a Mineral food. Apoplast and simplast

you are: a Mineral food

Mineral substances in the form of ions together with water are absorbed by a plant from soil through root hairs and others epidermalnye the cages located near to a tip of a root. During migration on a plant the absorbed ions can diffundirovat through apoplast or simplast (fig. 7.14). The structure apoplasta includes damp walls of all cages of a plant and intercellular space. Walls of adjacent cages are in physical contact, and, except for several specialised sites, such as corbels of Kaspari, all of them form a continuous zone through which water and ions can diffundirovat freely, without meeting on the way of barriers of permeability. In this connection such cellular walls name sometimes free space though their negative charge can influence relative movement of ions.

Plazmalemma surrounding each protolayer, separates apoplast from simplasta. Last includes:) the cytoplasm connected with a membrane vakuolizirovannyh cages; the bridges connecting the majority of cages of the higher plant with each other, and transport cages floemy. Recollect that tsitoplazmaticheokie the bridges named plazmodesmami, penetrate cellular walls (sm, fig. 2.33), giving the chance to those molecules to migrate from one protolayer to another, without crossing plazmalemmy and not diffundiruja through cellular walls.

We do not have enough extensive and exact information on relative use simplastnogo and apoplastnogo ways of movement of separate molecules or ions. Our conclusions lean mainly against the indirect data, i.e. Are based on inductive thinking. Walls of many cages participating in an intensive exchange metabolitami, contain numerous plazmodeomy, for example walls between mezofillom and cages of a facing of vascular bunches at some С4-растений (see gl. 4). It is supposed that movement of ions and metabolitov in such zones occurs on plazmodesmam.

At water plant Vallisneria as s6Rb + (analogue To +), and 36S1 - are transported on simplastu. After drawing of these radioactive isotopes on one end of the cut off part of the sheet floating on water, they have been found out on its other end without label loss in a surrounding solution. As at Vallisneria is absent kutikula and continuous cellular walls adjoin to surrounding solution, the conclusion is made that movement of ions at this plant floating on water occurs exclusively on simplastu.

Rather simplastnogo transport for land plants is available less data. Definition by the ultrastructural methods would be one of ways of reception of such information, what molecules are localised in plazmodesmah. Similar experiences on the plants infected with viruses show that virus particles move on plazmodesmam. At the majority of vegetative viruses diameter of particles fluctuates from 20 to 80 nanometers. External diameter plazmodesmy is in these limits (fig. 2.33 see). Therefore small virus particles can pass on opened plazmodesme, without changing its structure. However larger particles, apparently, change the sizes and the form plazmodesm, located between plant-owner cages.

The Decision of a question on, whether dvizhutsja the dissolved molecules on plazmodesmam, is connected with extremely big technical difficulties. The clamps used for preparation of vegetative fabrics for the purpose of their studying by means of an electronic microscope, are unsuitable in this case as they can dissolve and move many ions. To bypass this difficulty, it is possible to process a fabric a reactant besieging some ions before fixing. For example, silver nitrate (AgNOs) it is possible to use ion chloride as Ag +, co-operating with С1- in a fabric for fixing of a site, forms the insoluble salt AgCl possessing in high electronic density. This method ions С1- managed to be found out in plazmodeomah Limonium (fig. 7.15).

Other methods developed recently include fast freezing of a vegetative fabric at very low temperature on purpose immobilizatsii ions which then can be found out and localised by means of an electronic microscope. In this device the fabric is exposed to bombardment by a bunch elektronov with high energy. When the elements activated thus come back again to the initial power level, they let out X-rays. Frequency of these beams is characteristic for each element. Therefore the quantitative analysis of let out X-rays allows to find out any element. Similar methods were already used for supervision over movement kalija and chlorine in closing cages and from them at opening and closing ustits (fig. 6.14), and also in motor cages of moving leaves see and from them (fig. 12.8 see). Application of these methods at ultrastructural level would give the quantitative information on movement of ions on apoplastu and simplastu.

We know about the forces operating movement of ions on plazmodesmam a little. Some physiologists of plants believe that plazmodesmy represent an open time, through which ions quickly diffundirujut, and speed and a direction are net-movement defined exclusively by the sizes of a time and a difference of concentration of those or other ions inside and outside of cages. Now it is impossible to check up, whether this interpretation is correct. At some plants plazmodesma has two separate channels (fig. 2.33 see): an internal part central desmotrubochki and the cytoplasm lying between desmotrubochkoj and a membrane delimiting plazmodesmu. It is not known, whether diffundirujut solutions on both these channels and what role of each channel.

Until recently only few physiologists of plants recognised importance simplastnogo transport. It speaks mainly that before occurrence of an electronic microentrenchment plazmodesmy it was difficult to see. Now researchers in many laboratories give a great attention to such aspects, as ultrastructure and electric properties plazmodesm, and also localisations in them of ions. We have the right to expect that in the near future our understanding simplastnogo transport becomes much deeper.