Physiology and Molecular Biology of Stress ... - KHAM PHA MOI

204
K. Janardhan Reddy
Table 14. Effect of molybdenum on dry matter yield, nitrogen uptake and
molybdenum content in groundnut*
Molybdenum Dry matter Nitrogen Molybdenum content
Treatment (g/ha) (Kg/ha) uptake (kg/ha) (µg/g dry wt.)
Seeds Nodules Shoots
0 2685 70 0.02 0.4 0.02
200 (Soil application) 3413 90 0.02 1.5 0.20
200 (Foliar application) 3737 101 0.05 3.7 0.53
*Adapted from Rebafka et al., 1993
An appropriate concentration of Mo has to be used not only to boost the
agricultural yields but also to avoid reaching toxic levels of Mo accumulation in the
forage plants. Whenever Mo content is above 5mg/kg dry weight of forage, it will
induce toxicity in animals commonly known as “molybdenosis”.
13. BORON
Boron is best considered as intermediate in properties between metals and non metals.
Although soils contain around 20 to 200 ppm of boron (B), much of it is not available to
plants because of it is held firmly by soil organic matter. Boron is commonly referred as
an inert anion and available as boric acid. At lower pH it occurs as undissociated boric
acid (B(OH 3
)) and at higher pH it present as tetrahedral borate anion [B(OH) - 4 ].
A significant proportion of total boron is complexed in cis-diol configuration
in the cell walls of higher plants (Thellier et al., 1979). Boron forms cross links with
certain polyhydroxy polymers like galactomannan (Loomis and Durst, 1991). Boron
plays a key role in sugar transport and carbohydrate metabolism, cell wall synthesis,
nucleic acid metabolism, phenol metabolism, IAA metabolism and membrane stability.
For a long time it was suggested that boron plays an important role in the transport of
sugars by the formation of sugar-borate complex (Dugger, 1983). This hypothesis is no
longer acceptable since sucrose, the important sugar which is transported in the phloem,
forms very weak complex with boron. Apart from this boron is not involved in the
phloem loading of sucrose during sugar transport. The role of boron in cell wall synthesis
and membrane integrity is evident in pollen tube growth (Vaughan, 1977). After
germination new cell wall material is deposited at the tip of the pollen tube. If growing
pollen tubes are deprived of boron, there was abnormal swelling in the tip region within
2-3 minutes of removal. Boron also influence fertilization by increasing the pollen grain