The expression of photosynthetic traits during and following severe chilling stress of European and tropical maize genotypes

Four genotypes of maize ( Zea mays L.), one from North-West Europe, two from tropical highlands and one from tropical lowlands were grown at 24°C until full expansion of the second leaf. Seedlings were then subjected to 2 days at 5°C, with or without a previous conditioning phase at 10°C for 4 days. After stress plants were allowed to recover at 24°C for 5 days. Previous conditioning protected the tropical lowland variety to some extent against losses in chlorophyll content and ribulose I,5-bisphosphate carboxylase (EC 4.1.1.39) activity by severe chilling stress. Without conditioning heavy losses occurred during the recovery phase. In all other genotypes the values of these traits decreased more with than without conditioning. Phosphoenolpyruvate carboxylase (EC 4.1.1.31) activity was little affected during stress, but declined considerably during the recovery phase, with the exception of the North-West European genotype. Previous conditioning prevented or retarded losses of NADP⁺ malate dehydrogenase (EC 1.1.1.82) activity during the stress phase. However, at the end of the recovery phase losses were as high with as without conditioning, and activity was very low in one tropical highland genotype and in the tropical lowland genotype. In summary, the chilling stability of photosynthetic traits was better in the North-West European genotype than in the tropical ones.     

Pre-treating paclobutrazol enhanced chilling tolerance of sweetpotato

The objective of this work was to study changes in low molecular weight antioxidants and antioxidative enzymes in chilling-stressed sweetpotato, as affected by paclobutrazol (PBZ) pre-treatment 24 h prior to exposure to chilling conditions. Sweetpotato ‘TN71’ and ‘TN65’ were treated with 300 mg PBZ/5 ml/plant, after which plants were subjected to 7°C/7°C (day/night) for periods of 1, 3 and 5 days, followed by a 3-day recovery period at 24°C/20°C (day/night). A factorial experiment in completely randomized design with four replications was used in this study. Young fully expanded leaves at each temperature and period of time were clipped for antioxidative system measurement. We concluded that different varieties displayed variations in their oxidative system, and the differential expressions of each genotype were associated with chilling stress response. Plants with various antioxidative systems responded differently to chilling stress according to the duration of the chilling period and subsequent re-warming period. ASA, GSH and GSSG contents were enhanced in TN71 prior to chilling stress. Increased APX, GR, ASA and MDA activities accounted for chilling tolerance in TN65. Furthermore, our results indicate that the elevated levels of the antioxidative system observed after PBZ pre-treatments afforded the sweetpotato leaf improved chilling-stress tolerance. The levels of ASA and GSSG of both TN71 and TN65 under chilling were significantly raised by pre-treating with PBZ. PBZ pre-treatment exhibited the important function of enhancing the restoration of leaf oxidative damage under chilling stress and increasing the chilling tolerance of plants to mitigate chilling stress effects.     

Ultrastructural alterations in mesophyll and bundle sheath chloroplasts of two maize cultivars in response to chilling at high irradiance

Maize (Zea mays L.) seedlings of two cultivars (cv. Bastion adapted to W. Europe, and cv. Batan 8686 adapted to the highlands of Mexico), raised in a glasshouse (19–25 °C), were transferred to 4.5 or 9 °C at photon flux density (PPFD) of 950 μmol m⁻² s⁻¹ with 10-h photoperiod for 58 h and then allowed to recover at 22 °C for 16 h (14 h dark and 2 h at PPFD of 180 μmol m⁻² s⁻¹). The ultrastructural responses after 4 h or 26 h at 4.5 °C were the disappearance of starch grains in the bundle sheath chloroplasts and the contraction of intrathylakoid spaces in stromal thylakoids of the mesophyll chloroplasts. At this time, bundle sheath chloroplasts of cv. Batan 8686 formed peripheral reticulum. Prolonged stress at 4.5 °C (50 h) caused plastid swelling and the dilation of intrathylakoid spaces, mainly in mesophyll chloroplasts. Bundle sheath chloroplasts of cv. Batan 8686 seedlings appeared well preserved in shape and structure. Batan 8686 had also higher net photosynthetic rates during chilling and recovery than Bastion. Extended leaf photobleaching developed during the recovery period after chilling at 4.5 °C. This was associated with collapsed chloroplast envelopes, disintegrated chloroplasts and very poor staining.     

Genotype-specific differences in chilling tolerance of maize in relation to chilling-induced changes in water status and abscisic acid accumulation

Four inbred maize lines differing in chilling tolerance were used to study changes in water status and abscisic acid (ABA) levels before, during and after a chilling period. Seedlings were raised in fertilized soil at 24/22°C (day/night), 70% relative humidity. and a 12-h photoperiod with 200 μmol m⁻² s⁻¹ from fluorescent tubes. At an age of 2 weeks the plants were conditioned at 14/12°C for 4 days and then chilled for 5 days at 5/3°C. The other conditions (relative humidity, quantum flux, photoperiod) were unchanged. After the chilling period the plants were transferred to the original conditions for recovery. The third leaves were used to study changes in leaf necrosis, ion efflux, transpiration, water status and ABA accumulation. Pronounced differences in chilling tolerance between the 4 lines as estimated by necrotic leaf areas, ion efflux and whole plant survival were observed. Conditioning significantly increased tolerance against chilling at 5/3°C in all genotypes. The genotypes with low chilling tolerance had lower water and osmotic potentials than the more tolerant genotypes during a chilling period at 5/3°C. These differences were related to higher transpiration rates and lower diffusive resistance values of the more susceptible lines. During chilling stress at 5/3°C ABA levels were quadrupled. Only a small rise was measurable during conditioning at 14/12°C. However, conditioning enhanced the rise of ABA during subsequent chilling. ABA accumulation in the two lines with a higher chilling tolerance was triggered at a higher leaf water potential and reached higher levels than in the less tolerant lines. We conclude that chilling tolerance in maize is related to the ability for fast and pronounced formation of ABA as a protective agent against chilling injury.     

Increased Rubisco content in maize mitigates chilling stress and speeds recovery

Many C₄ plants, including maize, perform poorly under chilling conditions. This phenomenon has been linked in part to decreased Rubisco abundance at lower temperatures. An exception to this is chilling‐tolerant Miscanthus, which is able to maintain Rubisco protein content under such conditions. The goal of this study was to investigate whether increasing Rubisco content in maize could improve performance during or following chilling stress. Here, we demonstrate that transgenic lines overexpressing Rubisco large and small subunits and the Rubisco assembly factor RAF1 (RAF1‐LSSS), which have increased Rubisco content and growth under control conditions, maintain increased Rubisco content and growth during chilling stress. RAF1‐LSSS plants exhibited 12% higher CO₂ assimilation relative to nontransgenic controls under control growth conditions, and a 17% differential after 2 weeks of chilling stress, although assimilation rates of all genotypes were ~50% lower in chilling conditions. Chlorophyll fluorescence measurements showed RAF1‐LSSS and WT plants had similar rates of photochemical quenching during chilling, suggesting Rubisco may not be the primary limiting factor that leads to poor performance in maize under chilling conditions. In contrast, RAF1‐LSSS had improved photochemical quenching before and after chilling stress, suggesting that increased Rubisco may help plants recover faster from chilling conditions. Relatively increased leaf area, dry weight and plant height observed before chilling in RAF1‐LSSS were also maintained during chilling. Together, these results demonstrate that an increase in Rubisco content allows maize plants to better cope with chilling stress and also improves their subsequent recovery, yet additional modifications are required to engineer chilling tolerance in maize.     

Expression of Vernalization Genes in Near-isogenic Wheat Lines: Effects of Night Temperature

Four near-isogenic lines of wheat (Triticum aestivum L.em Thell)were used to compare selected night temperatures for their effectivenessas vernalizing temperatures. All treatments (conducted withina phytotron) had a common day temperature of 20 °C for 12h and night temperatures were 4, 7, 10, 13 and 20 °C. Interpretationof results for reproductive development was confounded by threeinteracting factors, their relative importance varying withgenotype. Firstly, development rate was generally slower atlower night temperatures. Secondly, in contrast, there was atendency for lower night temperatures to hasten developmentrate if vernalization requirements were satisfied. Thirdly,the lower night temperatures provided a more favourable environmentfor leaf production such that for some genotypes, vernalizedplants had higher final leaf numbers than unvernalized plants.Only for the genotype with the strongest vernalization response(vrn1 vrn2) did hastening of development due to vernalizationoverride any delaying effects. For this genotype, 4, 7 and 10°C were vernalizing temperatures. For the other three genotypes,any hastening of development due to vernalization was outweighedby delaying effects of lower night temperatures. Spikelet numberand days to anthesis were positively correlated in three ofthe four genotypes. It appeared that differences in spikeletnumber were a direct result of night temperature influencingthe duration of the spikelet phase and/or rate of spikelet initiation.Plant size at flowering was determined by the differential effectsof night temperature on growth and development rates. Triticum aestivum L., wheat, vernalization, night temperature, isogenic lines     

Saccharum × Miscanthus intergeneric hybrids (miscanes) exhibit greater chilling tolerance of C4 photosynthesis and postchilling recovery than sugarcane (Saccharum spp. hybrids)

Although commercial sugarcane (Saccharum spp. hybrid) produces large biomass yields, its lack of cold tolerance limits its cultivation to the tropics and subtropics. In contrast, sugarcane's close relative, Miscanthus, tolerates low temperatures. We studied 18 miscane genotypes, derived from hybridizations between two genotypes of sugarcane and two genotypes of Miscanthus (one each of M. sinensis and M. sacchariflorus). In an initial greenhouse experiment on long‐duration chilling stress (12–13°C day/7–9°C night), photosynthetic rates of the Miscanthus parents were significantly higher than the sugarcane parents after 7 days of chilling and were more than double by 14 days. The Miscanthus also retained more of their prechilling (22–25°C day/13–15°C night) photosynthetic rates (68%–72% 7 days, 64%–66% 14 days) than the sugarcanes (27% 7 days, 19%–20% 14 days). Seven of 18 miscanes exhibited higher photosynthetic rates than their sugarcane parents after 7 days of chilling, whereas after 14 days only four miscane genotypes had significantly higher photosynthetic rates than their sugarcane parents, but notably two of these did not differ from their highly tolerant Miscanthus parents. In a subsequent growth chamber experiment to evaluate short‐duration chilling stress and postchilling recovery, three miscanes representing the range of responses observed in the greenhouse experiment were compared with their parents. After 4 days of chilling (12/7°C day/night), the miscanes retained between 45% and 60% of their prechilling photosynthetic rate, with the best entry not significantly different from its Miscanthus parent (66%), and all three miscanes performed significantly better than the sugarcane parents (32%–33% for sugarcanes). After 7 days of postchilling recovery (26/18°C day/night), the Miscanthus parents and two of the miscanes fully recovered their prechilling photosynthetic rates but the sugarcane parents only recovered 69%–73% of their prechilling rates. Thus, genes from Miscanthus can be used to improve chilling tolerance of sugarcane via introgression.     

Development of the Fifth Leaf is Indicative for Whole Plant Performance at Low Temperature in Tomato

In the search for early-detectable selection criteria for growthat low temperature conditions in tomato, first the initiationand growth of individual leaves was analysed. Scanning electronmicroscopy revealed that the first four primordia had alreadydeveloped during the germination period at 25°C. The primordiumof the fifth leaf, however, was initiated after the transferof seedlings to the experimental conditions. The increase inlength of the first three leaves, and to a lesser extent ofthe fourth leaf, was considerably smaller in comparison withthat of later formed leaves. Moreover, the morphology of thefirst three to four leaves was deviant, whereas the others showedthe normal compound leaf architecture. All these results indicatedthat the fifth leaf was the earliest formed leaf with growthcharacteristics that might reflect the growth potential of thewhole plant. Development of the fifth leaf was tested as a marker for wholeplant growth. At three temperature, 18, 15 and 12°C, growthresponses of the fifth leaf were similar to that of whole plantsin four tomato genotypes: Line A, Line B, Premier and MXXIV-13.Significant differences in relative growth rate of dry weightof whole plants and fifth leaves (RGR_W)and of leaf area of thefifth leaves (RGR_LA between two fast growing and two slow growinggenotypes were found. No genotype by temperature interactionfor RGR_W and RGR_LA was found, indicating that the effect oftemperature decrease was similar for the four genotypes. The structure of the mature fifth leaf of one fast and one slowgrowing genotype, Line A and MXXIV-13, was analysed. For bothgenotypes, leaves were small and thick at low temperature, 12°C.The total number of epidermis and palisade parenchyma cellsper leaf was smaller but the size of the cells developed at12°C was larger than at 18°C. Consequently, the slowgrowth at 12°C was due to a low rate of cell division. Atboth temperatures, the fifth leaf to MXXIV-13 was smaller comparedto that of line A. Since the size of the cells were similar,the smaller leaf size was due to lower number of leaf cells. The results confirm the suitability of the growth, especiallyexpressed as RGR_LA , of the fifth leaf as a nondestructive marketfor vegetative development of tomato at low temperature. Growthdifferences between genotypes were mainly reflected by differencesin cell number of leaves, which might be correlated with geneticallydetermined differences in cell number of leaf primordia.Copyright1993, 1999 Academic Press Lycopersicon esculentum Mill. genotypes, plant growth, selection criteria, low temperature, leaf initiation, leaf development, RGR, leaf structure, cell expansion     

Field Studies of Cereal Leaf Growth: I. INITIATION AND EXPANSION IN RELATION TO TEMPERATURE AND ONTOGENY

Measurements of leaf initiation, appearance, and expansion arepresented for winter wheat and spring barley crops. For winterwheat, these processes occurred during periods of several weekswhen fluctuating temperatures influenced process rates. Analysisof these measurements was facilitated by plotting variablesagainst the time integral of temperature above an appropriatebase temperature (O °C), here called thermal time with unitsof °C d. Leaf primordial number and appearance stage increasedlinearly with thermal time for both winter wheat and springbarley which initiated 12 and 9 leaves respectively. When plottedagainst thermal time 90% of laminar and leaf length growth and80% of laminar width growth was satisfactorily described bya straight line for both species. This enabled an average extensionrate and duration of linear growth to be defined for each leaf.When expressed in thermal time, wheat leaves had a similar durationof linear growth (210 °C d; s.d. 30 °C d) with insolationexerting a negligible influence. The first seven barley leaveshad a shorter duration of linear growth (151 °C d; s.d.8 °C d). For wheat, final leaf length and laminar widthincreased with leaf number and were not apparently associatedwith changes in apical development stage. Changes of barleyleaf dimensions with leaf number were more complex.     

Effect of Temporary Chilling on Foliar and Caulinary Growth and Productivity in Soybean (Glycine max)

The effects of chilling on the apical bud were investigatedin soybean. A temperature of 6 °C was applied to leavesI and 2 and the apex for 12 d as soon as leaf 2 appeared. Thegrowth of the foliar and internodal series was analysed usingNelder's logistics. Chilling decreased the number of neoformed phytomers as wellas their rate of appearance. Schematically, the effect of chillingon growth is characterized by an inhibition of leaves and astimulation of internodes. However, the changes in the adultdimensions, and in the kinetic parameters (various durationsand growth rates), depend on the rank of the organ on the axis.Branching was markedly decreased at some nodes. The overallfructification of the main axis remained unchanged, but thedistribution of the seeds on the axis and the repartition betweenaxis and branches were markedly modified. These data clearly show (i) the importance of the position effecton the nature and the intensity of the responses and (ii) theabsence of any return to a normal growth pattern. Thus, a transientand relatively localized thermal stress can disturb the wholeontogenesis. Glycine max, soybean, growth kinetics, development, leaf, shoot, chilling     

The Responses of Leaf Water Potential and Leaf Diffusive Resistance to Abscisic Acid, Water Stress and Low Temperature in Hibiscus esculentus: The Effect of Water Stress and ABA Pre-Treatments

Intact plants of okra (Hibiscus esculentus) were chilled at6°C in the light, and leaf diffusion resistance (LDR) andleaf water potential measured. The response of the LDR of excisedleaves to fresh weight loss and, separately, exogenous abscisicacid (ABA) supply, was also studied at 6°C and 30°C.The influence of two pre-treatments upon these measurementswas studied. The two pre-treatments consisted of the impositionof a period of water stress at 30°C prior to measurement(followed by re-watering) or the spraying of the leaves withABA. It was found that plants that had been grown in a highhumidity environment continuously (designated control plants)had stomata that were very unresponsive to both water loss fromthe leaves and to exogenous ABA at both temperatures (6°Cand 30°C). Chilling the control plants resulted in rapidwilting and concomitant decline in leaf water potential. A pre-treatmentof water stress prior to chilling did not alter or reduce therate of development of chilling injury, nor did the pre-treatmentincrease the responsiveness of stomata at 6°C to water lossor exogenous ABA. However, spraying the leaves with ABA priorto chilling reduced the severity and delayed the onset of chillinginjury. Stomatal response to water stress and exogenous ABAwas increased by the spraying pre-treatment. These results arediscussed in relation to previous studies of the phenomenonof stomatal locking open at low temperature and the effect ofpre-treatments upon the development of chill-resistance. Key words: Water stress, chilling, stomata     

Thermoluminescence Investigation of Low Temperature Stress in Maize

The thermoluminescence (TL) emission of photosynthesising materials originates from the recombination of charge pairs created by a previous excitation. Using a recently described TL set-up the effect of chilling stress on TL bands occurring at positive temperatures (AG, B, and HTL) was investigated in intact leaves. The far-red irradiation of leaves at low, but non-freezing temperatures induced a TL band peaking at around 40–45 °C (AG band), together with a B band peaking between 20 and 35 °C. Low temperature stress first caused a downshift and a temporary increase in the AG band after 4 h at 0 °C in the light, then a decrease in the AG and B TL bands after 1 d at 0 °C in the light. This decrease was less pronounced in cold-tolerant genotypes and in those grown at acclimating temperatures. Furthermore, an additional band appeared above 80 °C after severe cold stress. This band indicates the presence of lipid peroxides. Thus TL is a useful technique for studying the effects of low temperature stress.     

Different root low temperature response of two maize genotypes differing in chilling sensitivity

Here we report on the root hydraulic properties of intact and excised root systems of two maize genotypes differing in chilling sensitivity (Z7, tolerant and Penjalinan, sensitive) subjected for 3 d to 5 °C. When root hydraulic conductance (L) was measured under a hydrostatic force using an excised root system in a pressure chamber, an initial decrease of L was observed in both genotypes. However, the value of L increased in the chilling tolerant genotype after 30 h at 5 °C; in the chilling sensitive Penjalinan genotype there was no such increase. Osmotic root hydraulic conductance was measured in excised root systems exuding under atmospheric pressure. We observed a progressive decline during the chilling treatment of the osmotic root hydraulic conductance in the chilling sensitive Penjalinan plants; however, after 54 h at 5 °C, the chilling tolerant Z7 plants had a significantly higher osmotic hydraulic conductance. Moreover, in the chilling tolerant plants we found an increase in the inhibition caused by HgCl₂ of the osmotic hydraulic conductance during the chilling treatment, indicating a possible increase in the contribution of aquaporins to root hydraulic conductance in the chilling tolerant Z7 plants during chilling treatment.     

ABA Levels and Effects in Chilled and Hardened Phaseolus vulgaris

Leaf abscisic acid (ABA) levels of chilled P. vulgaris weremeasured after 18 h chilling at 5°C, at a saturation deficitof 1.24 g m^–3 (SD), and after chilling in a water-saturatedatmosphere. Changes were also followed during a chill hardeningperiod of 4 d at 12°C, 2.1 g m^–3 SD. It was foundthat hardening resulted in an almost 5. fold increase in ABAlevels after 3 d at 12°C, and this decreased to approximatelycontrol levels on the fourth day. Subsequent chilling of hardenedplants produced no change in ABA levels from that of controlplants (22° C). In contrast, non-hardened plants chilledat 1.24 g m^–3 SD had ABA levels almost 3 times the levelof control plants. However, chilling in a water-saturated atmosphereresulted in a decrease in ABA levels. In addition, the response of leaf diffusion resistance (LDR)to exogenous ABA fed via the transpiration stream was measuredat 5 ° C and 22° C in hardened and non-hardened plants.Use of tritium-labelled ABA was made to calculate the stomatalsensitivity to ABA. It was found that exogenous ABA caused anincreased in LDR at 22°C in both hardened and non-hardenedplants. However, the sensitivity of the hardened plants to ABAwas greater in terms of rate of closure and amount of ABA requiredto close the stomata. At 5°C, however, ABA caused stomatalopening and the maintainance of open stomata in non-hardenedplants. In hardened plants, ABA caused stomatal closure at 5°C.These results are discussed in relation to the locking-openresponse of chilled P. vulgaris stomata. Key words: Chilling, Stomata, ABA, Phaseolus vulgaris     

Comparison of Maize Inbred Lines Differing in Low Temperature Tolerance: Effect of Acclimation at Suboptimal Temperature on Chloroplast Functioning

Acclimation to optimal or suboptimal temperature may influencephotosynthetic properties of different maize genotypes in distinctways. In this study, leaf growth and chloroplast functioningof the second leaves of Penjalinan, an inbred line used in warmtropical regions (CS) and Z7, an inbred line adapted to cooltemperate regions (CT), acclimated at near optimal (24/22°C)and suboptimal (15/13°C) temperature, were compared. Whenacclimated at 24/22°C, there was little difference betweenCT and CS in pigment content, in photosynthetic capacity, measuredas the maximum quantum yield of oxygen evolution under lightlimiting conditions (     

Physiological responses to water stress following a conditioning period in berseem clover

Berseem clover (Trifolium alexandrinum L.) is an important crop in semi-arid regions; its herbage and seed yields are often reduced by water stress. Our objectives were (i) to determine the effect of water stress, applied after a conditioning period, on water relations, proline accumulation and plant dry weight, and (ii) to investigate if some physiological responses differed in varieties of berseem. Five cultivars (Axi, Bigbee, Lilibeo, Sacromonte and Saniros) were grown in a controlled environment, and subjected to four irrigation treatments (T1, T2, T3 and T4 referring to plants irrigated to field capacity every 1, 2, 3 or 4 d, respectively) during a conditioning period (12 d). T1 treatment indicated the well-watered control, whereas T2, T3 and T4 treatments represented the conditioned plants. Leaf water potential (Ψ), osmotic potential (Ψπ), relative water content (RWC), gravimetric soil water content (GSWC) and leaf proline concentration were recorded during the conditioning period and a subsequent water deficit period (3 d) applied at early flowering growth stage. The conditioned plants subjected to subsequent water deficit maintained higher values of Ψ, Ψπ, RWC and GSWC, and lower values of leaf proline concentration. Reductions in parameter values were inversely related to the water stress severity that plants had previously experienced. At the end of the experiment, T1 showed 42%, 58% and 31% lower values for Ψ, Ψπ and RWC, respectively, than those of T4. Conditioned plants were also shorter and accumulated less leaf, stem and total dry weight. The conditioning treatments did not affect the relation between Ψ and Ψπ since conditioned plants show similar values of Ψπ as the control at the same Ψ value. Thus, drought acclimation in berseem clover contributed to water stress tolerance by the maintenance of tissue hydration. The berseem cultivars examined showed differences in plant growth parameters, but they were very similar for physiological responses to water deficit. The main genetic difference was recorded for turgor maintenance capacity. This revised version was published online in June 2006 with corrections to the Cover Date.     

Chilling tolerance and early vigour-related characteristics evaluated in two Miscanthus genotypes

A long growing season, mediated by the ability to grow at low temperatures early in the season, can result in higher yields in biomass of crop Miscanthus. In this paper, the chilling tolerance of two highly productive Miscanthus genotypes, the widely planted Miscanthus × giganteus and the Miscanthus sinensis genotype ‘Goliath’, was studied. Measurements in the field as well as under controlled conditions were combined with the main purpose to create basic comparison tools in order to investigate chilling tolerance in Miscanthus in relation to its field performance. Under field conditions, M. × giganteus was higher yielding and had a faster growth rate early in the growing season. Correspondingly, M. × giganteus displayed a less drastic reduction of the leaf elongation rate and of net photosynthesis under continuous chilling stress conditions in the growth chamber. This was accompanied by higher photochemical quenching and lower nonphotochemical quenching in M. × giganteus than that in M. sinensis ‘Goliath’ when exposed to chilling temperatures. No evidence of impaired stomatal conductance or increased use of alternative electron sinks was observed under chilling stress. Soluble sugar content markedly increased in both genotypes when grown at 12°C compared to 20°C. The concentration of raffinose showed the largest relative increase at 12°C, possibly serving as a protection against chilling stress. Overall, both genotypes showed high chilling tolerance for C₄ plants, but M. × giganteus performed better than M. sinensis ‘Goliath’. This was not due to its capacity to resume growth earlier in the season but rather due to a higher growth rate and higher photosynthetic efficiency at low temperatures.     

Effects of growth temperature and chilling duration on leaf phenology of <Emphasis Type="Italic">Fauria crista</Emphasis>-<Emphasis Type="Italic">galli</Emphasis> subsp. <Emphasis Type="Italic">japonica</Emphasis>, an alpine snowbed geophyte

I examined the effects of growth temperature and winter duration on the leaf phenology of Fauria crista-galli plants, which have an indeterminate growth habit. After a 220-day chilling treatment, the leaf expansion and green periods of plants maintained at 25/20°C were much longer than those of plants maintained at 15/10°C and of plants at the natural habitat obtained in a previous study. The results indicate that early growth cessation and early leaf senescence in the natural habitat are not only due to endogenous rhythm but determined to some extent by cool summer temperatures. When grown at 15/10°C, the green period of individual leaves and plants was much shorter after a long chilling treatment (220 days) than after a short chilling treatment (110 days). The plants sprouted during or immediately after the termination of chilling treatment, suggesting that the decrease in the green period results partly from an advance of endogenous developmental stages during the chilling treatment and that the timing of snowmelt potentially affects the time of leaf senescence in the natural habitat.     

Determination of the Length of the Vegetative and Pre-floral Stages in the Day-Neutral Plant Helianthus annuus by Chilling Pulses

Helianthus annuus seedlings grown in an 18 h day at 28 ?C wereexposed to one 6 d chilling pulse of 12 ?C, at spaced timesduring the first 21 d from sowing. At 2 d intervals, the terminalbuds of 5 plants were dissected to determine leaf number andto score the vegetative or flowering state of the shoot apex.It was found that, while the rate of leaf initiation was reducedequally by each chilling pulse, pulses commencing on days 9or 12 reduced the total leaf number from 30 to 26, while pulsesapplied earlier had little effect. This variation is interpretedin terms of the time available for leaf production. The apicesof control plants commenced the visible transition to flowering16 d after sowing. Chilling pulses applied from days 3 or 6delayed this transition by about 5 d, whereas later pulses causedonly a 1•5 d delay. In a second experiment, where the chillingwas reduced to 2 d duration, it was again found that chillingdelayed flowering during the first 8 d and was progressivelyless effective when applied later. From this variation in temperaturesensitivity it is proposed that chilling sunflower plants immediatelyafter sowing delays flowering by extending the vegetative phaseof growth and so delaying the attainment of a ‘ripenessto flower’ state that appears to coincide with the expansionof the first pair of leaves. From day 8 onwards processes leadingto flowering that are relatively temperature insensitive apparentlybecome dominant in the apex and result in visible signs of flowering8 d later, although during this transitional stage leaf primordiacontinue to be initiated on the flanks of the apex.