|
Today is :
Archives (2003)
Understanding
salt tolerance in corn
by Likha C. Cuevas |
October-December
2003
|
 “Salinity
in soil? That's ridiculous! Only sea water is the saline environment
I know," one might say.
Au contraire, salinity is widespread and
about 344 million hectares of land in the world are affected.
However, 230 million of the total hectarage are not extremely
saline and can be planted with salt-tolerant species.
The ability of crops to tolerate saline environment
has been studied by researchers since it has been increasingly
necessary to boost crop production using available land that
we have. Many studies have focused on understanding the fundamental
mechanism for salt tolerance and breeding crops like corn
that are suitable for saline soils.
Researchers from the Institute of Plant Breeding
(IPB), University of the Philippines Los Baños (UPLB),
headed by Dr. Flordeliza Faustino and Dr. Roberta N. Garcia,
banked on the knowledge that these studies generated in identifying
corn genotypes that are saline-tolerant, the growth responses
of corn such as dry matter production, ion uptake, and nitrate
reductase (NR) and phosphoenolpyruvate carboxylase (PEPC)
activities in response to saline conditions towards revealing
the mechanism of salt tolerance in corn.
Causes of salinity
First of all, what is salinity? "Salinity," according
to Crop Growth in Saline Environment by Dr. S.M. Alam and
M.U. Shirazi, "is a concentration of dissolved mineral
salts present in water and soils on a unit volume or weight
basis." The major dissolved solutes comprising mineral
salts are the cations of sodium, calcium, magnesium, and potassium
and the anions chloride, sulfate, bicarbonate, carbonate,
and nitrate.
The problem of salinity is evident in: irrigation
and drainage waters, saline and sodic soils, saline groundwater,
seawater intrusion and brines from natural salt deposits.
The primary source of salts in water and soils is the chemical
weathering of earth materials, i.e. minerals that are constituents
of rocks and soils. Evaporative salinization, like evaporation
of water and transpiration by plants, and rainfall, snowmelt
waters and irrigation waters, affect the concentration of
dissolved mineral salts. Natural secondary sources of salts
include: atmospheric deposit of oceanic salts along coastal
areas, seawater intrusion into estuaries due to tidal events,
seawater intrusion into groundwater basins in coastal areas
due to overdraft, saline water from rising groundwater. Agriculture
and related activities that contribute to the salinity include
irrigation and drainage waters, soil and water amendments,
animal manure and waste, chemical fertilizer, sewage sludge
and effluent, and oil and gas field components.
What does salinity
do to crops?
Salinity has harmful effects on plants and these are caused
by two primary mechanisms. One is osmotic stress due to dehydration
of the plant. Osmosis is the diffusion of salt molecules through
a semi-permeable membrane of plant cells from a place of higher
concentration to a place of lower concentration until the
concentration on both sides is equal. The other mechanism
is the accumulation of toxic ions, primarily chloride and
sodium in plant cells, which can impede important physiological
processes.
Plants that suffer from salinity stress have
reduced vegetation, scorched leaf tips or margins, leaf premature
discoloration and abscission (shedding of leaves following
formation of scar tissue in a plant), and underdeveloped and
discolored roots.
Since salinity is a bad condition for important
agricultural crops like corn, there is a need to develop new
varieties that could withstand saline environments. However,
before corn varieties that could stand salt stress are developed
in the future, the IPB researchers needed to understand first
the mechanisms involved in salt toleration.
Effects of increasing
saline conditions on developing corn
The researchers obtained yellow corn inbred lines from the
Cereals Division of IPB to screen for corn breeds that are
salt-tolerant. The inbreds were subjected to salinity treatment
during the seedling stage and the reproductive stage. Twelve
tolerant, two moderately tolerant, and six sensitive breeds
were identified. Pi 11 and SMCE 21-28 (both sensitive at seedling
stage) and Pi 21 and Pi 31 (rated tolerant at both seedling
and reproductive stage) were chosen after the physiological
and biochemical studies performed on these inbreds. The growth
responses, accumulation of charged particles of salt (Na+
and Cl-), and NR and PEPC enzyme analyses were then performed
on the inbreds.
The researchers observed that the salt-sensitive
variety decreased by 30% in plant dry weight while the tolerant
variety decreased by 25%. The researchers attributed the observed
reduction in plant growth to the reduced surface area available
for photosynthesis, changes in plant water status and sodium
toxicity effects on leaf elongation. The comparison of dry
matter accumulation between salt-sensitive and salt-tolerant
inbreds suggested the adaptive mechanism of the two types.
"During the period of water deficit," Faustino et
al. noted, "dry matter content of the tolerant and sensitive
inbreds was lessened within acceptable levels while dry matter
accumulation was reduced (even resulting to death of the sensitive
plants) during the period of salt toxicity corresponding to
exposures to high salt levels." On the other hand, the
tolerant inbreds were able to sustain root growth at higher
levels of salt.
In the salt particle (ion) accumulation test,
the researchers observed that there is consistently low sodium
uptake combined with high potassium absorption in the tolerant
inbreds. "The high potassium content in the cells of
the salt-tolerant inbred corn may contribute to its ability
to adapt to salt stress by compensating the ionic imbalance
caused by the excess sodium and chlorine ions," the researchers
concluded. This means a better growth performance of the tolerant
inbreds.
The researchers observed that root NR of the
salt-tolerant Pi 31 showed higher activities while Pi 21 gave
the highest leaf NR activity in most salt levels. The results
indicate that the tolerant Pi 31 can withstand wider range
of salinity compared to the sensitive Pi 11. The researchers
concluded that, "the capability of NR and PEPC enzymes
to function effectively under saline conditions may have also
provided a mechanism for the enhanced assimilation and production
of organic acids for plant growth."
Mechanisms for
salinity tolerance
Faustino and her team concluded that high potassium concentration
in the cell and enhanced NR and PEPC activities might have
provided a mechanism to the tolerant inbred lines to adapt
to salt stress. These two enzymatic processes, "may have
contributed positively to the ability of the tolerant inbreds
to grow robustly at higher salt concentrations than the sensitive
ones." The researchers suggested that further studies
be done on the differential potassium, sodium, and chlorine
uptake processes and NR and PEPC molecular properties of salt-tolerant
and salt-sensitive corn varieties. 
References:
Faustino, F.C., Garcia, R.N., Agtarap, M.L., Tecson-Mendoza,
E.M.T., and Lips, S.H. Salt Tolerance in Corn: Growth Responses,
Ion Accumulation, Nitrate Reductase, and PEP-Carboxylase Activities.
Philipp. J. Crop. Sci. 2000. 25(1) 17-26
S.M. Alam and M.U. Sshirazi. Crop Growth in SalineEnvironment.
www.pakistaneconomist.com
Effects of Salinity to Crops.www.agrisupportonline.com/Articles/salinity/salinity.
"Osmosis", "homeostasis", and"abscission”
[More
2003 Articles]
|
