P.R. Van Hasselt

Long-term chilling of young tomato plants under low light and subsequent recovery : I. Growth, development and photosynthesis.

The influence of unfavourable climatic conditions at the onset of the growth period on chilling-sensitive tomato (Lycopersicon esculentum Mill., cv. Abunda) was studied by exposing young plants to combinations of low temperature and low light (60–100 μmol quanta · m−2 · s−1) for several weeks. When the temperature did not decrease below a critical point (8 ° C) no loss of developmental capacity of the plants was detected. However, while new leaves were readily formed upon return to normal growth conditions (22/18 °C, day/night, in a greenhouse), net accumulation of biomass showed a lag phase of approximately one week. This delay was accompanied by a strong, irreversible inhibition of photosynthesis in the fully expanded leaves which had been exposed to the chilling treatment. When plants were subjected to temperatures below 8 ° C, survival rates decreased after three weeks at 6 ° C and irreversible damage of apical meristematic tissue occurred. Drought-hardening prior to chilling ensured survival at 6 ° C and protected the plants against meristem loss.

Long-term chilling of young tomato plants under low light and subsequent recovery : II. Chlorophyll fluorescence, carbon metabolism and activity of ribulose-1,5-bisphosphate carboxylase/oxygenase.

To identify possible reasons for the persisting impairment of photosynthesis after long-term chilling, young tomato (Lycopersicon esculentum Mill.) plants were exposed to 6–10° C for two weeks under low illumination during the daily light period (60–100 μmol quanta · m−2 · s−1). The time courses of leaf carbohydrate contents, phosphorylated intermediates and chlorophyll-fluorescence parameters were followed. While starch formation was impaired during chilling at 6° C, soluble sugar contents increased from the first day onwards and reached up to eightfold the values found in unchilled plants within two weeks. At 8 and 10° C, a less drastic increase in soluble-carbohydrate contents was observed. During chilling, glucose-6-phosphate and fructose-6-phosphate accumulated up to 16 mM (assuming they are restricted to the cytoplasm). At the same time, non-photochemical quenching of chlorophyll fluorescence had increased and did not return to control values during the first week of recovery. The 3-phosphoglyceric acid/triose phosphate ratio remained nearly unaffected by the chilling treatment, indicating that the assimilatory power of the plants was still high even at the low temperatures. As a consequence of the chilling treatment, ribulose-1,5-biphosphate carboxylase/oxygenase (Rubisco) activity in the chilled leaves was irreversibly decreased. It is suggested that, in addition to a possible (orthophosphate-mediated) feedback inhibition by internal sugar accumulation, the low activity of Rubisco can play a significant role in the strong decrease of photosynthetic capacity during long-term chilling in tomato.

Impact of Suboptimal Temperature on Growth, Photosynthesis, Leaf Pigments and Carbohydrates of Domestic and High-altitude Wild Lycopersicon Species

The impact of near-optimal (25/20 degrees C) and suboptimal (16/14 degrees C) day/night temperatures on growth, photosynthesis, pigment composition and carbohydrate content was compared between domestic and high-altitude wild Lycopersicon species. When related to the relative shoot growth rate (RSGR) measured at optimal temperature, genotypes of the domestic tomato (L. esculentum (L.) Mill. cv. Abunda and cv. Large Red Cherry (LRC) showed a stronger inhibition of RSGR at suboptimal temperature than the high-altitude wild species L. peruvianum Mill. LA 385 and L. hirsutum Humb. & Bonpl. LA 1777. The initiation rare of new leaves was 2.1-fold faster in all species at 25/20 degrees C than at 16/14 degrees C. In contrast to the other genotypes, the leaf area of suboptimally grown Abunda plants was 28 % smaller than the area of leaves that were fully expanded at optimal temperature. In all species, specific leaf area (SLA) at 16/14 degrees C was 17-26 % lower than at 25/20 degrees C. The percentage of leaf dry matter increased in response to growth ar suboptimal temperature. This increase was higher in L. esculentum genotype Abunda (99 %) than in genotype LRC (38 %), and the wild species L. peruvianum (50 %) and L. hirsutum (38 %), which could be attributed to inter- and intra-specific differences in starch accumulation of 16/14 degrees C-grown leaves. Only in both L. esculentum genotypes, net photosynthetic rate at growth irradiance (A(225)) and at light saturation (A(sat)) was 14 to 30 % lower in leaves grown and measured at suboptimal temperature, compared with leaves grown and measured at optimal temperature (25 degrees C). Chlorophyll (Chl) a fluorescence measurements indicated that the decrease of A225 in leaves of suboptimally grown L. esculentum plants was paralleled by a decrease in the quantum yield of photosystem II electron transport (Phi(PSII)), which could be mainly attributed to a decrease in the photochemical quenching component (q(P)). In all species, the nonphotochemical quenching component (NPQ) was 2 to 4-fold higher at 16/14 degrees C. Growth temperature hardly affected Chi content on a leaf area basis, whereas the content of xanthophyll cycle pigments (violaxanthin + antheraxanthin + zeaxanthin) on a Chi basis was ca. 1.5-fold higher in 16/14 degrees C-grown leaves. The epoxidation state of the xanthophyll cycle pool was only slightly lower in suboptimal leaves due to the moderate growth irradiance.

Effect of acclimation to suboptimal temperature on chilling-induced photodamage: comparison between a domestic and a high-altitude wild Lycopersicon species

Plants of a domestic (Lycopersicon esculentum [L.] Mill. cv. Abunda) and a high-altitude wild Lycopersicon species (L. peru6ianum Mill. LA 385) were grown at near-optimal (25:20°C) or suboptimal (16:14°C) temperature. Leaf discs from just fully expanded leaves were exposed to an irradiance of 1000 mmol m 2 s 1 at 5°C for 48 h. The effect of growth temperature on the susceptibility to photoinhibition of photosystem II (PSII) and its recovery, degradation of leaf pigments, chlorophyll (Chl) fluorescence quenching and xanthophyll cycle activity were examined. Leaves of L. esculentum and L. peru6ianum plants grown at optimal temperature, were similarly susceptible to photodamage. Suboptimal-grown leaves of both species showed a higher tolerance to photoinhibition than optimal-grown leaves. In both species, recovery of photoinhibited PSII was more complete in leaves grown at suboptimal than at optimal temperature. In contrast to L. esculentum, suboptimal-grown leaves of L. peru6ianum exhibited faster kinetics of recovery from photoinhibition than optimal-grown leaves. Light-induced degradation of leaf pigments in leaves grown at 16:14°C was 2.3- and 2.7-times slower in L. esculentum and L. peru6ianum, respectively, when compared with leaves grown at 25:20°C. Non-photochemical quenching (NPQ) of Chl fluorescence developed faster in leaves of suboptimalgrown plants, and steady-state levels were20% higher than in leaves of optimal-grown plants of both species. An increased pool size of xanthophyll cycle pigments together with a slightly higher conversion state, resulted in a 1.5- (L. esculentum) or 3-fold (L. peru6ianum) higher maximal zeaxanthin content in suboptimal-, as compared with optimal-grown leaves. These results demonstrate that acclimation to suboptimal temperature increased the capacity to resist chilling-induced photodamage in both the domestic and the high-altitude wild Lycopersicon species. However, the acclimatory response was more pronounced in L. peru6ianum than in L. esculentum, indicating a greater ability of the high-altitude wild species to acclimate its photosynthetic apparatus to suboptimal temperatures.

The effect of low root temperature on growth and lipid composition of low temperature tolerant rootstock genotypes for cucumber

In the framework of research directed to diminish energy consumption of glasshouse cucumber production, three low temperature tolerant rootstock genotypes for cucumber were compared. Firstly, growth at low root temperature of one Cucurbita ficifolia and two Sicyos angulatus genotypes was studied to determine which was the one most suitable as a cucumber rootstock at suboptimal root temperatures. Secondly, differences in lipid composition were examined. Thirdly, lipid composition of these rootstock genotypes was compared with that of low temperature sensitive cucumbers (Cucumis sativus L.; cv. Farbio and CPO inbred lines 79345 and 81354) studied earlier, to determine whether observed differences in lipid composition were consistent with differences in growth at low temperature of related species. Plants were grown at an air temperature of 20-degrees-C d/12-degrees-C n and at constant root temperatures of 20-degrees-C, 16-degrees-C or 12-degrees-C. Although growth decreased at 12-degrees-C for all genotypes, both Sicyos angulatus genotypes were more tolerant to low root temperature than Cucurbita ficifolia. Low root temperature affected root lipid composition only. An increased phospholipid and a markedly lower sterol and sterol ester level resulted in a strongly decreased sterol/phospholipid ratio at 12-degrees-C. Growth at low root temperature was inversely related with the sterol/phospholipid ratio but no correlation between growth and sterol/phospholipid ratio of the rootstock genotypes within one temperature regime was observed. So, this ratio is not an appropriate selection criterion for growth capacity.

Growth under UV-B radiation increases tolerance to high-light stress in pea and bean plants

Pea (Pisum sativum L.) and bean (Phaseolus vulgaris L.) plants were exposed to enhanced levels of UV-B radiation in a growth chamber. Leaf discs of UV-B treated and control plants were exposed to high-light (HL) stress (PAR: 1200 μmol m−2 s−1) to study whether pre-treatment with UV-B affected the photoprotective mechanisms of the plants against photoinhibition. At regular time intervals leaf discs were taken to perform chlorophyll a fluorescence and oxygen evolution measurements to assess damage to the photosystems. Also, after 1 h of HL treatment the concentration of xanthophyll cycle pigments was determined. A significantly slower decline of maximum quantum efficiency of PSII (Fv/Fm), together with a slower decline of oxygen evolution during HL stress was observed in leaf discs of UV-B treated plants compared to controls in both plant species. This indicated an increased tolerance to HL stress in UV-B treated plants. The total pool of xanthophyll cycle pigments was increased in UV-B treated pea plants compared to controls, but in bean no significant differences were found between treatments. However, in bean plants thiol concentrations were significantly enhanced by UV-B treatment, and UV-absorbing compounds increased in both species, indicating a higher antioxidant capacity. An increased leaf thickness, together with increases in antioxidant capacity could have contributed to the higher protection against photoinhibition in UV-B treated plants.

NaCl effects on root plasma membrane ATPase of salt tolerant wheat

Wheat seedlings of a salt tolerant cultivar were grown hydroponically in presence and absence of 100 mM NaCl. Roots were harvested, and the plasma membrane (PM) fraction was purified. PM ATPase required a divalent cations for activity (Mg > Mn > Ca > Co > Zn > Ni > Cu), and it was further stimulated by monovalent cations (K > Rb > NH4 > Li > choline > Cs). The pH optima were 6.0 and 5.6 in absence and presence of 25 mM KCl, respectively. The enzyme was sensitive to vanadate and DCCD but insensitive to azide, oligomycine and nitrate. The enzyme displayed a high preference for ATP but was also able to hydrolyze other nucleotide tri- and diphosphates. The enzyme activity showed a simple Michaelis-Menten kinetics for the substrate Mg2+-ATP in both control and salt exposed roots. The polypeptide patterns of control and salt stressed PM fractions, detected by SDS-PAGE, were very similar. NaCl substantially reduced the PM ATPase specific activity, whereas it had little effect on the apparent Km for Mg2+-ATP. Since the root PM ATPase of salt sensitive and resistant genotypes responded similarly to salinity stress, it seems unlikely that the mechanism of salt tolerance in wheat is primarily based on differences in PM ATPase characteristics.

GROWTH TEMPERATURE AND LIPID-COMPOSITION OF CUCUMBER GENOTYPES DIFFERING IN ADAPTATION TO LOW-ENERGY CONDITIONS

Summary Total lipid, phospholipid and fatty acid composition of leaf and root as well as leaf PG fatty acid composition were determined for two low temperature tolerant inbred lines of cucumber ( Cucumis sativus L.) and a control variety, all grown at two temperature regimes. Hardly any genotypic differences were observed at either temperature regime. At low temperature the degree of unsaturation of fatty acids increased. There was no correlation between fatty acid composition of PG and genotypical differences in growth. However, at low temperature the inbred lines reacted by lowering the amount of PG without changing the proportion of relatively saturated PG fatty acids, while the control variety reacted by lowering the proportion of relatively saturated PG fatty acids at a stable PG content. In general, no valid selection criterion based on lipid composition was found, indicating that lipid composition at present shows no promise as a suitable selection criterion for genetic adaptation to low temperature conditions.

Combined electron-spin-resonance, X-ray-diffraction studies on phospholipid vesicles obtained from cold-hardened wheats. 2: The role of free sterols

The contents of free sterols and phospholipids in leaves of wheat, Triticum aestivum L., cultivars of different frost resistances, as well as the physical state of isolated phospholipids in the presence and absence of sterols, were compared before and after hardening. There was an inverse relationship between the sterol/phospholipid ratio and frost tolerance as a consequence of both a decrease in the free sterol, and an increase in the total phospholipid content. Sterol-sterol interactions were investigated using wide angle X-ray diffraction, while the phase behaviour of phospholipid vesicles was studied using the electron-spin-resonance (ESR) technique. No sterol-sterol interactions at-10° C were detected in vesicles obtained from the hardened most cold-tolerant cultivar (Miranovskaja 808), containing sterols in a ratio (0.08) found in the original lipid extracts. In contrast, when the sterol-phospholipid ratio in the vesicles was set to the level (0.39) found in the extracts of the most sensitive cultivar, Penjamo 62, the appearance of sharp reflexion rings at 4.5·10-1, 4.8·10-1 and 5.0·10-1 nm indicated strong sterol-sterol interactions. The temperatures for the onset of phase separation for vesicles of identical sterol/phospholipid ratios found in lipid extracts of hardened Miranovskaja 808 were almost the same as those measured in purified phospholipids (-15 vs.-16° C). In contrast, the temperature for the onset of phase separation of vesicles with a sterol/phospholipid ratio characteristic of hardened Penjamo 62 was shifted upwards (from-6 to-2° C). Phase separation was not completed in the vesicles of Miranovskaja 808 in the temperature range scanned (-30° C) but was shifted from-22 to-18° C in the presence of sterols in the case of Penjamo 62. The results are discussed in terms of the composition and physical state of membranes in relation to survival at freezing temperatures.

Short-term exposure to atmospheric ammonia does not affect frost hardening of needles from three- and five-year-old Scots pine trees

The effect of atmospheric ammonia on frost hardening of needles from 3- and 5-year-old Scots pine trees was investigated. Trees were exposed to various concentrations of NH(3) during different hardening stages under laboratory conditions and in experiments with open-top chambers under a natural hardening regime during winter. Under laboratory conditions, exposure to 250 nL.L(-1) NH(3) did nor affect frost tolerance, whereas hardening was increased in needles of trees exposed to 500 nL.L(-1) NH(3). In trees that were hardened in open-top chambers, frost hardiness was not influenced by exposure to atmospheric ammonia up to 106 nL.L(-1) Needle nitrogen concentrations in the open-top chambers increased with atmospheric NH(3) concentration. Needle concentrations of potassium, calcium, magnesium and phosphorus were not affected by NH(3) and were within a normal range. The role of the nutrient status of the needles on the response of frost hardening of Scots pine needles towards exposure to atmospheric ammonia is discussed.