight. We also identified two pleiotropic major quantitative trait loci that acted indirectly on seed weight via their effects on pod length. RNA‐seq results showed that genes related to pod development and hormones were significantly differentially expressed in the pod wall; genes related to development, cell division, nutrient reservoir and ribosomal proteins were all up‐regulated in the seeds of the large‐seed pool. Finally, we proposed a potential seed weight regulatory mechanism that is specific to rapeseed and novel in plants. The results demonstrate a causal link between the size of the pod (mother, source) and the seed (offspring, sink) in rapeseed, which provides novel insight into the maternal control of seed weight and will open a new research field in plants.     

Differential proline metabolism in vegetative and reproductive tissues determine drought tolerance in chickpea

Proline is emerging as a critical component of drought tolerance and fine tuning of its metabolism under stress affects the plants sensitivity and response to stress. Thus the study was carried out to analyse the effect of water deficit on the proline content and principal enzymes involved in its synthesis (Δ¹-pyrolline-carboxylate synthetase) and catabolism (proline dehydrogenase) at different developmental stages and in different organs (roots, nodules, leaves, pod wall, and seeds) of two chickpea (Cicer arietinum L.) cultivars differing in drought tolerance (drought tolerant ICC4958 and drought sensitive ILC3279). It was observed that increased Δ¹-pyrolline-carboxylate synthetase activity under moderate stress in roots and nodules of ICC4958 caused an increase in proline content during initiation of reproductive development whereas increased proline dehydrogenase activity in nodules and leaves at this period helped to maintain reducing power and energy supply in tissues and proper seed development as seed biomass increased consistently up to maturity. On the other hand, roots and nodules of ILC3279 responded to stress by increasing proline content after the developmental phase of reproductive organs was over (near maturity) which negatively affected the response of pod wall to stress. Concurrent increase in activities of Δ¹-pyrolline-carboxylate synthetase and proline dehydrogenase in pod wall of ILC3279 aggravated the oxidative stress and affected seed development as seed biomass initially increased rapidly under stress but was unaffected near maturity.     

Nitrogen nutrition and metabolic interconversions of nitrogenous solutes in developing cowpea fruits

Budgets for import and utilization of ureide, amides, and a range of amino acids were constructed for the developing first-formed fruit of symbiotically dependent cowpea (Vigna unguiculata [L.] Walp. cv Vita 3). Data on fruit total N economy, and analyses of the xylem and phloem streams serving the fruit, were used to predict the input of various solutes while the compositions of the soluble and protein pools of pod, seed coat, and embryo were used to estimate the net consumption of compounds. Ureides and amides provided virtually all of the fruit's N requirements for net synthesis of amino compounds supplied inadequately from the parent plant. Xylem was the principal source of ureide to the pod, while phloem was the major source of amides to pod and seed. All fruit parts showed in vitro activity of urease (EC 3.5.1.5), allantoinase (EC 3.5.2.5), asparaginase (EC 3.5.11), ammonia-assimilating enzymes and aspartate and alanine aminotransferases (EC 2.61.1 and EC 2.6.1.1.2). Asparagine:pyruvate aminotransferase (EC 2.6.1.14) was recovered only from the pod. The pod was initially the major site for processing and incorporating N; later seed coats and finally embryos became predominant. Ureides were broken down mainly in the pod and seed coat. Amide metabolism occurred in all fruit organs, but principally in the embryo during much of seed growth. Seed coats released N to embryos mainly as histidine, arginine, glutamine, and asparagine, hardly at all as ureide. Amino compounds delivered in noticeably deficient amounts to the fruit were arginine, histidine, glycine, glutamate, and aspartate, while seeds received insufficient arginine, histidine, serine, glycine, and alanine. Quantitatively based schemes are proposed depicting the principal metabolic transformation accompanying N-flow between seed compartments during development.     

Day/night variations in turgor pressure in individual cells of Mesembryanthemum crystallinum L.

Summary Using a pressure probe, turgor pressure was directly determined in leaf-mesophyll cells and the giant epidermal bladder cells of stems, petioles and leaves of the halophilic plant Mesembryanthemum crystallinum. Experimental plants were grown under non-saline conditions. They displayed the photosynthetic characteristics typical of C₃-plants when 10 weeks old and performed weak CAM when 16 weeks old. In 10 week old plants, the turgor pressure (P) of bladder cells of stems was 0.30 MPa; of bladder cells of petioles 0.19 MPa, and of bladder cells of leaves 0.04 MPa. In bladder cells from leaves of 16 week old plants, marked changes in turgor pressure were observed during day/night cycles. Maximum turgor occurred at noon and was paralleled by a decrease in the osmotic pressure of the bladder cell sap. Similar changes in the cell water relations were observed in plants in which traspirational water loss was prevented by high ambient relative humidity. Turgor pressure of mesophyll cells also increased during day-time showing macimum values in the early morning. No such changes in turgor pressure and osmotic pressure were observed in bladder and mesophyll cells of the 10 week old plants not showing the diurnal acid fluctuation typical of CAMAbbreviations CAM crassulacean acid metabolism - V volume of the cells (mm³) - P turgor pressure (MPa) - volumetric elastic modulus (MPa) - ⁱ osmotic pressure of the cell sap (MPa) - T _1/2 half-time of water exchange (s) - Lp hydraulic conductivity of the cell membrane (m·s^-1·MPa^-1) - A surface area of cells (mm²) - P pressure changes (MPa) - V volume changes (mm³) - nocturanal nighttime - diurnal daytime     

Regulation of monocarpic senescence of Brassica campestris by the developing pods

Senescence of Brassica campestris L. cv. B-9 was studied with regard to seed maturation and source-sink relationships. In normal control plants leaf senescence (as determined by the change in chlorophyll level) started and proceeded in a progressive manner from base to apex during the period of early pod setting. Complete yellowing of the leaves occurred well before the seed maturation and pod wall senescence. The pod wall always senesced before the attainment of final seed weight. In two different sets of acrocarpous plants containing 65 pods and 10 pods, respectively, leaf senescence was delayed during the pod filling period. It started non-sequentially after complete yellowing and senescence of the pod wall. The degree of leaf senescence at the post-pod filling stage was almost proportional to the number of pods present. When peduncles of the acrocarpous 10-podded plants were removed after the pod filling stage of the plant, leaf senescence was delayed compared to plants whose pedicels were removed, although the senescence pattern of the upper three leaves was nonsequential in both cases. Defruiting at an early stage of development delayed leaf senescence, although the pattern of such senescence remained unaltered (i.e. nonsequential). Defoliation hastened the seed-filling process and pod wall senescence. Plants containing fewer pods had higher average seed weight, although yield per plant was reduced.
These results suggest that the pod wall serves as a temporary as well as intermediary storage organ and that foliar senescence is not directly related to seed maturation. The possible cause of uncoupling between foliar senescence and seed development is discussed.     

Regulation of Turgor Pressure by Suspension-Cultured Tobacco Cells

Turgor regulation and effects of high NaCl and water deficiton growth and internal solutes were studied after transferringtobacco cells from control culture medium (osmotic pressure= 0.13–0.15 MPa at time of transfer) to culture mediumcontaining either 82 mol m^–3 NaCl or 150 mol m^–3melibiose (osmotic pressure of media = 0.62 MPa). Followingtransfer to media with higher osmotic pressure, expansion rateand turgor pressure were reduced. Within 24 h of imposing thewater deficit, expansion rate had returned to that of cellsin control culture medium. However, by 24 h, turgor pressurehad only risen from 0.2 MPa to 0.65 MPa in the NaCl treatmentand to 0.53 MPa in the melibiose treatment, while it was 0.73MPa in the control treatment. Furthermore, turgor pressure remainedwithin 0.05 MPa of these respective values for the rest of the(75 h) experiment. These results suggest differences in bothcell wall properties (extensibility and/or threshold turgor)and the level at which turgor is maintained for cells in thevarious treatments. Solutes contributing nearly all (82–97%) of the osmoticpressure in cells were identified. The initial (up to 24 h)increases in turgor pressure were mainly due to increases insolute concentrations caused by relatively slow expansion rates.However, increased Na⁺ and Cl ^– uptake contributed toincreased turgor pressure in the NaCl treatment and caused turgorpressure of cells in this treatment to increase faster thanin the melibiose treatment. Likewise, expansion rate rose morequickly in the NaCl than in the melibiose treatment. After 24h, maximum expansion rate was reached and concentrations ofmost internal solutes began to decrease. Nevertheless, turgorpressure remained relatively constant. The constancy of turgorpressure was due to increased glucose uptake rates relativeto controls, with consequent increases in concentrations ofsucrose, glucose and fructose and, in cells in the melibiosetreatment, of organic acids. Glucose uptake was slower in theNaCl than in the melibiose treatment but higher turgor pressurewas maintained in the NaCl treatment due to high uptake of Na⁺and Cl^–. Glucose uptake appears to respond to a systemof turgor regulation, but further experiments are required toconfirm this and to determine whether Na⁺ and Cl^– uptakealso respond to a system of turgor regulation. Key words: Salinity, water deficit, growth     

The Response of Groundnut (Arachis hypogaea L.) to Timing of Irrigation: II. 14C-PARTITIONING AND PLANT WATER STATUS

Finite quantities of water were applied at different growthstages of groundnut stands (Arachis hypogaea L.) grown in controlledenvironment glasshouses. Soil moisture deficits were imposedbetween sowing and pod initiation or between pod initiationand final harvest by withholding or applying water. Effectson assimilate production and partitioning and plant water relationswere examined. Leaves were the primary sites of ¹⁴CO₂ fixation, though theircontribution generally declined late in the season, whereasfixation by stems was initially low but increased sharply whenstress was released in the late-irrigated stands. ¹⁴C-fixationby stem apices and pegs also rose sharply following irrigationof the late-stressed stands. Leaves were the primary source of assimilates, though translocationtended to decrease as the season progressed, even in the late-irrigatedstands. Stems were initially the major sinks, but their sinkactivity disappeared almost completely when stress was releasedin the late-irrigated stands. Assimilate import by stem apicesdeclined progressively and pod sink activity was negligiblein the late-stressed stand, but both increased markedly whenearly-season stress was released. Leaf water status showed marked diurnal variation, whereas pegsshowed less variation and maintained much higher turgor levels,largely because of their lower solute potentials. Marked osmoticadjustment occurred in expanding but not in mature leaves, allowingthem to maintain higher turgor levels during periods of severestress. This adjustment was rapidly lost when stress was released.The observed changes in assimilate production and partitioningpreceded detectable changes in bulk turgor levels. Implications for growth, development and yield are discussed. Key words: Groundnut, irrigation, partitioning, water status     

Changes in leaf water relations, gas exchange, growth and flowering quality in potted geranium plants irrigated with different water regimes

Geranium plants are an important part of urban green areas but suffer from drought, especially when grown in containers with a limited volume of medium. In this experiment, we examined the response of potted geraniums to different irrigation levels. Geranium (Pelargoniumxhortorum L.) seedlings were grown in a growth chamber and exposed to three irrigation treatments, whereby the plants were irrigated to container capacity (control), 60% of the control (moderate deficit irrigation, MDI), or 40% of the control (severe deficit irrigation, SDI). Deficit irrigation was maintained for 2 months, and then all the plants were exposed to a recovery period of 112 month. Exposure to drought induced a decrease in shoot dry weight and leaf area and an increase in the root/shoot ratio. Height and plant width were significantly inhibited by the SDI, while flower color parameters were not affected by deficit treatment. The number of wilting and yellow leaves increased, coinciding with the increase in the number of inflorescences and open flowers. Deficit irrigation led to a leaf water potential of about -0.8MPa at midday, which could have caused an important decrease in stomatal conductance, affecting the photosynthetic rate (Pn). Chlorophyll fluorescence (Fvm) values of 0.80 in all treatments throughout the experiment demonstrate the lack of drought-induced damage to PSII photochemistry. Pressure-volume analysis revealed low osmotic adjustment values of 0.2MPa in the SDI treatment, accompanied by increases in the bulk tissue elastic modulus (epsilon, wall rigidity) and resulting in turgor loss at lower leaf water potential values (-1.38MPa compared with -1.0MPa for the control). Leaf water potential values throughout the experiment below those for Psitlp were not found at any sampling time. By the end of the recovery period, the leaf water potential, stomatal conductance and net photosynthesis had recovered. We infer from these results that moderate deficit irrigation in geranium reduced the consumption of water, while maintaining the good overall quality of plants. However, when SDI was applied, a reduction in the number of flowers per plant was observed.