O

10 20 30 40 0 10 20 30 40 50 Days from sowing

Fig. 3.15 Effects of copper supply (left = low; right = high) and shading on the copper and nitrogen content of the oldest leaf of wheat. Key: O, unshaded; shaded. (From Hill et al.,

1979a.)

be accelerated by shading and this is associated with a more rapid remobilization of both nitrogen and copper; in copper-deficient plants most of the copper can then be remobilized. Nitrogen deficiency, like shading, also enhances copper remobilization (Hill etal., 1978). The same is true for zinc (Hill etal., 1979b). These relationships may in part be responsible for the results of field experiments showing a particularly high copper demand in plants supplied with high levels of nitrogen fertilizers and the corresponding delay in leaf senescence (Section 6.4).

The relatively high remobilization rates of micronutrients during reproductive growth as compared with vegetative growth stage are presumably the result of fruit- and seed-induced leaf senescence, in which changes in hormonal balance play an important role (Mauk and Nooden, 1992; Section 5.6). The development of sulfur-deficiency symptoms in either old or young leaves depends on the level of nitrogen supply (Loneragan etal., 1976) and is most likely also related to leaf senescence.

Remobilization of mineral nutrients requires several steps: (a) mobilization within individual leaf cells; (b) short-distance transport in the symplasm to the phloem; (c) phloem loading; and (d) phloem transport. Discrepancies between high or intermediate phloem mobility (Table 3.9) and low rates of remobilization, particularly during the vegetative growth stage, are most likely caused by insufficient mobilization within the leaf cells. A large proportion of micronutrients is incorporated into cell structures and high-molecular-weight organic compounds (e.g., enzymes). During reproductive growth seed- and fruit-induced leaf senescence overcomes the most limiting step (step a) of remobilization for most micronutrients. This is most likely the reason that, despite the high to moderate phloem mobility of the micronutrients iron, zinc, copper, molybdenum and also boron, deficiency symptoms of these micronutrients during the vegetative growth first appear in young leaves and the shoot apex. These vegetative growth sinks obviously lack the capacity to produce a 'signal' strong enough to induce leaf senescence and, thus, enhanced mobilization of these mineral nutrients within leaf cells.

The extent of remobilization of mineral nutrients is attracting increasing attention in connection with the selection and breeding of genotypes of high 'nutrient efficiency'. Genotypes that grow well on soils of low nutrient availability not only may have a higher rate of uptake and translocation of a particular mineral nutrient but may also show higher nutrient efficiency at the cellular level (compartmentation, binding stage, etc.), including high rates of remobilization from older to younger leaves, seeds, and storage organs.

3.5.5 Period Before Leaf Drop (Perennials)

Remobilization of mineral nutrients (except calcium and manganese) from the leaves to woody parts is a typical feature of perennial species before leaf drop in temperate climates, and is closely related to the discoloration of leaves in the autumn. As a rule, and similar to annual species, the extent of remobilization is high for nitrogen, potassium, phosphorus, and zinc, whereas the leaf contents of calcium, boron, iron and manganese increase until leaf drop (Sanchez-Alonso and Lachica, 1987a). During this period, typical visible deficiency symptoms are often observed, indicating that during the growing period there might have been a latent deficiency of a particular mineral nutrient. In plants growing on saline substrates, preferential remobilization of certain mineral nutrients (Table 3.19) often gives rise to toxicity symptoms in leaf margins, indicating a further shift toward extreme ionic imbalance before leaf drop.

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