Mechanical pruning and soil organic amendments in vineyards of ‘Syrah’: effects on wine mineral composition

The interaction of mechanized pruning systems and soil organic amendment can affect vine vegetative and reproductive growth. However, since organic amendments supply several mineral elements, namely heavy metals, this study aimed to understand the effects of the interaction between these two practices on the mineral composition of wine. Two field trials were implemented in ‘Syrah’ vineyards in two Portuguese wine regions (Lisboa and Tejo). Mechanical hedge pruning was compared with hand spur pruning and four different organic amendments were tested: biochar, municipal solid waste compost, cattle manure and sewage sludge. The nitrogen (N), phosphorus (P) and potassium (K) wine contents were significantly reduced by mechanical pruning while calcium (Ca) and magnesium (Mg) contents were tendentially higher in this pruning system. Mechanical pruning also reduced the content of some minor elements, such as arsenic (As), molybdenum (Mo) and nickel (Ni). In 2014, the year with the higher reproductive growth, some other elements also decreased as a consequence of the mechanical pruning (gallium - Ga; lithium – Li; rubidium - Rb, thallium – Tl; yttrium - Y). Concerning the organic amendments, sewage sludge was associated with the wines with the lowest P and iron (Fe) content. Ca content was tendentially higher in municipal solid waste compost and sewage sludge treatments. Mechanical pruning and organic amendments had different effects on the mineral composition of wine, according to each specific element. However, the legal limits, recommended by OIV and established by European Union, as well as the technical limits, adopted by winemakers, were never exceeded and the interaction of both practices does not seem to be a problem in what concerns to the mineral composition of the produced wines.

The influence of transpiration on foliar accumulation of salt and nutrients under salinity in poplar (Populus × canescens)

Increasing salinity is one of the major drawbacks for plant growth. Besides the ion itself being toxic to plant cells, it greatly interferes with the supply of other macronutrients like potassium, calcium and magnesium. However, little is known about how sodium affects the translocation of these nutrients from the root to the shoot. The major driving force of this translocation process is thought to be the water flow through the xylem driven by transpiration. To dissect the effects of transpiration from those of salinity we compared salt stressed, ABA treated and combined salt- and ABA treated poplars with untreated controls. Salinity reduced the root content of major nutrients like K+, Ca2+ and Mg2+. Less Ca2+ and Mg2+ in the roots resulted in reduced leaf Ca2+ and leaf Mg2+ levels due to reduced stomatal conductance and reduced transpiration. Interestingly, leaf K+ levels were positively affected in leaves under salt stress although there was less K+ in the roots under salt. In response to ABA, transpiration was also decreased and Mg2+ and Ca2+ levels decreased comparably to the salt stress treatment, while K+ levels were not affected. Thus, our results suggest that loading and retention of leaf K+ is enhanced under salt stress compared to merely transpiration driven cation supply.

Evidence for symplastic involvement in the radial movement of calcium in onion roots

The pathway of Ca(2+) movement from the soil solution into the stele of the root is not known with certainty despite a considerable body of literature on the subject. Does this ion cross an intact, mature exodermis and endodermis? If so, is its movement through these layers primarily apoplastic or symplastic? These questions were addressed using onion (Allium cepa) adventitious roots lacking laterals. Radioactive Ca(2+) applied to the root tip was not transported to the remainder of the plant, indicating that this ion cannot be supplied to the shoot through this region where the exodermis and endodermis are immature. A more mature zone, in which the endodermal Casparian band was present, delivered 2.67 nmol of Ca(2+) mm(-1) treated root length d(-1) to the transpiration stream, demonstrating that the ion had moved through an intact endodermis. Farther from the root tip, a third zone in which Casparian bands were present in the exodermis as well as the endodermis delivered 0.87 nmol Ca(2+) mm(-1) root length d(-1) to the transpiration stream, proving that the ion had moved through an unbroken exodermis. Compartmental elution analyses indicated that Ca(2+) had not diffused through the Casparian bands of the exodermis, and inhibitor studies using La(3+) and vanadate (VO(4)(3-)) pointed to a major involvement of the symplast in the radial transport of Ca(2+) through the endodermis. It was concluded that in onion roots, the radial movement of Ca(2+) through the exodermis and endodermis is primarily symplastic.

The exodermis: a variable apoplastic barrier

The exodermis (hypodermis with Casparian bands) of plant roots represents a barrier of variable resistance to the radial flow of both water and solutes and may contribute substantially to the overall resistance. The variability is a result largely of changes in structure and anatomy of developing roots. The extent and rate at which apoplastic exodermal barriers (Casparian bands and suberin lamellae) are laid down in radial transverse and tangential walls depends on the response to conditions in a given habitat such as drought, anoxia, salinity, heavy metal or nutrient stresses. As Casparian bands and suberin lamellae form in the exodermis, the permeability to water and solutes is differentially reduced. Apoplastic barriers do not function in an all-or-none fashion. Rather, they exhibit a selectivity pattern which is useful for the plant and provides an adaptive mechanism under given circumstances. This is demonstrated for the apoplastic passage of water which appears to have an unusually high mobility, ions, the apoplastic tracer PTS, and the stress hormone ABA. Results of permeation properties of apoplastic barriers are related to their chemical composition. Depending on the growth regime (e.g. stresses applied) barriers contain aliphatic and aromatic suberin and lignin in different amounts and proportion. It is concluded that, by regulating the extent of apoplastic barriers and their chemical composition, plants can effectively regulate the uptake or loss of water and solutes. Compared with the uptake by root membranes (symplastic and transcellular pathways), which is under metabolic control, this appears to be an additional or compensatory strategy of plants to acquire water and solutes.

The pathways of calcium movement to the xylem

Calcium is an essential plant nutrient. It is acquired from the soil solution by the root system and translocated to the shoot via the xylem. The root must balance the delivery of calcium to the xylem with the need for individual root cells to use [Ca2+]cyt for intracellular signalling. Here the evidence for the current hypothesis, that Ca2+ travels apoplastically across the root to the Casparian band which it then circumvents via the cytoplasm of the endodermal cell, is critically reviewed. It is noted that, although Ca2+ channels and Ca2+-ATPases are present and could catalyse Ca2+ influx and efflux across the plasma membrane of endodermal cells, their transport capacity is unlikely to be sufficient for xylem loading. Furthermore, there seems to be no competition, or interactions, between Ca2+, Ba2+ and Sr2+ for transport to the shoot. This seems incompatible with a symplastic pathway involving at least two protein-catalysed transport steps. Thus, a quantity of purely apoplastic Ca2+ transport to the xylem is indicated. The relative contributions of these two pathways to the delivery of Ca2+ to the xylem are unknown. However, the functional separation of symplastic Ca2+ fluxes (for root nutrition and cell signalling) and apoplastic Ca2+ fluxes (for transfer to the shoot) would enable the root to fulfil the demand of the shoot for calcium without compromising intracellular [Ca2+]cyt signals. This is also compatible with the observed correlation between transpiration rate and calcium delivery to the shoot.

Transport across plant roots

The plant root is a complex system that has evolved under the constraints of a number of functions. It is a pressure-probe that can penetrate the soil; it is a scavenger of nutrients that may be either tightly bound to soil particles or in low concentrations in the soil solution; it is an absorber of water from the soil. The tip of the root contains the region of dividing cells from which the root is formed, and is pushed through the soil by the extension of these cells some 10–20 times as they develop and differentiate.

Cortical cell fluxes and transport to the stele in excised root segments of Allium cepa L. : III. Magnesium

From compartmental analysis of radioisotope elution measurements, concentrations and fluxes of Mg(2+) were estimated for cortical cells in root segments of onion, Allium cepa L., relative to a complete nutrient solution containing 0.25 mM Mg(2+). Five compartments for Mg(2+) in the cortex were found and, in order of increasing rates of exchange, identified with the vacuoles and the cytoplasm of the cortical parenchyma, the Donnan free space, the water free space, and the superficial film of solution on the segments. With the Ussing-Teorell flux ratio equation as the criterion, it was concluded that Mg(2+) entered the cytoplasm passively and was actively pumped back across the plasmalemma. Mg(2+) concentration in the vacuole could be estimated only as lying between wide limits (1.3 to 14.3 μeq ml(-1)), but whatever the concentration within this range, it was concluded that Mg(2+) was passively distributed across the tonoplast. Net flux was zero and the vacuolar concentration commensurate with this was found to be 6.6 μeq ml(-1). The transported fraction of total efflux, appearing at the segment cut ends, was estimated separately. Magnesium was found to be transported almost exclusively in the basipetal direction.

Simultaneous uptake and translocation of magnesium and calcium in barley (Hordeum vulgare L.) roots

The patterns of uptake and translocation of magnesium in different regions of the root are very similar to those of calcium. Once the endodermis has become suberized translocation of either ion to the shoot is greatly reduced and it is concluded that magnesium, like calcium, appears to move across the root cortex largely in the free space.