Differential effects of temperature on fruit development and bean quality of contrasting genotypes of cacao (Theobroma cacao)

The effects of temperature and light integral on fruit growth and development of five cacao genotypes (Amelonado, AMAZ 15/15, SCA 6, SPEC 54/1 and UF 676) were studied in semi‐controlled environment glasshouses in which the thermal regimes of cacao‐growing regions of Brazil, Ghana and Malaysia were simulated. Fruit losses because of physiological wilt (cherelle wilt) were greater at higher temperatures and also differed significantly between genotypes, reflecting genetic differences in competition for assimilates between vegetative and reproductive components. Short‐term measurements of fruit growth indicated faster growth rates at higher temperatures. In addition, a significant negative linear relationship between temperature and development time was observed. There was an effect of genotype on this relationship, such that time to fruit maturation at a given temperature was greatest for the clone UF 676 and least for AMAZ 15/15. Analysis of base temperatures, derived from these relationships indicated genetic variability in sensitivity of cacao fruit growth to temperature (base temperatures ranged from 7.5°C for Amelonado and AMAZ 15/15 to 12.9 for SPEC 54/1). Final fruit size was a positive function of bean number for all genotypes and a positive function of light integral for Amelonado in the Malaysia simulated environment (where the temperature was almost constant). In simulated environments where temperature was the main variable (Brazil and Ghana) increases in temperature resulted in a significant decrease in final pod size for one genotype (Amelonado) in Brazil and for two genotypes (SPEC 54/1 and UF 676) in Ghana. It was hypothesised that pod growth duration (mediated by temperature), assimilation and bean number are all determinants of final pod size but that under specific conditions one of these factors may override the others. There was variability between genotypes in the response of bean size and bean lipid content to temperature. Negative relationships between temperature and bean size were found for Amelonado and UF 676. Lipid concentration was a curvilinear function of temperature for Amelonado and UF 676, with optimal temperatures of 23°C and 24°C, respectively. The variability observed here of different cacao genotypes to temperature highlights the need and opportunities for appropriate matching of planting material with local environments.     

Genotypic variation in photosynthesis in cacao is correlated with stomatal conductance and leaf nitrogen

Variation in photosynthetic parameters was observed between eight contrasting cacao (Theobroma cacao) genotypes. Net photosynthetic rate (P_N) ranged from 3.4 to 5.7 μmol(CO₂) m⁻² s⁻¹ for the genotypes IMC 47 and SCA 6, respectively. Furthermore, genotypic differences were detected in quantum efficiency ranging from 0.020 to 0.043 μmol(CO₂) μmol⁻¹(photon) for UF 676 and AMAZ 15/15, respectively. Differences in PN were correlated with both stomatal conductance (g_s) and leaf nitrogen per unit area. Some variation in water use efficiency was observed between genotypes, both intrinsic (P_N/g_s) and instantaneous (P_N/transpiration rate). Both measures of water use efficiency were a negative function of specific leaf area. Evidence was found for a trade-off mechanism between cacao genotypes in photosynthesis and leaf structure. High photosynthetic rate, expressed on a mass basis was associated with smaller leaves. Furthermore, thinner leaves were compensated for by a higher nitrogen content per unit mass.     

Chlorophyll fluorescence parameters characterizing the development of two cacao genotypes infected by witches' broom

Greenhouse-grown susceptible 20-d-old seedlings of Theobroma cacao genotypes Catongo and tolerant genotype SCA6xCatongo were inoculated with a mixture of isolates of Crinipellis perniciosa, the causal agent of witches' broom. The characteristics of chlorophyll a fluorescence emission were monitored during leaf ontogeny using a portable system PAM-2000. In both inoculated and non-inoculated genotypes, significant differences were found for the effective quantum yield values of photosystem (PS) 2 (F/Fm') at the B (7 to 14-d-old), D (21 to 30-d-old), and E (>30-d-old) stages of leaf development, and in quantum yield of the non-cyclic photosynthetic electron transport between PS2 and PS1 [qp(Fv/Fm)] and quencher efficiency [(Fm-Ft)/F0] at the B, C (15 to 20-d-old) and D stages. Intergenotypic differences were found only for the [qp(Fv/Fm)] and [(Fm-Ft)/F0] values at the E stage, and for fluorescence quenching (Fm-Ft) at the B and E stages. Highly significant inter- and intragenotype relationships were found between the rate of photosynthetic electron transport to PS2 (Amax) and maximum fluorescence during actinic irradiation (Fm'). Also, each of the highly significant relationships between (Fm-Ft) and Amax, [(Fm-Ft)/F0] and F/Fm', and between [(Fm-Ft)/F0] and Amax were represented by a general model, independent of treatments. Therefore, alterations in energy distribution in the radiant energy collector complex interior of PS2 and reduction in absorption of photosynthetically active radiation were observed in the infected plants, mainly in the hybrid at the C stage. Also, variations were found in the noncyclic photosynthetic electron transport at the B and C stages in the infected Catongo.     

Photosynthetic pigments,morphology and leaf gas exchange during ex vitro acclimatization of micropropagated CAM Doritaenopsis plantlets under relative humidity and air temperature

The effect of relative humidity (RH) and temperature on CO₂ assimilation (An), stomatal conductance (Sc), transpiration rate (Tr), chlorophyll content, fresh and dry weight, leaf length, leaf area, leaf width, formation of new root and survival rate have been assayed in Doritaenopsis in growth chamber after 1 month of acclimatization. Reduced growth was observed at below and above 25 °C whereas it was increased with increasing humidity. Relative water content (RWC) was decreased at 50% and 70% humidity after second day of transfer and recovered completely with the progression of acclimatization. RWC also reduced at high temperature but recovered slowly and a gradual decrease of RWC was observed at 15 °C. A visual symptom of severe leaf tip burn was observed at 50–70% humidity and at 35 °C during acclimatization. At 15 °C and 50% humidity sudden decrease of photosynthetic efficiency (F_v/F_m) was observed, which could not recover in temperature treated plantlets during acclimatization period. Chlorophyll content increased with increasing humidity and at 15 and 35 °C chlorophyll content was decreased compared to 25 °C. Chlorophyll a/b ratio was unchanged while total chlorophyll/carotenoids ratio was increased from low to high temperature. Exposure of plantlets to high temperature led to a noticeable decrease in An, Sc and Tr, and at 15 °C they were more decreased whereas significant differences were not observed in the parameters tested under humidity after 25 days of acclimatization. During daytime at 15 °C, increase in An, Sc and Tr indicates the plantlets adaptability in the new environment. The peroxidase activity remained unaffected in all humidity stress whereas low temperature increased the peroxidase activity compared to high temperature. These finding suggests that photosynthetic properties was greatly affected by air temperature conditions with a reduction of An, Sc and Tr at 15 and 35 °C compared to humidity stress that played a greater role in limiting photosynthesis.     

Apparent absence of viruses in most symptomless field-grown sweet potato in Uganda

Heat tolerance of groundnut (Arachis hypogaea L.) genotypes was evaluated by solute leakage and chlorophyll fluorescence techniques in heat-hardened and non-hardened plants. To determine the appropriate hardening treatment, 1-month-old plants of two groundnut genotypes, ICGV 86707 and Chico were conditioned at five combinations of hardening (37°C) and non-hardening (30°C) air temperatures over a 5-day period. Heat injury, was assessed through measurements of electrolyte leakage after stressing leaf discs to 55°C for 15 min. The relative injury was significantly influenced by the conditioning temperatures and by the temperature during 24 h prior to measurement if those involved non-hardening conditions. Relative injury and chlorophyll fluorescence were measured after stressing leaves of six genotypes at a range of temperatures between 49°C and 55°C. Significant genotype × hardening treatment interactions were observed in relative injury and chlorophyll fluorescence. Chico was susceptible to heat stress, the relative injury test identified ICGV 86707 as tolerant, and the chlorophyll fluorescence test identified ICGV 86707 as tolerant under hardened conditions and ICGV 87358 as tolerant when non-hardened. When expressed as percentage of control values, the relative injury and chlorophyll fluorescence measurements over the 49–53°C stress temperature range were strongly correlated. Chlorophyll concentrations were increased by hardening in all genotypes except Chico. In Chico, chl_b concentration was decreased and the chl_a/b ratio increased by hardening, and chlorophyll concentrations were correlated with chlorophyll fluorescence parameters. Chlorophyll concentration may therefore provide an alternative means of screening for heat tolerance.     

Gene discovery and microarray analysis of cacao (Theobroma cacao L.) varieties

The cacao bean harvest from the relatively under developed tropical tree cacao (Theobroma cacao L.) is subject to high losses in potential production due to pests and diseases. To discover and understand the stability of putative natural resistance mechanisms in this commodity crop, essential for chocolate production, we undertook a gene-discovery program and demonstrated its use in gene-expression arrays. Sequencing and assembling bean and leaf cDNA library inserts produced a unique contig set of 1,380 members. High-quality annotation of this gene set using Blast and MetaFam produced annotation for 75% of the contigs and allowed us to identify the types of gene expressed in cacao beans and leaves. Microarrays were constructed using amplified inserts of the uni-gene set and challenged with bean and leaf RNA from five cacao varieties. The microarray performed well across the five randomly chosen cacao genotypes and did not show a bias towards either leaf or bean tissues. This demonstrates that the gene sequences are useful for microarray analysis across cacao genotypes and tissue types. The array results, when compared with real-time PCR results for selected genes, showed a correlation with differential gene-expression patterns.We intend that the resultant DNA sequences and molecular microarray platform will help the cacao community to understand the basis, likely stability and pathotype resistance range of candidate cacao plants.     

Comparative response of maize and rice genotypes to heat stress: status of oxidative stress and antioxidants

In the present study, two genotypes each of maize and rice were compared for their response to varying degrees of temperature stress (35/30, 40/35, 45/40°C) with controls growing at 30/25°C. At elevated temperatures of 40/35 and 45/40°C, the rice genotypes were inhibited to a significantly higher extent, especially for their shoot growth compared to maize genotypes. The stress injury measured as damage to membranes, loss of chlorophyll and reduction in leaf water status was significantly higher in rice plants, especially at 45/40°C. The components of oxidative stress particularly the level of malondialdehyde was significantly greater in rice plants while the differences for hydrogen peroxide concentrations were small at 40/35 and 45/40°C. The expression of enzymatic antioxidants like catalase, ascorbate peroxidase and glutathione reductase was found to be higher in maize plants compared to rice plants while no variations existed for superoxide dismutase at 45/40°C. In addition, the non-enzymatic antioxidants like ascorbic acid, glutathione and proline were maintained at significantly greater levels at 45/40°C in maize than in rice genotypes. These findings suggested that maize genotypes were able to retain their growth under high-temperature conditions partly due to their superior ability to cope up with oxidative damage by heat stress compared to rice genotypes. Since, maize and rice belong to C₄ and C₃ plant groups, respectively, these observations may also reflect the relative sensitivity of these plant groups to heat stress.     

Molecular,Physiological and Biochemical Responses of Theobroma cacao L. Genotypes to Soil Water Deficit

Six months-old seminal plants of 36 cacao genotypes grown under greenhouse conditions were subjected to two soil water regimes (control and drought) to assess, the effects of water deficit on growth, chemical composition and oxidative stress. In the control, soil moisture was maintained near field capacity with leaf water potentials (ΨWL) ranging from −0.1 to −0.5 MPa. In the drought treatment, the soil moisture was reduced gradually by withholding additional water until ΨWL reached values of between −2.0 to −2.5 MPa. The tolerant genotypes PS-1319, MO-20 and MA-15 recorded significant increases in guaiacol peroxidase activity reflecting a more efficient antioxidant metabolism. In relation to drought tolerance, the most important variables in the distinguishing contrasting groups were: total leaf area per plant; leaf, stem and total dry biomass; relative growth rate; plant shoot biomass and leaf content of N, Ca, and Mg. From the results of these analyses, six genotypes were selected with contrasting characteristics for tolerance to soil water deficit [CC-40, C. SUL-4 and SIC-2 (non-tolerant) and MA-15, MO-20, and PA-13 (tolerant)] for further assessment of the expression of genes NCED5, PP2C, psbA and psbO to water deficit. Increased expression of NCED5, PP2C, psbA and psbO genes were found for non-tolerant genotypes, while in the majority of tolerant genotypes there was repression of these genes, with the exception of PA-13 that showed an increased expression of psbA. Mutivariate analysis showed that growth variables, leaf and total dry biomass, relative growth rate as well as Mg content of the leaves were the most important factor in the classification of the genotypes as tolerant, moderately tolerant and sensitive to water deficit. Therefore these variables are reliable plant traits in the selection of plants tolerant to drought.     

Effects of air temperature, photoperiod and leaf age on foliar senescence of western larch (Larix occidentalis Nutt.) in environmentally controlled chambers

We induced foliar senescence in western larch (Larix occidentalis Nutt.) needles (100 to 400 d old) in chambers set to combinations of photoperiod (8 or 16 h) and air temperature (8 or 15°C). We used regression analysis to estimate the effects of air temperature, photoperiod, provenance and leaf age on the date of decline in chlorophyll, photosynthesis, and chlorophyll a/b ratio. Increased air temperature delayed the decline in chlorophyll and photosynthesis without affecting chlorophyll a/b: seedlings with 200-d-old needles placed under 8h days and 8°C lost chlorophyll, photosynthesis and chlorophyll a/b in 22, 37 and 63d, respectively, while those senescing under 15°C lost them in 54, 61 and 63d. Extended photoperiod delayed the decline in chlorophyll but not photosynthesis or chlorophyll a/b ratio: the 200-d-old needles placed in 8°C and 16 h photoperiod lost chlorophyll, photosynthesis, and chlorophyll a/b in 34, 37 and 63 d, compared with 22, 37 and 63 d for seedlings in 8 h days. Our analysis shows that it is possible to quantify the extent to which the timing and process of senescence vary with environmental conditions.     

A Comparative Study ofMiscanthus floridulus(Labill) Warb andM. transmorrisonensisHayata: Photosynthetic Gas Exchange, Leaf Characteristics and Growth in Controlled Environments

The relationship between temperature and the distribution ofMiscanthusfloridulus(Labill) Warb andM. transmorrisonensisHayata alongaltitudinal gradients in central Taiwan was examined. Responsesof biomass accumulation, leaf characteristics and photosyntheticgas exchange to growth temperature (from 10 to 30 °C) ofM.floridulusfrom an altitude of 390 m and ofM. transmorrisonensisfrom2700 m were determined. There were differences between the twospecies in above-ground biomass, CO₂uptake characteristics andleaf chlorophyll contents in response to growth temperature.The optimal temperatures for biomass accumulation were 30/25(day/night temperature) and 25/20 °C forM. floridulusandM.transmorrisonensis,respectively. Light saturated photosyntheticrates (A_max) were largest in plants grown at the optimal temperature.Growth at 15/10 and 10/10 °C compared to the optima reducedaccumulated biomass, leaf chlorophyll content and photosyntheticrate in both species with a greater reduction inM. floridulusthaninM. transmorrisonensis.We concluded that growth ofM. floridulusathigh altitude is limited by an inability to grow at temperatureslower than 15 °C, whileM. transmorrisonensisis able to growin chilling temperatures at higher altitudes.Copyright 1998Annals of Botany Company Miscanthus floridulus;M. transmorrisonensis; C₄plants; chlorophyll content; leaf growth; photosynthetic gas exchange; biomass accumulation; temperature response.     

Effects of temperature on growth and water relations of cacao (Theobroma cacao var.Comum) seedlings

Growth of 55-day-oldTheobroma cacao var.Comum seedlings varied with temperature regimes, various plant parts, growth parameters, and time of harvesting. Over a 60-day period the optimal day-temperature regimes were near 33.3°C for dry weight increase and relative growth rates of seedlings and leaves; 30.5°C for increase in leaf area, height growth, and leaf abscission; 22.2°C for dry weight increase of stems or roots, stem diameter growth, and root-shoot ratio. The rates of increase in dry weights of stems or roots as well as root-shoot ratios declined progressively at temperatures above 22.2°C Partitioning of dry matter was affected by temperature regime, with proportionally more photosynthate retained by shoots and less translocated to roots at high temperatures. The progressive decrease in the root-shoot ratio at temperatures above 22.2°C may decrease drought tolerance of seedlings because roots will be less capable of absorbing endugh water to replace transpirational losses. This was shown by more negative shoot water potentials at high temperatures. Research supported by the College of Agricultural and Life Science. University of Wisconsin, Madison, Wi, USA and the Centro de Pesquisas do Cacau (CEPLAC/CEPEC), 45600 Itabuna, Ba, Brazil.     

Somatic embryogenesis and plant regeneration from floral explants of cacao (Theobroma cacao L.) using thidiazuron

Summary A procedure for the regeneration of cacao (Theobroma cacao) plants from staminode explants via somatic embryogenesis was developed. Rapidly growing calli were induced by culturing staminode explants on a DKW salts-based primary callus growth (PCG) medium supplemented with 20 g glucose per L, 9 μM 2,4-D, and thidiazuron (TDZ) at various concentrations. Calli were subcultured onto a WPM salts-based secondary callus growth medium supplemented with 20 g glucose per L, 9 μM 2,4-D, and 1.4 nM kinetin. Somatic embryos were formed from embryogenic calli following transfer to a hormone-free DKW salts-based embryo development medium containing sucrose. The concentration of TDZ used in PCG medium significantly affected the rate of callus growth, the frequency of embryogenesis, and the number of somatic embryos produced from each responsive explant. A TDZ concentration of 22.7 nM was found to be the optimal concentration for effective induction of somatic embryos from various cacao genotypes. Using this procedure, we recovered somatic embryos from all 19 tested cacao genotypes, representing three major genetic group types. However, among these genotypes, a wide range of variation was observed in both the frequency of embryogenesis, which ranged from 1 to 100%, and the average number of somatic embryos produced from each responsive explant, which ranged from 2 to 46. Two types of somatic embryos were identified on the basis of their visual appearance and growth behavior. A large number of cacao plants have been regenerated from somatic embryos and established in soil in a greenhouse. Plants showed morphological and growth characteristics similar to those of seed-derived plants. The described procedure may allow for the practical use of somatic embryogenesis for clonal propagation of elite cacao clones and other applications that require the production of a large number of plants from limited source materials.     

Study of the defensive mechanism against drought in French bean (<Emphasis Type="Italic">Phaseolus vulgaris</Emphasis> L.) varieties

The response to drought of six green- and yellow-podded varieties of green bean was investigated in a phytotron. These varieties were also grown without irrigation in field experiments and analysis of leaf samples was carried out three times; before and under flowering and pod ripening, respectively. At first, the carotene and raffinose contents of leaves increase during mild drought (30/15°C). High temperature (35/25°C) combined with water deficiency resulted in a considerable decrease in leaf weight, chlorophyll b, maltose content, as well as the level of water-soluble antioxidants of the leaves, while the content of lipid-soluble antioxidants and raffinose content increased when compared with the control. The yellow-podded varieties of green beans responded to drought much more sensitively than the green-podded ones. The results suggested that a selection method based on the activity of antioxidants in the leaves can be used for testing the adaptability of numerous bean genotypes to drought.     

Acclimation by suboptimal growth temperature diminishes photooxidative damage in maize leaves

Leaves of Zea mays L. seedlings which developed at optimal (25°C) or suboptimal (15°C) temperature were exposed to high irradiance (1000 μmol m^?2 s^?1) and a severe chilling temperature (5°C) for up to 24 h to investigate their ability to withstand photooxidative stress. During this stress, the degradation of the endogenous antioxidants ascorbate, glutathione and α-tocopherol was delayed and less pronounced in 15°C leaves. Similarly, the decline in chlorophyll a, chlorophyll b, β-carotene and lutein was slower throughout the stress period. Faster development and a higher level of non-photochemical quenching (NPQ) of chlorophyll fluorescence, related to a stronger de-poxidation of the larger xanthophyll cycle pool in 15°C leaves, could act as a defence mechanism to reduce the formation of reactive oxygen species during severe chilling. Furthermore, plants grown at suboptimal temperature exhibited a higher amount of the antioxidants glutathione and α-tocopherol. The higher α-tocopherol content in leaves (double based on leaf area; 4-fold higher based on chlorophyll content) which developed at suboptimal temperature may play an especially important role in the stabilization of the thylakoid membrane and thus prevent lipid peroxidation.     

Heat susceptibility of grain filling in wheat (<Emphasis Type="Italic">Triticum aestivum</Emphasis> L.) linked with rapid chlorophyll loss during a 3-day heat treatment

Brief heat events (1–3 days, >30 °C) commonly reduce wheat (Triticum aestivum L.) grain size and consequently yield. To identify mechanisms of tolerance to such short heat events, 36 wheat genotypes were treated under day/night temperatures of 37 °C/27 °C for 3-days in a growth chamber, at 10 days after anthesis, and a range of developmental, chlorophyll and yield-related traits monitored. The degree of flag leaf chlorophyll loss during the treatment was the variable that showed the highest correlation to grain weight loss (r = 0.63; p < 0.001), identifying chlorophyll stability during this brief period as a potential determinant or indicator of grain weight stability under heat. Variables summarizing the combined during- and post-heat chlorophyll losses showed similar or lower correlations with heat tolerance of grain filling, despite the fact that genotypes varied in their ability to resume normal chlorophyll loss rates after the heat treatment. Additionally, heat tolerance of grain size showed no correlation with grain filling duration or traits relating to utilization of stem carbon reserves under heat stress. Measurement of chlorophyll loss over a forecasted heat wave was thereby identified as a potential basis for developing tools to help breeders select heat tolerant genotypes.     

Heat-stress induced inhibition in growth and chlorosis in mungbean (<Emphasis Type="Italic">Phaseolus aureus</Emphasis> Roxb.) is partly mitigated by ascorbic acid application and is related to reduction in oxidative stress

The rising temperatures (>35°C) are proving detrimental to summer-sown mungbean genotypes that experience inhibition of vegetative and reproductive growth. In the present study, the mungbean plants growing hydroponically at varying temperatures of 30/20°C (control), 35/25, 40/30, and 45/35°C (as day/night 12 h/12 h) with (50 μM) or without ascorbic acid (ASC) were investigated for effects on growth, membrane damage, chlorophyll loss, leaf water status, components of oxidative stress, and antioxidants. The ASC-treated plants showed significant improvement in germination and seedling growth especially at 40/30 and 45/35°C. The damage to membranes, loss of water, decrease in cellular respiration, and chlorophyll were significantly prevented by ASC treatment to plants growing at these temperatures. The oxidative stress measured as malondialdehyde and hydrogen peroxide content was observed to be significantly lower at high temperatures with ASC application. The activities of superoxide dismutase, catalase, ascorbate peroxidase, and glutathione reductase increased at 40/30°C but decreased at 45/35°C in the absence of ASC while with its application, the activities of these enzymes were appreciably resorted. Among all the antioxidants, the endogenous ASC content decreased to the greatest extent at 45/35°C grown plants indicating its vital role in affecting the response of mungbean to heat stress. Exogenously applied ASC raised its endogenous content along with that of glutathione and proline at 45/35°C. The findings indicated that heat stress-induced inhibition in growth and chlorosis was associated with decrease in leaf water status and elevation of oxidative stress, which could partly be prevented by exogenous application of ASC. Its role in imparting protection against heat stress is discussed.     

Variation in growth attributes of contrasting populations of Trifolium repens

A range of growth attributes was measured in seedlings of 10 Trifolium repens populations, differing in leaf size and origin, grown in three temperature and two glasshouse environments. Growth rates of large leaf types of Mediterranean origin were higher than those of smaller leaf types at 10°C. However, the greater temperature response of the smaller leaf types resulted in higher growth rates for S.100 and S.184 than for a large leaf type from Israel at 20°C. The increase of growth rate with temperature was associated with changes in leaf area ratio and net assimilation rate between 10° and 15°C but only with changes in net assimilation rate between 15° and 20°C. Within each temperature environment, population differences in growth rate were related to differences in net assimilation rate rather than leaf expansion. At low temperature a greater proportion of dry matter was distributed to leaf tissue in large leaf types particularly those of Mediterranean origin but they showed a proportionately smaller increase in allocation to leaves with increasing temperature compared with small leaf types. In the glasshouse environments growth rates in spring were more than double those in the autumn. This difference was associated with net assimilation rates which were about five times greater in the spring environment. However, leaf area ratios in the spring were only half those in the autumn. These differences in leaf area ratio between the glasshouse environments were closely related to differences in specific leaf area but not to differences in distribution of dry matter to leaf tissue which was greater in the spring environment.     

Genotypic variation in cold tolerance influences the yield of Miscanthus

When grown in Europe, Miscanthus genotypes often produce yields lower than their potential due to late emergence of shoots in the spring or to damage from late frosts when shoots emerge too early. Here, we investigate genotypic variation in the base temperature (T_b) for shoot emergence and in the lethal temperature for shoots (LT₅₀) in four Miscanthus genotypes. In all genotypes, lowering temperature increased the time to shoot emergence, with T_b ranging from 8.6°C in Sac‐5 to 6°C in Sin‐H9. Frost treatments below ?8°C resulted in a marked reduction in growth in all four genotypes. Sin‐H9 was the most frost tolerant with an LT₅₀ of ?9.3°C. There was little variation found in leaf osmotic potential, but leaf moisture content was significantly lower in Sin‐H9 than in the other genotypes. The lower thermal requirement for emergence and lower LT₅₀ seen in Sin‐H9 was incorporated into a model of Miscanthus production. The model showed an extended growing season that was predicted to increase yields by up to 25%.     

Influence of low temperatures on the growth and photosynthetic activity of industrial chicory, <Emphasis Type="Italic">Cichorium intybus</Emphasis> L. partim

The cold stress effect on early vigour and photosynthesis efficiency was evaluated for five industrial chicory varieties with contrasting early vigour. The relationships between the growth and physiological parameters were assessed. The varieties were examined at three growth temperatures: 16 (reference), 8 (intermediate) and 4 °C (stress). The effect was measured using physiological processes (growth, photosynthesis, chlorophyll a fluorescence), and pigment content. The analysis of the measured growth parameters (dry leaf and root mass, and leaf area) indicated that temperature had a significant effect on the varieties, but the overall reaction of the varieties was similar with lowering temperatures. The photosynthesis and chlorophyll a fluorescence measurements revealed significant changes for the photosynthesis (maximum net photosynthesis, quantum efficiency, light compensation point and dark respiration) and chlorophyll a fluorescence parameters (photochemical and non-photochemical quenching) with lowering temperatures for Hera and Eva, two extremes in youth growth. No significant differences could be found between the extremes for the different temperatures. The pigment content analysis revealed significant differences at 4 °C in contrast to 16 and 8 °C, especially for the xanthophyll/carotenoid pool, suggesting a protective role. Subsequently, the relationship between the physiological processes was evaluated using principal component analysis. At 4 °C, 2 principal components were detected with high discriminating power for the varieties and similar classification of the varieties as determined in the growth analysis. This provides a preview on the possible relationships between photosynthesis and growth for industrial chicory at low temperatures.     

The size and germinability of Scots pine pollen in different temperatures in vitro

The effect of genotype, the origin of genotype, and germination temperature on Scots pine pollen grain size, hydration rate, germinability, and tube growth was studied in vitro. The mean sizes of dry and germinated pollen grains varied among pollen genotypes in different ways, thus the hydration rate varied among genotypes. Pollen from Scots pine that originates in northern Finland hydrated more than pollen from a population in southern Finland. Germination temperature had no effect on the hydration rate. Germinability and tube growth rate of northern genotypes were higher at 20 °C than at 15 °C. Differences among southern genotypes were not significant. At 15 °C, the germinability and pollen tube growth rate of northern genotypes were lower than southern genotypes. At 20 °C, the differences were not significant. It appears that germination and growth of pollen from northern populations are enhanced at higher temperatures whereas pollen from southern populations is unaffected.