Cold Stress (Plant Biology)

In nature, plants are often exposed to multiple
biotic and abiotic stresses, severely affecting their growth
and development and reducing their productivity. Future
predicted adverse climatic changes might threaten the very
sustainability of crop production worldwide. Various
approaches ought to be explored to deal with the challenges
of sustained crop production under such conditions. In this
review, we explore the potential of osmotin, a stressresponsive
multifunctional pathogenesis-related (PR)-5c
protein from tobacco, in improving adaptability of crop
plants to climatic changes. As osmotin plays an important
role in salt and drought tolerance as well as in cold tolerance
and in protecting plants against some fungal pathogens, the
relevance of osmotin in improving tolerance to abiotic and
biotic stresses in plants with focus on strawberry, a salt-sensitive plant that is also
susceptible to several fungal pathogens.

Analysis of the partitioning of absorbed light energy within PSII into fractions utilised by PSII photochemistry
(FPSII), thermally dissipated via DpH- and zeaxanthin-dependent energy quenching (FNPQ) and constitutive
non-photochemical energy losses (Ff,D) was performed in control and cold-stressed maize (Zea mays L.) leaves. The
estimated energy partitioning of absorbed light to various pathways indicated that the fraction of FPSII was twofold lower, whereas the proportion of thermally dissipated energy throughFNPQ was only 30% higher, in cold-stressed plants compared with control plants. In contrast,Ff,D, the fraction of absorbed light energy dissipated by additional quenching mechanism(s), was twofold higher in cold-stressed leaves. Thermoluminescence measurements revealed that the changes in energy partitioning were accompanied by narrowing of the temperature gap (DTM) between S2/3QB-
and S2QA- charge recombinations in cold-stressed leaves to 8oC compared with 14.4oC in control maize plants. These observations suggest an increased probability for an alternative non-radiative P680+QA- radical pair recombination pathway for energy dissipation within the reaction centre of PSII in cold-stressed maize plants. This additional quenching mechanism might play an important role in thermal energy dissipation and photoprotection when the capacity for the primary, photochemical (FPSII) and zeaxanthin-dependent non-photochemical quenching (FNPQ) pathways are thermodynamically restricted in maize leaves exposed to cold temperatures.

Traditional architecture of the historical cities
of Iran contains valuable lessons related to architecture and
urban design. A group of these strategies are those used in
outdoor urban spaces in desert cities providing a safe and
sustainable microclimate for pedestrian. This research paper
will highlight some of these strategies by doing a field study
research in hot summer and cold winter in Kashan, a
historical city of Esfahan Province. The weather data are
collected in 11-12 July 2011 and 11-12 Jan 2012 by a mobile
Kestrel Personal Weather station in the traditional part of the
city. The collected data are analyzed for human thermal
condition by UTCI (Universal Thermal Climate Index) on
the psychrometric chart. The different thermal zones on this
chart are compared with people’s behaviour according to
their exposure time, clothing and activity. The collected
weather data of observation days are compared in four levels
of long-term meso climate, short-term meso climate, local
climate and microclimate. To speed up the analyzing process
a new software is designed called SIKRON. The result of
this research has shown the effect of architectural strategies
on modifying the microclimate condition in hot summer and
cold winter for outdoor living in hot-arid climate.

Concentrations of citronellol and geraniol in essential oil from rose geranium (Pelargonium hybrid) were monitored monthly over four years using gas chromatography-mass spectrometry. Geraniol concentrations fell in winter and citronellol concentrations increased in spring. The citronellol/geraniol (C:G) ratio was a more sensitive indicator of change in oil composition than the concentration of either alcohol in isolation. C:G ratios rose sharply if the minimum air temperature (Tmin) fell below 5.58C for any time during the 5 d prior to monthly harvesting. A nonlinear model is proposed to predict the C:G ratio at any Tmin. High C:G ratios indicated cold stress in geranium plants.

Plants respond to low temperatures with changes in their pattern of gene expression and protein products. Many species of tropical or subtropical origin are injured or killed by non-freezing low temperature, and exhibit various symptoms of chilling injury such as chlorosis, necrosis, or growth retardation. In contrast, chilling tolerant species are able to grow at such cold temperatures. Conventional breeding methods have met with limited success in improving the cold tolerance of important crop plants through inter-specific or inter-generic hybridization. Recent studies involving full genome profiling/sequencing, mutational and transgenic plant analyses have provided a deep insight of the complex transcriptional mechanism that operates under cold stress. The alterations in expression of genes in response to cold temperatures are followed by increases in the levels of hundreds of metabolites, some of which are known to have protective effects against the damaging effects of cold stress. Various low temperature inducible genes have been isolated from plants. Most appear to be involved in tolerance to cold stress and the expression of some of them is regulated by C-repeat binding factor/dehydration-responsive element binding (CBF/DREB1) transcription factors. Numerous physiological and molecular changes occur during cold acclimation reveals that the cold resistance is more complex than perceived and involves more than one pathway. The findings summarized in this review have shown potential practical applications for breeding cold tolerance in crop and horticultural plants suitable to temperate geographical locations.

The basis of the increased resistance to photoinhibition upon growth at low temperature was investigated. Photosystem II (PSII) excitation pressure was estimated in vivo as 1 - qp (photochemical quenching). We established that Chlorella vulgaris exposed to ei- ther 5'C/150 pmol m-'s-l or 27°C/2200 pmol m-'s-l experi- enced a high PSll excitation pressure of 0.70 to 0.75. In contrast, Chlorella exposed to either 27"C/150 pmol m-, s-l or 5'C/20 pmol m-'s-l experienced a low PSll excitation pressure of 0.10 to 0.20. Chlorella grown under either regime at high PSll excitation pressure exhibited: (a) 3-fold higher light-saturated rates of O, evolution; (b) the complete conversion of PSlh centers to PSllp centers; (c) a 3-fold lower epoxidation state of the xanthophyll cycle intermediates; (d) a 2.4-fold higher ratio of chlorophyll Jb; and (e) a lower abundance of light-harvesting polypeptides than Chlorella grown at either regime at low PSll excitation press...

Winter wheat (Trificum aesfivum L. cv Monopol), spring wheat
(Trificum aesfivum L. cv Katepwa), and winter rye (Secale cereale 1. cv Musketeer) grown at 5°C and moderate irradiance (250 micromol m-2 s -1 ) (5/250) exhibit an increased tolerance to photoinhibition at low temperature in comparison to plants grown at 20°C and 250 micrpmol m-2s-1 (20/250). However, 5/250 plants exhibited a higher
photosystem II (PSII) excitation pressure (0.32-0.63) than 20/250 plants (0.18-0.21), measured as 1 - qp, the coefficient of photochemical quenching. Plants grown at 20°C and a high irradiance (800 micromol m-2s-1) (20/800) also exhibited a high PSll excitation pressure (0.32-0.48). Similarly, plants grown at 20/800 exhibited a comparable tolerance to photoinhibition relative to plants grown at 5/250. In contrast to a recent report for Chlorella vulgaris (D.P. Maxwell, S. Falk, N.P.A. Huner [1995] Plant Physiol 107: 687-694), this tolerance to photoinhibition occurs in winter rye with minimal
adjustment to polypeptides of the PSll light-harvesting complex, chlorophyll a/b ratios, or xanthophyll cycle carotenoids. However, Monopol winter wheat exhibited a 2.5-fold stimulation of sucrosephosphate synthase activity upon growth at 5/250, in comparison to Katepwa spring wheat. We demonstrate that low-temperatureinduced
tolerance to photoinhibition is not a low-temperaturegrowth
effect per se but, instead, reflects increased photosynthetic
capacity in response to elevated PSll excitation pressure, which may be modulated by either temperature or irradiance.

We studied the influence of two irradiances on
cold acclimation and recovery of photosynthesis in Scots pine
(Pinus sylvestris L.) seedlings to assess mechanisms for quenching the excess energy captured by the photosynthetic apparatus. A shift in temperature from 20 to 5 °C caused a greater decrease in photosynthetic activity, measured by chlorophyll fluorescence and oxygen evolution, in plants exposed to moderate light (350 μmol m– 2 s –1) than in shaded plants (50 μmol m– 2 s –1). In response to the temperature shift, maximal photochemical efficiency of photosystem II (PSII), measured as the ratio of variable to maximal chlorophyll fluorescence (Fv/Fm) of dark-adapted samples, decreased to 70% in ex posed seedlings, whereas shaded seed lings maintained Fv/Fm close to initial values. After a further temperature decrease to –5 °C, only 8% of initial Fv/Fm remained in exposed plants, whereas shaded plants retained 40% of initial Fv/Fm. Seven days after trans -
fer from –5 to 20 °C,  recovery of photochemical efficiency was
more complete in the shaded plants than in the ex posed plants (87 and 65% of the initial Fv/Fm value, respectively).
In response to cold stress, the estimated functional absorption cross section per remaining PSII reaction center increased at both irradiances, but the in crease was more pronounced in exposed seed lings. Estimates of energy partitioning in the needles showed a much higher dissipative component in the expoesd seed lings at low temperatures, pointing to stronger development of non-photochemical quenching at moderate irradiances. The de-epoxidation state of the xanthophyll cycle pigments in creased in exposed seedlings at 5 °C, contributing to the quenching capacity, whereas significant de-epoxidation in the shaded plants was observed only when temperatures decreased to –5 °C. Thermoluminescence (TL) measurements of PSII revealed that charge recombinations between the second oxidation state of Mn-cluster S2 and the semi-reduced secondary electron acceptor quinone QB– (S2QB–) were shifted to
lower temperatures in cold-acclimated seedlings compared
with control seed lings and this effect depended on irradiance. Concomitant with this, cold-acclimated seedlings demonstrated a significant shift in the S2 recombination with primary acceptor QA– (S2QA–) characteristic TL emission peak to higher temperatures, thus narrowing the redox potential gap between S2QB– and S2QA–, which might result in increased probability for non-radiative radical pair recombination between the PSII reaction center chlorophyll a (P680+) and QA– (P680+QA–) (reaction center quenching) in cold-acclimated seedlings. In Scots pine seedlings, mechanisms of quenching excess light energy in winter therefore involve light-dependent regulation of reaction center content and both reaction center-based and
antenna-based quenching of excess light energy, enabling
them to withstand high excitation pressure under northern
winter conditions.