Abstract
Temperature and daylength act as environmental signals that determine the length of the growing season in boreal evergreen conifers. Climate change might affect the seasonal development of these trees, as they will experience naturally decreasing daylength during autumn, while at the same time warmer air temperature will maintain photosynthesis and respiration. We characterized the down-regulation of photosynthetic gas exchange and the mechanisms involved in the dissipation of energy in Jack pine (Pinus banksiana) in controlled environments during a simulated summer-autumn transition under natural conditions and conditions with altered air temperature and photoperiod. Using a factorial design, we dissected the effects of daylength and temperature. Control plants were grown at either warm summer conditions with 16-h photoperiod and 22 degrees C or conditions representing a cool autumn with 8 h/7 degrees C. To assess the impact of photoperiod and temperature on photosynthesis and energy dissipation, plants were also grown under either cold summer (16-h photoperiod/7 degrees C) or warm autumn conditions (8-h photoperiod/22 degrees C). Photosynthetic gas exchange was affected by both daylength and temperature. Assimilation and respiration rates under warm autumn conditions were only about one-half of the summer values but were similar to values obtained for cold summer and natural autumn treatments. In contrast, photosynthetic efficiency was largely determined by temperature but not by daylength. Plants of different treatments followed different strategies for dissipating excess energy. Whereas in the warm summer treatment safe dissipation of excess energy was facilitated via zeaxanthin, in all other treatments dissipation of excess energy was facilitated predominantly via increased aggregation of the light-harvesting complex of photosystem II. These differences were accompanied by a lower deepoxidation state and larger amounts of beta-carotene in the warm autumn treatment as well as by changes in the abundance of thylakoid membrane proteins compared to the summer condition. We conclude that photoperiod control of dormancy in Jack pine appears to negate any potential for an increased carbon gain associated with higher temperatures during the autumn season.
Highlights
Temperatureand daylengthact as environmentalsignalsthatdeterminethe lengthof the growingseasonin borealevergreen conifers.Climatechange might affectthe seasonal developmentof these trees,as they will experiencenaturallydecreasing daylengthduringautumn,while at the same time warmerair temperaturewill maintainphotosynthesisand respiration.We characterizedthe down-regulationof photosyntheticgas exchangeand the mechanismsinvolved in the dissipationof energy in Jack pine (Pinus banksianai)n controlled environmentsduring a simulated summer-autumntransitionunder natural conditionsand conditionswith alteredair temperatureand photoperiod.Using a factorialdesign, we dissectedthe effectsof daylengthand temperatureC. ontrolplantswere grownat eitherwarmsummerconditionswith 16-hphotoperiodand22°Cor conditionsrepresentinga cool autumnwith 8 h/7°C
As evergreen conifers keep their needles during winter,the cold hardeningprocessinvolves a reorganization of the photosyntheticmachinery(Ensminger et al, 2004),as the needles retaina substantialamount of chlorophyll(Chi) and proteins and thereforecontinue to absorblight.Absorptionof lightunderwinter conditions can cause photooxidative damage of the photosynthetic apparatus, created by an imbalance between the photochemical generation of electrons and theirdiminishedutilizationdue to decreasingsink capacity,i.e. the down-regulationof metabolismand growth in winter (Oquistand Huner,2003).Tomaintain the balance between light capture and energy utilizationunderconditionswith alteredsinkcapacity, energy flow and photosynthesishave to be adjusted by a processdefinedas photostasis(OquistandHuner, 2003)
Photosynthesisthereforeappearsto be temperaturedependentbut is stronglyinfluencedby thelengthof thephotoperiod.In conifers,shorteneddaylength acts as a signal for the inductionof terminalbudsetand thecessationof growth (Repoetal, 2001).Incombinationwithlow temperature, this is an earlyautumnprerequisiteto induce freezing resistanceand thereforean importantmechanism to prepare for the harsh winter conditions in northern forest environments(Bigraset al., 2001)
Summary
Needle level gas exchangewas performedon samples of seedlingsof all fourtreatments(Fig.).At 1,000 /xmolphotonsm~2s"1,we observedthehighestrateof light-saturatednet assimilation (Asat)in the summer conditionswith 16-hphotoperiodand 22°C(LD/HT) treatment. In the natural autumn control with 8-h photoperiod/7°C(SD/LT),Asatwas decreasedby 19%. Withinthe othertwo treatments,Asatwas considerably lower, with the warm autumn conditions with 8-h photoperiod/22°C treatment(SD/HT) showing 41% lower values than LD/HT and 28%lower values than SD/LT. This pattern was observed when net assimilationwas measuredat growth light conditions (Fig. 1). Low temperatureimposed an additionaleffecton darkrespiration,beingresponsible fora furtherdecreasein cold summerconditionswith 16-h photoperiod/7°C (LD/LT) and SD/LT needles. These results are valid when data are expressed on a fresh weight basis because the ratio of fresh weight to leaf area (grams per meter) only changes minimallybetweentreatmentsoverthe durationof the. Experiment(LD/HT,369 ± 40;SD/HT, 342 ± 19;LD/ LT,341 ± 21;SD/LT,366 ± 37)
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