Plant water content

Here, the fresh weight approximates the plant water content, assuming that 1 g fresh weight is equivalent to cm since the plant volume is largely a function of the water content. Some computations of surface expansion are given in Table 2.1. As can be seen from the table, surface expansion is determined to some extent by the water status of the tissue (see Walter, 1926). 

Another group of effects consists in blocking the channels of losing water from the soil layer, i.e., the hydraulic conductivity responsible for the gravitational flow, and of physical evaporation. All these effects provide an increase of the water content of the soil and, consequently, improve the water supply of plants, which is reflected in the three last columns in Table 8. According to the data of various authors, an increase in the soil water content (AW) in sandy soils lies in the range of 10-35% at doses up to 0.2% in a number of cases [10, 11, 58, 131-133] the dependencies of AW on the doses of the hydrogels added have been studied. 

In many cases, where one is concerned with the effects of specific environmental factors it is appropriate to replace the general term stress by the appropriate quantitative measure (e.g. soil water content or water potential) together with an appropriate measure of the plant response (e.g. growth rate). 

Fig. 8. ABA accumulation in detached root tops (apical 20-30 mm) of Commelina as a function of root tip turgor. Root tips were excised from well-watered plants and dried in air at 23 °C in the dark until different percentages of fresh weight had been lost. This process took between 5 and 20 min. The samples were then maintained at the various water contents for 7 h prior to measurements of water relations and ABA content. Points are means s.e. of at least four measurements. Modified from Zhang Davies (1987).

Fig. 11. Bulk soil water content and root ABA content in consecutive 10 cm soil layers on days 3, 9,12 and 18 after withholding water (A) from maize plants growing in 1 m deep soil columns. Well-watered plants (A) received water daily throughout the experimental period. Points are means of five measurements. Modified from Zhang Davies (1989).

Fig. 12. Relationships between root ABA content and bulk soil water content for maize plants growing in drying soil columns. Data are from Fig. 11, but do not include soil water contents less than 0.1 g cm in which many roots were non-living. Modified from Zhang Davies (1989).

There are representatives of desiccation tolerant species amongst all of the major plant divisions. The water content of many bacterial and fungal spores is low (<25%) and they exhibit great tolerance of desiccation (see Ross Billing, 1957 Bradbury et ai, 1981). Desiccation tolerant cyanobacteria are found in a diverse range of drought-prone habitats. 

Heyser, J.W. Nabors, M.W. (1981). Growth, water content and solute accumulation of two tobacco cell lines cultured on sodium chloride, dextran and polyethylene glycol. Plant Physiology, 68,1454-9. 

The second requirement is that enforcement methods for food must be validated by an independent laboratory [independent laboratory validation (ILV)]. The sample set is identical with the general sample set (see Section 4.1). If the method is identical for all four crop groups (mentioned at the beginning of the section), it may be sufficient to perform the ILV for plant materials with a minimum of two matrices, one of them with a high water content. In the case of food of animal origin, the ILV should be performed with at least two of the matrices milk, egg, meat, and, if appropriate, fat. 

Plant material water contents range from high (>90%, e.g. vegetables) to low (< 10%, e.g. straw, herbs, tea, hops, etc.). Thus the ratio between the analytes (residues) and the organic matter potentially interfering with the analysis is very different for, e.g., cucumber and camomile tea. Other ingredients in plant materials such as acids, oil, sugars, starch or substances typically for the taste and effect of plant materials may have properties similar to those of the analytes and thus interfere in or influence the cleanup procedures. 

El Extraction and subsequent Uquid/liquid partition Plant material and foodstuffs with a water content exceeding 70g/100g and a fat content below 2.5 g/100 g Fruit, vegetables, juices... 

E7 Extraction in the presence of large amounts of fat Plant and animal fats with low water content, if the Umit of determination is not sufficient with E6, and dry food with a fat content exceeding 2.5 g/100 g Edible fats and oUs, wheat and rye germs, oats, nuts, oUseed... 

The lack of understanding of the effect of the allelopathic agents on whole plant photosynthetic processes, namely, changes in stanatal opening, membrane permeability, water content and marry other processes that affect the overall photosynthetic processes. 

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