Chilling sensitivity of the frost-tolerant potato Solanum commersonii

Frost tolerance has been reported in the shoots of wild, tuberiferous potato species such as Solanum commersonii when the plants are grown in either field or controlled conditions. However, these plants can survive as underground tubers and avoid unfavorable environmental conditions altogether. As such, leaf growth and photosynthesis at low temperature may not be required for survival of the plants. In order to determine the temperature sensitivity of S. commersonii shoots, we examined leaf growth, development and photosynthesis in plants raised at 20/16°C (day/night). 12/9°C and 5/2°C. S. commersonii leaves grown at 5°C exhibited a marked decrease in leaf area and in total chlorophyll (Chl) content per leaf area when compared with leaves grown at 20°C. Furthermore, leaves grown at 5°C did not exhibit the expected decrease in either water content or susceptibility to low-temperature-induced photoinhibition that normally characterizes cold acclimation in frost-tolerant plants. Measurements of CO₂-saturated O₂ evolution showed that the photosynthetic apparatus of 5°C plants was functional, even though the efficiency of photosystem II photochemistry was reduced by growth at 5°C. A decrease in the resolution of the M-peak in the slow transients for Chl a fluorescence in leaves grown at 12 and 5°C and in all leaves exposed to high light at 5°C indicated that low temperature significantly affected processes on the reducing side of Q_A, the primary quinone electron acceptor in photosystem II. Thus S. commarsonii exhibits the characteristics of a plant that is limited by chilling temperatures. Although S. commersonii can tolerate light frosts, its sensitivity to chilling temperatures may result in shoot dieback in winter in its native habitat. The plants may avoid both chilling and freezing temperatures by overwintering as underground tubers.     

Growth, photosynthesis and assimilate partitioning in Lolium temulentum exposed to chilling temperatures

Vegetative plants of Lolium temulentum L, grown at 20°C with an 8 h photoperiod were transferred to either 5 or 2°C (8 h photoperiod) at 4th leaf maturity. Measurement of dry weight gain indicated a marked decline in relative growth rate below 5°C, with growth being reduced as much between 5 and 2°C as between 20 and 5°C. This reduction was not associated with increased mortality and was reversible if plants were returned to 20°C. Tissue explants from cold-treated plants retained the ability to extend if returned to 20°C Rates of extension in explants were less temperature sensitive than the relative growth rates observed in intact plants. Measurements of photosynthetic capacity, and of the patterns of accumulation of reserve carbohydrate in plants exposed to different temperatures, suggested that the inhibition of growth caused by chilling is not caused by an inability of the plants to fix sufficient carbon.     

Mode of high temperature injury to wheat during grain development

High temperature stress adversely affects wheat growth in many important production regions, but the mode of injury is unclear. Wheat ( Triticum aestivum L. cv. Newton) was grown under controlled conditions to determine the relative magnitude and sequences of responses of source and sink processes to high temperature stress during grain development. Regimes of 25°C day/15°C night, 30°C day/20°C night, and 35°C day/25°C night from 5 days after anthesis to maturity differentially affected source and sink processes. High temperatures accelerated the normal decline in viable leaf blade area and photosynthetic activities per unit leaf area. Electron transport, as measured by Hill reaction activity, declined earlier and faster than other photosynthetic processes at the optimum temperature of 25/15 °C and at elevated temperatures. Changes in RUBP carboxylase activities were similar in direction but smaller in magnitude than changes in photosynthesic rate. Increased protease activity during senscence was markedly accentuated by high temperature stress. Specific protease activity increased 4-fold at 25/15 °C and 28-fold at 35/25 °C from 0 to 21 days after initiation of temperature treatments. Grain-filling rate decreased from the lowest to the highest temperature, but the change was smaller than the decrease in grain-filling duration at the same temperatures. We concluded that a major effect of high temperature is acceleration of senescence, including cessation of vegetative and reproductive growth, deterioration of photosynthetic activities, and degradation of proteinaceous constituents.     

Low temperature delays development of photosystem II activity in winter rye leaves

The ability of leaves to acclimate photosynthetically to low temperature was examined during leaf development in winter rye plants ( Secale cereale L. cv. Puma) grown at 20°C or at 6°C. All leaves grown at 6°C exhibit increased chlorophyll (Chl) levels per leaf area, higher rates of uncoupled, light-saturated photosystem I (PSI) electron transport, and slower increases in photosystem II (PSII) electron transport capacity, when compared with 20°C leaves. The stoiehiometry of PSI and PSII was estimated for each leaf age class by quantifying Chl in elcctrophorctic separations of Chl-protein complexes. The ratio of PSII/PSI electron transport in 20°C leaves is highly correlated with the ratio of core Chl a -proteins associated with PSII (CPa) to those associated with PSI (CP1). In contrast, PSII/PSI electron transport in 6°C leaves is not as well correlated with CPa/CP1 and is related, in part, to the amount and organization of light-harvesting Chl a/b -proteins associated with PSII. CPa/CP1 increases slowly in 6°C leaves, although the ratio of CPa/CP1 in mature 20°C and 6°C leaves is not different. The results suggest that increased PSI activity at low temperature is not related to an increase in the relative proportion of PSI and may reflect, instead, a regulatory change. Photosynthetic acclimation to low environmental temperature involves increased PSI activity in mature leaves shifted to 6°C. In leaves grown entirely at 6°C, however, acclimation includes both increased PSI activity and modifications in the rate of accumlation of PSII and in the organization of LHCII.     

Photosystem II Excitation Pressure and Development of Resistance to Photoinhibition (II. Adjustment of Photosynthetic Capacity in Winter Wheat and Winter Rye)

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

Regulation of frost resistance during cold de-acclimation and re-acclimation in oilseed rape. A possible role of PSII redox state

A possible role of photosynthetic apparatus during cold de-acclimation was studied in oilseed rape ( Brassica napus var. oleifera ). Plants of spring (Star) and winter (Górczañski) cultivars were cold acclimated at + 5°C, and de-acclimated during 4 weeks at combinations of + 12 and + 20°C operating in the light or/and dark, with a 12-h photoperiod. Evidence is presented that the photosynthetic apparatus may be involved in temperature perception during de-acclimation. De-acclimation was faster under a 20/12°C (day/night) treatment than under the reverse 12/20°C (day/night). De-acclimation rate was constant when the day temperature was constant, irrespective of the night temperature both under cold day temperature regimes (12/20, 12/12°C (day/night) and warm-day treatments (20/12, 20/20°C (day/night). The fast decrease in frost resistance observed under warm-day de-acclimation was always accompanied by an acceleration of elongation growth. In the spring cultivar, elongation growth increased starting from the second week of de-acclimation, regardless of temperature conditions. Once elongation growth had commenced during de-acclimation, it continued throughout the period necessary for re-acclimation to low temperature. Re-acclimation to the initial freezing tolerance level was only possible when plant elongation was reduced. In addition re-acclimation of the photosynthetic apparatus to low temperature was impossible in fast growing plants. A possible relationship between PSII, growth rate and frost resistance during cold acclimation and de-acclimation is discussed.     

Photosynthetic acclimation of Tradescantia albiflora to growth irradiance: morphological, ultrastructural and growth responses

Tradescantia albiflora (Kunth), a trailing ground species naturally occurring in deep shade in rainforests, has an unusual photosynthetic acclimation profile for growth irradiance. Although capable of increasing its capacity for electron transport, photophosphorylation and carbon fixation when grown in full sunlight, Tradescantia has constant chlorophyll alb ratios, photosystem reaction centre stoichiometry and pigment-protein composition at all growth irradiances (Chow et al. 1991. Physiol. Plant. 81: 175–182). To gain an insight into the compensatory strategies which allow Tradescantia to grow in both high and low lights, plants were grown under shade cloth (100 to 1.4% relative growth irradiance) and leaf and chloroplast attributes were compared. While shade Tradescantia chloroplasts had three times more chlorophyll per chloroplast and twice the length of thylakoid membranes compared to plants grown in full sunlight, the ratios of appressed to nonappressed thylakoid membranes were constant. The average net surface charge density of destacked thylakoids was the same for plants grown at moderate and low-irradiance, consistent with their similar stacking profiles. Tradescantia plants grown in direct sunlight had 10-times more fresh and dry weight per plant compared to plants grown in shade, despite a lower photosynthetic capacity on a leaf area basis with partial photoinhibition. We conclude that having a light-harvesting apparatus permanently locked into the "shade-plant mode " does not necessarily prevent a plant from thriving in high light. Analyses of leaf growth at different irradiances provide a partial explanation of the manner in which Tradescantia compensates for very low photosynthetic capacity per unit leaf in sunlight.     

Photosystem II Excitation Pressure and Development of Resistance to Photoinhibition (I. Light-Harvesting Complex II Abundance and Zeaxanthin Content in Chlorella vulgaris)

The basis of the increased resistance to photoinhibition upon growth at low temperature was investigated. Photosystem II (PSII) excitation pressure was estimated in vivo as 1 - qp (photochemical quenching). We established that Chlorella vulgaris exposed to either 5[deg]C/150 [mu]mol m-2 s-1 or 27[deg]C/2200 [mu]mol m-2 s-1 experienced a high PSII excitation pressure of 0.70 to 0.75. In contrast, Chlorella exposed to either 27[deg]C/150 [mu]mol m-2 s-1 or 5[deg]C/20 [mu]mol m-2 s-1 experienced a low PSII excitation pressure of 0.10 to 0.20. Chlorella grown under either regime at high PSII excitation pressure exhibited: (a) 3-fold higher light-saturated rates of O2 evolution; (b) the complete conversion of PSII[alpha] centers to PSII[beta] centers; (c) a 3-fold lower epoxidation state of the xanthophyll cycle intermediates; (d) a 2.4-fold higher ratio of chlorophyll a/b; and (e) a lower abundance of light-harvesting polypeptides than Chlorella grown at either regime at low PSII excitation pressure. In addition, cells grown at 5[deg]C/150 [mu]mol m-2 s-1 exhibited resistance to photoinhibition comparable to that of cells grown at 27[deg]C/2200 [mu]mol m-2 s-1 and were 3- to 4-fold more resistant to photoinhibition than cells grown at either regime at low excitation pressure. We conclude that increased resistance to photoinhibition upon growth at low temperature reflects photosynthetic adjustment to high excitation pressure, which results in an increased capacity for nonradiative dissipation of excess light through zeaxanthin coupled with a lower probability of light absorption due to reduced chlorophyll per cell and decreased abundance of light-harvesting polypeptides.     

Low growth temperature-induced changes to pigment composition and photosynthesis in Zea mays genotypes differing in chilling sensitivity

The effects on pigment composition and photosynthesis of low temperature during growth were examined in the third leaf of three chilling-tolerant and three chilling-sensitive genotypes of Zea mays L. The plants were grown under a controlled environment at 24 or 14 °C at a photon flux density (PFD) of 200 or 600 μ mol m^–2 s^–1. At 24 °C, the two classes of genotypes showed little differences in their photosynthetic activity and their composition of pigments. At 14 °C, photosynthetic activity was considerably reduced but the chilling-tolerant genotypes displayed higher photosynthetic rates than the chilling-sensitive ones. Plants grown at 14 °C showed a reduced chlorophyll (Chl) a + b content and a reduced Chl a / b ratio but an increased ratio of total carotenoids to Chl a + b . These changes in pigment composition in plants grown at low temperature were generally more pronounced in the chilling-sensitive genotypes than in the tolerant ones, particularly at high PFD. Furthermore, at 14 °C, all the genotypes showed increased ratios of lutein, neoxanthin and xanthophyll-cycle carotenoids to Chl a + b but a reduced ratio of β -carotene to Chl a + b , especially at high PFD. At 14 °C, the chilling-tolerant genotypes, when compared with the sensitive ones, were characterized by higher contents of β -carotene and neoxanthin, a lower content of xanthophyll-cycle carotenoids, a lower ratio of xanthophylls to β -carotene, and less of their xanthophyll-cycle carotenoid pool in the form of zeaxanthin. These differences between the two classes of genotypes were more pronounced at high PFD than at low PFD. The results are discussed in terms of the relationship that may exist in maize between pigment composition and the capacity to form an efficient photosynthetic apparatus at low growth temperature.     

Photosynthetic acclimation to high temperatures associated with heat tolerance in creeping bentgrass

Photosynthetic responses to increasing temperatures play important roles in regulating heat tolerance. The objectives of this study were to determine photosynthetic acclimation to increasing temperatures for creeping bentgrass (Agrostis stolonifera L.) and to examine changes in major photosynthetic components (photosynthetic pigments, photochemical efficiency, ribulose-1,5-bisphosphate carboxylase/oxygenase (rubisco) activity, and activation state of rubisco) involved in heat responses of photosynthesis. 'Penncross' was exposed to 20, 25, 30, and 35 degrees C for 7d at each temperature (acclimated) before being exposed to 40 degrees C for 28d or directly exposed to 40 degrees C for 28d from 20 degrees C (non-acclimated) in growth chambers. Leaf net photosynthetic rate (Pn), photochemical efficiency, rubisco activity, rubisco activation state, chlorophyll content, and carotenoid content decreased when grasses were subjected to severe heat stress at 40 degrees C for 28d. The declines in rubisco activity and activation state were most dramatic among different photosynthetic components examined in this study. Heat-acclimated plants were able to maintain significantly higher Pn, the content of chlorophyll and carotenoid, and the level of rubisco activity and activation state during subsequent exposure to severe heat stress, compared to non-acclimated plants. These results suggested that photosynthetic acclimation to increasing temperatures contributed to creeping bentgrass tolerance to severe heat stress, which was associated with the maintenance of both higher light-harvesting capacity and carbon fixation activity during heat stress.     

A field study of photosynthetic temperature acclimation in Carex eleocharis Bailey

Abstract. Seasonal patterns in photosynthetic temperature acclimation and growth were investigated in the sedge, Carex eleocharis Bailey, a species which has demonstrated a marked capacity for shifts in the photosynthetic temperature optimum in previous growth chamber studies. The seasonal production of new leaves was 90% complete by the earliest study date, June 3. Shifts in the photosynthetic temperature optimum of 10°C (from 15 to 25°C) were observed during the months of June and July. These results indicate that in situ acclimatory adjustments in C. eleocharis occur in existing leaf tissue, rather than new leaves which are produced as the season progresses. Despite the 10°C increase in the temperature optimum, mean mid-day leaf temperatures were higher than the optimum throughout the summer. A broad temperature response appeared to be more important than the acclimation adjustments in maintaining near-maximum photosynthesis rates during the mid-day period. Seasonal shifts in the photosynthetic temperature optimum were not as great as those previously observed in growth chamber studies. This discrepancy arises because of the capacity for growth chamber grown plants to produce new leaves with temperature response characteristics closely tuned to the growth temperature regime. In field-grown plants the production of 90% of the leaves during the cool portion of the season places limitations on the potential for acclimation to the warmer midsummer temperatures.     

List of members August, 1962

We investigated the acclimation of Chondrus crispus to growth at 5°C and 20°C in the laboratory. We were specifically interested in the responses of light-limited photosynthesis to temperature and the effects of short-term thermal changes (of the order of minutes). Thermal acclimation to constant temperatures over 3–4 weeks had significant effects on the light-use characteristics of this species such that in comparison with those grown at 5°C, 20°C-grown plants had higher concentrations of chlorophyll a and total phycobilins, which were associated with larger photosynthetic unit sizes. Plants grown at the higher temperature had greater photosynthetic efficiencies (α) and higher rates of light-limited photosynthesis at a given photon flux density than did plants acclimated to 5°C. Plants acclimated to 20°C were less sensitive to short-term temperature changes than were 5°C-acclimated plants. These results are discussed in terms of (1) the effects of growth temperature on light harvesting and (2) the implications of exposure to constant temperature for short-term thermal responses.     

Low-temperature damage and subsequent recovery of fab1 mutant Arabidopsis exposed to 2 degrees C.

The fab1 mutant of Arabidopsis thaliana, which contains increased levels of saturated fatty acids, was indistinguishable from the wild type when it was grown at 22 or 12 degrees C. During the first 7 to 10 d after transfer to 2 degrees C, the growth and photosynthetic characteristics of the fab1 plants remained indistinguishable from the wild type, with values for the potential quantum efficiency of photosystem II decreasing from 0.8 to 0.7 in plants of both lines. Whereas wild-type plants maintained quantum efficiency of photosystem II at approximately 0.7 for at least 35 d at 2 degrees C, this parameter declined rapidly in the mutant after 7 d and reached a value of less than 0.1 after 28 d at 2 degrees C. This decline in photosynthetic capacity was accompanied by reductions in chlorophyll content and the amount of chloroplast glycerolipids per gram of leaf. Electron microscopic examination of leaf samples revealed a rapid and extensive disruption of the thylakoid and chloroplast structure in the mutant, which is interpreted here as a form of selective autophagy. Despite the almost complete loss of photosynthetic function and the destruction of photosynthetic machinery, fab1 plants retained a substantial capacity for recovery following transfer to 22 degrees C. These results provide a further demonstration of the importance of chloroplast membrane unsaturation to the proper growth and development of plants at low temperature.     

Interaction of temperature and irradiance effects on photosynthetic acclimation in two accessions of Arabidopsis thaliana

The effect of temperature and irradiance during growth on photosynthetic traits of two accessions of Arabidopsis thaliana was investigated. Plants were grown at 10 and 22?°C, and at 50 and 300?μmol photons?m(-2)?s(-1) in a factorial design. As known from other cold-tolerant herbaceous species, growth of Arabidopsis at low temperature resulted in increases in photosynthetic capacity per unit leaf area and chlorophyll. Growth at high irradiance had a similar effect. However, the growth temperature and irradiance showed interacting effects for several capacity-related variables. Temperature effects on the ratio between electron transport capacity and carboxylation capacity were also different in low compared to high irradiance grown Arabidopsis. The carboxylation capacity per unit Rubisco, a measure for the in vivo Rubisco activity, was low in low irradiance grown plants but there was no clear growth temperature effect. The limitation of photosynthesis by the utilization of triose-phosphate in high temperature grown plants was less when grown at low compared to high irradiance. Several of these traits contribute to reduced efficiency of the utilization of resources for photosynthesis of Arabidopsis at low irradiance. The two accessions from contrasting climates showed remarkably similar capabilities of developmental acclimation to the two environmental factors. Hence, no evidence was found for photosynthetic adaptation of the photosynthetic apparatus to specific climatic conditions.     

Photosynthetic apparatus of pea thylakoid membranes : response to growth light intensity

We investigated the effect of growth light intensity on the photosynthetic apparatus of pea (Pisum sativum) thylakoid membranes. Plants were grown either in a growth chamber at light intensities that ranged from 8 to 1050 microeinsteins per square meter per second, or outside under natural sunlight. In thylakoid membranes we determined: the amounts of active and inactive photosystem II, photosystem I, cytochrome b/f, and high potential cytochrome b₅₅₉, the rate of uncoupled electron transport, and the ratio of chlorophyll a to b. In leaves we determined: the amounts of the photosynthetic components per leaf area, the fresh weight per leaf area, the rate of electron transport, and the light compensation point. To minimize factors other than growth light intensity that may alter the photosynthetic apparatus, we focused on peas grown above the light compensation point (20-40 microeinsteins per square meter per second), and harvested only the unshaded leaves at the top of the plant. The maximum difference in the concentrations of the photosynthetic components was about 30% in thylakoids isolated from plants grown over a 10-fold range in light intensity, 100 to 1050 microeinsteins per square meter per second. Plants grown under natural sunlight were virtually indistinguishable from plants grown in growth chambers at the higher light intensities. On a leaf area basis, over the same growth light regime, the maximum difference in the concentration of the photosynthetic components was also about 30%. For peas grown at 1050 microeinsteins per square meter per second we found the concentrations of active photosystem II, photosystem I, and cytochrome b/f were about 2.1 millimoles per mol chlorophyll. There were an additional 20 to 33% of photosystem II complexes that were inactive. Over 90% of the heme-containing cytochrome f detected in the thylakoid membranes was active in linear electron transport. Based on these data, we do not find convincing evidence that the stoichiometries of the electron transport components in the thylakoid membrane, the size of the light-harvesting system serving the reaction centers, or the concentration of the photosynthetic components per leaf area, are regulated in response to different growth light intensities. The concept that emerges from this work is of a relatively fixed photosynthetic apparatus in thylakoid membranes of peas grown above the light compensation point.     

Susceptibility to low-temperature photoinhibition and the acquisition of freezing tolerance in winter and spring wheat: The role of growth temperature and irradiance

Five winter and five spring wheat ( Triticum aestivum L.) cultivars were grown under either control conditions (20°C/250 photosynthetic photon flux density (PPFD) [μmol m⁻² s⁻¹]), high irradiance (20°C/800 PPFD) or at low temperature (either 5°C/250 PPFD or 5°C/50 PPFD). To eliminate any potential bias, the wheat cultivars were arbitrarily chosen without any previous knowledge of their freezing tolerance or photosynthetic competence. We show that the differential susceptibilities to photoinhibition exhibited between spring and winter wheat cultivars, as assessed by chlorophyll fluorescence cannot be explained on the basis of either growth irradiance or low growth temperature per se. The role of excitation pressure is discussed. We assessed the correlation between susceptibility to low-temperature photoinhibition, maximum ribulose 1,5-bisphosphate carboxylase-oxygenase (EC 4.1.1.39) and NADP-dependent malate dehydrogenase (EC 1.1.1.82) activities, chlorophyll and protein concentrations and freezing tolerance determined by electrolyte leakage. Susceptibility to photoinhibition is the only parameter examined that is strongly and negatively correlated with freezing tolerance. We suggest that the assessment of susceptibility to photoinhibition may be a useful predictor of freezing tolerance and field survival of cereals.     

The interrelationship of growth and frost tolerance in winter rye

The reduction in growth of winter cereals that occurs in the fall is thought to be required for the development of frost resistance. In the present study, the interrelationship of freezing tolerance and growth was examined by raising winter rye ( Secale cereale cv. Puma) plants at 20/16°C (day/night) and at 5/3°C under 8-, 16- and 24-h daylengths to vary growth rates and frost tolerance. Temperature and irradiance were quantified as thermal time, photothermal time and photosynthetic photon flux and examined by multiple linear regression in order to determine their effects on growth and frost tolerance of rye shoots. At low temperature, both growth and frost tolerance were markedly influenced by daylength and irradiance. Plants grown at 5/3°C with a short daylength accumulated shoot dry weight and increased frost tolerance at a greater rate per unit photothermal time or photon flux than plants grown at longer daylengths. Moreover, 5/3°C plants grown with a 16-h day grew more slowly and were less frost tolerant than plants grown with a 24-h day. We conclude that the interrelationship between growth and frost tolerance is a quantitative one. Frost tolerance is induced only by low temperature, but the development of forst tolerance is dependent upon both irradiance, which affects the amount of photoassimilate available, and daylength, which may affect the partitioning of photoassimilates between growth and frost tolerance.     

Temperature inducible protochlorophyllide reduction in darkness in a pigment mutant of Scenedesmus obliquus

Accumulation of chlorophyll and protochlorophyllide (PChlide) was followed during beterotrophic growth of the pigment mutant C-2A' of Scenedesmus obliquus L. in the darkness at 30 and 20°C. At 30°C the cells remained yellow with accumulation of protochlorophyllide, whereas they became green at 20°C with only traces of protochlorophyllide. The capacity of mutant cells to reduce PChlide to chlorophyllide (Chlide) in the dark with or without addition of 5-aminolevulinic acid as measured in isolated membranes, was high in cells grown at 20°C but negligible at 30°C. The high capacity to reduce PChlide created in cells growing at 20°C was only slightly diminished by exposure of cells to 38°C for 3 h. Mechanisms of temperature-sensitive chlorosis in algae and higher plants are discussed in relation to the results with pigment mutant C-2A' of Scenedesmus obliquus . It is assumed that either an activator of NADPH protochlorophyllide oxidoreductase (EC 1.6.99.1) or a different enzyme system can be activated by lower temperature as by light.     

Membrane lipids in Ceratodon purpureus protonemata grown at high and low temperatures

Ceratodon purpureus (Hedw.) Brid. was grown at two temperatures, 20 and 4°C. The protonemata grown at 4°C fixed more CO₂ at low temperatures; but their frost tolerance, tested as the recovery of photosynthesis after frost treatment, was not better than in the protonemata grown at 20°C. The effects of the growth temperature were studied on the membrane lipids of intact protonemata and on the lipid and protein contents of isolated thylakoid membranes. A large proportion, 70 to 90%, of the thylakoid membrane lipids was lost unless precautions were taken to inhibit the lipid-degrading enzyme activities. The lipid content of the thylakoid membranes of protonemata grown at 20 and 4°C was 3.9 and 4.8 mol (mol chlorophyll)⁻¹, respectively. Only minor differences were found in the lipid class composition. Monogalactosyldi-acylglycerol constituted more than 50 mol-% of the thylakoid membrane lipids at both 20 and 4°C. However, each lipid class had a higher average number of double bonds per lipid molecule in cold growth conditions. The protein content of the thylakoid membranes was low at both 20 and 4°C. These characteristics of the thylakoid membranes may be a prerequisite for the observed ability of protonemata to photosynthesize even at subzero temperatures.     

Effect of Temperature on the Formation of Microsclerotia of Verticillium dahliae

Microsclerotium formation by six isolates of Verticillium dahliae was studied at different temperatures both in vitro and in Arabidopsis thaliana . In vitro mycelial growth was optimal at 25°C, but microsclerotium formation was greatest at 20°C (two isolates) or 15–20°C (one isolate). Seedlings of A. thaliana were root-dipped in a conidial suspension, planted, and either placed at 5, 10, 15, or 25°C, or left at 20°C until the onset of senescence, after which some of the plants were placed at 5, 10, 15, or 25°C. The amount of microsclerotia per unit of shoot weight was assessed in relation to isolate and temperature. The optimal temperature for production of microsclerotia was 15–25°C. Two isolates each produced about 10 times more microsclerotia than each of the other four isolates. For these isolates, high R ²_adj.-values of 0.77 and 0.66 were obtained, with temperature and its square as highly significant (P   < 0.001) independent variables. R ²_adj.-values for the other isolates varied between 0.28 and 0.39. Moving plants to different temperatures at the onset of senescence led to microsclerotial densities that were intermediate between densities on plants that had grown at constantly 20°C and plants grown at other temperatures. This suggests that vascular colonization rate and rate of microsclerotium formation are similarly affected by temperature. The senescence rate of plants appeared unimportant except for plants grown at 25°C, which showed the highest amounts of microsclerotia per unit of plant weight in the most rapidly senescing plants.