Ir

37 ± 5a

9 ± 3b

820 ± 135a

556±98b

57.4 ± 4.0b

83.7 ± 7.3a

9.9 ± 1.2b

21.0 ± 4.5a

Data of two Zn supply levels within each genotype and parameter followed by the same letter are not significantly different (P< 0.05). Definition of Zn-efficient genotypes refers to a classification based on the ability of plants to grow under low Zn supply with less Zn deficiency symptoms and less reduction in dry matter production compared with Zn-inefficient genotypes (Hajiboland 2000)

Data of two Zn supply levels within each genotype and parameter followed by the same letter are not significantly different (P< 0.05). Definition of Zn-efficient genotypes refers to a classification based on the ability of plants to grow under low Zn supply with less Zn deficiency symptoms and less reduction in dry matter production compared with Zn-inefficient genotypes (Hajiboland 2000)

a close linkage between nutritional status of leaves and spectral characteristics seems unlikely (Adams et al. 2000). However, there are reports on significant reduction of maximum quantum efficiency of PSII (Wang and Jin 2005) and severe damage to the ultrastructure of chloroplasts in plants subjected to inadequate Zn supply (Chen et al. 2007). Photooxidative damage in Zn-deficient leaves can be expected as a result of impaired photosynthetic CO2 fixation and reduced activity of SOD. Reduction in photosynthesis induced by Zn deficiency is associated with a decrease in intercellular CO2 concentration and stomatal conductance (Hajiboland and Amirazad 2010a). Sharma et al. (1995) reported a significant role of Zn in the regulation of the stomatal aperture. This role of Zn was ascribed to maintenance of a high K concentration in guard cells. A decrease in carbonic anhydrase activity due to Zn deficiency is well known (Hajiboland 2000), and may be a factor contributing to reduced photosynthesis (Cakmak and Engels 1999) . Inhibition of photosynthesis in Zn-deficient plants can also be a consequence of a Zn-deficiency-induced reduction in phloem sap sink demand. Additionally, in Zn-deficient plants there is an enhanced accumulation of carbohydrates, possibly resulting from either impaired phloem export of carbohydrates or decreased sink demand (Marschner et al. 1996). Impairment in utilization of electrons and absorbed light energy for CO2 fixation in Zn-deficient plants may accentuate photogeneration of ROS and pho-tooxidative damage to chloroplasts (Fig. 16.1).

4.3 Zinc Deficiency-Enhanced

Susceptibility to Excess Light

Enhancements in chlorosis and necrosis due to increased light intensity are very typical in Zn-deficient source leaves, reflected in a massive accumulation of sucrose and starch (Marschner and Cakmak 1989) causing a high potential for photooxidative damage of chloroplast constituents. In accordance with this suggestion, enhancements in light intensity markedly stimulated appearance of leaf chlorosis under Zn deficiency, but not at adequate Zn supply. Also, partial shading of Zn-deficient leaves prevented or strongly delayed appearance of chlorosis in the shaded areas (Cakmak 2000) . Increased severity of leaf chlorosis under high light intensity in Zn-deficient conditions is not caused by lower Zn concentration in leaves but is a consequence of photooxidative damage to chloroplast pigments catalyzed by ROS. Photooxidative damage of the chloroplast constituents under Zn deficiency can also be aggravated by reduced activity of enzymes scavenging Oi" and H2O2 in chloroplasts (Hajiboland and Amirazad 2010b»).

whole plant photosynthesis following stomatal limitation. In addition, under low Zn and drought stress only a small part of Zn taken up by plants is transported into leaves due to significantly lower stomatal opening and transpiration (Hajiboland and Amirazad 2010b).

4.4 Zinc Deficiency-Induced

Susceptibility to Drought Stress

It was reported that Zn deficiency is prone to occur in arid and semi-arid regions where soils, particularly top soil, are usually deficient in water (Cakmak et al. 1996). Under drought conditions, Zn mobility in the soil is extremely low, therefore, Zn uptake is usually reduced by low water availability in the substrate. In addition, strongly inhibited root growth in Zn-deficient plants reduces markedly the soil volume exploited by roots and impairs nutrients uptake particularly those are dependent more on spatial availability such as Zn (Marschner 1995). Within plants, there are also some interactions between Zn nutritional status and water relations. It was shown that the ability of plants to cope with water stress during early vegetative stage could be enhanced with adequate Zn supply (Grewal and Williams 2000). Sensitivity to Zn deficiency stress became more pronounced when plants were drought-stressed (Bagci et al. 2007). Impairment of growth in Zn-deficient plants was markedly higher when they were subjected to drought stress and in turn, the effect of drought stress on the inhibition of dry matter production was greater in Zn-deficient compared with Zn-sufficient plants (Hajiboland and Amirazad 2010b).

Because of the effect of Zn deficiency on increasing stomatal limitation (Sharma et al. 1995), plants under low Zn supply are more conservative in relation to water economy than sufficient plants when grown under drought stress as indicated by lower water loss, greater water and osmotic potential (Hajiboland and Amirazad 2010b). However, higher growth impairment under combinative effects of Zn deficiency and drought stress is due to damage to photosynthesis apparatus, greater ROS production and remarkable reduction of

4.5 Reduction of Plants Resistance to Flooding

It was reported that symptoms of Zn deficiency normally appear shortly after flooding (Van Breemen and Castro 1980). Flooding conditions may not only influence Zn availability in soil but also alter plants performance under Zn starvation. In soils, flooded conditions lead to a high concentration of bicarbonate and organic matters which in turn reduce the Zn concentration in soil solution. On the contrary, under flooding conditions concentration of Zn in soil solution decreases through formation of insoluble Zn sulfide (Marschner 1995). Alcohol dehydrogenase (ADH), an enzyme which contains two Zn atoms per molecule, involves in the reaction of acetaldehyde to ethanol. In Zn-deficient plants particularly under anaerobic conditions, ADH activity decreased, which might lead to accumulation of acetaldehyde up to toxic levels (Marschner 1995). Moore and Patrick (1988) reported a decreased root ADH activity in flooded rice plants that was correlated with Zn concentration in the roots and resulted in less ATP production, thereby reducing vital metabolic activities of the roots. Accordingly, a correlation would be expected between susceptibility of plants to Zn deficiency and flooding. In a work with four rice genotypes, however, it was found that ADH activity rather increased in some genotypes under low Zn supply irrespective to their Zn-efficiency trait (Table 16.3). It could be suggested that, Zn availability at molecular level was not necessarily a limiting factor for ADH holoenzyme in these genotypes. The main cause for negative effect of flooding may be disruption in the function of antioxidant defense system required particularly for Zn-deficient plants under waterlogged conditions. Accordingly, a close correlation has been observed between flooding tolerance and accumulation of ROS in rice genotypes

Table 16.3 Plants dry matter production and activity of alcohol dehydrogenases (ADH) in roots of Zn-efficient (IR34 and IR36) and Zn-inefficient (IR26 and IR54) rice genotypes grown in hydroponic medium with adequate (130 pM free Zn2+ activity) and low (9 pM free Zn2+ activity) Zn supply with (control) or without (hypoxia) aeration

Table 16.3 Plants dry matter production and activity of alcohol dehydrogenases (ADH) in roots of Zn-efficient (IR34 and IR36) and Zn-inefficient (IR26 and IR54) rice genotypes grown in hydroponic medium with adequate (130 pM free Zn2+ activity) and low (9 pM free Zn2+ activity) Zn supply with (control) or without (hypoxia) aeration

Was this article helpful?

0 0
Berry Boosters

Berry Boosters

Acai, Maqui And Many Other Popular Berries That Will Change Your Life And Health. Berries have been demonstrated to be some of the healthiest foods on the planet. Each month or so it seems fresh research is being brought out and new berries are being exposed and analyzed for their health giving attributes.

Get My Free Ebook


Post a comment