Management of agricultural produce respiration during storage and aspects of mineral root nutrition

The need to manage respiration arises during the storage of grain, fruits, vegetables, and root crops. The process of respiration releases energy in the form of heat, carbon dioxide, and water; organic substances are consumed, resulting in a decrease in the mass of the produce.

For the proper storage of cereals and grain legumes, as well as the seed of oilseed crops, the following humidity standards have been established:

At higher humidity, the intensity of respiration increases, which is accompanied by the appearance of free water in the tissues of caryopses and seed, and this, in turn, causes an intensification of metabolism. With an increase in seed humidity, heat accumulation occurs, which causes self-heating. One of the methods for managing respiration is air-thermal heating of seed.

Mineral nutrition is as unique a plant property as photosynthesis. It is these two functions that form the basis of the autotrophy of a plant organism, i. e., the ability to build its body from inorganic substances. Moreover, managing plant root nutrition is significantly easier than regulating aerial nutrition – the assimilation of CO2.

Ion uptake by roots is a combination of two processes:

• passive absorption, where ions move due to mass flow and diffusion through the apoplast;
• active absorption or active transport, during which ions are transferred into cells against a concentration gradient through respiration.

N. Green, W. Stout, D. Taylor, 1990

Plant roots absorb water and mineral nutrition elements from the soil, serve as a kind of anchor holding the above-ground part of the plant, synthesize organic compounds necessary for the plant organism, and release metabolic products. Roots must be capable of performing all these functions without restricting plant growth.

The root has a complex morphological and anatomical structure. In accordance with the functions it performs, the root structure is divided into a root cap and four zones: division, growth, absorption, and conduction (Fig. 41; Yakovlev G.P., Chelombitko V.A., 1990). It is important to know the significance and functions of these root zones, as they determine the parameters of agricultural practices: the depth of primary and Pre-sowing tillage">pre-sowing tillage, the depth and method of fertilizer application, and the depth and width of cultivation during plant maintenance.

The functions of the root cap are to protect the growth point and ensure positive geotropism for the roots. Adjacent to the root cap is the division zone, composed of meristem cells. During mitotic division, the meristem deposits cells inward, ensuring root growth, and outward, replenishing the cells of the root cap.

The growth zone follows the division zone. The cells of the growth zone practically do not divide but are capable of stretching in a longitudinal direction, pushing the root tip deep into the soil. They are characterized by high turgor, which contributes to the active pushing apart of soil particles. Within the growth zone, the differentiation of primary conductive tissues occurs.

Beyond the growth zone lies the absorption zone. In this zone, numerous root hairs are formed from the cells of the primary integumentary tissue – the epiblema – which serve as a support for the growing root tip and perform the function of absorbing water and mineral nutrition elements from the soil.

Fig. 41. General view (A) and longitudinal section (B) of a root tip: I – root cap; II – growth and elongation zone; III – root hair zone, or absorption zone; IV – beginning of the conduction zone (lateral roots also originate in this zone): 1 – developing lateral root; 2 – root hairs on the epiblema; 3 – epiblema; 3a – exodermis; 4 – primary cortex; 5 – endodermis; 6 – pericycle; 7 – central cylinder

Root hairs are thin outgrowths of epiblema cells, 60–1500 µm in length and 5–70 µm in diameter. The total length of root hairs produced during a plant's lifetime is enormous. For example, the total length of rye roots is approximately equal to the distance from Moscow to St. Petersburg, and with root hairs included – the distance from Moscow to Khabarovsk. The number of root hairs reaches several hundred per 1 mm² of root surface in the absorption zone, which increases the absorbing surface of the root system by 5–20 times. They release metabolic products into the soil – mineral and organic substances that facilitate the dissolution of poorly soluble soil compounds and positively influence the development of microflora. The lifespan of root hairs does not exceed 10–20 days. The absorption zone is several centimeters long; it is here that roots absorb the bulk of water and dissolved salts. Cells in the absorption zone cannot move through the soil, as longitudinal cell elongation has ceased in this zone, and the root hairs "fuse" with soil particles. Nevertheless, the absorption zone constantly moves through the soil. This is explained by the continuous inclusion of new young cells from the growth zone and the simultaneous exclusion of aging cells, which transition into the conduction zone. It begins above the absorption zone, where root hairs disappear. Through this part of the root, water and dissolved mineral nutrients are transported to the plant's aerial organs.

The root system of plants is a dynamic organ that constantly moves through the soil in search of moisture and nutrients. As the absorption zone grows, the plant explores new micro-areas of the soil and gains access to unused reserves. The direction of this growth depends directly on the distribution of fertilizer.

For an agronomist, this feature provides an opportunity to manage root development in the field or in a greenhouse. It is sufficient to know how plants react to the concentration of the soil solution. Depending on external conditions, the root system exhibits two types of response:

• Positive chemotropism — roots grow vigorously towards zones with available nutrients.
• Negative chemotropism — root growth slows down or stops in zones with a high, unfavorable salt concentration.

How to prevent root suppression during fertilizer application

With proper distribution of nutrients, the root system is capable of developing colossal absorption capacity. The agronomist's task is to stimulate this process from the very first stages of the growing season. The physiological characteristics of the absorption apparatus allow the crop to use soil resources with maximum efficiency.