Scaling Up: the Essence of Effective Agricultural Research
J.B.Passioura,
CSIRO Plant Industry
GPO Box 1600,
Canberra, 2601
Australia
Abstract
Successful scaling up from laboratory research to application in the field depends on its
practitioners being aware of the constraints and other interactions that arise as scaling up
proceeds. Although exploration of promising ideas are often of intrinsic scientific interest,
such ideas fail the test of utility if they do not get adopted by agronomists or plant breeders
and if practices and cultivars based on such ideas do not get adopted by farmers. This notion
of scaling up is explored here using salinity tolerance of crops as a case study, with examples
drawn from gene expression, tissue culture, controlled environment studies of plants grown in
hydroponics and in pots, and the behaviour of plants at the field scale. The most effective
research in this arena has resulted from a culture of collegiate dialogue between scientists
working at different scales.
Keywords
salinity, prebreeding, wheat, biological organisation, abiotic stress
Introduction
Plant scientists organise their thoughts by dividing the broad sweep of their subject matter into
conceptual layers: community, whole plant, organ, tissue, cell, organelle, membrane, molecule
(metabolite, protein) gene. These layers are essentially structural, but implied in them are a
wide range of time scales of processes and interactions that are predominantly peculiar to each
structural scale (Osmond and Chow 1988). We usually think of these layers as a nested
hierarchy of systems and sub-systems, but it is also informative to view them as a loop in which
one can travel in either direction (Fig.1).
Fig.1. Levels of organisation in crop
plants, represented as a loop in which
clockwise flow represents reduction, the
search for mechanistic understanding at
finer and finer scales, and anti-clockwise
flow represents functional integration, the
roles of various structures and processes in
transmitting genes to the next generation.
In the sense that selection of individual
genotypes at the crop level (or, in natural
systems, the community) determines the
genetic makeup of the next generation, the
“loop” is a helix when viewed across
generations.
1
Clockwise flow denotes the familiar search for mechanistic understanding at finer and finer
scales (reduction). Anticlockwise flow denotes the search for functional significance
(integration). It is notable that clockwise flow essentially stops at gene. It is anticlockwise
flow that captures the essence of biology, for unless the loop is closed none of the structures
and processes that comprise it would have evolved. This point is an underlying sentiment of
Functional Plant Biology.
In natural systems, selection pressures among organisms in communities, spiced by
occasional mutations, determines what genes are transmitted to the next generation, and at
what frequency. In agricultural systems, selection takes place in breeders’ plots and in
farmers’ fields. Only cultivars that do well remain current. The rest are, at best, relegated to
banks for genetic resources. The prediction of higher order behaviour from lower level
information (“reductionism from below” (Weiner 1996)) has been predominantly
unsuccessful.
At least ostensibly, much research in the plant sciences deals with improving the performance
of the major crop plants. In trying to improve the performance of crops, the final arbiter is the
farmer. New agronomic techniques or new cultivars will only be adopted if farmers find them
effective. Many laboratory scientists who think that they have a great idea for improving
crops find themselves eventually deeply disappointed. This is often because they try to cut
across the loop in Fig.1 without taking their idea anticlockwise around it first.
There are some exceptions, which relate not to improving the performance of crops
intrinsically, but to knocking out possible impediments. Bt (a gene whose product kills
caterpillars) and herbicide resistance of crops are clear examples of successful genetic
modification in which the path between gene and plant shown in Fig.2 works well. The
persistence of the success however, depends on appropriate agronomic management to ensure
that the pests and weeds do not become resistant to these measures.
There have also been successes in improving the nutritional quality of seeds by following the
direct path in Fig. 2. Examples include: direct selection of natural variation (Canola, produced
from oilseed rape by selecting lines low in the undesirable erucic acid and glucosinolates);
knock-out mutation breeding (Linola, produced by blocking the conversion of the doubleunsaturated linoleic acid into the triple-unsaturated linolenic acid (Green 1986)); and genetic
transformation (Golden Rice, engineered to contain provitamin A).
Fig.2. A modification of Fig.1 to illustrate
scaling up (anticlockwise flow) from fine
levels of organisation to whole plants and
crops. The direct path from gene to plant
works well for genes that are not involved in
vital processes, such as Bt, herbicide
resistance, and nutritional quality of seeds.
Adaptation to stress requires anticlockwise
flow of ideas through several levels of
organisation to attain success at the level of
the crop. Many ideas that do not explicitly
consider the problems of scaling up fail, as
depicted by the arrows leaving the loop – see
text for examples.
2
The history of improving the performance of crops, whether in yield, productivity, or
environmental viability, has shown that almost all of the direct success has come from
cleverly focussed empirical breeding and new agronomic techniques. But we are now faced
with a slow down in the rate of genetic progress by empirical means (Fischer and Edmeades
2010), and we have the challenge before us of making use of our knowledge of the workings
of plants to modify and to incorporate specific traits into breeders’ lines (see, for example
Passioura (2007b)). Such traits can relate to any of the levels of organisation in Fig.1
Before looking at how best to do that, it is worthwhile looking for, and learning from,
common patterns of success and failure in the past. Just as genes can head for oblivion during
selection among genotypes, ideas can head for oblivion (Fig.2) as we try to scale up by
moving anticlockwise around the loop in the hope of improving productivity.
Salinity tolerance, the central theme of this special issue of Functional Plant Biology, affords
a wide range of examples of attempts to translate research at lower levels of organisation to
performance of whole plants in controlled environments, and, most difficult of all, to crops in
the field. Several such examples are discussed below, against a backdrop of the most
important physiological features and behaviours of salt-affected wheat and barley, briefly
summarised as follows.
The sequence of events following exposure of a plant to salt is first, that leaf expansion slows
immediately. It may even stop completely before recovering somewhat to a value
substantially less than that of the controls. This is primarily an osmotic effect, and is
essentially a result of water stress (Passioura and Munns 2000). Within a day or two,
however, the osmotic effect is overtaken by a signal from the roots which maintains slow leaf
expansion (Munns et al. 2000). Meanwhile, transporters limit the amount of sodium entering
the roots, pump much of what gets in back out of the roots into the medium, and remove
sodium from the transpiration stream as it flows to the shoot (Munns and Tester 2008; Zhu
2002). After some time, the remaining sodium that reaches the fully expanded leaf blades
slowly accumulates until the leaves starts to senesce, then die. This process is delayed by the
extent to which sodium can be sequestered in the vacuoles. Eventually, however, the loss of
photosynthetic area greatly reduces the growth of the plant, which in severe cases will die.
These processes, together with other important features affecting salinity tolerance are
summarised, with additional references, in Table 1.
3
Table 1 Some features or behaviours at various scales pertinent to growth response of saltaffected wheat and barley
Organisational
level
Fully expanded leaf
Feature, process
References
Na steadily accumulates in leaves
James et al. (2002)
until they can hold no more and lose
photosynthetic activity
Leaf expansion zone Slow leaf growth
Negligible sodium or chloride
Greenway and Munns (1980)
Munns et al. (1982)
Substantial supply of photosynthate
Likely elongation control by signals
from roots
Termaat et al. (1985); PérezAlfocea et al. (this volume)
Na sequestered in vacuole (cheap
osmoticum to help maintain turgor)
Flowers et al. (1977);Blumwald,
et al. (2000)
Compatible solutes in cytoplasm
Wyn Jones and Storey (1978)
Enzymes
No evidence of differences in
properties of major enzymes
between glycophytes and
halophytes
Greenway and Osmond (1972)
Transporters
Cell specific expression of
transporter genes that control traffic
of sodium (Na exclusion by roots,
sequestration of Na in leaf sheath
parenchyma)
Moller et al. (2009); Tester and
Davenport (2003)
Genes involved
HKT1;4 (Nax1), HKT1;5 (Nax2,
Kna1), SOS1, NHX1, HvNax3
Huang et al. (2006); Byrt et al.
2007); Martinez-Atienza et al.
(2007); Shavrukov et al. (2010)
Cells
Salinity tolerance: successes and failures in scaling up from research in laboratories and
controlled environments to behaviour of crops in the field
Gene expression
There has been much interest in using functional genomics and related technology to discover
genes that could help plants cope better with salt, but little of practical importance has
emerged, primarily because of the multigenic nature of salinity tolerance (Flowers 2004).
Much of this research has, in the past, involved suddenly exposing plants to such high
concentrations of salt (> 100 mM) that the cells of the roots plasmolyse (Munns et al. 2002),
i.e. the membranes surrounding the cell contents are torn off the cells’ walls as the contents
are shrunk by the surrounding salt sucking out the water; this induces major trauma that rarely
if ever occurs in nature. Exploring gene expression in such circumstances is irrelevant to how
plants behave in the field. Further, constitutive promoters have typically been used in
transgenics, whereas cell-specific expression (Tester and Davenport 2003) and stressinducible promoters (Kasuga et al. 1999; Flowers 2004) are more appropriate. Unfamiliarity
with cell biology may lead to major misleading artefacts.
4
One can overcome the trauma of osmotic shock by increasing the concentration of salt
gradually, in several small steps over a few days rather than in one large step, but even this
may uncover little useful genetic variation in salt tolerance. The problem is that it can take
weeks for such variation to become evident. This is because the main defence of plants
against salt is to exclude almost all of it from the water that they take up, and it is variation in
how much salt gets through and then builds up to damaging concentrations in the leaves that
strongly influences a plant’s salt tolerance (Munns 2005). Laboratory experiments on salinity
that are uninformed by processes occurring in whole plants over several weeks have little
hope of being useful in improving productivity.
Tissue culture
Similarly, a few decades ago there was much interest in trying to select salt tolerance at the
cellular level in cell culture. Cells that survived the challenge of salinity could be used to
regenerate plantlets. But conditions in tissue culture do not duplicate the conditions within
plants, and cells can be much more tolerant of salinity than whole plants (Flowers et al.
1985). The regenerated plantlets turned out to be no more salt tolerant than normal,
unselected, plants (Dracup 1993).
Analysis of leaves
Schachtman and Munns (1992) observed that it is the slow build up of salt in the expanded
leaves over days to weeks that eventually damages those leaves, leading to major loss of
photosynthetic activity. From this knowledge, Munns et al. (2002) developed an assay for salt
tolerance of whole plants based on measuring the salt concentration in emerged leaves of tenday old plants grown in gravel culture – essentially supported hydroponics – with constant
salt concentrations in the medium and realistic conditions of evaporation. The rationale was
that the measurements, essentially of rates of uptake of Na+ into emerged leaves, would be
good predictors of the timing of the onset of the later damage.
This assay proved to be so effective that it led to the discovery of two novel transporter genes
in durum wheat, Nax1 and Nax2 (Table 1). Durum wheat cultivars have been notoriously
more sensitive to salinity than bread wheat, because genes for protective transporters in bread
wheat reside on its D genome, which is lacking in durums. The discovery of a salt tolerant
durum genotype and its crossing with salt-sensitive durum cultivars generated populations
segregating for salt tolerance (Munns et al. 2003). Using the leaf assay on these populations
grown in gravel culture revealed the presence of two major genes for salt tolerance called Nax
1 and Nax2 (Lindsay et al. 2004). Both Nax1 and Nax2 markedly reduced the amount of salt
arriving in the xylem to the leaves. Nax1 removed Na+ from the xylem in roots and the base of
leaves, Nax2 was confined to the roots (James et al. 2006). Nax1 was subsequently cloned by
fine mapping as the Na+ transporter HKT1;4, and Nax2 as HKT1;5 (Byrt et al. 2007)). This
success was based on a simple and effective assay whose invention was inspired by
knowledge of the slow development of salt damage in the mature leaves.
Plants in potted soil
The roots of plants grown in hydroponic systems, whether suspended in solution or anchored
in frequently-flushed coarse sand or gravel, experience uniform conditions. Salinised soil,
however, is typically heterogeneous in space and in time. One can set up pots containing
uniformly distributed sodium chloride, but as soon as the roots start taking up appreciable
amounts of water, the distribution of the salt starts to vary. When roots take up water, almost
all the salt in that water is excluded from entering the roots and thus remains in the soil where
5
it concentrates in the diminishing soil solution. Because roots are never uniformly distributed
in a pot, the concentration of salt in the soil solution increases most where the uptake of water
is greatest, usually towards the top of the pot, unless the pot is supplied with fresh water.
If a pot is not watered, the concentration of salt in the soil solution keeps increasing until it
becomes so great that the roots can no longer extract the water. The plant experiences
increasing osmotic stress, which may override any specific effects of sodium chloride
(Rengasamy 2010, this issue). If the pot is rewatered the roots may experience the opposite:
fresh water added to the top of a pot will create a zone with no or little salinity, and a cline
will develop with the soil solution becoming increasingly concentrated with depth. This
scenario may underlie the observation by (Tavakkoli et al. 2010) that barley is much less
affected by the seemingly same exposure to salinity in soil than in solution culture.
Equally important heterogeneity may also occur at the much finer scale of the rhizosphere,
where there is strong, albeit circumstantial, evidence that a build-up of solutes and thence
osmotic pressure can occur at the surface of roots exposed to strong nutrient solution
(Stirzaker and Passioura 1996).
Finally, there is the complication in soil but not in hydroponics that the exchangeable ions on
the soil particles interact with those of added salts to produce a soil solution whose
composition may be substantially different from what was aimed at (Tavakkoli et al. 2010).
The heterogeneous nature of soil and the distribution of salt within it open up the possibility
that roots may be able to reduce their exposure to the salt by preferentially growing into less
salty areas, thereby decreasing the great influence that the Nax genes have in hydroponics.
Field-grown plants – spatial heterogeneity
Moving to the field brings with it another rich collection of interactions. The recent discovery
of widespread salinity in the commonly occurring sodic subsoils in the Australian cropping
areas (Rengasamy 2002; Rengasamy 2010, this volume) has sparked great interest in breeding
crops, especially wheat and barley, that cope better with salinity. This discovery was
facilitated by observations in the mid 1990s by a farmer and a breeder who were puzzling
about why durum wheat would grow as well as bread wheat in some seasons, but was
dismally worse in other seasons (Tom Cootes and Tony Rathjen, personal communication).
Knowing that durum wheat was much less tolerant of salinity than bread wheat, they
augmented their observations by using an electromagnetic conductivity meter, which detected
subsoil salinity.
The presence of subsoil salinity in the field amplifies some of the issues pertaining to pots,
and brings in others. Subsoil salinity is often accompanied by sodicity and boron toxicity
(Adcock et al. 2007). With sodicity can come the danger of hypoxia, for sodic soils are finely
porous and can become almost saturated with water in wet conditions, leaving little space for
air; hypoxia can worsen the exposure to salinity (Barrett-Lennard 2003; Wetson and Flowers,
this issue).
The amount and pattern of the seasonal water supply can markedly influence the dependence
of crops on water in the subsoil, as evidenced by the differential responses of durum and
bread wheats mentioned above, and as explored with a simulation model by Rodriguez et al.
(2006).
6
The discussion above has centred on vertical and temporal variation in salinity. Spatial
variation across a field can also be large, especially in irrigated environments. Richards
(1983) has argued that, where such variation is large and includes a substantial area not
strongly affected by salt, it is better for breeders to select for high yield in benign conditions
rather than for salinity tolerance, for the yields of crops in the benign areas of a highly
variable field typically account for most of the total yield from such a field. This argument
does not apply, though, where all parts of a field are affected by salinity (Flowers and Yeo
1995). The primary salinity that is common in sodic subsoils in Australia, often varies widely
within a paddock. Farmers have been using electromagnetic conductivity meters to map this
variation, which strongly influences the amount of water available to the plants, and they can
alter their agronomic management accordingly (Whitbread et al. 2008) – for example, by
applying less fertilizer where subsoil salinity in evident.
Patterns of success and failure in scaling up
Problems with the performance of crops are first recognised at the level of the plant or crop:
disease, pests, weeds, nutrition, waterlogging, water deficits, toxicities (e.g. salinity, acidity,
boron), lodging, heat damage, floral sterility, and so on. It is tempting for laboratory scientists
to think of cellular or molecular solutions for solving some of these problems. There have
been some outstanding successes using this approach. Incorporating genes whose products are
toxic to certain pests (e.g. Bt) or that block the effects of herbicides (e.g. glyphosate
resistance) has been enormously successful. Similarly, resistance to many important diseases
involve one or a few genes which are likely to be amenable to genetic transformation or
amenable to easy selection in breeding programs with the help of molecular markers (e.g.
resistance to cereal cyst nematode (Ogbonnaya et al. 2001)).
A distinguishing feature of these genes is that they target alien organisms or molecules. They
are not involved directly in the major metabolic machinery of the plant nor in the large scale
processes that determine how crop plants cope productively with highly variable
environments; nor do they impose major metabolic loads on the plants. Thus the shortcut
between gene and plant shown in Fig. 2 can work well.
Abiotic stresses, however, impinge on plants at different scales of time and space and involve
elaborate processes occurring at all levels of organisation. Salinity provides a clear example,
as described above. The deep understanding we now have of how plants deal with salinity at
the levels of genes, transporters, cells, tissues, organs, whole plants and crops has enabled
much recent progress in developing crops that are more salt tolerant.
Operationally (sociologically?), the main ingredient of this success has been mutually
illuminating dialogue between people working at different levels of organization in the plant.
The important interactions are illustrated in Table 2, in which “Level N” represents any level
in the loop in Fig.1, and the central practitioner is in dialogue with colleagues both above and
below (Passioura 1979):
Table 2. General interactions across levels of organisation
Biological significance
Level N+1
Level N
Phenomenon
Mechanistic explanation
Level N-1
7
There are as many of these triads as there are levels in Fig.1 (at the “gene” end the
progression is, say, into the physical chemistry of epigenetic interactions), and the most
fruitful research requires ideas to be running continually, both clockwise and anticlockwise,
around the loop.
To facilitate such movement is not trivial, for dialogue across levels requires the learning of
other languages. Several examples are given earlier. The term “plasmolysis” does not exist in
the lexicon of genomics, and familiarity with cell biology is needed to understand (and avoid)
the trauma arising from severe osmotic shock. Similarly cell biologists whose experience is
restricted to cell culture need familiarity with the disparate functions of various organs and
tissues , the purview of plant physiology, if they are to ask questions pertinent to the
behaviour of whole plants.
Learning another language is best done on a daily basis. Thus, the practitioner at the central
level in the above table is best off when sharing facilities with colleagues working at the
adjacent levels. But what of those colleagues? Who will teach them other languages, and
thereby help them hone their ideas and generate new and better ones? It is rare for a wide
spectrum of biological interests to be housed together in an interactive way, but the socioscientific networks that abound these days can provide the wherewithal for such conversations
if the interest, resolve, and recognition of the requirement are there.
In agriculture the most important conversations are those between farmers and field scientists
– agronomists, breeders, and sometimes crop physiologists. Given the general observation
that emerging problems, and sometimes emerging opportunities, are first seen at the level of
the plant or crop, these conversations form the basis of many innovations. As von Hippel
(2005) has argued, innovations are frequently made by those who are the immediate
beneficiaries of them. Farmers are no exception to this – one of von Hippel’s examples is that
of a farmer who created the first centre pivot irrigator by coupling his motor bike, with its
throttle set, to the end of a long length of irrigation pipe with the other end fixed on a spindle.
Figure 3 illustrates the richness of these conversations between farmers and agronomists/
breeders by means of the boxes above and below the main axis. Similar interactions are
implied by the other arrows. An essential feature of scaling up is that the higher level brings
with it a range of constraints and interactions that affect operations at that level but are
generally unfamiliar to practitioners at lower levels. Plasmolysis, control of the traffic of salt
around the plant, and spatial variability in salinity, are examples given earlier.
8
Fig.3. Schematic depiction of the types of interactions between farmers and field scientists
(agronomists, plant breeders). Similar interactions across other levels are implied by the
arrows to the left of the farmers and to the right of the agronomists/breeders.
General discussion
Although I have focussed on salinity in the above, a similar set of scaling arguments apply to
other abiotic problems, such as drought, discussed in some detail in Passioura (2007a), and
waterlogging, which brings with it the danger of anoxia (Barrett-Lennard 2003; Colmer and
Voesenek 2009).
With the latter, anaerobic biochemistry, though of great intrinsic interest, is of secondary
importance to the development of aerenchyma, continuous open corridors within roots that
enable oxygen to diffuse for tens of centimetres within roots growing in waterlogged soil.
Another adaptation is the ability of the roots to recover rapidly from the transient
waterlogging that commonly occurs in many dryland cropping soils (Setter and Waters 2003).
There is genetic variation in these developmental processes that could one day be harnessed,
perhaps twenty years hence, to provide more tolerant genotypes. Meanwhile, when scaling up
to farmer’s fields, it is better to seek an agronomic solution than a genetic one. Preventing
waterlogging in the surface soil by growing plants on raised beds can substantially increase
yields in areas where the excess water can be drained away (Bakker et al. 2005).
Trait-based breeding
Plant breeders have made, and are continuing to make, substantial progress in improving
water-limited productivity. They have done so mostly by traditional empirical means, though
augmented over the past few decades by more powerful statistics that enable them to
distinguish promising breeding lines more clearly, and by selection based on molecular
markers. The only agronomic or physiological traits they use routinely are flowering time,
which is essential to fit the variety to the target environment, and height.
Meanwhile, there is very large R&D expenditure on abiotic pre-breeding around the world.
Australia spends about as much on pre-breeding as on breeding of wheat and barley, and there
is concern that little of this effort has resulted in new varieties (Anon 2009). A major
difficulty is that there has not been a well-developed procedure for scaling-up, an issue that
was explored in a recent workshop which brought together agronomists, breeders, and a range
of pre-breeders from molecular biologists to crop physiologists (Passioura 2007b).
Scaling up at this advanced level is exceedingly difficult. An essential tool in this process is
the setting up of field phenotyping sites to explore and expedite the introduction of novel
traits into advanced breeding lines in realistic environments using breeding lines adapted to
the target environments. In the context of getting a radical new cultivar into the hands of
farmers it is salutary to remember that it took 8 years of shuttle breeding, with two
generations per year, to produce the first commercial cultivars of the semi-dwarf wheats that
were the basis (together with semi-dwarf rice) of the Green Revolution (Reitz and Salmon
1968) – this, despite the seemingly simple task of incorporating a single dwarfing gene that
could be easily selected for.
Scaling up of agricultural R&D
9
How then can we best facilitate the scaling up of agricultural R&D, whether within the R&D
community, or between it and farmers and their advisers? One impediment is the prevailing
language, which has many terms implying one-way flows of information rather than
interactions across scales. “Technology transfer” is the most potent of these, but several
others, such as “extension” and “delivering outcomes” have similar connotations. All imply
that R&D professionals produce solutions to agricultural problems which are then “delivered”
to receptive farmers, or perhaps thrust upon, or seductively sold, to recalcitrant ones. In fact,
one can argue that what agricultural research mainly produces is useful management options
and cultivars for farmers to choose among when dealing with their highly variable
environments as each season, or run of seasons, develops.
This language of “technology transfer” is appropriate in many circumstances, but gives no
hint of the mutually stimulating dialogues that takes place between many farmers and
agricultural scientists and that greatly expedite scaling up. This two-way flow of information
alerts the scientists to problems and operational constraints that may accompany an increase
in scale, and helps train the intuition of the farmers by giving them a deeper understanding of
the processes going on in their crops and pastures and in the ground under their feet – after all,
it is the activity of well-informed inventive farmers that leads to many agricultural
innovations.
Dialogues between agricultural scientists working at different scales are as important as
wellsprings of innovation as those between farmers and scientists. The challenge is to create
R&D institutions, of which farmers and their advisers are an essential part, in which the
informal dialogues that stimulate sensible scaling up can be fostered across all scales from
molecular genetics to farms. The notion of the value-chain is well recognised beyond the farm
gate. It is of equal importance in fostering innovation before the farm gate.
Acknowledgements
I am indebted to Tim Flowers, John Kirkegaard, Rana Munns, Richard Richards, Richard
Stirzaker, Anton Wasson and Michelle Watt for their penetrating comments on the manuscript
and for countless highly illuminating conversations.
10
References
Adcock D, McNeill AM, McDonald GK, Armstrong RD (2007) Subsoil constraints to crop
production on neutral and alkaline soils in south-eastern Australia: A review of current
knowledge and management strategies. Australian Journal of Experimental Agriculture 47,
1245-1261.
Anon (2009) Australian Winter Cereals Pre-Breeding Alliance.
http://www.grdc.com.au/director/events/grdcpublications/prebreedingalliance.cfm.
Bakker DM, Hamilton GJ, Houlbrooke DJ, Spann C (2005) The effect of raised beds on soil
structure, waterlogging, and productivity on duplex soils in Western Australia. Australian
Journal of Soil Research 43, 575-585.
Barrett-Lennard EG (2003) The interaction between waterlogging and salinity in higher
plants: Causes, consequences and implications. Plant and Soil 253, 35-54.
Blumwald E, Aharon GS, Apse MP (2000) Sodium transport in plant cells. Biochimica et
Biophysica Acta-Biomembranes 1465, 140-151.
Byrt CS, Platten JD, Spielmeyer W, James RA, Lagudah ES, Dennis ES, Tester M, Munns R
(2007) HKT1;5-like cation transporters linked to Na+ exclusion loci in wheat, Nax2 and Kna1.
Plant Physiology 143, 1918-1928.
Colmer TD, Voesenek LACJ (2009) Flooding tolerance: Suites of plant traits in variable
environments. Functional Plant Biology 36, 665-681.
Dracup M (1993) Why does in vitro cell selection not improve the salt tolerance of plants? In
'Genetic aspects of plant mineral nutrition. The Fourth International Symposium on Genetic
Aspects of Plant Mineral Nutrition, 30 September-4 October 1991, Canberra, Australia.' pp.
137-142. (Kluwer Academic Publishers)
Fischer RA, Edmeades GO (2010) Breeding and cereal yield progress. Crop Science 50, S85S98.
Flowers TJ (2004) Improving crop salt tolerance. Journal of Experimental Botany 55, 307319.
Flowers TJ, Lachno DR, Flowers SA, Yeo AR (1985) Some effects of sodium-chloride on
cells of rice cultured in vitro. Plant Science 39, 205-211.
Flowers TJ, Troke PF, Yeo AR (1977) Mechanism of salt tolerance in halophytes. Annual
Review of Plant Physiology and Plant Molecular Biology 28, 89-121.
Flowers TJ, Yeo AR (1995) Breeding for salinity resistance in crop plants: Where next?
Australian Journal of Plant Physiology 22, 875-884.
Green AG (1986) A mutant genotype of flax (Linum usitatissimum L.) containing very lowlevels of linolenic acid in its seed oil. Canadian Journal of Plant Science 66, 499-503.
11
Greenway H, Munns R (1980) Mechanisms of salt tolerance in nonhalophytes. Annual Review
of Plant Physiology 31, pp. 149-190.
Greenway H, Osmond CB (1972) Salt responses of enzymes from species differing in salt
tolerance. Plant Physiology 49, 256-&.
Huang SB, Spielmeyer W, Lagudah ES, James RA, Platten JD, Dennis ES, Munns R (2006)
A sodium transporter (HKT7) is a candidate for Nax1, a gene for salt tolerance in durum
wheat. Plant Physiology 142, 1718-1727.
James RA, Davenport RJ, Munns R (2006) Physiological characterization of two genes for
Na+ exclusion in durum wheat, Nax1 and Nax2. Plant Physiology 142, 1537-1547.
James RA, Rivelli AR, Munns R, von Caemmerer S (2002) Factors affecting CO2
assimilation, leaf injury and growth in salt-stressed durum wheat. Functional Plant Biology
29, 1393-1403.
Kasuga M, Liu Q, Miura S, Yamaguchi-Shinozaki K, Shinozaki K (1999) Improving plant
drought, salt, and freezing tolerance by gene transfer of a single stress-inducible transcription
factor. Nature Biotechnology 17, 287-291.
Lindsay MP, Lagudah ES, Hare RA, Munns R (2004) A locus for sodium exclusion (Nax1), a
trait for salt tolerance, mapped in durum wheat. Functional Plant Biology 31, 1105-1114.
Martinez-Atienza J, Jiang XY, Garciadeblas B, Mendoza I, Zhu JK, Pardo JM, Quintero FJ
(2007) Conservation of the salt overly sensitive pathway in rice. Plant Physiology 143, 10011012.
Moller IS, Gilliham M, Jha D, Mayo GM, Roy SJ, Coates JC, Haseloff J, Tester M (2009)
Shoot Na+ exclusion and increased salinity tolerance engineered by cell type-specific
alteration of Na+ transport in Arabidopsis. Plant Cell 21, 2163-2178.
Munns R (2005) Genes and salt tolerance: Bringing them together. New Phytologist 167, 645663.
Munns R, Greenway H, Delane R, Gibbs J (1982) Ion concentration and carbohydrate status
of the elongating leaf tissue of Hordeum vulgare growing at high external NaCl. 2. Cause of
the growth reduction. Journal of Experimental Botany 33, 574-583.
Munns R, Guo J, Passioura JB, Cramer GR (2000) Leaf water status controls day-time but not
daily rates of leaf expansion in salt-treated barley. Australian Journal of Plant Physiology 27,
949-957.
Munns R, Husain S, Rivelli AR, James RA, Condon AG, Lindsay MP, Lagudah ES,
Schachtman DP, Hare RA (2002) Avenues for increasing salt tolerance of crops, and the role
of physiologically based selection traits. Plant and Soil 247, 93-105.
Munns R, Rebetzke GJ, Husain S, James RA, Hare RA (2003) Genetic control of sodium
exclusion in durum wheat. Australian Journal of Agricultural Research 54, 627-635.
12
Munns R, Tester M (2008) Mechanisms of salinity tolerance. Annual Review of Plant Biology
59, 651-681.
Ogbonnaya FC, Subrahmanyam N C, Moullet O, DeMajnik J, Eagles H A, Brown J S,
Eastwood RF, Kollmorgen J, Appels R, Lagudah ES (2001) Diagnostic DNA markers for
cereal cyst nematode resistance in bread wheat. Australian Journal of Agricultural Research
52, 1367-1374.
Osmond CB, Chow WS (1988) Ecology of photosynthesis in the sun and shade - summary
and prognostications. Introduction. Australian Journal of Plant Physiology 15, 1-9.
Passioura J (2007a) The drought environment: Physical, biological and agricultural
perspectives. Journal of Experimental Botany 58, 113-117.
Passioura JB (1979) Accountability, philosophy, and plant physiology. Search 10, 347-350.
Passioura JB (2007b) Workshop on pre-breeding for better performance under drought.
http://www.grdc.com.au/uploads/documents/droughtworkshopreport1007.pdf
Passioura JB, Munns R (2000) Rapid environmental changes that affect leaf water status
induce transient surges or pauses in leaf expansion rate. Australian Journal of Plant
Physiology 27, 941-948.
Reitz LP, Salmon SC (1968) Origin history and use of Norin 10 wheat. Crop Science 8, 686689.
Rengasamy P (2002) Transient salinity and subsoil constraints to dryland farming in
Australian sodic soils: An overview. Australian Journal of Experimental Agriculture 42, 351361.
Richards RA (1983) Should selection for yield in saline regions be made on saline or nonsaline soils? Euphytica 32, 431-438.
Rodriguez D, Nuttall J, Sadras VO, Van Rees H, Armstrong R (2006) Impact of subsoil
constraints on wheat yield and gross margin on fine-textured soils of the southern Victorian
mallee. Australian Journal of Agricultural Research 57, 355-365.
Schachtman DP, Munns R (1992) Sodium accumulation in leaves of Triticum species that
differ in salt tolerance. Australian Journal of Plant Physiology 19, 331-340.
Setter TL, Waters I (2003) Review of prospects for germplasm improvement for waterlogging
tolerance in wheat, barley and oats. Plant and Soil 253, 1-34.
Shavrukov Y, Gupta NK, Miyazaki J, Baho MN, Chalmers KJ, Tester M, Langridge P,
Collins NC (2010) HvNax3—a locus controlling shoot sodium exclusion derived from wild
barley (Hordeum vulgare ssp. spontaneum). Functional and Integrative Genomics 10, 277291.
Stirzaker RJ, Passioura JB (1996) The water relations of the root-soil interface. Plant, Cell
and Environment 19.
13
Tavakkoli E, Rengasamy P, McDonald GK (2010) The response of barley to salinity stress
differs between hydroponics and soil systems. Functional Plant Biology in press.
Termaat A, Passioura JB, Munns R (1985) Shoot turgor does not limit shoot growth of NaClaffected wheat and barley. Plant Physiology 77, p869-872.
Tester M, Davenport R (2003) Na+ tolerance and Na+ transport in higher plants. Annals of
Botany 91, 503-527.
von Hippel E (2005) 'Democratizing innovation.' (MIT Press: Cambridge, Massachusetts)
http://web.mit.edu/evhippel/www/books.htm
Weiner J (1996) Problems in predicting the ecological effects of elevated CO2. In 'Carbon
dioxide, populations and communities.'. (Eds C Körner and FA Bazzaz) pp. 431-441.
(Academic Press, San Diego)
Whitbread W, Llewellyn R, Gobbett D, Davoren B (2008) EM38 and crop-soil simulation
modelling can identify differences in potential crop performance on typical soil zones in the
mallee. . In 'Proceedings of 14th Agronomy Conference 21-25 September 2008, '. Adelaide,
South Australia
Wyn Jones RG, Storey R (1978) Salt stress and comparative physiology in the Gramineae. 2.
Glycinebetaine and proline accumulation in two salt-stressed and water-stressed barley
cultivars. Australian Journal of Plant Physiology 5, 817-829.
Zhu JK (2002) Salt and drought stress signal transduction in plants. Annual Review of Plant
Biology 53, 247-273.
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