Some Accessions of Amazonian Wild Rice (Oryza glumaepatula) Constitutively Form a Barrier to Radial Oxygen Loss along Adventitious Roots under Aerated Conditions

A barrier to radial oxygen loss (ROL), which reduces the loss of oxygen transported via the aerenchyma to the root tips, enables the roots of wetland plants to grow into anoxic/hypoxic waterlogged soil. However, little is known about its genetic regulation. Quantitative trait loci (QTLs) mapping can help to understand the factors that regulate barrier formation. Rice (Oryza sativa) inducibly forms an ROL barrier under stagnant conditions, while a few wetland plants constitutively form one under aerated conditions. Here, we evaluated the formation of a constitutive ROL barrier in a total of four accessions from two wild rice species. Three of the accessions were wetland accessions of O. glumaepatula, and the fourth was a non-wetland species of O. rufipogon. These species have an AA type genome, which allows them to be crossed with cultivated rice. The three O. glumaepatula accessions (W2165, W2149, and W1183) formed an ROL barrier under aerated conditions. The O. rufipogon accession (W1962) did not form a constitutive ROL barrier, but it formed an inducible ROL barrier under stagnant conditions. The three O. glumaepatula accessions should be useful for QTL mapping to understand how a constitutive ROL barrier forms. The constitutive barrier of W2165 was closely associated with suberization and resistance to penetration by an apoplastic tracer (periodic acid) at the exodermis but did not include lignin at the sclerenchyma.

Xylem sap analysis reveals new facts of salt tolerance in rice genotypes

Salinity damage in rice and other salt-sensitive species is due to excessive transport of NaCl through the root system to the leaves and consequently low salt transport to the shoot can be a major trait determining salt resistance. Since the rapid uptake of sodium ions is such a crucial part of the response of rice to salinity, physiological experiments were carried out to compare bypass flow in two genotypes of rice (IR4630 and IR15324) differing in salt tolerance, because it has been suggested that an apoplastic pathway, bypass flow, is a major contributory pathway for Na+ entrance into rice plants. Experiments on the youngest fully expanded photosynthetic leaf (the third from the base), using PTS as a tracer for apoplastic movement and Philaenus spumarius (a xylem-feeding insect) as a means to sample the xylem sap, did not demonstrate any apparent difference in bypass flow between the two lines. The similarity of Na+ concentration in the xylem sap of both genotypes paralleled the results of PTS (a fluorescent dye used as an apoplastic tracer for the transpiration stream) measurements. Despite the similarity of Na+ concentration in the xylem sap of the third leaves, the Na+ concentration in the bulk of these leaves of IR15324 plants (the sensitive line) was about twice that of IR4630 (the tolerant line). Measurements of transpiration over 8 d of salinisation showed the similarity of rates in both lines providing evidence that the greater accumulation of NaCl in IR15324 than in IR4630 plants was unlikely to be due to a difference in the delivery of salt to the leaves by an apoplastic route. Results of the current work suggest that the difference in salt tolerance might be a consequence of damage to leaves 1 and 2 of IR15324 that allowed Na+ to leak into the phloem - and consequently move to leaf 3.

Beneficial effects of direct foliar water uptake on shoot water potential of five chasmophytes

Five chasmophytic species growing as wall-fissure plants on vertical retaining walls of a castle in the town of Patras, Greece, were examined for their ability to benefit from direct absorption of foliar surface water. Epifluore scence microscopy and application of an apoplastic tracer of water movement indicated that sprayed water on leaf surfaces penetrated into the mesophyll of all species, and in some cases was also detected within conducting tissues of the leaf. Water potential (Ψw) of sprayed detached shoots was improved compared with unsprayed controls when they were slowly losing water under laboratory conditions. The beneficial effect of leaf water absorption on Ψw was found either on leaves that were originally fully turgid or on leaves that had undergone a considerable water content reduction. Sprayed leaves maintained their Ψw above turgor loss point for a time period of 10-90 min depending on species and degree of water deficiency. The relevance of the results to the importance of dew uptake from chasmophytes growing under water shortage in wall fissures in inhabited areas is discussed.Key words: chasmophytes, leaf water absorption, wall vegetation, drought resistance.

A rapid fluorescence technique to probe the permeability of the root apoplast

The alkaloid berberine is useful as a mobile apoplastic tracer. It is readily precipitated by thiocyanate, forming bright yellow, needle-like, fluorescent crystals. When berberine hemisulphate and potassium thiocyanate are applied sequentially to plant tissue, the crystals form in unmodified walls and in the lumina of dead cells such as tracheary elements. Lignified and suberized walls stain with berberine but do not develop crystals. Regions of the plant that have been penetrated by the chemicals can be located by preparing freehand sections, mounting them in potassium thiocyanate to inhibit dissolution of the crystals, and examining them with a fluorescence microscope. When the two chemicals are sequentially introduced into the xylem of onion, corn, and broad bean roots, the tracer does not pass the endodermal Casparian band, indicating that the system traces apoplastic pathways. The chemical concentrations that produce sufficient crystals so that berberine can be used as an apoplastic tracer in primary roots are 0.05% berberine hemisulphate and 0.09 M potasium thiocyanate. These concentrations were not toxic to cells of onion bulb epidermis. They reduced the growth rates of corn and broad bean roots but did not kill them. Berberine–thiocyanate is a useful apoplastic tracer provided care is taken not to exceed the limits of berberine toxicity to the tissue. Key words: apoplastic tracer, berberine hemisulphate, potassium thiocyanate, roots, toxicity tests, corn, onion, broad bean.

Apoplastic and ultrastructural characterizations of the trichomes from the carnivorous bromeliad Brocchinia reducta

The ultrastructure of the trichomes from the carnivorous bromeliad Brocchinia reducta was studied. The foliar trichome occurred in a cutinized epidermal crypt and was differentiated into cap, stalk, and foot cells. Unlike members of other bromeliad genera, the approximately 30 cells in the cap were alive at maturity and possessed an unusual labyrinthine-like wall organization. Irregularly shaped, dense wall regions that were separated by electron-transparent interstices extended from a thin, primary wall. This system appeared to constitute an elaborate system of channels specialized for nutrient uptake, a role which is consistent with the observed localization of the apoplastic tracer lanthanum. Moreover, cuticular gaps were noted where the cap cells face the tank cavity. These openings were the only site of solute entry into the gland. Cap cells possessed much plasmalemma blebbing and contained numerous coated vesicles and coated plasmalemma regions, all suggesting specializations for absorption and possibly secretion. Experiments using lanthanum as a tracer indicated that an apoplastic continuum exists from the cap cells to the underlying mesophyll, and that this system might provide a diffusive pathway for nutrient uptake.