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Journal ArticleDOI

Canopy temperature of high-nitrogen water-stressed cotton

01 May 2020-Crop Science (Wiley)-Vol. 60, Iss: 3, pp 1513-1529

Abstract: Australian cotton (Gossypium hirsutum L.) farmers are adopting canopy temperature (Tc)‐based irrigation scheduling as a decision support tool to improve on‐farm production. High N supply, characteristic of the high‐yielding, furrow‐irrigated cotton system of Australia, might alter cotton Tc with implications for irrigation. We examined growth, physiological, and biochemical traits and changes in Tc of well‐watered and water‐stressed cotton plants supplied with high to excessive levels of N under glasshouse conditions. We also examined Tc, lint yield, and fiber quality of furrow‐irrigated cotton crop supplied with high N. In the glasshouse and under well‐watered conditions, high N supply stimulated plant growth and increased stomatal conductance and photosynthesis, resulting in cooler Tc. Under water deficit stress, high N also stimulated growth, increasing plant water demand and thus vulnerability to water stress, which manifested as warmer Tc. Water‐stressed plants supplied high N also showed reduced stomatal conductance, lower leaf water potential, and greater accumulation of leaf and xylem sap abscisic acid. Furrow‐irrigated crops supplied higher N also had higher Tc, but there was no gain in lint yield and fiber quality. The influence of high N on cotton Tc suggests that the need for accurate and reliable Tc‐based irrigation scheduling is paramount.
Topics: Stomatal conductance (57%), Irrigation scheduling (51%), Irrigation (50%)

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Running title: Canopy temperature of high N cotton
Canopy temperature of high-nitrogen water-stressed cotton
Onoriode Coast
*
, Steven Harden, Warren C. Conaty, Rose Brodrick, and Everard J. Edwards
Affiliations:
O. Coast, W.C. Conaty, and R. Brodrick, CSIRO Agriculture & Food, Locked Mail Bag 59,
Narrabri NSW 2390, Australia; O. Coast, current address, ARC Centre of Excellence in Plant
Energy Biology, Research School of Biology, The Australian National University, 134
Linnaeus Way, Canberra, ACT 2601, Australia; R. Brodrick, current address, CSIRO
Agriculture & Food, Black Mountain, Canberra, ACT 2601, Australia; S. Harden, NSW
Department of Primary Industries, Tamworth Agricultural Institute, 4 Marsden Park Road,
Calala, New South Wales 2340, Australia; E.J. Edwards, CSIRO Agriculture & Food, Locked
Bag 2, Glen Osmond, South Australia, 5064, Australia.
*
Corresponding author
(onoriode.coast@anu.edu.au).
Abbreviations:
ABA, abscisic acid; ABA-GE, abscisic acid glucose ester; ACRI, Australian Cotton Research
Institute; DAS, days after sowing; HVI, High Volume Instrument; GAM, generalised additive
model; NUE, nitrogen use efficiency; REML, residual maximum likelihood; Tc, canopy
temperature; VPD, vapour pressure deficit; WUE, water use efficiency; ψ
leaf
, leaf water
potential.

This article is protected by copyright. All rights reserved.
ABSTRACT
Australian cotton (Gossypium hirsutum L.) farmers are adopting canopy temperature (Tc)
based irrigation scheduling as a decision support tool to improve on-farm production. High
nitrogen supply, characteristic of the high-yielding, furrow irrigated cotton system of
Australia, might alter cotton Tc with implications for irrigation. We examined growth,
physiological and biochemical traits and changes in Tc of well-watered and water-stressed
cotton plants supplied high to excessive levels of nitrogen under glasshouse conditions. We
also examined Tc, lint yield and fibre quality of furrow irrigated cotton crop supplied with
high nitrogen. In the glasshouse and under well-watered conditions, high nitrogen supply
stimulated plant growth, increased stomatal conductance and photosynthesis resulting in
cooler Tc. Under water deficit stress high nitrogen also stimulated growth, increasing plant
water demand and thus vulnerability to water stress, which manifested as warmer Tc. Water
stressed plants supplied high nitrogen also showed reduced stomatal conductance, lower leaf
water potential, and greater accumulation of leaf and xylem sap abscisic acid. Furrow
irrigated crops supplied higher nitrogen also had higher Tc, but there was no gain in lint yield
and fibre quality. The influence of high nitrogen on cotton Tc suggests the need for accurate
and reliable Tc-based irrigation scheduling is paramount.
INTRODUCTION
Irrigated cotton production in Australia is intensive, broadacre cropping, characterised by
high nitrogen fertiliser application. Despite the recommendation of 200 kg N ha
-1
to achieve
optimal crop nitrogen use efficiency (NUE) (Rochester, 2011), almost 50% of farmers apply
more than 50 kg N ha
-1
above the recommendation (Roth, 2013). Roth (2013) in a 2013

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survey of irrigated cotton farms reported that nitrogen rates varied between 93 to 370 kg N
ha
-1
with 46% of farmers applying 250 kg N ha
-1
or more. The main reason for the high
nitrogen application is as insurance against nitrogen-deficit related yield loss (Roth, 2013).
This over application of nitrogen unnecessarily increases the cost of production, nitrogen
leaching (Macdonald et al., 2016c), runoff losses (Silburn and Hunter, 2009; Macdonald et
al., 2017), and the potential for greenhouse gas emissions (Macdonald et al., 2016a, b), as
well as reducing NUE (Rochester, 2011). In contrast to high nitrogen application, water use
in the Australian cotton production system is constrained by its scarcity and increasing
competition from other sources (Richards et al., 2008). This has compelled farmers to adopt
more efficient use of water and concerted efforts to further improve water use efficiency
(WUE). There are several projects the Australian cotton industry has invested in to optimise
WUE and NUE in order to maximise their profits. For reviews of such projects see Roth et al.
(2013) and Rochester (2011), respectively.
One method for improving WUE, which is gaining acceptance by the industry, is to adopt
canopy temperature (Tc) based irrigation scheduling (Conaty et al., 2012, 2015). This
approach is favoured because it is plant-based, equipment required is relatively cheap and
easy to operate, it provides continuous data on plant water status and is suited to the long
irrigation intervals (>5 days) characteristic of Australian furrow irrigated systems. Most
(92%) cotton farms in Australia are furrow irrigated (Roth, 2015). Canopy temperature-based
irrigation scheduling can also be easily incorporated with other currently used irrigation
scheduling techniques, which are mainly soil-based (soil moisture capacitance probes and
neutron soil moisture probes, used by 57 and 22% of farmers, respectively) (Roth 2011).

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Under non-water limiting conditions, plants transpire water from open stomata, mostly on
leaf surfaces. The loss of water and latent heat from plant leaves and the crop canopy cools
the leaves and canopy. In contrast, under water deficit conditions, plants minimise water loss
from leaves by inducing gradual stomatal closure and reduction in stomatal conductance to
water, which in turn limits evaporative cooling. This limitation causes a rise in leaf and
canopy temperature. Researchers have exploited this knowledge to develop sensors that
monitor Tc and protocols for optimising irrigation scheduling based on Tc. The initial
application of Tc for irrigation scheduling used Tc derived indices including stress degree
day (Idso et al., 1977; Jackson et al., 1977), crop water stress index (Idso et al., 1981; Jackson
et al., 1981), temperature stress day (Gardner et al., 1981), and canopy temperature variability
(Clawson and Blad, 1982). Later the stress time temperature threshold approach was
developed for irrigating cotton in parts of the USA (Wanjura et al., 1995; Upchurch et al.,
1996; Wanjura and Upchurch, 1997; Wanjura et al., 2004). This approach was based on
comparing Tc against a pre-determined threshold temperature and triggering irrigation when
Tc exceeds the threshold for a specified period of time provided atmospheric conditions will
allow for transpirational cooling to occur, i.e. cumulative Tc stress duration (Mahan et al.,
2005). The threshold Tc for cotton was taken as 28 to 29°C, the optimum for cotton
enzymatic and physiological function (Mahan et al., 2005; O'Shaughnessy and Evett, 2010;
Conaty et al., 2012). In some other research fields, the principles underlining changes in Tc
have been similarly applied i.e. Tc of the subject of interest is compared against a
predetermined reference to assess crop health and maturity. This form of application of Tc
data might be inappropriate if there are other factors that influence Tc independently, such as
growth environment temperature, vapour pressure deficit (VPD), and nitrogen status of the
plant. Whilst the latter is amenable to manipulation for positive outcomes, some others are

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not. Radin and Ackerson (1981) showed that nitrogen deficiency reduced stomatal
conductance by inducing stomatal closure. Inhibition of stomatal conductance results in
warmer Tc.
Abscisic acid (ABA) is a multifunctional plant hormone readily occurring in vascular
tissue, as well as parenchyma cells outside vascular bundles. It plays roles in germination,
seasonal growth patterns, and importantly in stress responses including water deficit,
salinity, cold temperatures and frost (Vishwakarma et al., 2017). ABA production results in
responses that help protect plants from these stressors. During prolonged periods of drought
stress, catabolism of ABA occurs continuously, but is balanced by de novo biosynthesis to
maintain high ABA levels until the stress is alleviated (Harrison and Walton, 1975; Ren et al.,
2007). The phytohormones ABA is catabolised through either conjugation to produce ABA-
glucose ester (ABA-GE) or oxidation to form phaseic acid, which is further metabolised to
inactive dihydrophaesic acid (Sharkey and Raschke, 1980; Zeevaart, 1980). Phaseic acid can
trigger similar plant responses to ABA, including stomatal closure; however, its bioactivity in
terms of stomatal behaviour is much weaker than ABA and unlike ABA the phaseic acid
response can vary significantly across species (Sharkey and Raschke, 1980). Stomatal
conductance is influenced by a range of factors including water and nutrient status, and it is
linked to concentration of ABA (Chaves et al., 2002; Pantin et al., 2012). For example, water
stress increases endogenous ABA concentration, which acts directly on the guard cells,
causing stomatal closure (Sharkey and Raschke, 1980; Zeevaart and Creelman, 1988). Also,
nitrate (the predominant form of soil available nitrogen to plants) deficiency increases ABA
concentration resulting in decreased stomatal conductance (Radin et al., 1982; Wilkinson et
al., 2007).

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