Handouts Physiology of salt tolerance
Current scenario of world population and food security
Humans primarily consume plants. Humans also consume animals that only consume
plants. Humans get 85% energy directly from plants (Chrispeels and Sadava, 2003). It is
a fact that increasing human population puts considerable pressure on the food production
system because directly or indirectly (as animal feed) plants are the basis of human
nutrition (Chrispeels and Sadava, 2003). According to various estimates, it is expected
that human population will increase over 8 billion by the year 2020 that will worsen the
current food insecurity. According to a report by the FAO (2003) over past 50 years,
world food supplies increased up to 20% per person due to improved crop productivity
and proportion of food-insecure peoples living in developing countries reduced from 57
to 27% of the total population. Regardless of these astounding achievements, 800 million
people are still hungry and malnourished in the developing world. Of these malnourished
or hungry people, greater numbers of people are in India, sub-Saharan Africa, China (UN
Millennium Project, 2003). It is predicted that at least 10 billion people will be hungry
and malnourished in the world by the end of this century (FAO, 2003). A few years back,
in the first 5 chapters of an edited book by Chrispeels and Sadava (2003) [Plants, Genes
and Crop Biotechnology, 2nd edition] different scientists discussed the economic,
Fig. 2.1 Projections of world population and annual growth rate. This is the median projection from
the United Nations as of 2000. Source: Data from United Nations produced after Sadava, D. E.
2003. Human population growth: lessons from demography In: Plants, genes and crop
biotechnology, M.J. Chrispeels and D.E. Sadava. (eds). 2nd (Ed), Jones and Bartlett Publishers,
Massachusetts, pp. 1-21.
Fig. 2.2. Food Index: Food production per person. The amount of food produced per person has
remained the same in the developed regions for the past 25 years, whereas food production increases
have outstripped the population rise in the developing regions. For purposes of illustration, production
levels in 1970 are arbitrarily set at 100 source data from US Department of Agriculture. Produced after
Sadava, D. E. 2003. Human population growth: lessons from demography In: Plants, genes and crop
biotechnology, M. J. Chrispeels and D. E. Sadava. (eds). 2 nd (Ed), Jones and Bartlett Publishers,
Massachusetts, pp. 1-21.
sociological and ethical aspects of feeding world in the present scenario and reported that
growth rate of increasing human population is continuously decreasing due to decreased
fertility rate and improved human health. Moreover, two agricultural economists Pardey
and Wright (2003) critically analyzed the data for human world population, growth rate
and world food production, and reported that world agricultural productivity dramatically
increases during the 20th century and exactly matches the human population, particularly
in under-developed countries. Sadava (2003) and Cohen (2003) pointed out various
sociological and ethical issues to pinpoint the problem, “if increases in human population
have been matched by increases in food supply, why hunger still persists”. They
highlighted various reasons, of which include policies for inequitable distribution of food,
ethnic or racial discrimination, lack of political power on the part of hungry people,
violent conflict, lack of jobs, environmental degradation. For example, Cohen (2003)
pointed out that in January, 2001 the Indian government debated whether to dump surplus
wheat and rice in sea to make a place for a new harvest, while 208 million Indians were
suffering from chronic under nutrition. He further emphasized this thing by quoting
words of Hindustan Times newspaper, “Grain, Grain everywhere, but not enough to
eat”. Similarly, during 2007 and 2008 in Pakistan, high amounts of wheat, sugar and rice
were exported to its neighbor countries by the smugglers as well as by the Govt. officials
itself due to policy makers, while 140 million Pakistanis suffered from a chronic shortage
of
these
food
items
particularly
of
wheat
flour
(http://www.pak-
times.com/2008/05/18/food-crisis-in-pakistan/;
http://www.dawn.com/2008/06/09/ebr5.htm Accessed on 16th August, 2008). Thus, the
biggest challenge to the food supply comes from our market driven economic system,
which is poor income growth and not population growth (Chrispeels and Sadava, 2003;
Cohen, 2003; Pardey and Wright, 2003; Sadava, 2003).
Here question arises, “if the availability of food is not a major problem, then why
is increased agricultural productivity an important part of solution?” During the past half
century, increase in food production and availability was due to development of new
wheat and rice varieties –Green revolution (Cohen, 2003; Sadava, 2003). As the human
population is growing, for plant scientists, it is necessary to make efforts to improve crop
production. Secondly, various abiotic and biotic stresses markedly reduce the crop
production; and of them soil erosion, salinity and water shortage are of serious concern
for the future (Chrispeels and Sadava, 2003; Cohen, 2003). Thus, to reduce the food
insecurity, crop production will have to be doubled, and produced in more
environmentally sustainable ways (Borlaug and Dowswell, 2005). Therefore, it was
suggested that appropriate policies are needed to address the underlying causes of hunger
in addition to steps taken to improve crop production.
World food security and crop improvement
Increase in crop productivity can be achieved by expanding cultivable land area or by
increasing per hectare crop productivity. However, it is well evident from the history of
the past century that enhancement in crop production due to expansion in growing area
was only observed in the first half of the twentieth century (Slafer and Satorre, 1999;
Sadava, 2003). Furthermore, during the 2nd half of the past century, rise in per hectare
crop productivity was due to improved or high yield potential (Sadava, 2003; Araus et
al., 2004). Although a number of methods could be used to improve crop production such
as screening and selection, breeding, use of fertilizer, agronomic practices, irrigation, and
molecular biology, conventional selection and breeding technique proved to be more
useful. For example, Bell et al. (1995) estimated that relative contribution of use of
conventional breeding techniques in total yield increase was 30% in Mexico. However, in
some other countries increase in total yield potential by breeding was 50% (Araus et al.
2004; Tollenaar and Lee, 2004). Thus, 80% of required future increase in crop production
must come from increase in crop yield potential. Overall, it is advisable to focus on
genetic gain to improve crop productivity.
Crop productivity as affected by various abiotic stresses
In developing countries, a number of factors cause a substantial decrease in crop
productivity and hence food insecurity. Of them, less availability of agricultural land,
depletion of fresh water resources, ever-increasing biotic and abiotic stresses, and low
economic activity in agricultural sector are the most important factors. However, it is
generally believed that abiotic stresses are considered to be the main factors for yield
reduction (Boyer, 1982; Rehman et al., 2005; Munns and Tester, 2008; Reynolds and
Tuberosa, 2008). The estimated potential yield losses are 17% due to drought, 20% due
to salinity, 40% due to high temperature, 15% due to low temperature and 8% by other
factors (Rehman et al., 2005; Ashraf et al., 2008; Athar and Ashraf, 2009; Ashraf, 2010).
Salinity – a major menace for crop production throughout
the history of civilization world over
Salinity is one of the major abiotic stresses that affect various aspects of human
lives of one third world population including human health and agricultural productivity.
The problem of salinity existed long before the human beings and start of agricultural
practices. From the historical record of the last 6000 years of civilization, it is evident
that people were unable to continue their colonization due to salinity-induced destruction
of resources (Athar and Ashraf, 2009; Ashraf, 2010). For example, it was found that
increase in salinity level over 700 years from 2400 B.C to 1700 B.C. caused a decline in
agricultural productivity, e.g., 29 bushels per acre of barley to 10 bushels per acre
(Gelburd, 1985; Ashraf, 1994). Although a progressive increase in salinity has caused
degradation of arable land over many hundred-years period, cultivated land could have
been degraded due to salinity during less than 100 years. For example, in California 4.5
out of 8.6 Mha irrigated agricultural land has become salt affected during the last century
(Lewis, 1984; Ashraf, 1994). At present, its extent throughout the world is increasing
regularly in extent (Schwabe et al., 2006) and it has now become a very serious problem
for crop production (Munns and Tester, 2008; Ashraf, 2009; 2010), particularly in arid
and semi-arid regions.