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درسی - 1-3

درسی

دوشنبه 19 اردیبهشت 1390

1-3

نویسنده: رضا کوشکی   

 

 

Abstract Temperature thresholds for a range of crops from cereal crops to horti-


cultural crops and to legum crops were identified through an extensive literature
review. Identification of temperature thresholds provides a basis for quantifying the
probability of exceeding temperature thresholds which is a very important aspect of
climate change risk assessment. The effects of extreme temperatures on yield and
yield components were then reviewed and summarised. Through these processes,
critical phenophases were defined based on the sensitivity of crop yield and/or
yield components to extreme high temperatures which were imposed on various
phenophases. Information on the direction and degree of the impact of extreme
temperature on yield/yield components can contribute to the improvement of crop
models in which the effects of extreme temperature on crop production have not
been adequately represented at this stage. Identification of critical phenophases at
which crops yield and/or other economic characteristics are sensitive to extreme
temperatures will help scoping appropriate adaptation options.

1 Introduction

Temperature (T) is one of the major environment factors affecting the growth,
development and yields of crops especially the rate of development. On one hand,
crops have basic requirement for T to complete a specific phenophase or the whole
life cycle. On the other hand, extremely high and low Ts can have detrimental effects
on crop growth, development and yield particularly at critical phenophases such as

Climatic Change

anthesis. Wheeler et al. (2000) pointed out that the effects of hot T episodes close
to the time of anthesis were of more importance to the yield of many crops than
the effects of the increase in mean seasonal T of about 2◦ C. Cardinal T (i.e. base T
(Tbase), optimum T (Topt1 and Topt2), and failure point T (Tfp)) and lethal T (e.g.
lethal minimum T (Tlmin) and lethal maximum T (Tlmax)) are typical Ts associated
with crop production. Crops grow and develop ideally within the range of Topts and
at a slower rate beyond the range (the so-called sub/supra-Topt). It was found that
the rate of many development processes is a positive linear function of T between
Tbase and Topt and a negative linear function of T between Topt2 and Tfp (Roberts
and Summerfield 1987; Wheeler et al. 2000). Figure 1 depicts the relative position of
these temperatures. Tfp represents failure (ceiling) T at which grain yield fails to zero
yield (Hatfield et al. 2008). The difference between lethal Ts and cardinal Ts is that
recovery of function is possible within the range of cardinal Ts but is irrecoverably
lost beyond the lethal limits (Porter and Gawith 1999). Cardinal T and lethal T are
associated with thresholds. What they relate to climate change risk assessment is how
often and much T thresholds will be crossed and what the effects of exceeding those
T thresholds might be in relation to crop yield and yield components including impact
direction and magnitude. Under global warming scenarios, the chances and extent of
crossing T thresholds may be higher and more than those under current temperature
regime.
It seems that there is an urgent need to identify temperature thresholds and the
effects of extreme temperature on crop production. Identification of temperature
thresholds will provide a starting point for assessing extreme temperature related
risks and this will provide a pathway toward to exploring adaptation options. Iden-
tification of the effects of extreme temperature on crop production across various
phenophases will help to define critical phenophases so that impact assessment and
adaptation evaluation/implementation are focus-oriented. Information on the effects
of extreme temperature on crop production under various field/controlled environ-
mental conditions can be used to improve crop models for accurate quantification
of the impacts of temperature change on crop production at regional level. The
impacts of mean T on crop production were represented in some crop models by
growing degree days (GDD). However, the effect of extreme Ts on crop production
is lacking in many models which may bias the projection of the impacts of climate

 

Fig. 1 Relative position of temperature types. Data source: Porter and Gawith (1999). The cardinal
temperatures (Tbase, Topt and Tmax) are associated with terminal spikelet initiation stage

 

change on crop production. This indicates that crop models need to be improved to
accommodate the effects of extreme T on crop production.

From what argued above, this paper aims to identify temperature thresholds and
the effects of exceeding temperature thresholds on crop yield and yield components
for various phenological stages and across a range of crops from cereal crops to
legume crops and to horticultural crops based on an extensive literature review
and to identify the key phenophases which are most sensitive to the exceedance of
temperature thresholds. This kind of information will be very useful for assessing
temperature change impacts and for scoping appropriate adaptation options either
for a specific industry or for a specific region where a range of crops grow.

2 Temperature thresholds

2.1 Cereal crops

Wheat Temperature thresholds in relation to wheat were well defined. Porter
and Gawith (1999) extensively reviewed temperature thresholds across different
components (root, leaf, culm) and phenophases based on worldwide experimental
studies. Key findings from that literature were given in Table 1. It is found that
cardinal temperatures increase as wheat growth and development progress (Porter
and Gawith 1999; Slafer and Rawson 1995). This was demonstrated in Fig. 2.

 

Barley Prasil et al. (2007) evaluated the tolerance of 39 barley cultivars and breed-
ing lines to low temperature by using four direct methods in the Czech Republic: (1)
field survival after five winters 1999–2004; (2) winter survival in a provocation pot
test under natural conditions; (3) lethal temperature (LT50) of plants taken from a
field in winter; and (4) LT50 of plants grown and hardened in a growth chamber. It
was found that barley has a Tlmin50 (at which 50% of samples are killed) of −17.3 ∼
−12.9◦ C across 20 cultivars which is close to the Tlmin (−18 ∼ −16◦ C) of wheat as
given in Table 1.

Maize Several studies found that temperatures of above 35◦ C are lethal to maize
pollen viability (Herrero and Johnson 1980; Schoper et al. 1987; Dupuis and Dumas
1990). Leaf photosynthesis rate of maize has a high Topt of 33◦ C to 38◦ C (Crafts-
Brandner and Salvucci (2002).

Rice The response of rice to temperature has been well studied. Leaf-appearance
rate increases with temperature from a Tbase of 8◦ C, until reaching 36–40◦ C, the
thermal threshold of survival (Alocilja and Ritchie 1991; Baker et al. 1995) with
biomass increasing up to 33◦ C (Matsushima et al. 1964). However, the Topt for grain
formation and yield is lower (25◦ C) (Baker et al. 1995). High percentages of rice
spikelet sterility occur if temperatures exceed 35◦ C at anthesis and last for more than
1 h (Yoshida 1981).

Sorghum The vegetative development of sorghum has a Tbase of 8◦ C and Topt of
34◦ C (Alagarswamy and Ritchie 1991) while reproductive development of sorghum
has a Topt of 31◦ C (Prasad et al. 2006). Maiti (1996) reported that sorghum vegetative
growth has a Topt of 26 ∼ 34◦ C while reproductive growth has a Topt of 25 ∼ 28◦ C.

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