Ionic Balance of Root-Shoot Nitrate Transfer in Dwarf Bean

The Dijkshoorn-Ben Zioni model about potassium recycling in higher plants proposes a stoichiometric coupling between the upward transport of nitrate and potassium, and therefore a 1:1 ratio of these ions in the xylem sap. Phaseolus vulgaris grown in media with different NO₃: K⁺ ratios, however, only rarely showed equimolar concentrations of the two ions in the bleeding sap. Ratios between 0.2 and 11 were observed and the ratios between concentrations of the ions in the exudate and in the external solution were positively correlated. An analysis of the major inorganic constituents of the exudate revealed an ionic balance that requires negligible amounts of charges from organic molecules.     

Respiration of the Roots of Flood-Tolerant and Flood-Intolerant Senecio Species: Affinity for Oxygen and Resistance to Cyanide

The affinity of respiration for oxygen in the roots of six Senecio species studied was low compared with the affinity of cytochrome oxidase for oxygen. Half saturation values of approximately 22 μM oxygen were measured. Root respiration was to a large extent insensitive to cyanide in flood-tolerant as well as in flood-sensitive species. The evidence presented suggests that high activity of salicylhydroxamic acid (SHAM)-sensitive oxidase in Senecio roots was the basis for the low oxygen affinity and for the high cyanide-insensitivity of root respiration in the Senecio species. Methods are described to determine the in vivo activity of the SHAM-sensitive oxidase. It was estimated that it contributed 70% to the total root respiration. The presence of SHAM-sensitive oxidase activity could explain a higher efficiency of root growth respiration under a low oxygen tension if this alternate oxidase was inhibited at a low oxygen concentration in the root medium. However, the SHAM-sensitive oxidase was not specifically involved in either growth respiration or maintenance respiration. Its significance in regulation of the redox state of the cells is discussed.     

Kinetics of Integration of Xyloglucan into the Walls of Suspension-Cultured Rose Cells

To study the kinetics of synthesis, wall-binding and degradationof xyloglucan, we incubated suspension-cultured rose cells for0–5–24 h in L-[1-³H]arabinose. >95% of the [³H]arabinosewas taken up within 2 h. UDP-Pentoses were maximally labelledwithin 0–5 h and had lost most of their ³H by 2 h afterthe addition of [³H]arabinose. Therefore, the 24 h experimentresembled a pulse-chase rgime. The [³H]xyloglucan formed wasfractionated into four cellular pools [detergent-extractable(interpreted as cytoplasmic), and guanidinium thiocyanate-,06 M NaOH- and 60 M NaOH-extractable (interpreted as progressivelymore firmly wall-bound)]; soluble extracellular xyloglucan wascollected as a fifth pool. All five pools of xyloglucan hadstarted accumulating ³H at their respective maximal rates by     

Effect of Plant Growth Regulators on Nitrate Utilization by Roots of Nitrogen-Depleted Dwarf Bean

We examined the effect of pretreatments (18 h at 5 µmoldm^–3) with abscisic acid, the ethylene-releasing substance‘Ethephon’, gibberellic acid, indoleacetic acid,kinetin and zeatin on nitrate uptake and in vivo nitrate reductaseactivity (NRA) in roots of nitrogen-depleted Phaseolus vulgarisL. Nitrate uptake showed an apparent induction pattern witha steady state after about 6 h, in all treatments. The nitrateuptake rate after 6 h was unaffected or at most 30% lower aftertreatments with the plant growth regulators. Gibberellic acid, kinetin and zeatin induced substantial NRAin roots in the absence of nitrate, whereas Ethephon enhancedNRA only during nitrate nutrition. Kinetin-induced NRA (Ki-NRA)was maximal after a pretreatment at 1 µmol dm^–3,and showed a lag phase of 6–8 h. Ki-NRA was additive tonitrate-induced NRA (NO^–₃-NRA) for at least 24 h, independentof the induction sequence. After full induction, Ki-NRA approximated20% of NO-₃-NRA. Abscisic acid counteracted the developmentof Ki-NRA, but not of NO^–₃-NRA. Cycloheximide and tungstatewere equally effective to suppress the development of nitratereductase activity after supply of kinetin or NO^–₃. Our data are consistent with the operation of two independentenzyme fractions (Ki-NRA and NO^–₃-NRA) with apparentlyidentical properties but with separate control mechanisms. Theabsence of major effects of plant growth regulators on the time-courseand rate of nitrate uptake suggests that exogenous regulators,and possibly endogenous phytohormones are of minor importancefor initial nitrate uptake. The differential effect of someregulators on nitrate uptake and root NRA furthermore indicatesthat the processes of uptake and reduction of NO^–₃ arenot obligatory or exclusively coupled to each other.     

Efficiency and regulation of root respiration in a legume: Effects of the N source

A comparison was made of energy metabolism of nodulated N₂ fixing plants and non-nodulated NO₃-fed plants of Lupinus albus L. Growth, N-increment, root respiration (O₂ uptake and CO² production) and the contribution of a SHAM-sensitive oxidative pathway (the alternative pathway) in root respiration were measured. Both growth rate and the rate of N-increment were the same in both series of plants. The rate of root respiration, both O₂ uptake and CO₂ production, and the activity of the SHAM-sensitive pathway were higher in NO₃-fed plants than in N₂ fixing plants. The rate of ATP production in oxidative phosphorylation was computed also to be higher in NO₃-fed plants. It is concluded that both carbohydrate costings and ATP costings for synthesis + maintenance of root material were lower in N₂ fixing than in NO₃-fed plants. The respiratory quotient of root respiration was 1.6 in N₂-fixing plants and 1.4 in NO₃-fed plants. These values were slightly higher than the values calculated on the basis of CO₂ output due to N-assimilation and the experimental values of O₂ uptake, but showed the same trend: highest in N₂ fixing plants. Root respiration of NO₃-fed plants showed a diurnal pattern (both O₂ uptake, CO₂ production and the activity of the SHAM-sensitive pathway), whilst no diurnal variation in root respiration was found in N₂ fixing plants. However, C₂H₂ reduction did show a diurnal rhythm, which is suggested to be related to the diurnal variation in transpiration. Addition of NO₃ to N₂ fixing plants increased the rate of root respiration and the activity of the alternative pathway. This treatment did not decrease C₂H₂ reduction and H₂ evolution within 4 days. Withdrawal of NO₃-supply from NO₃-fed plants decreased the rate of root respiration but had no effect on the relative activity of the alternative pathway. It is suggested that the higher rate of root respiration and the higher activity of the SHAM-sensitive pathway in NO₃-fed plants is due to a larger supply of carbohydrates to the roots, partly due to a better photosynthetic performance of the shoots and partly due to a higher capacity of the roots to attract carbohydrates.     

Direct Measurements of Turgor Pressure Potentials of Guard Cells: II. THE MECHANICAL ADVANTAGE OF SUBSIDIARY CELLS, THE SPANNUNQSPHASE, AND THE OPTIMUM LEAF WATER DEFICIT

Evidence of the mechanical advantage of subsidiary cells wasobtained by simultaneous measurements of turgor pressure potentialsin adjacent subsidiary and guard cells using injection circuitswith two separate needles. In Tradescantia virginiana the mechanicaladvantage approaches two. Using the same technique evidencewas obtained that the Spannungsphase is, in the first place,a turgor relations phenomenon due to the mechanical advantageof epidermal or subsidiary cells. In addition, the evidenceindicated that the elastic properties of guard cell walls mayundergo changes during the Spannungsphase when potassium iontransport commences. During these measurements it was confirmedthat the optimum leaf water deficit for maximum stomatal openingoccurs when the epidermal turgor is near zero. Under these conditionsthe width of the stomatal pore is a function of the turgor pressureof the guard cells, since at zero turgor of the subsidiary cellstheir mechanical advantage has disappeared.     

A new species of Selaginella from Hainan Island, China, with a key to the Hainan species

Selaginella hainanensis X.C.Zhang & Noot. sp. nov. , a new species from Hainan Island, South China, is described and illustrated. Diagnostic notes and a key to the species distributed in Hainan are also provided.  © 2005 The Linnean Society of London, Botanical Journal of the Linnean Society , 2005, 148 , 323–327.     

The ecdysteroid receptor

Summary

The steroid molting hormone of insects and other arthropods regulates gene activity in target tissues through its association with a specific, high affinity receptor protein. In this review we summarize recent advances in several areas of ecdysteroid receptor research, including efforts to characterize and purify the receptor protein, cytochemical studies of its tissue distribution and subcellular localization during development, and current molecular genetic studies ecdysteroid action.     

High relative air humidity and continuous light reduce stomata functionality by affecting the ABA regulation in rose leaves

Plants developed under high (90%) relative air humidity (RH) have previously been shown to have large, malfunctioning stomata, which results in high water loss during desiccation and reduced dark induced closure. Stomatal movement is to a large extent regulated by abscisic acid (ABA). It has therefore been proposed that low ABA levels contribute to the development of malfunctioning stomata. In this study, we investigated the regulation of ABA content in rose leaves, through hormone analysis and β‐glucosidase quantification. Compared with high RH, rose plants developed in moderate RH (60%) and 20 h photoperiod contained higher levels of ABA and β‐glucosidase activity. Also, the amount of ABA increased during darkness simultaneously as the ABA‐glucose ester (GE) levels decreased. In contrast, plants developed under high RH with 20 h photoperiod showed no increase in ABA levels during darkness, and had low β‐glucosidase activity converting ABA‐GE to ABA. Continuous lighting (24 h) resulted in low levels of β‐glucosidase activity irrespective of RH, indicating that a dark period is essential to activate β‐glucosidase. Our results provide new insight into the regulation of ABA under different humidities and photoperiods, and clearly show that β‐glucosidase is a key enzyme regulating the ABA pool in rose plants.