Soil Water Deficit Conditions Research Articles

Drought-Induced Leaf Proteome Changes in Switchgrass Seedlings.

Switchgrass (Panicum virgatum) is a perennial crop producing deep roots and thus highly tolerant to soil water deficit conditions. However, seedling establishment in the field is very susceptible to prolonged and periodic drought stress. In this study, a “sandwich” system simulating a gradual water deletion process was developed. Switchgrass seedlings were subjected to a 20-day gradual drought treatment process when soil water tension was increased to 0.05 MPa (moderate drought stress) and leaf physiological properties had expressed significant alteration. Drought-induced changes in leaf proteomes were identified using the isobaric tags for relative and absolute quantitation (iTRAQ) labeling method followed by nano-scale liquid chromatography mass spectrometry (nano-LC-MS/MS) analysis. Additionally, total leaf proteins were processed using a combinatorial library of peptide ligands to enrich for lower abundance proteins. Both total proteins and those enriched samples were analyzed to increase the coverage of the quantitative proteomics analysis. A total of 7006 leaf proteins were identified, and 257 (4% of the leaf proteome) expressed a significant difference (p < 0.05, fold change <0.6 or >1.7) from the non-treated control to drought-treated conditions. These proteins are involved in the regulation of transcription and translation, cell division, cell wall modification, phyto-hormone metabolism and signaling transduction pathways, and metabolic pathways of carbohydrates, amino acids, and fatty acids. A scheme of abscisic acid (ABA)-biosynthesis and ABA responsive signal transduction pathway was reconstructed using these drought-induced significant proteins, showing systemic regulation at protein level to deploy the respective mechanism. Results from this study, in addition to revealing molecular responses to drought stress, provide a large number of proteins (candidate genes) that can be employed to improve switchgrass seedling growth and establishment under soil drought conditions (Data are available via ProteomeXchange with identifier PXD004675).

Soil water deficit and climate conditions during the dry season along the coastal-inland gradient in Castelporziano forest, central Italy

Summer precipitation and temperature regimes have been compared along the coastal-inland gradient in three sites (Tor Paterno and Castello in Castelporziano forest and Collegio Romano in inner Rome, central Italy) during the last 15 years (1999–2013). Soil water budget using rainfall, evapotranspiration and maximum water capacity of the soil was calculated according to Thornthwaite–Mather approach for the two forest sites. Meteorological conditions during the dry season were found significantly different in the three sites with minimum temperatures increasing from the coastal site (Tor Paterno) to the inland area (Castello and Rome). Maximum temperatures showed a less marked coastal-inland gradient. Cumulative summer rainfalls (accounting for less than the 10 % of annual precipitation) were comparable in the three sites along with a high variability. Although soil water budget variables (available water content and water deficit) were similar in the two forest sites, the aridity index was higher in the inland site than in the coastal site. Despite based on a relatively short climatic time-series, implications of the climate variations observed along the coastal-inland gradient in Castelporziano are discussed in the light of conservation of the residual pristine flat oakwood stands and the traditional agro-forest landscape mosaic.

Improvement of FAO-56 Model to Estimate Transpiration Fluxes of Drought Tolerant Crops under Soil Water Deficit: Application for Olive Groves

Agro-hydrological models are considered an economic and simple tool for quantifying crop water requirements. In the last two decades, agro-hydrological physically based models have been developed to simulate mass and energy exchange processes in the soil-plant-atmosphere system. Although very reliable, because of the high number of required variables, simplified models have been proposed to quantify crop water consumes. The main aim of this paper is to propose an amendment of the Food and Agricultural Organization (FAO) of the United Nations FAO-56 spreadsheet program to introduce a more realistic shape of the stress function, valid for mature olive orchards (Olea europaea L.). The modified model is successively validated by means of the comparison between measured and simulated soil water contents and actual transpiration fluxes. These outputs are finally compared with those obtained with the original version of the model. Experiments also allowed assessing the ability of simulated crop water stress coefficients to explain the actual water stress conditions evaluated on the basis of measured relative transpirations and midday stem water potentials. The results show that the modified model significantly improves the estimation of actual crop transpiration fluxes and soil water contents under soil water deficit conditions, according to the RMSEs associated with the revised model, resulting in significantly higher than the corresponding values obtained with the original version.

Assessment of soil water deficit for the middle reaches of Yarlung-Zangbo River from optical and passive microwave images

The middle reaches of Yarlung-Zangbo River (YZR) and its two tributaries (Lhasa River and Nianchu River) is a main agricultural region in central Tibet Autonomous Region. Soil water deficit (SWD) estimation has significant relevance to local crop growth monitoring, crop yield assessment and disaster monitoring. It also has great theoretical importance for understanding the local energy and water balance status. In this paper, AVHRR and MODIS data on 14 April 2003, 16 October 2003 under nearly clear weather conditions are selected as the spring and autumn cases. Land surface parameters, such as land surface temperature, surface albedo, Normalized Difference Vegetation Index, and emissivity, have been derived from different algorithms for AVHRR and MODIS data. In combination with meteorological data, the soil water deficit index is determined by applying Surface Energy Balance System. The R square values between SWDI and AMSR-E soil moisture are ranging from 0.457 to 0.607, with spring SWD being much more severe than that in autumn. The limited river runoff (less than 5% of the annual total) is the dominant factor for spring SWD. This study also reveals that the derived spring SWD from AVHRR and MODIS data is quite different on the same day. This phenomenon is caused by different satellite overpass times which influence the melting frozen soil. This also confirms that the soil moisture may have diurnal variations. The spatial variations of SWD conditions in the middle reaches of YZR and its two tributaries have been clearly identified.

The photosynthetic response of tobacco plants overexpressing ice plant aquaporin McMIPB to a soil water deficit and high vapor pressure deficit

We investigated the photosynthetic capacity and plant growth of tobacco plants overexpressing ice plant (Mesembryanthemum crystallinum L.) aquaporin McMIPB under (1) a well-watered growth condition, (2) a well-watered and temporal higher vapor pressure deficit (VPD) condition, and (3) a soil water deficit growth condition to investigate the effect of McMIPB on photosynthetic responses under moderate soil and atmospheric humidity and water deficit conditions. Transgenic plants showed a significantly higher photosynthesis rate (by 48 %), higher mesophyll conductance (by 52 %), and enhanced growth under the well-watered growth condition than those of control plants. Decreases in the photosynthesis rate and stomatal conductance from ambient to higher VPD were slightly higher in transgenic plants than those in control plants. When plants were grown under the soil water deficit condition, decreases in the photosynthesis rate and stomatal conductance were less significant in transgenic plants than those in control plants. McMIPB is likely to work as a CO2 transporter, as well as control the regulation of stomata to water deficits.

Modelling eco-physiological response of table olive trees (Olea europaea L.) to soil water deficit conditions

The knowledge of crop response to water stress is crucial to predict transpiration reductions under limited soil water conditions and for a rational scheduling of irrigation.In order to assess whatever water stress model, it is necessary to estimate critical thresholds of soil water status, below which plant transpiration starts to decrease.The main objective of the work is to identify the shape and to determine the parameters of table olive orchards (Olea europaea, var. Nocellara del Belice) water stress function, assessed according to relative transpiration or leaf/stem water potential.In order to assess different water stress functions describing the eco-physiological field response to soil water status, an experimental campaign was carried out in a farm located in South-West coast of Sicily. Meteorological data and soil and crop water status were monitored during irrigation seasons 2008 and 2009.A value of soil matric potential of about −40m was identified as the threshold below which actual transpiration decreases with decreasing soil water content. For values of soil matric potential higher than the critical threshold, actual transpiration resulted almost constant. A similar behavior was observed when the xylematic leaf/stem water potentials were used to quantify the crop water stress. Investigation also showed that the non-linear models better reproduced the initial phase of the transpiration reduction process; for the examined crop, in fact, convex shape models, typical of xerophytes, better reproduce the reductions of actual transpiration under the soil water deficit conditions recognized in the field.

Productivity and seed health of husked oats (Avena sativa L.) grown under different soil moisture conditions

The present study investigated the effect of different soil moisture content levels (60 - 70% SWC (soil water capacity) - control; 30 - 35% SWC - water stress) on yields, gas exchange parameters, seed health, and protein fractions of husked oat grain. The study showed that water deficit resulted in a decrease in grain weight per plant and a reduction in the gas exchange rates, primarily the photosynthesis and transpiration rates. &lt;i&gt;Cladosporium cladosporioides&lt;/i&gt; was the dominant species on oat kernels in both experimental treatment options and in both years of the study. The presence of &lt;i&gt;Fusarium poae&lt;/i&gt; was also found. Higher contents of prolamin, albumin and globulin fractions were found in the oat grain harvested from plants grown under soil water deficit conditions.

Effects of soil water deficit on physiological causes of rice grain-filling

Aims Our objective was to elucidate the physiological causes of the influence of soil water deficit on grain-filling and rice (Oryza sativa) yield by evaluating enzyme activities involved in sucrose-starch synthesis during the grain-filling period. Methods Two rice cultivars (‘Shanyou 63’ and ‘Pc311/Zaoxiandang’) were grown in cement containers. Controlled soil water deficit was imposed at 9th day after flowering because the division of endosperm cells is sensitive to water deficit. The activities of key enzymes involved in the metabolism of sucrose to starch during grain-filling were evaluated. Important findings The 1000-grain weight and rice yield under soil water deficit were significantly decreased. The key enzyme activities, including sucrose synthase, adenine diphosphoglucose pyrophosphorylase, soluble starch synthase and starch branching enzyme, involved in the metabolism of sucrose to starch were significantly increased under soil water deficit during 10–20 days after anthesis. However, the activities of these enzymes under soil water-deficit were decreased rapidly 20–30 days after flowering compared to well-watered treatments. Active grain-filling periods of the water-deficit treatments were shortened as a result of the decrease of starch accumulation. Therefore, the rapid decline of the key enzyme activities involved in sucrose to starch metabolism and the shortage of assimilation supply at 20–30 days after anthesis are reasons for decreases in grain weight and yield under soil water deficit conditions.

Hydrotime Analysis of Ethiopian Mustard (Brassica carinata A. Braun) Seed Germination Under Different Temperatures

Ethiopian mustard (Brassica carinata A. Braun) is an oil-seed species that has recently become an object of great interest as a promising crop for energy purposes (biodiesel and biolubricant production). In semi-arid regions of South Italy, soil moisture at sowing time is often inadequate, delaying and reducing seed germination. In this study, the effect of reduced water potential on seed germination in three cultivars of B. carinata (ISCI 7, CT 180 and Sincron) was investigated in the laboratory. Germination behaviour at constant temperatures under low water availability was also analysed through the hydrotime model. Six water potentials (ψs) in PEG solution (0, −0.2, −0.4, −0.6, −0.8 and −1.0 MPa) and three temperatures (10, 20 and 30 °C) were used for the germination tests. A thermo-inhibiting effect was observed at the highest levels of water stress. The hydrotime analysis revealed that the increase of temperature to 30 °C reduced predicted hydrotime (θH) by hastening the rate of germination, but shifted median base water potential (ψb(50)) to higher values (less negative), whereas the lowering of temperature increased θH. These observations may have a great agronomic impact because although fewer seeds germinated at 30 °C at the highest levels of water stress, they may germinate faster in rapidly drying soil. However, genetic differences were observed among cultivars in terms of estimated θH and ψb(50). The hydrotime modelling approach offered in this study may help predict B. carinata seed germination under soil water deficit conditions occurring under early or late autumnal sowing.

EFFECT OF N, K, CI ON PHOTOSYNTHESIS AND WATER RELATIONS OF OPEN POLLINATED TALL (typica) COCONUT SEEDLINGS

CO2 assimilation and plant water status were measured in sand-culture-grown amputatedcoconut seedlings treated with three levels ofN, K, and CI and subjected to different soil waterdeficit conditions. Application of increased levels of N and K enhanced the assimilation of C02while individual and combined effects of N,K, and CI improved the water economy of coconutseedlings. Under the soil water deficit conditions, adequate supply o fN helped to maintain highleaf water potential by the accumulation of solutes like proline and sugars. Whereas, adequatesupply of K and Cl themselves acted as osmotica by increased accumulation of these nutrients inleaf tissues, although other solute concentrations such as those of sugars and proline were reduced.Our data show that maintenance of sufficient levels of N and K contribute to the growth of thecoconut seedling through improved gaseous exchange, CO2 assimilation and better partitioningof assimilated carbon into shoot and roots, potassium and C1 are further important for maintenanceof water status of coconut seedlings by improved stomatal regulation, water uptake and osmoticadjustment of tissues under water deficit conditions.

Content of carotenoids at different ripening stages in processing tomato in relation to soil water availability

The influence of 2 water regimes (a fully irrigated treatment receiving 100% of evapotranspiration for the whole growing season and an unirrigated control watered up to plant establishment only) on lycopene and β-carotene accumulation during fruit ripening in a field-grown processing tomato was studied. Since a strong effect of irrigation treatments on fruit water content was expected, carotenoid content on both a fresh and dry matter basis was studied. Regardless of ripening stage and adopted parameter unit (fresh or dry matter), higher amounts of lycopene were measured in the well watered treatment. Positive and no effects of water stress were reported on β-carotene content when expressed, respectively, on a fresh and dry weight basis. Both experimental factors influenced the β-carotene/lycopene ratio mostly in the first 2 ripening stages and there is evidence to suggest that, under soil water deficit conditions, the carotenoid biosynthetic pathway is more ‘β-carotene accumulation’ oriented, especially at the beginning of the fruit ripening process. Appropriateness of adopting both a fresh and dry basis calculation, in order to better evaluate the role of water stress on carotenoid content, is emphasised. The possibility of reducing the irrigation water supply without drastically decreasing the studied fruit quality characteristics is suggested.

Réponse au climat du sapin pectiné (Abies alba Mill.) et de l’épicéa commun (Picea abies Karst.) dans le Réseau National de suivi à long terme des Écosystèmes Forestiers (RENECOFOR)

This paper explores the growth/climate relationships in earlywood, latewood and total ring-width chronologies of five Norway spruce (Picea abies) and six silver fir (Abies alba) stands sampled in the French permanent plot network (RENECOFOR) (327 trees). The relationships between climate and ring widths were analyzed using extreme growth years, simple correlations and response functions analysis (bootstrapped coefficients). Monthly climatic regressors were derived by a physiological water balance model that used daily climatic data and stand parameters to estimate soil water deficits. Pointer years underline the high sensitivity to winter frosts (1956, 1986) and exceptional annual droughts (1962, 1976, 1991) for both species. For those years, growth variations were higher for Abies alba than for Picea abies. For each species, the climate information of tree ring series is not modified by local site characteristics (altitude, slope, aspect, soil water reserve). Moreover, strong specific differences appear among species. Earlywood and latewood spruce growth mainly depends on current summer soil water deficit conditions. For silver fir, winter and early summer temperatures, as well as the water supply of the previous year (August to October) play a major role for the production of earlywood, after which the current early summer water supply influences mainly latewood width.

Alterações anatômicas e fisiológicas em Setaria anceps Stapf ex Massey e Paspalum paniculatum L. sob condições de déficit hídrico

Ambas as espécies Setaria anceps (setária) e Paspalum paniculatum (paspalo) são tolerantes a condições de deficiência hídrica e inundação do solo. O objetivo desse trabalho foi avaliar alterações anatômicas, ecofisiológicas e metabólicas em folhas e raízes de setária e paspalo quando submetidas a condições de deficiência hídrica do solo. Aos onze dias de restrição hídrica, pouca variação foi observada na densidade e tamanho dos estômatos em setária, houve diminuição da espessura do limbo foliar, feixe vascular e córtex radicular e nas taxas de transpiração. O conteúdo de proteínas (P), açúcares solúveis totais (AST), açúcares redutores (AR) e amido decresceu em folhas e AST e amido, em raízes. Em paspalo, ocorreu variações no tamanho e densidade dos estômatos, redução no diâmetro do metaxilema e córtex radicular, aumento no tamanho de células buliformes e na concentração foliar de aminoácidos, AST, AR e amido.

Indole acetic acid (IAA) induced changes in growth, relative water contents and gas exchange attributes of barley (Hordeum vulgare L.) grown under water stress conditions

Effect of different concentrations of indole acetic acid (IAA) under varying soil water deficit conditions on two barley cultivars viz. B-99094 and Jau-87 was investigated in soil filled earthen pots. There were six treatments including control each with four replicates. Three concentrations of IAA (0, 15 and 30 mg l−1) were applied as foliar spray 30 days after germination. After hormone application, half of the pots were subjected to one cycle of water stress (withholding of water till incipient wilting), followed by regular watering. Plant height, photosynthetic rate, transpiration rate, stomatal conductance, water use efficiency relative water content, dry biomass, and grain yield/plant were significantly reduced by water stress. However, IAA treatments alleviated the adverse effect of water stress and successful in enhancing the plant growth and yield of barley cultivars. Barley cultivar Jau-87 performed better than B-99094. IAA application␣was effective in enhancing growth and photosynthetic efficiency of barley both under normal and water stress conditions.

Drought resistance of potato cultivars with contrasting canopy architecture

There has been little research in potatoes ( Solanum tuberosum L.) to define the ideal canopy structure for obtaining high and stable tuber yields. Two field experiments were conducted on a loamy sand soil (Haplic Luvisol) near Braunschweig in the North German Plain. The eight medium late potato cultivars grown in the year 2000 under non-limiting conditions differed significantly ( P < 0.05) for leaf and stem dry weight, leaf/stem mass ratio, leaf number, leaf area, and other canopy structure parameters. Genotypes characterised by a high leaf/stem ratio (leaf-types) produced a lower shoot biomass than those with a low leaf/stem ratio (stem-types). The cultivars Producent and Tomba, representing a leaf- and a stem-type, respectively, were chosen to study the relationship between shoot and tuber yield under rainfed and irrigated conditions during the years 2001–2003. Different seasonal rainfall together with varying levels of extra water from irrigation created a wide range of soil moisture levels. The detrimental effects of drought on light interception were much greater for the leaf-type than for the stem-type. Under rainfed conditions, the relative tuber dry weight of the stem-type as compared with the leaf-type amounted to 99% in the absence of drought (2002), 109% under moderate drought (2001), and 148% under severe drought (2003). Obviously, the ability of stem-types to form a greater aboveground biomass is especially advantageous under soil water deficit conditions. High availability of water under irrigation in the years 2002 and 2003 produced a canopy size in the stem-type in excess of that required for maximum light interception. Consequently, the tuber dry weight of the stem-type displayed an optimum relationship with the shoot dry weight whereas the tuber yield of the leaf-type increased linearly with the shoot yield.

Estimation of Winter Wheat Evapotranspiration under Water Stress with Two Semiempirical Approaches

Winter wheat (Triticum aestivum L.) is one of most important crops in the North China Plain. However, soil water deficit (SWD) often occurs due to lack of precipitation in its growing season. In this study, we introduce two semiempirical approaches, a recharge model and the crop coefficient (Kc)–reference evapotranspiration (ET0) approach, to estimate wheat actual evapotranspiration (ETa) under no SWD and slight and severe SWD conditions. The recharge model allocated ET0 to reference evaporation and reference transpiration as a function of leaf area index. In the model, ETa is limited by soil water content, and crop water extraction for ETa is distributed through the soil profile as exponential functions of soil and root depth. The Kc–ET0 approach regarded ETa under the SWD condition as a logarithmic function of soil water availability. Under no SWD condition, the recharge model simulated 10‐d ETa with a root mean square error (RMSE) of 5.58 mm and a bias of 0.95 mm compared with measurements from a large‐scale weighing lysimeter. The two approaches both estimated seasonal evapotranspiration (ET) well compared with the adjusted ET (from the soil water balance and the recharge model–simulated deep drainage). The recharge model, which simulated the seasonal ET with the RMSE of 27.8 mm and the bias of −8.0 mm, was better than the Kc–ET0 approach (RMSE = 31.7 mm and bias = −33.1 mm). The seasonal pattern of soil water stress coefficient (Ks) showed that there were faster water losses at grain‐filling stage than at other stages.

Changes in the photosynthetic characteristics of Plantago major plants caused by soil drought stress

Changes in net photosynthetic rate (PN), stomatal conductance (gs), intercellular CO2 concentrations (Ci), transpiration rate (E) and water use efficiency (WUE) were measured in Plantago major L. plants grown under sufficient soil water supply or under soil water stress conditions. The plants had high PN in a wide range of soil water potential and temperature regimes. Soil water had little effect on PN under ambient CO2 concentrations, which was explained by a high carboxylation rate, but increased the dark respiration rate. Carboxylation activity at low Ci depended on RuBP regeneration, whereas at high Ci it depended on the phosphate regeneration rate. The gs and E values were low in plants under stress as compared to the controls that resulted in an increase of WUE. The results obtained show that Plantago major plants have different ways of adaptation to soil water deficit conditions.

Varietal Differences in the Growth and Dry-Matter Production during Vegetative Growth in Rice under Upland Cultivation Conditions with Water Stress. A comparison of Japonica, Indica and Japonica-Indica hybrid cultivars.

Since rice is highly susceptible crops to water stress, it is important to improve drought resistance to increase and stabilize the rice yield under upland cultivation conditions. This study was conducted to elucidate the morphological and physiological characteristics related to drought resistance in diverse rice germplasm. Six cultivars including Japonica, Indica and Japonica-Indica hybrid cultivars were grown in a pot, suffering the same soil water deficit conditions at a vegetative growth stage. The reduction in leaf area growth and in dry matter production under water stress was smaller in IR36 and two Japonica -Indica hybrid cultivars, Suweon 287 and Suweon 290 than Japonica and Indica upland cultivars, Toyohatamochi and IRAT109. The reduction in leaf photosynthetic rate was also smaller in Suweon 287 and Suweon 290. IR36, Suweon 287 and Suweon 290 showed a semi dwarf morphological characteristic with a large tiller number. The partitioning of the net production directed much to roots and less to leaves under the water stress conditions in all cultivars tested. IR36 and Suweon 287 partitioned more net production to leaf sheaths. Suweon 287 showed good recovery after rewatering and partitioned much assimilates to roots after released from water stress.

Improved biomass productivity and water use efficiency under water deficit conditions in transgenic wheat constitutively expressing the barley HVA1 gene

The ABA-responsive barley gene HVA1, a member of group 3 late embryogenesis abundant (LEA) protein genes, was introduced into spring wheat ( Triticum aestivum L.) cv. Hi-Line using the biolistic bombardment method. High levels of expression of the HVA1 gene, regulated by the maize ubi1 promoter, were observed in leaves and roots of independent transgenic wheat plants and were inherited by offspring generations. T 3 progenies of four selected transgenic wheat lines were tested under greenhouse conditions for tolerance of soil water deficit. Potted plants were grown under moderate water deficit and well-watered conditions, respectively. Two homozygous and one heterozygous transgenic lines expressing the HVA1 gene had significantly ( P<0.01) higher water use efficiency values, 0.66–0.68 g kg −1, as compared to 0.57 and 0.53 g kg −1, respectively, for the non-expressing transgenic and non-transgenic controls under moderate water deficit conditions. The two homozygous transgenic plant lines also had significantly greater total dry mass, root fresh and dry weights, and shoot dry weight compared to the two controls under soil water deficit conditions. Results of this study indicate that growth characteristics were improved in transgenic wheat plants constitutively expressing the barley HVA1 gene in response to soil water deficit.

Rate of leaf production in response to soil water deficits in field pea

Plant leaf area is critical for predicting the amount of radiation intercepted by a crop, and thereby, for estimating dry matter production. Under soil water deficit conditions, plant leaf expansion is reduced as a result of both a reduction in the rate of leaf production (RLP) and in the rate of individual leaf expansion. Quantifying the effect of soil water deficits on plant leaf expansion depends in part on predicting its effects on the timing of leaf production. The effect of soil water deficits on RLP was examined for three pea cultivars in greenhouse and field experiments. The level of soil water deficit was characterized as the fraction of transpirable soil water (FTSW). A quantitative function between RLP and FTSW was established in greenhouse experiments and was tested in independent pot and field experiments. A good consistency in this relationship across a diversity of experimental conditions and cultivars was shown. The logistic function obtained represents an effective way to simulate the effects of soil water deficits on RLP, especially as FTSW could be estimated from a soil water balance. RLP was reduced only for FTSW<0.2, and consequently, RLP was less sensitive to soil water deficits than transpiration and leaf expansion. Soil water deficit induced a slight rise in canopy temperature due to stomatal closure. However, this rise in temperature for FTSW<0.4 cannot account for maintaining RLP compared to the drop of transpiration and of leaf expansion rates observed for FTSW between 0.4 and 0.2. RLP can be considered independent of soil water content if FTSW>0.2. In the field, such level of soil water deficit inducing a decrease of RLP occurs generally only after the end of leaf production during the last reproductive stages of pea crop. Thus, except in situations of extreme soil water deficit and on shallow soils, leaf production depends solely on air temperature.