Effects of Desiccation on some Activated Oxygen Processing Enzymes and Anti-Oxidants in Mosses

The possibility that desiccation tolerance in mosses may bedependent on an ability to process species of activated oxygenwas investigated using the tolerant sand-dune moss Tortula ruraliformis(Besch.) Grout and the sensitive minerotrophic flush speciesDicranella palustris (Dicks.) ex. E. F. Warb (D. squarrosa (Starke)Schp.). Mosses were desiccated in low and high irradiance andresponses of these plants compared to those of dark-desiccatedand hydrated control plants. Both desiccated and undesiccatedplants of T. ruraliformis had a higher superoxide dismutase(SOD) activity than D. palustris, but had similar, or loweractivities of the chloroplastic H₂O₂-processing enzymes peroxidaseand ascorbate peroxidase. In T. ruraliformis, desiccation inboth light and dark led to a significant increase in SOD activity,but did not consistently stimulate the activities of peroxidaseand ascorbate peroxidase. In D. palustris, desiccation in combinationwith irradiance led to a decrease in peroxidase activity, buthad little affect on the activities of other activated oxygen-processingenzymes. Catalase, an extra-chloroplastic enzyme, was up to7-fold more active in hydrated T. ruraliformis than in D. palustris,but desiccation resulted in significant decreases in the activityof this enzyme in both species. Regardless of irradiance level,there was a depletion of the anti-oxidant ascorbic acid in bothspecies when desiccated. Only in T. ruraliformis was there asynthesis of     

Analysis of the decrease in photosynthesis on desiccation of mosses from xeric and hydric environments

Three moss species [ Tortula ruraliformis (Besch.) Grout. Bryum pseudotriquetrum (Hedw.) Schaegr and Dicranella palustris (Dicks.) Crund. ex. E. F. Warb. ( D. squarrosa (Starke) Schp.] collected from a range of habitats differing in water availability were desiccated in controlled conditions. All species became photosynthetically inactive when dried below a water content of 100–200% dry weight. Only Tortula ruraliformis , a moss from arid sand dunes. was able to recover fully to pre-desiccated rates of photosynthetic electron transport during subsequent rehydration. The rate of recovery was influenced by irradiance during desiccation. Mosses from hydric habitats showed some resumption of photosynthetic electron transport (following rehydration) if dried in the dark. but did not do so if dried even in low light. In these circumstances the mosses showed evidence of lasting photoinhibition of photosynthesis after rehydration. The desiccation-tolerant T. ruraliformis became significantly photoinhibited only when continually exposed to high irradiance (1200 μmol m⁻² s⁻¹) in the hydrated state. If allowed to desiccate whilst exposed to high irradiance this species showed less evidence of photoinhibition after rehydration, and was not at all affected by desiccation in low irradiance. Photon flux absorption in dry moss was 50–60% less than that in hydrated moss as a result of leaf curling. However, the reduction in absorption of photosynthetically active radiation cannot account for the total loss of photosynthetic oxygen evolution and variable chlorophyll fluorescence observed in the desiccated mosses.     

ABA Increases the Desiccation Tolerance of Photosynthesis in the Afromontane Understorey Moss Atrichum androgynum

The effect of pretreatment with abscisic acid (ABA) on the physiologyof the moss Atrichum androgynum during a desiccation–rehydrationcycle was examined. During rehydration following desiccationfor 16 h, net CO₂fixation recovered much more slowly than photosystemII (PSII) activity, conditions conducive to the formation ofreactive oxygen species (ROS) in the photosynthetic apparatus.Pretreatment with ABA increased the rate of recovery of photosynthesisand PSII activity, and also doubled non-photochemical quenching(NPQ). Increased NPQ activity will reduce ROS formation, andmay explain in part how ABA hardens the moss to desiccation.In ABA-pretreated, but not untreated mosses, desiccation significantlyincreased the concentration of soluble sugars. Sugar accumulationmay promote vitrification of the cytoplasm and protect membranesduring desiccation. Starch concentrations in freshly collectedA. androgynum were only approx. 40 mg g^-1dry mass; they roseslightly during desiccation but were only slightly affectedby ABA pretreatment. ABA did not reduce chlorophyll breakdownduring desiccation. Copyright 2001 Annals of Botany Company Moss, desiccation, abscisic acid, photosynthesis, chlorophyll fluorescence     

Radical Formation and Accumulation In Vivo,In Desiccation Tolerant and Intolerant Mosses

Water loss in a desiccation-sensitive moss resulted in destruction of chlorophyll, loss of carotenoids and increased lipid peroxidation, indicating the presence of damaging forms of activated oxygen. These effects were exaggerated when the plants were desiccated at high light intensities. During water-deprivation there was a build up of a free radical, detected in vivo, with a close correlation between molecular damage and radical accumulation. In contrast, in a desiccation-tolerant moss there was almost no indication of molecular (oxidative) damage. However a stable radical similar in type and concentration to that found in the desiccation-sensitive species accumulated, particularly under high irradiances. The stable radical appears to be one of the end-products of a process initiated by environmental stress, desiccation and high irradiance: its association with molecular damage depending on the degree to which the species is tolerant of desiccation. Identification of the radical in intact tissue from EPR and ENDOR studies, suggests that this is not a short-lived proxy-radical but instead is relatively stable and carbon-centred.     

Recalcitrant Seed Storage Physiology in Three Aquatic Grasses (Zizania palustris, Spartina anglica and Porteresia coarctata)

The moisture content/probit viability relationship for storedseeds of Zizania palustris L. and Spartina anglica C. E. Hubbardwas linear and independent of the rate of embryo drying. Theseresults provide firm evidence of recalcitrant storage physiologyin these taxa. Preliminary tests strongly suggest that freshseeds of Porteresia coarctata (Roxb.) Tateoka are also intolerantof desiccation In Z. palustris apparent differences in desiccation tolerancebetween individuals can be partly explained by wide variationin individual embryo moisture contents during desiccation. Long-termstorage experiments in solutions of polyethylene glycol 6000(PEG) suggest that the actual variation in desiccation toleranceis confined to a narrow range of embryo water potentials inthe range –2 to –3 MPa. Despite the presence of prolonged dormancy in seeds of Z. palustrisand S. anglica there is no evidence of a significant effectof dormancy or storage period (up to the point of visible germination)on the limits of desiccation tolerance Aquatic grasses, seeds, storage, desiccation intolerance     

Radical Formation and Accumulation In Vivo, In Desiccation Tolerant and Intolerant Mosses

Water loss in a desiccation-sensitive moss resulted in destruction of chlorophyll, loss of carotenoids and increased lipid peroxidation, indicating the presence of damaging forms of activated oxygen. These effects were exaggerated when the plants were desiccated at high light intensities. During water-deprivation there was a build up of a free radical, detected in vivo, with a close correlation between molecular damage and radical accumulation. In contrast, in a desiccation-tolerant moss there was almost no indication of molecular (oxidative) damage. However a stable radical similar in type and concentration to that found in the desiccation-sensitive species accumulated, particularly under high irradiances. The stable radical appears to be one of the end-products of a process initiated by environmental stress, desiccation and high irradiance: its association with molecular damage depending on the degree to which the species is tolerant of desiccation. Identification of the radical in intact tissue from EPR and ENDOR studies, suggests that this is not a short-lived proxy-radical but instead is relatively stable and carbon-centred.     

Changes in chlorophyll a fluorescence, photosynthetic CO2 assimilation and xanthophyll cycle interconversions during dehydration in desiccation-tolerant and intolerant liverworts

The interactions among water content, chlorophyll a fluorescence emission, xanthophyll interconversions and net photosynthesis were analyzed during dehydration in desiccation-tolerant Frullania dilatata (L.) Dum. and desiccation-intolerant Pellia endiviifolia (Dicks) Dum. Water loss led to a progressive suppression of photosynthetic carbon assimilation in both species. Their chlorophyll fluorescence characteristics at low water content were: low photosynthetic quantum conversion efficiency, high excitation pressure on photosystem II and strong non-photochemical quenching. However, dissipation activity was lower in P. endiviifolia and was not accompanied by a rise in the concentration of de-epoxidised xanthophylls as F. dilatata. The photosynthetic apparatus of F. dilatata remained fully and speedily recuperable after desiccation in as indicated by the restoration of chlorophyll fluorescence parameters to pre-desiccation values upon rehydration. A lack of recovery upon remoistening of P. endiviifolia indicated permanent and irreversible damage to photosystem II. The results suggest that F. dilatata possesses a desiccation-induced zeaxanthin-mediated photoprotective mechanism which might aid photosynthesis recovery when favourable conditions are restored by alleviating photoinhibitory damage during desiccation. This avoidance mechanism might have evolved as an adaptative response to repeated cycles of desiccation and rehydration that represent a real threat to photosynthetic viability. Received: 12 January 1998 / Accepted: 14 July 1998     

Studies on Protein Synthesis in Tortula ruralis: POLYRIBOSOME REFORMATION FOLLOWING DESICCATION

Desiccation of Tortula ruralis was achieved rapidly by placingthe moss on the laboratory bench, or more slowly by placingit in desiccators with atmospheres of high relative humidities.Unlike the rapidly desiccated moss, the slower desiccated mossretained no polyribosomes in the dehydrated state, althoughpolyribosome reformation and protein synthesis resumed on reintroductionof the moss to water. Protein synthesis commenced on rehydrationof the slower desiccated moss at a greater rate than on rehydrationof the faster desiccated moss. A lack of correlation betweenendogenous ribonuclease activity and polyribosome levels extractedfrom the moss suggests that the observed reduction in polyribosomesduring desiccation was not due to their degradation but wasmore likely a consequence of stress-induced restriction on reinitiationof existing messenger RNA. The observed protein synthesis onrehydration of the moss was largely independent of any priorRNA synthesis.     

Mate limitation does not explain the lack of capsules in a Pseudocalliergon trifarium population

Abstract

(1) Net assimilation and respiration rates were measured at intervals after re-moistening, following various periods of desiccation, in Hookeria lucens, Hylocomium splendens, Neckera crispa, Plagiochila spinulosa, Plagiothecium undulatum, Rhacomitrium lanuginosum, Rhytidiadelphus loreus, Saccogyna viticulosa, Scorpiurium circinatum and Tortula ruraliformis.

(2) Rhacomitrium lanuginosum was extremely resistant, recovering apparently normally after 239 days' desiccation at 32% R.H.; Plagiochila spinulosa and Hookeria lucens were the most sensitive.

(3) Rhacomitrium lanuginosum and Tortula ruraliformis were most quickly damaged at the highest humidity (76%) and Plagiothecium undulatum at the lowest humidity tested (32%).

(4) Saccogyna viticulosa and Scorpiurium circinatum combined relatively rapid impairment and slow recovery of assimilation with the capacity to survive long dry periods.

(5) Dark respiration was relatively slow (commonly c. 5–20% of net assimilation). It usually showed a slight initial stimulation and a longer-term build-up following moderate or prolonged desiccation.

(6) Desiccation responses of bryophytes can be characterized in terms of parameters defining rate of loss of photosynthetic capacity with desiccationtime, rate of recovery after short periods of desiccation, and survival.     

Effects of Desiccation on Phosphorus and Potassium Acquisition by a Desiccation-tolerant Moss and Lichen

The hypothesis that desiccation-tolerant mosses and lichensmay be more responsive to nutrient inputs accompanying intermittentdesiccation than mesophytic forest species was investigatedemploying species from semi-arid grassland in Hungary. Shootapices of the moss Syntrichia ruralis and marginal lobes ofthe lichen Cladonia convoluta were maintained for 7 weeks undercontrolled conditions. They were cultivated with or withouta weekly application of the major inorganic macronutrients,and either under constant hydration or with one or two 24 hperiods of desiccation each week. Growth of S. ruralis was stimulatedby nutrient additions, but lower weight increments were achievedwith increasing frequency of desiccation. All samples of thelichen showed positive growth, and no significant treatmenteffects were detected. A large net uptake of P occurred in nutrient-treatedmaterial of both species that was unaffected by the impositionof desiccation treatments. A smaller net uptake of K into theintracellular fraction was also observed when nutrients wereapplied, but in the moss this was against a baseline of decreasingK content. In contrast, more of the original K content was retainedin C. convoluta. In neither species was any clear evidence foundfor inhibition of nutrient uptake by the desiccation episodes.It is suggested that the lack of growth response in the lichenarises from an inability to bring together the additional nutrients,presumably mainly absorbed by the mycobiont, with photosynthateproduced by the photobiont. Copyright 2000 Annals of BotanyCompany Cladonia convoluta, Syntrichia ruralis, desiccation tolerance, mineral nutrition, phosphorus, potassium     

Desiccation of Phaseolus vulgaris Seeds During and Following Germination, and its Effect Upon the Translatable mRNA Population of the Seed Axes

Misra, S. and Bewley, J. D. 1986. Desiccation of Phaseolus vulgansseeds during and following germination, and its effect uponthe translatable mRNA population of the seed axes.—J.exp. BoL 37: 364–374. After imbibition and germination, seeds of P. vulgaris passfrom a stage where they are insensitive to desiccation to astage where they are sensitive. Desiccation of seeds duringthe sensitive stage results in an almost total impairment ofprotein synthesis upon subsequent rehydration. Seeds desiccatedduring the desiccation-tolerant stage, however, resume proteinsynthesis at almost control levels. The protein patterns obtained following in Vitro translationof bulk RNA from fresh imbibed, desiccated, and desiccated-rehydratedseed axes were qualitatively similar at 5 HAI (the desiccation-tolerant stage). The drying treatment resulted in increasedintensity of extant proteins at 5 and 12 HAI. At 12 HAI (thetransition stage between the desiccation-tolerant and desiccation-intolerantphases) desiccation and subsequent rehydration triggered synthesisof a unique set of proteins-the rehydration proteins. At 20HAI (the desiccation-intolerant stage) desiccation resultedin an overall decline in the intensity of proteins synthesizedin vitro. Also the rehydration proteins were not synthesizedin response to a drying and rehydration treatment at this time. Key words: Seed germination, desiccation, mRNA, in vitro translation, Phaseolus vulgaris     

Responses of the resurrection plant <Emphasis Type="Italic">Haberlea rhodopensis</Emphasis> to high irradiance

The effect of high irradiance (HI) during desiccation and subsequent rehydration of the homoiochlorophyllous desiccation-tolerant shade plant Haberlea rhodopensis was investigated. Plants were irradiated with a high quantum fluence rate (HI; 350 μmol m⁻² s⁻¹ compared to ca. 30 μmol m⁻² s⁻¹ at the natural rock habitat below trees) and subjected either to fast desiccation (tufts dehydrated with naturally occurring thin soil layers) or slow desiccation (tufts planted in pots in peat-soil dehydrated by withholding irrigation). Leaf water content was 5 % of the control after 4 d of fast and 19 d of slow desiccation. Haberlea was very sensitive to HI under all conditions. After 19 d at HI, even in well-watered plants there was a strong reduction of rates of net photosynthesis and transpiration, contents of chlorophyll (Chl) and carotenoids, as well as photosystem 2 activity (detected by the Chl fluorescence ratio R_Fd). Simultaneously, the blue/red and green/red fluorescence ratios increased considerably suggesting increased synthesis of polyphenolic compounds. Desiccation of plants in HI induced irreversible changes in the photosynthetic apparatus and leaves did not recover after rehydration regardless of fast or slow desiccation. Only young leaves survived desiccation.     

Effects of Intermittent Desiccation on Nutrient Economy and Growth of Two Ecologically Contrasted Mosses

The mossesBrachythecium rutabulum (Hedw.) B., S. & G. andPseudoscleropodiumpurum (Hedw.) Fleisch. were cultivated for more than 50 d ina growth cabinet with or without weekly drying interludes of24 h. Some plants also received applications of a dilute NPKnutrient solution at weekly intervals. The continuously hydratedplants showed appreciably more biomass production than thosereceiving intermittent desiccation. Desiccation led to somebleaching of the green tissues inB. rutabulum but not inP. purumwhich appeared more desiccation-tolerant. NPK addition causeda further significant growth stimulation in continuously hydratedplants, but not in intermittently desiccatedB. rutabulum. Pseudoscleropodiumpurum showed NPK-induced growth stimulation even when intermittentlydesiccated. Net uptake of N was similar in desiccated and hydratedplants in both species. Considerable net uptake of P and K⁺occurredin continuously wetB. rutabulum , but uptake was much reducedin intermittently desiccated plants. Net uptake of P and K⁺byP.purum was similar in desiccated and hydrated samples. IntracellularK⁺, leaked from the cells during the desiccation treatment,was retained by cation exchange on the negatively charged cellwalls in both species. Levels of intracellular K⁺and Mg²⁺inthe new growth were maintained at the expense of the pool ofexchangeable cations. The growth stimulation and the net uptakeof nutrients under intermittent desiccation was greatest whenthe NPK application was made at the start of rehydration, possiblybecause of accentuated uptake in the early stages of recovery.The results support the hypothesis thatP. purum has a lowernutrient requirement thanB. rutabulum and highlight the importanceof continuous hydration for the latter's more productive plantlife strategy. The data also show that considerable new growthof bryophyte tissues is possible without additional nutrientabsorption. Brachythecium rutabulum ; Pseudoscleropodium purum ; mineral nutrition; desiccation; solute leakage; plant life strategies     

Desiccation and Free Radical Mediated Changes in Plant Membranes

Senaratna, T., McKersie, B. D. and Borochov, A. 1987. Desiccationand free radical mediated changes in plant membranes.—J.exp. Bot. 38: 2005-2014. In vitro treatment of microsomal membranes from the axes ofsoybean (Glycine max (L.) Merr.) seeds with free radicals simulatesthe type of membrane injury observed following a lethal desiccationstress—the accumulation of free fatty acids in the membranebilayer, the loss of lipid-P, and the formation of gel phasedomains. The major phospholipids in the microsomal fractionwere phosphatidylcholine, phosphatidylethanolamine and phosphatidylinositol.Although these treatments induced an extensive loss of totalphospholipid from the microsomal fraction following desiccation,the ratio of the major phospholipids remained unchanged. Neitherlysophosphatides nor phosphatidic acid accumulated in the fraction,but free fatty acid levels increased. Therefore, cleavage ofboth acyl chains from the phospholipid molecule occurred followingdesiccation of the axes and in vitro free radical treatmentof the membrane. Both treatments also promoted formation of gel phase domainsas shown by wide angle x-ray diffraction and increased microviscosityas determined by the fluorescent probe, DPH (1,6-diphenyl-1,3,5-hexatriene).This could be simulated in liposomes prepared from the totalmicrosomal lipid fraction by the addition of saturated freefatty acids (16:0 and 18:0) at the levels observed followingstress. In contrast, the addition of unsaturated fatty acidsperturbed the bilayer and reduced microviscosity. The inclusionof both saturated and unsaturated free fatty acids as observedin vivo promoted a response similar to that observed with onlythe addition of the saturated free fatty acids. Desiccation of the axes also promoted a loss of microsomal protein,which was recovered in the 165 000 x g supernatant, and an apparentloss of thiol groups from the membrane as determined by a thiolspecific fluorescence probe, dansylaziridine. This loss of thiolgroups could also be simulated by exposure of the membranesto gamma irradiation, which was used as a non-enzymatic sourceof free radicals. Collectively, these data support the hypothesisthat membrane disassembly following desiccation stress is mediatedby a free radical mechanism, and that the consequent de-esterificationof membrane phospholipid and accumulation of saturated freefatty acids alter the physical properties of the membrane. Key words: Membrane microviscosity, membrane fluidity, free fatty acids     

Predicted Patterns of Photosynthetic Production in Maritime Antarctic Mosses

The microclimate of a moss turf of Polytrichum alpestre Hoppeand a moss carpet of Drepanocladus uncinatus (Hedw.) Warnst.is described for two sites in the maritime Antarctic. Thesemicro-climatic data, together with some additional data, arethen used in the solution of multiple regression equations describingnet photosynthetic response to variation in radiant flux densityand temperature as measured in Antarctic material of the twomosses grown under controlled conditions in the U.K. Predictionsare made of daily and seasonal courses of net photosynthesisat the Antarctic field site. The basis of acclimation of netphotosynthesis to growth temperatures through a season is discussedand the physiological basis of growth under snow is examined. Polytrichum alpestre. Hoppe, Drepanocladus uncinatus (Hedw.) Warnst., mosses, Bryophyta photosynthesis, productivity, Antarctic     

Mineral Depletion and Leaf Senescence in Soya Bean as Influenced by Foliar Nutrient Application During Seed Filling

Senescence, as judged by the time courses of leaf lamina photosynthesis,soluble protein and chlorophyll contents, was studied in relationto mineral redistribution in field-grown soya beans [Glycinemax (L.) Merr] to investigate the hypothesis that the depletionof nutrients m the leaves by the developing seeds is the causeof soya bean senescence. A mineral nutrient solution was appliedto the canopy during the seed-filling period, and the effectson senescence and mineral depletion of the leaves were determinedin three cultivars, at two leaf positions, weekly from beginningof seed filling through physiological maturity. The onset of senescence occurred shortly after the beginningof rapid seed filling Photosynthetic rate declined about 60per cent within 3 weeks. Protein dropped by 52 per cent andchlorophyll by 48 per cent over the same period. Foliar nutrient application, at a rate previously shown to givesignificant yield increases in soya beans, increased the concentrationsof N, P and K in the leaf laminae, but tended only to delaytheir decline and failed to either delay the onset or alterthe course of senescence. The results of this experiment seem to indicate that, undernormal growth conditions, the events of senescence in the soyabean are not causally related to the N, P or K concentrationsof the leaf laminae Glycme max (L.) Merr., soya bean, nitrogen, phosphorus, potassium, leaf protein, chlorophyll, photosynthesis, foliar nutrient application, mineral depletion, leaf senescence     

Respiration in Relation to Adenosine Triphosphate Content during Desiccation and Rehydration of a Desiccation-tolerant and a Desiccation-intolerant Moss

O₂ consumption by the desiccation-tolerant moss Tortula ruralis and the desiccation-intolerant Cratoneuron filicinum increased markedly during the latter stages of desiccation. ATP content of the mosses during desiccation was not correlated with O₂ consumption, but was influenced by the rate at which the mosses lost water. The more rapid the water loss, the more ATP that was present in the dry mosses. The pattern of O₂ consumption on rehydration also was influenced by the previous rate of desiccation. After rapid desiccation of T. ruralis O₂ consumption upon rehydration was considerably elevated, and for up to 24 hours. After very slow desiccation the elevation was small and brief. Normal O₂ consumption did not occur in C. filicinum after rapid desiccation, but did so within a few hours of rehydration after slower speeds of drying. ATP levels in T. ruralis returned to normal within 5 to 10 minutes of rehydration. In C. filicinum, increases in ATP were closely correlated with O₂ consumption. These observations are considered to be related to differential damage caused to mitochondria and to cellular integrity by different speeds of water loss. The desiccation-tolerant moss appears to be able to repair the severe damage imposed by rapid desiccation whereas the desiccation-intolerant moss cannot.     

Leaf Characteristics and Net Gas Exchange of Diploid and Autotetraploid Citrus

The effect of tetraploidy on leaf characteristics and net gasexchange was studied in diploid (2x ) and autotetraploid (4x) ‘Valencia’ sweet orange (Citrus sinensis (L.)Osb.) and ‘Femminello’ lemon (Citrus limon (L.)Burm. f.) leaves. Comparisons between ploidy levels were madeunder high irradiance (I) in a growth chamber or low total Iin a glasshouse. Tetraploids of both species had thicker leaves,larger mesophyll cell volume and lower light transmittance thandiploids regardless of growth I. Mesophyll surface area perunit leaf area of 2x leaves was 5–15% greater than on4x leaves. Leaf thickness and mesophyll cell volume were greaterin high I leaves than low I leaves. In high I, average leafarea was similar for 2x and 4x leaves, whereas in low I it was30% greater in 4x than in 2x leaves. Nitrogen and chlorophyllconcentration per cell increased with ploidy level in both growthconditions. The ratio of chlorophyll a:b was 25% greater in2x than in 4x leaves. When net CO₂assimilation rate (A_CO2) wasbased on leaf area, 4x orange leaves had 24–35% lowerA_CO2than their diploids. There were no significant differencesin A_CO2between 2x and 4x orange or lemon leaves when expressedon a per cell basis. Overall, lower A_CO2per unit leaf area oftetraploids was related to increase in leaf thickness, largermesophyll cell volume, the decrease in mesophyll area exposedto internal air spaces, and the lower ratio between cell surfaceto cell volume. Such changes probably increased the resistanceto CO₂diffusion to the site of carboyxlation in the chloroplasts. Cell volume; chlorophyll; irradiance; leaf thickness; nitrogen; photosynthesis; ploidy; Citrus limon ; C. sinensis ; ‘Valencia’ sweet orange; ‘Femminello’ lemon     

An Intermediate Category of Seed Storage Behaviour?II. EFFECTS OF PROVENANCE, IMMATURITY, AND IMBIBITION ON DESICCATION-TOLERANCE IN COFFEE

Seeds of four cultivars of arabica coffee (Coffea arabica L.)received from three continents survived desiccation to between7-2% and 11-3% moisture content (wet basis), i.e. to seed waterpotentials of –90 MPa to –150 MPa, but further desiccationreduced germination (criterion, normal seedling development)in all seed lots. Only a few individuals from four of the lotsgerminated after being dried to 4–5% moisture content.Differences in desiccation sensitivity were apparent among lotswithin each cultivar. Desiccation sensitivity in these lotswas similar to that observed in seeds of orthodox species whichhave begun to germinate. Seeds extracted from fruits of intermediatematurity (yellow) were able to tolerate greater desiccationthan those from either ripe (red) or immature (green) fruits.Imbibed storage increased desiccation sensitivity. The resultsare compatible with the view that arabica coffee seeds are unableto tolerate extreme desiccation because germination has beeninitiated before harvest.     

Embryo Water Status and Survival in the Recalcitrant Species Quercus robur L: Evidence for a Critical Moisture Content

The responses of Q. robur L. fruits, seeds and embryonic axesto desiccation are characterized and discussed in relation tocurrent knowledge of recalcitrant seed behaviour. A relationshipbetween viability and seed moisture content is described. Thisrelationship was unaffected by rate of drying, year of harvestor presence of the pericarp. Desiccation sensitivity did notincrease with storage. Excised embryonic axes survived to lower moisture contents thanintact seeds. However, in the intact seed, loss of viabilityappeared to be determined by a critical moisture content inthe cotyledons. Consequently, the level of desiccation tolerancewithin the axis attached to cotyledons was not determined byaxis drying rate. A link is drawn between the difference in the desiccation toleranceof embryonic axes and of cotyledons, and estimates of theirdifferent levels of matrix-bound water. The results presentedare consistent with a critical moisture content for survivalwhich is determined by the loss of all free cellular water.This hypothesis takes account of the differential desiccationsensitivity of seed tissues and differences in desiccation tolerancebetween species.