Roots, Dicotyledon Monocotyledon

Despite increased citrate accumulation in roots of Zn-deficient rice plants, root exudation of citrate was not enhanced. However, in distinct adapted rice cultivars, enhanced release of citrate could be observed in the presence of high bicarbonate concentrations in the rooting medium, a stress factor, which is frequently associated with Fe and Zn deficiency in calcareous soils (235) (Hajibo-huid, unpublished). This bicarbonate-induced citrate exudation has been related to improved Zn acquisition in bicarbonate-tolerant and Zn-efficient rice genotypes (Fig. 9) (23S). Increased exudation of sugars, amino acids, and phenolic compounds in response to Zn deficiency has been reported for various dicotyledonous and monocotyledonous plant species and seems to be related to increased... 

I The distinction between mono-and dicotyledonous plants is quite simple monocotyledons are flowering plants which have only one seed leaf, and usually have parallel-veined leaves, flower parts in multiples of three, and no secondary growth in stems and roots, whereas dicotyledons are flowering plants with two seed leaves (cotyledons), net-veined leaves, flower parts in fours and fives, and in woody plants have cambium, a layer of formative cells between the wood and the bark the cells increase by division and differentiate to form new wood and bark. 

While wood forms in the stems and roots of both the cone bearing plants (the gymnosperms popularly known as softwoods) and many dicotyledonous flowering plants (the angiosperms also called hardwoods), monocotyledonous flowering plants such as the palms have solid stems and roots but the material in them is not true wood and has very different properties. 

Lachance et al. [50] also conducted a suite of experiments to evaluate the potential of HMX to accumulate in several plant species. Consistent with other reports, the highest concentrations were found in the leaves, followed by roots and stems. The bioaccumulation potential for HMX was lower than that for RDX, and was independent of soil concentration. Indeed, Lachance et al. [50] showed that [14C]HMX was accumulated by different plant species with maximum BAF of 2.2. No distinction could be made regarding clear differences between monocotyledonous and dicotyledonous species in bioaccumulation potential under these conditions (Table 10.8). 

The uptake and translocation of dalapon is similar in sensitive and tolerant plants, so the toxicity of the compound to weeds must have another basis. Despite rapid absorption the phytotoxic action is manifested slowly. The leaves of sensitive plants curl and, starting from the margins, become yellow. The development of the plants stops, buds and roots cease to grow, and finally, the plants wither in 3-4 weeks. Se germination of dicotyledonous weeds is barely inhibited by dalapon, and even that of monocotyledons only at a high concentration. 

Both herbicides are absorbed mainly through the roots and kill monocotyledon-ous and dicotyledonous weeds efficiently. Owing to their lack of selectivity, they are primarily used on industrial and non-crop lands at a rate of 10 kg active ingredient/ha. 

Castanospermine (36) (1,6,7,8-tetrahydroxyoctahydroin-dolizine) is a potent phytotoxin for dicotyledonous plants at 300 ppb, and for monocotyledonous plants at 200 ppb. Swainsonine is ineffective as an inhibitor of root elongation. (Stevens and Molyneux, 1988). 

In the stem of dicotyledons, the xylem and phloem tubes are usually found in a ring near the outside of the stem with phloem on the outside and xylem on the inside, separated by a cambium layer. In monocotyledons the vascular bundles are more randomly arranged within the stem. In the root, the xylem and phloem tubes form separate bundles and are found near the centre. 

This is found on all grasses and cereals, and it is the main root system of most monocotyledons. The primaiy root is soon replaced by adventitious roots, which arise from the base of the stem (Fig. 1.12). These roots can, in fact, develop from any part of the stem, and they are found on some dicotyledons as well (but not as the main root system), e.g. underground stems of the potato. 

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