E

4.6±0.1a

3.1 ±0.6b

4.4±0.4a

2.1±1.1b

gs

0.97±0.1a

0.35±0.1b

0.76 ± 0.2a

0.19±0.1b

Water potential

-0.06 ± 0.01a

-0.18±0.01b

-0.15±0.02b

-0.27 ± 0.02c

Data of each row followed by the same letter are not significantly different (P < 0.05) (Hajiboland and Farhanghi 2011)

Data of each row followed by the same letter are not significantly different (P < 0.05) (Hajiboland and Farhanghi 2011)

of shoot-root translocation of B under drought conditions caused reduction of shoot B content under drought stress (Hajiboland and Farhanghi 2011) . The transport of B from root to shoot is mainly driven by transpiration in the leaves, and the distribution of B in the shoot follows the gradient of transpiration rate in leaves (Brown and Shelp 1997) . There has been an active debate concerning a relationship between the ion and water transport in plants (Dalton et al. 2000). Water loss through transpiration in B-deficient plants has been studied in various plant species. In mustard (Brassica campestris L.), B-deficient plants had lower water potentials, decreased sto-matal pore opening, and reduced transpiration relative to B-sufficient plants (Sharma and Ramchandra 1990) . Reduction of stomatal conductance has been reported in other plant species under B deficiency conditions (Hajiboland and Farhanghi 2011). Interestingly, though a more conservative behavior of B-deficient plants regarding water economy following elevated sto-matal limitations, water potential was lower in B-deficient plants irrespective to the watering regime (Hajiboland and Farhanghi 2011) (Table 16.4). It implies that disturbance in water economy was due to lower water uptake and not greater water loss.

Lowered stomatal conductance by B deficiency and drought results in reduction of net CO2 assimilation rate (Han et al. 2008). Mechanism of a role for B in stomatal opening has not been investigated. Boron is required for integrity of membranes (Cakmak and Romheld 1997), and function as well as activity of H+ATPase (Roldan et al. 1992). Therefore, it is plausible that B deficiency causes reduction of K+ uptake into guard cells and following loss of membrane integrity, stimulates passive leakage of K+ from guard cells. Increased K+ leakage from leaf tissues was observed in B-deficient turnip plants (Hajiboland and Farhanghi 2010). Not only root-shoot transport but also retranslocation of B is influenced by drought conditions. Lower phloem transport of B under drought conditions was reflected in significantly lower ratio of B content of young to old leaf (Hajiboland et al. unpublished results). A dramatic effect of drought on reduction of B retranslocation evidenced that water-stressed plants had significantly lower B remobilization from old to young growing leaves. It was reported that B-deficient Norway spruce plants were not able to retranslocate B from other plant parts to the meristematic tissues and developing buds (Mottonen et al. 2005). The visible damage to the apical growth of the B-deficient plants observed after drought period may have been caused by inadequate B supply to the developing buds of mature Norway spruce trees growing in soil poor in B (Mottonen et al. 2005) .

8.1.3 Boron-Deficiency Induced Photoinhibition Under Drought Conditions

In addition of stomatal limitation, nonstomatal limitation of photosynthesis has been reported under combination of B-deficiency and drought stress that could be explained by inhibited leaf photochemistry as well as metabolic impairment. Photochemical quenching decreased significantly in plants subjected to combination of B deficiency and drought stresses (Hajiboland and Farhanghi 2011). Reduction of photochemical quenching could be related to photoinhibition rather than to a direct damage to PSII (Baker and Bowyer 1994). In addition, leaves under B deficiency and drought showed lower capacity for heat dissipation as indicated by significantly lower nonphoto-chemical quenching (Table 16.4). The activation of the carbon reduction cycle may be especially sensitive to drought (Reddy et al. 2004). Reduction of the activities of ribulose-1, 5-bisphosphate carboxylase/oxygenase, NADP-glyceraldehyde-3-phosphate dehydrogenase, and stromal fruc-tose-1,6-bisphosphatase in B-deficient citrus leaves has been reported (Han et al. 2008).

8.1.4 Boron-Deficiency Induced Oxidative Stress Under Drought Conditions

A significant rise of H2O2 titer and increased lipid peroxidation were detected under combinative effect of B deficiency and drought (Hajibland et al. unpublished data) implied occurrence of oxidative damage. However, proline accumulation was observed in B-starved plants when subjected to drought stress (Hajiboland et al. unpublished results). Boron deficiency-induced accumulation of proline may be regarded as a strategy for plants to counteract with the oxidative stress provoked after subjection of plants to the second stress factor such as drought.

8.2 Boron Deficiency and Plant

Responses to Low Temperatures

Low temperature is a common threat faced by crop species, particularly those of tropical or subtropical origin (Huang et al. 2005), fruit trees, eucalyptus, ash, and Norway spruce (Raisanen et al. 2007). Cultivation areas of many crop species of tropical or subtropical origin have been expanded into temperate regions for growth in the warm season. These plants encounter chilling stress in early spring and/or in late autumn. In addition, for perennial species, freezing damage in wintering buds and dieback of apical shoots is a common problem in forestry in boreal climates (Raisanen et al. 2009) .

Boron deficiency with characteristic symptoms, dieback of leaders and a bushy appearance of crown, is often reported in forest trees. Dieback of leaders has been observed after winter and therefore, winter hardiness is assumed to be impaired in B deficiency (Raisanen et al. 2006a). The susceptibility to summer frost of fruit trees, ash and spruce seedlings (Raisanen et al. 2007), and many cereals (Huang et al. 2005) decreased also when leaves were treated with borax solution some days before frost occurred. In chilling-sensitive crop species, but not in chilling-tolerant ones, exposure to low temperature induced severe B deficiency symptoms (Huang et al. 2005). In contrast to the interaction between drought stress and B deficiency, extensive studies have been performed on the chilling stress under low B supply. These studies suggested that there are interactions between chilling temperature and B nutrition at the organ level as well as the whole plant level.

8.2.1 Boron Deficiency-Induced

Impairment of Cold Acclimation

In plants supplied with adequate B and in which the buds are structurally normal, freezing tolerance increases properly during the cold acclimation in the autumn. However, if the buds are deformed because of B deficiency, they are not able to deep supercool and are unable to cold harden well accordingly (Raisanen et al. 2006b). The deformation is usually visible (under a ste-reomicroscope) as poor development of the primordial shoot, collenchymatic plate, and bud cavity in buds cut in half, even though all buds looked normal outward. Deep supercooling is the mechanism of survival of winter temperatures in most perennials in boreal forests. It means cooling of the buds to temperatures down to even -40°C without ice crystal formation in the primordial shoot. Deep supercooling of buds is dependent on the structures within the bud, as the collenchymatic plate in the bud axis functions as a barrier for the spread of ice (Raisanen et al. 2006b). Possible mechanisms that can affect the properties of this barrier include changes in the pectic compounds of the collenchymatic plate (Fleischer et al. 1999) or membrane-cell wall interactions (Bassil et al. 2004). Such changes might occur due to B deficiency even in buds that are not visibly deformed. Boron is a regulative element in pectin orientation in cell walls (Hu and Brown 1994) . In stems and buds with normally developed structure, pectin-rich ice barriers with small micropores prevent lethal ice invasion into the sensitive tissues in deep supercooling stems and buds (Wisnievski 1995). B-deficient cells have been found to be nonelas-tic with increased pore-sizes in cell walls (Fleischer et al. 1999). If the structure of micropores in cell walls is irregular, the deep-supercooling ability may decline. A decrease in photoperiod initiates the cold acclimation of trees, which further proceeds at low temperatures with several physiological and anatomical changes in cells. Chilling temperatures are also needed for hardening of roots (Raisanen et al. 2009).

8.2.2 Chilling-Induced Reduction of B Uptake, Root-Shoot Translocation, and Partitioning

In chilling-sensitive cassava plants, exposure to root temperature of 18°C for 28 days at a relatively high B supply induced severe B deficiency symptoms due to the inhibition of B absorption its transport into the shoot. Plants grown at 22, 28, and 33°C, by contrast, did not show any

B deficiency symptoms at the same level of B supply (Huang et al. 2005) . However, in chilling-tolerant species, such as wheat and oilseed rape exposure of roots to 10°C did not decrease B uptake (Huang et al. 2005) . In addition, reduction of B uptake under low temperatures was the result of impairment in root growth. In contrast to susceptible species, in the chilling-tolerant species, root growth was tolerant to low root temperature that led to an increased root: shoot ratio at 10°C in the root zone temperature (Huang et al. 2005). In turn, increased mortality of roots during soil freezing and thawing cycles due to low-B conditions and increase in the risk of freezing injury due to structural damages has been reported for Norway spruce roots (Raisanen et al. 2009). In a long-term field study in a Norway spruce stand with low B status, the proportion of dead fine roots decreased due to B fertilization (Mottonen et al. 2003).

8.2.3 Boron-Deficiency Induced

Disruption of Water Relations Under Chilling Stress

Variation among species and genotypes in chilling tolerance is closely related to the effects of chilling on water uptake by roots and transpiration by leaves (Bloom et al. 2004).

Transpiration

In chilling-sensitive species such as cucumber, tomato, and sunflower, dysfunction of stomatal control expressed as delayed closure or closure failure causes an excessive transpiration (Allen and Ort 2001) . Root low temperatures reduce B partitioning into new leaves and increase the sensitivity of growth of young leaves to low B supply (Ye et al. 2000).

Reduction of root hydraulic conductance in chilling-sensitive species like cassava, sunflower and tomato, reduction of root hydraulic conductance and water absorption under low temperatures are likely the cause of observed reduction in the B absorption and enhanced B deficiency. By contrast, chilling tolerant species such as oilseed rape are able to maintain a high root hydraulic conductance at similar root zone temperature (10-15°C) (Huang et al. 2005).

Reduction of Water and B Uptake via Water Channels in B-Deficient Plants

Water channels (aquaporins) in the plasma membrane of root cells play an important role in plant-water relationships particularly under changing environmental conditions (Tyerman et al. 2002) . Evidences suggest that closing of water channels in chilling-sensitive species occurs in response to root chills. Water channels in the plasma membrane may be also reversibly closed by 'OH radicals (Henzler et al. 2004) that are accumulated under B deficiency conditions (Cakmak and Romheld 1997). In addition, water channels also play an important role in the B uptake across the plasma membrane particularly when external B concentrations are low (Dordas et al. 2000). Severe B deficiency causes reduction in the amount of the plasma membrane water channel proteins (ZmPIP1 aquaporins) in the roots of maize and transgenic tobacco (Goldbach et al. 2002) .

Boron Xylem Loading

When external B levels are low, loading of B into the xylem requires the function of B transporters (Takano et al. 2002). The activity of B-transporters is sensitive to low temperature and chilling stress may inhibit active B transport across the pericy-cle into the stele of the root cylinder (Huang et al. 2005) .

Plasma Membrane Function and Permeability

Instability of cell membranes has been recently suggested to explain decreased chilling tolerance of crop species in B deficiency (Huang et al. 2005). Since the plasma membrane is a primary site of freezing injury, B deficiency may increase the susceptibility of cells to frost damage (Raisanen et al. 2007) . Changes in the proportions of sterols and longer chain fatty acids in the plasma membrane of root cells alters significantly B uptake in Arabidopsis thaliana mutants (Dordas and Brown 2000). Reduction in membrane fluidity and permeability of root cells following chilling stress may also contribute to the inhibition of B uptake in chilling-sensitive species such as Coffea arabica (Queiroz et al. 1998).

Function of H+ATPase

Low temperature (6°C) during acclimation period increases transcription and translation of plasma membrane H+ATPase gene (Ahn et al. 1999). Boron-deficiency-induced impairment of H+ATPase activity (Roldan et al. 1992), which needs to be more activated under cold conditions, could be one of the reasons for the exacerbating effect of B deficiency under low temperatures.

8.2.4 Boron-Deficiency Induced

Photoinhibition and Oxidative Stress Under Chilling Temperatures

Boron deficiency intensifies chilling-induced photooxidative damage and reduces plants anti-oxidant capacity during recovery from photoinhibition. In plants grown in low B soil, there is a relationship between chilling temperature and leaf tissue damage (bleached patches) (Ye 2005). One of the possible mechanisms is the role of B in the integrity and functions of thyalkoid membranes. In a chilling-sensitive cucumber cultivar, low temperatures enhanced effect of B-deficiency on membrane leakage (K+ ) and chloroplast disruption and plasmolysis of mesophyll cells (Wang et al. 1999). Another reason for B-deficiency induced photoinhibition under cold conditions is the substrate feedback inhibition and starch accumulation in chloroplasts (Huang et al. 2005).

In addition, suboptimal B nutrition reduces the activity of antioxidant enzymes and the level of antioxidants such as reduced form of ascorbic acid, SH-compounds and GR (Cakmak and Romheld 1997) . Moreover, B-deficiency-induced accumulation of soluble phenolics due to enhanced PAL activity and subsequent oxidation by polyphenol oxidase located in the thylakoid membrane and in cell walls (Cara et al. 2002) causes permanent damage to the cells and photo-synthetic membranes. In addition, chilling temperature increases production of H2O2, which with superoxide forms highly reactive 'OH radicals in leaf cells (Saruyama and Tanida 1995) . Accordingly, B deficiency increases the sensitivity of leaf cells to chilling through enhanced generation of ROS and weakened antioxidation capacity.

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