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Proceedings Crop Production in Northern Britain 2020 STABILISING AMINE UREA IN NITROGEN FERTILISER INCREASES LEAF CHLOROPHYLL CONTENT, TILLER BASE DIAMETER AND ROOT LENGTH OF WHEAT PLANTS

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Fertilisation of field crop plants with urea nitrogen is very inefficient because over half of this is degraded via hydrolysis and nitrification, releasing greenhouse gases and leaching nitrate into water systems. Technologies for stabilising urea N in fertiliser, and prolonging its availability for plants, have been developed. Here we investigate whether chemically stabilising urea amine N (in a product called 'Elona') in foliar fertiliser applied to pot-grown wheat, induces favourable physiological effects, compared to those of industry standard nitrogen fertilisers. All treatments contain identical amounts of nitrogen by weight, equivalent to a rate of 2.5 L/ha stabilised amine nitrogen (SAN) in 100L, and were applied every 3-4 weeks in March-June 2018, in a greenhouse in Preston, Lancashire, UK. The chlorophyll content of wheat leaves was significantly increased by SAN nutrition 3 and 10 days after the first treatment, initially at 4-5 tiller stage; and tillers were more upright. At 14-15 tiller stage tiller bases had an increased diameter. This gave rise to a higher tiller diameter-canopy height ratio. Three weeks later roots of SAN-treated plants were significantly longer, which gave rise to a larger root length-canopy height ratio. We discuss how these attributes relate to specific effects of ureic amine N on plant phenotype, and how they may affect yields in the longer term. We argue that genetic screening for high yield-linked phenotypic traits may be more effective when wheat is fertilised with stabilised urea.
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Proceedings Crop Production in Northern Britain 2020
STABILISING AMINE UREA IN NITROGEN FERTILISER INCREASES LEAF
CHLOROPHYLL CONTENT, TILLER BASE DIAMETER AND ROOT LENGTH OF WHEAT
PLANTS
Marks DJ, Weston AK and Wilkinson S
Levity Crop Science, Myerscough College and University Centre, Preston, PR3 0RY, UK
david@levitycropscience.com
Summary: Fertilisation of field crop plants with urea nitrogen is very inefficient
because over half of this is degraded via hydrolysis and nitrification, releasing
greenhouse gases and leaching nitrate into water systems. Technologies for
stabilising urea N in fertiliser, and prolonging its availability for plants, have been
developed. Here we investigate whether chemically stabilising urea amine N (in
a product called ‘Elona’) in foliar fertiliser applied to pot-grown wheat, induces
favourable physiological effects, compared to those of industry standard nitrogen
fertilisers. All treatments contain identical amounts of nitrogen by weight,
equivalent to a rate of 2.5 L/ha stabilised amine nitrogen (SAN) in 100L, and
were applied every 3-4 weeks in March-June 2018, in a greenhouse in Preston,
Lancashire, UK. The chlorophyll content of wheat leaves was significantly
increased by SAN nutrition 3 and 10 days after the first treatment, initially at 4-5
tiller stage; and tillers were more upright. At 14-15 tiller stage tiller bases had an
increased diameter. This gave rise to a higher tiller diameter canopy height
ratio. Three weeks later roots of SAN-treated plants were significantly longer,
which gave rise to a larger root length canopy height ratio. We discuss how
these attributes relate to specific effects of ureic amine N on plant phenotype,
and how they may affect yields in the longer term. We argue that genetic
screening for high yield-linked phenotypic traits may be more effective when
wheat is fertilised with stabilised urea.
INTRODUCTION
The effect of the form of nitrogen (N) within fertiliser on plant development and yielding has
often been overlooked. One reason for this is that all N forms (ammonium, urea, nitrite,
organic amines) are eventually degraded to nitrate (and gaseous pollutants) within hours to
weeks of application (dependent on environmental conditions) unless they are stabilised
(see Wilkinson et al. 2019a). When N form has, however, been studied on plant growth in
the field, this mainly relates to differences between effects of nitrate and ammonium nutrition
(e.g. Carlisle et al. 2012), which nevertheless show widely contrasting effects on many
physiological characteristics which influence yield, including in wheat. As it was originally
believed that non-leguminous plants could only take up these inorganic N forms from the
soil, effects of urea in plants have not been investigated to the same extent. However this is
a growing area of research, particularly as technologies for preventing urea degradation to
nitrate and pollutants are becoming available, which can increase crop nutrient use
efficiency and/or yield in some cases (e.g. Wang et al. 2015).
The effects of urea N, ammonium N and nitrate N on plants have frequently been compared
in highly controlled experimental systems, such as in hydroponics or agar-filled pouches.
Under these conditions plants grown in the presence of ammonium alone or urea alone can
exhibit reduced growth, and generate symptoms of toxicity compared to nitrate nutrition (see
Yang et al. 2015). When a detrimental effect of ammonium on hydroponic solution pH is
corrected, however, ammonium nutrition improves biomass and tillering of wheat in
comparison to nitrate (Chen et al 1998). Further increases in biomass and tillering occurred
when both N forms were supplied together. When two N forms are supplied in ratios, for
example 75-25 urea-nitrate compared to 25-75, nutritional effects on plants in experimental
systems can still be attributed to the dominant N form (Pompeiano and Patton, 2017), whilst
reflecting more closely conditions existing in the field. In the latter case both above and
below ground biomass was greatest under a ratio of 75-25 urea-nitrate in greenhouse grown
Zoysia grass. Different sets of genes are up-regulated when both N forms are present,
increasing the efficiency of total N uptake, assimilation and use (Pinton et al. 2016).
New strands of research are emerging showing that plants have evolved to take up urea
from soil, and possess highly conserved systems within root cells for doing so (Wang et al.
2016). We are gathering field and greenhouse data showing that the N form urea amine has
unique and beneficial effects on plant form and function in comparison to nitrate and
ammonium nutrition when it is stabilised. These effects can lead to greater, more uniform
tuber yields in potato (Marks et al. 2018; Wilkinson et al. 2019b), and to increased flowering
in ornamental species (Wilkinson et al 2019a). We have shown that, in the main, this is not
related to the stabilisation-induced maintenance of nitrogen concentration per se (by
preventing ammonification and nitrate leaching), although this positive effect may still
additionally occur in the field. Instead, favourable effects of urea amine (Wilkinson et al.
2019a, b), in comparison to conventional ammonium nitrate and/or un-stabilised urea
controls, can be some or all of the features of the specific phenotype generated by this N
form: increased root-shoot ratio during early development, increased root development per
se, reduced shoot extension rate, and increased chlorophyll content. At later developmental
stages, aboveground biomass increases over and above that of controls, lateral shoot
development is increased, and chlorophyll content remains high.
Here we describe the effects of foliar treatments of a range of N fertilisers, including
chemically stabilised amine nitrogen (SAN), on wheat growth and physiology. We aimed to
determine whether any of the above effects, several of which are known to contribute to
and/or proxy for increased yields in the field (Bai et al. 2013), occur in pot-grown seedlings of
this staple food crop. Experiments were conducted in compost in a greenhouse in Preston,
UK, in 2017-2018.
MATERIALS AND METHODS
Triticum aestivum L. cv Anapolis was used in greenhouse trials. Seeds were drilled in
modules in December 2017, in J. Arthur Bowers John Innes No. 2 compost (Westland
Horticulture Ltd., Co. Tyrone, UK), at a rate of one per 2.5 x 2.5 cm module sub-
compartment. Prior to tillering seedlings were transplanted singly to 5 L pots containing the
same compost. The pH of this is 5.5-6.0, and it initially provides appropriate macro- and
micro-nutrients to all plants. Foliar spray treatments with a range of nitrogenous compounds
occurred every 3-4 weeks from the onset of tillering, in a heated and ventilated greenhouse
under natural light (PPFD 200-1000 µmol m2 s-1), in Preston, northern England, UK. Night-
time temperature was 12-16oC, and day-time temperature was 16-32oC. Plants were
watered by hand to soil capacity as required. Each of the nitrogenous treatments comprised
of five replicate wheat plants randomised within an area of 2.5 x 1.0 m2.
Nitrogen (N) fertiliser treatments were applied as liquid formulations: a standard N-P-K
control, stabilised amine nitrogen (SAN) in a formulation called ‘Elona’ (supplied by Levity
Crop Science Ltd., Preston, UK), and a cereal-specific industry standard (IS). SAN was
applied at a rate of 2.5 L ha-1 in 100 L water. It contains 15 % N (by weight), and the control
and IS treatments were designed to provide the same amount of N to the plants (given that
IS contains 24 % N). Both controls and commercial IS treatments contain a mixture of ureic
and ammonium nitrate N. All plants were supplemented via the soil with standard N-P-K
(control treatment) at 50% recommended strength every 3-4 weeks, approximately mid-way
between main treatment dates, ensuring access to sufficient micronutrients and P-K. Main
treatments with N fertiliser occurred approximately every 3-4 weeks, as specified in Table 1,
at a rate of 20 cm3 per m2.
Leaf relative chlorophyll content, tiller angle, tiller basal diameter, canopy height and root
lengths were measured once or on several occasions at different developmental stages over
the course of the experiments (Table 1).
Relative chlorophyll content was measured in leaves as an index, with a FieldScout CM
1000 Chlorophyll Meter (Spectrum Technologies Inc., Illinois, USA). “Point-and-shoot”
technology instantly measures the reflectance of ambient and reflected 700 nm and 840 nm
light in a conical viewing area on the adaxial leaf surface 30-180 cm from the light receptor.
Laser guides outline the edges of the sampling area, allowing replication of the position of
this between plants. The light receptor comprises four photodiodes; two for ambient light and
two for reflected light from the leaf. Measurement units are calculated as an index of relative
chlorophyll content, 0-999 ± 5%. Leaf canopy height and root lengths were hand-measured
with a ruler at the times detailed in Table 1. Tiller angle of the three largest tillers, with the
soil surface as the horizontal plane, was measured using a protractor. Tiller basal diameter
was measured at its widest point with a digital calliper.
Means and standard errors of each measurement type per treatment are displayed as bar
charts. The significance of the differences between treatments was calculated using a one-
tailed t-test for two independent means, and where treatments are significantly different from
each other (at p<0.1), this is denoted by ‘a’, ‘b’, or ‘c’, above the appropriate column on the
graphic representations of the data.
Table 1. Time course detailing foliar nitrogen (N) application
occasions, and measurement activity, during experiments on
greenhouse-grown Triticum aestivum L. cv Anapolis beginning in
March 2018 (Preston, Lancashire, England, UK).
________________________________________________________________
Days from start Activity Figure no.
0 N treatment 1
3 Chlorophyll analysis 1A
5 Tiller angle measured 2
10 Chlorophyll analysis 1B
27 N treatment 2
36 Tiller diameter measured 3A
36 Canopy height measured 3B
48 N treatment 3
56 Root length measured 4A
56 Canopy height measured 4B
________________________________________________________________
RESULTS
Figure 1A shows that the chlorophyll content of wheat leaves was significantly increased by
SAN 3 days after the first foliar nitrogen treatment, at 4-5 tiller stage; by 11.6% in
comparison to the controls, and by 19% compared to the industry standard (IS) treated
plants. Figure 1B shows that the effect of SAN persists 7 days later. In between the
chlorophyll measurements, tillers were more upright in SAN treated plants (Fig 2A), with an
increased angle between the soil surface and the three largest tillers per plant (Fig 2B). The
increase in angle was 51.5% in comparison to controls, and 50% compared to IS treated
plants.
Figure 1. Effect of foliar SAN application on leaf chlorophyll content of wheat
plants 3 days after treatment (A), compared to conventionally fertilized control
and industry standard (IS) treated plants. Fig 1B shows the effects 7 days later.
a
b
a
200
220
240
260
280
300
320
340
360
Chlorophyll Index Units (1-999)
Control SAN IS
A
a
b
a
70
75
80
85
90
95
100
105
110
115
120
Chlorophyll Index Units (%
control)
Control SAN IS
B
At 14-16 tiller stage, tiller base diameter was significantly larger in SAN-treated plants (Fig
3A) than in both control and IS treatments. Thus there was a significantly higher tiller base
diameter-canopy height ratio (3B), as canopy height was similar among treatments (not
shown).
Four weeks later, after a total of 3 foliar N fertiliser treatments, root length below the pot was
also the highest in SAN treated plants (Fig 4A), as was root length-canopy height ratio (4B),
as again canopy height was similar among treatments.
DISCUSSION
Genes and/or agronomic practises relating to phenotypic variability in root architectural traits,
nutrient uptake and metabolism, photosynthesis and canopy longevity, nitrogen
remobilization and wheat grain N accumulation are being sought to improve field wheat
nitrogen uptake efficiency (N taken up per unit N supplied) and/or nitrogen utilisation
efficiency (grain yield per unit N taken up), and yield per se (Hawkesford 2014, 2017).
Several of these characteristics are displayed during pre-anthesis growth of wheat
seedlings, and have been linked to yielding of mature plants in the field. These have largely
been determined by germplasm screening in a range of experimental and field systems,
under a range of conditions including drought, heat, and low and high levels of N. However,
these studies are rarely carried out on the basis of the N form(s) of the applied fertiliser.
Here we demonstrate that some of these traits can be induced in greenhouse-grown wheat
seedlings by a simple change in nutritional N form; these being increased relative leaf
chlorophyll content (Figure 1) and increased root length (Figure 4). Furthermore increased
wheat tiller basal internode diameter (Figure 3) is closely related to lodging resistance and
grain yield (Tripathi et al. 2003, Khan et al 2019). Erect tillers (Figure 2) can enhance
photosynthesis and dry matter production through greater sunlight capture (Abichou et al.
2019).
Improvements in root architecture (lateral root proliferation near the soil surface and at
depth) and increases in root biomass have been viewed as promising targets for selection
for NUE and yield amongst wheat genotypes (Hawkesford 2014, 2017). However, it has also
been demonstrated that root development is largely dependent on genetic differences in
above-ground shoot biomass and tillering processes (Allard et al. 2013), such that there is
an argument that root traits may not be as important as selection targets for improved NUE
and grain yield as originally believed. Allard et al. (2013) used ammonium nitrate as basal
fertiliser in field trials, which can be assumed to have been converted to nitrate, which will
have then been the dominant form of N in the soil. Given our research (e.g. Wilkinson et al.
2019b), we would maintain that the study by Allard et al (2013) was one based on genetic
variation in nitrate use efficiency, rather than one based on wider nitrogen use efficiency per
se. Nitrate from soil or foliar sources is preferentially allocated to shoots for above ground
vegetative growth and tillering during early seedling development, at the expense of root
biomass growth (Andrews et al. 2013, Wilkinson et al. 2019a, b). Compared to ammonium N
and ureic amine N, this generates a phenotype with a reduced root-shoot ratio and relatively
low internal nitrogen utilisation efficiency (nitrate assimilation is comparatively resource
inefficient). Screening for root traits would thus have occurred within a narrowed phenotypic
range, in which variations in vegetative traits would have provided a wider target. Had the
authors used stabilised urea as basal or foliar fertiliser, which promotes the generation of the
resource use efficient, stress resistant phenotype (characterised by high root-shoot ratio,
initially reduced apical dominance, and increased leaf chlorophyll content), we propose that
they would have found a wider variation in root traits within a more productive phenotypic
range.
Given that increased photosynthesis (Figure 1), and increased rooting (Figure 4) do indeed
show promising links to improved yields in other species (potato - Wilkinson et al 2019b,
lettuce Wilkinson et al 2020, manuscript in preparation), and that we show that these traits
are easily altered by nutritional N form in wheat, we argue that wheat crop fertilisation in the
field with stabilised urea will increase grain yield via the generation of a specific urea-amine
phenotype.
Figure 2. Comparison between the effects of SAN, control and industry standard
(IS) foliar N fertilisation treatments on tiller angle.
SAN IS
A
a
b
a
20
30
40
50
60
70
80
90
Tiller angle from horizontal at soil
surface (degrees)
Control SAN IS
B
Figure 3. Comparison between the effects of SAN, control and industry standard
(IS) foliar N fertilisation treatments on tiller diameter (A) and tiller diameter
canopy height ratio (B).
b
c
a
2.2
2.4
2.6
2.8
3
3.2
3.4
3.6
3.8
Tiller Base Diameter (mm)
Control SAN IS
A
a
b
a
0.072
0.074
0.076
0.078
0.08
0.082
0.084
0.086
0.088
0.09
0.092
Tiller Diameter-Canopy Height
Ratio (mm cm-1)
Control SAN IS
B
Figure 4. Comparison between the effects of SAN, control and industry standard
(IS) foliar N fertilisation treatments on root length (4A) and root length canopy
height ratio (4B).
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Root Length - Canopy Height
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Purpose: Supplying plants with nitrogen in ammonium nitrate- or urea-based fertiliser is wasteful: much is degraded before acquisition, releasing environmental pollutants. Preventing urea degradation can reduce pollution and improve crop nitrogen use efficiency. We investigate benefits to ureic stabilisation, on flowering and stress tolerance, as organic nitrogen sources favourably alter biomass partitioning in this regard. Research Method: We test effects of adding chemically stabilised urea to soil, on the physical form and flowering of containerised, greenhouse-grown pelargonium, petunia, pansy and marigold, when transplanting seedlings to larger pots. Efficacies of stabilised urea, non-stabilised urea and industry standard fertiliser are compared under identical total nitrogen supply. The significance of treatment differences is calculated using a one-tailed t-test. Findings: Development is favourably altered by ureic stabilisation. Earliest changes measured are increased root lengths, leaf growth rates and chlorophyll concentrations. Plants then develop more shoots and 25-130% more flowers. Improvements arise partially through increased nitrogen longevity in soil, and partially through positive effects of urea itself on biomass partitioning between organs, and on plant physiology; giving rise to improved commercial attributes (more branches and flowers) and tolerance to stress (more root, less apical dominance, more chlorophyll). Research Limitations: Further research could measure leachate nitrogen content, and compare different methods of ureic stabilisation in more crops. Originality/Value: Urea stabilisation can increase fruit and flower yields, whilst reducing vulnerability to erratic climates, and fertiliser-derived pollution. We propose that urea’s effectiveness arises because plants have evolved strategies to proliferate whilst competing with micro-organisms for organic nitrogen.
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Although zoysiagrass (Zoysia spp.) responds to nitrogen (N) application in the field, especially during the establishment phase, no guidelines are available for the preferred form of N for these species. A greenhouse experiment was performed to see how zoysiagrass responds to nitrate and urea as N sources. Z. japonica cv. El Toro (fast-growing) and cv. Meyer (slow-growing) and Z. matrella cv. Zorro (fast-growing) and cv. Diamond (slow-growing) were chosen for this study. Plants were clonally propagated as phytomers, established in 21 cm deep cone-tainers filled with a sand-based growth medium. The treatment consisted of a modified, half-strength Hoagland's solution with five different nitrate : urea ratios (100 : 0, 75 : 25, 50 : 50, 25 : 75, 0 : 100). Plant responses were assessed in terms of biomass production (leaf, culm, roots, and total plant dry weight) and rooting characteristics (length, area, diameter, volume, tips, forks, crossings, and root volume ratio) 10 weeks after the initiation of the experiment. The differing sources of N resulted in changes in plant growth and development. The cultivars had different above and below-ground biomass and root architecture traits. El Toro had the highest total biomass production (2.083 g plant−1 DW), while Meyer and Zorro together averaged 0.734 g plant−1 DW (65% less than El Toro). Diamond was the least productive in terms of leaf, culm, and root biomass (0.278 g plant−1 DW; 87% less than El Toro). Furthermore, above- and below-ground DW production was greatest following treatment with 25 : 75 nitrate:urea, whereas 100% nitrate produced plants with the lowest DW. Zoysiagrass rooting traits were only minimally influenced by N source; as the concentration of urea increased, slight increases in root surface area and volume were observed, accompanied by a decline in the root volume ratio.
Article
Crop nutrient and especially nitrogen use efficiency (NUE) is both an economically and an environmentally highly desirable trait. It has been estimated that only a third of nitrogen inputs to cereal crop worldwide are recovered in grain for consumption, resulting in a huge waste of resource with major negative impacts on the environment. Most measures of NUE in wheat and other cereals are based on field assessments of crop yields at given N inputs, performance responses to added N fertilizer, or by quantifying N fertilizer recovery rates. However, NUE is a complex trait comprising two key major components, N uptake and N utilization efficiency, both also complex traits in themselves, each involving many physiological processes and biochemical pathways. A deeper understanding of the processes involved in NUE has been a target of the UK Wheat Genetic Improvement Network project (http://www.wgin.org.uk/). This has enabled the breakdown of characteristics contributing to NUE and an assessment of the variation present in those characteristics, predominantly in modern cultivars; a total of 13 years of data have been obtained to date. Significant but limited variation suggests a requirement for broader germplasm screening such as older varieties, landraces, and wild relatives.
Article
While nitrate acquisition has been extensively studied, less information is available on transport systems of urea. Furthermore, the reciprocal influence of the two sources has not been clarified, so far. In this review, we will discuss recent developments on plant response to urea and nitrate nutrition. Experimental evidence suggests that, when urea and nitrate are available in the external solution, the induction of the uptake systems of each nitrogen source is limited, while plant growth and N utilization is promoted. This physiological behavior might reflect cooperation among acquisition processes, where the activation of different N assimilatory pathways (cytosolic and plastidic pathways), allowing a better control on the nutrient uptake. Based on physiological and molecular evidence, plants might increase nitrogen metabolism promoting a more efficient assimilation of taken-up nitrogen. The beneficial effect of urea and nitrate nutrition might contribute to develop new agronomical approaches to increase the nitrogen use efficiency in crops.
Article
The efficiency of classical mineral NPK fertilizers is usually low because a major part of these fertilizers does not reach plant roots and ends up polluting groundwaters with nitrates and phosphates. Recently, a novel polymer-coated urea made from recycled plastics was proposed to enhance N availability in cereal production. To evaluate the efficiency of this polymer for rice production, we set up field plots, microplots, and pot experiments with 15N tracing. We compared rice yield, N uptake, and N loss between conventional three split applications of urea and a single basal application of four derivatives from the polymer-coated urea. The four derivatives included a blend with 70 % of N from 6 % (w/w) coated urea and 30 % from urea and three coated urea fertilizers with 6, 8, and 12 % coating at an identical N application rate during two rice-growing seasons. Results show that 6 % coated urea improved 15N recovery, reduced 15N loss, and increased grain yield slightly due to an initial 15N burst occurring at high field temperatures after basal fertilization; 8 or 12 % coated urea better met plant N demand from transplanting to heading, greatly enhanced 15N recovery, and decreased 15N loss and NH3 volatilization. Nevertheless, unlike a significant increase of yield for 12 % coated urea, 8 % coated urea did not increase yield due to 15N release and excessive 15N uptake by plants at ripening. Overall, our findings show that a single basal polymer-coated urea application improves N use efficiency and reduces N loss in rice agroecosystem.
Article
Urea is the worldwide most widespread nitrogen (N) fertilizer and rapidly degraded in soil to ammonium by urease. Ammonium is either taken up by plant roots or is further processed to nitrate by soil microorganisms. However, urea can be taken up by roots and is further degraded to ammonium by plant urease for assimilation. When urea is supplied under sterile conditions, it acts as a poor N-source for seedlings or adult Arabidopsis thaliana plants. Here, the gene expression of young seedlings exposed to urea and ammonium nitrate nutrition was compared. Several primary metabolism and transport genes, including those for nitrate and urea, were differentially expressed in seedlings. However, urease and most major intrinsic proteins were not differentially expressed, with the exception of NIP6;1, a urea permeable channel, which was repressed. Furthermore, little overlap with the gene expression with ammonium as the sole nitrogen source was observed, confirming that pure urea nutrition is not associated with the ammonium toxicity syndrome in seedlings. The direct root uptake of urea was increased under boron deficiency, both in the high and low affinity range. This activity was entirely mediated by the NIP5;1 channel, which was confirmed to transport urea when expressed in oocytes. The uptake of urea in the high and low affinity range was also determined for maize and wheat roots. The urea uptake by maize roots was only about half of that of wheat, but was not stimulated by boron deficiency or nitrogen deficiency in both species. This analysis identifies novel components of the urea uptake systems in plants, which may become agronomically relevant to urea uptake and utilization, as stabilized urea fertilizers become increasingly popular. © The Author 2015. Published by Oxford University Press on behalf of Japanese Society of Plant Physiologists. All rights reserved. For Permissions, please e-mail: journals.permissions@oup.com.