International Congress of Ecology (VIII INTECOL),Abstructs
Onno Muller, Kouki Hikosaka, Tadaki Hirose, and MarinusJ.A. Werger
Utrecht University, The Netherlands and Tohoku University,Japan
In temperate regions the temperature varies widely during theyear. Also the light conditions are changing with the seasonsespecially in the understory of a deciduous forest. Thephotosynthetic apparatus can acclimate to changing light andtemperature to improve its performance. It is well known that shadeleaves invest more nitrogen in the light-harvesting chl-proteincomplexes while sun leaves invest more in the Calvin cycle enzymes.Recently knowledge is accumulating on the temperature acclimation:several studies have suggested the similarity between photosyntheticacclimation to high light conditions and that to low temperatures.However, the implications under natural seasonal environments arestill unclear, particularly in nature when light and temperaturechange simultaneously in a year. This makes separating the twoeffects important. In the present study, to separate the effect oflight and temperature on the photosynthetic apparatus in naturalseasonal environment, we studied plants growing under different lightregimes. We conducted field measurements of photosyntheticcharacteristics of an evergreen understory shrub, Aucuba japonica, inNorthern Japan. Occurring both in gaps and under deciduous andevergreen forests, Aucuba experiences different light climates duringthe year. Photosynthetic characteristics, such as photosyntheticcapacity, leaf nitrogen content,ribulose-bisphosphate-carboxylase-oxygenase and chlorophyll a/bratio, were in general well correlated with both light intensity andtemperature that the leaves experienced. Multiple regressionsuggested that change in photosynthetic components is dependent ontemperature and light. We conclude that the interaction betweentemperature and light is responsible for the acclimation of thephotosynthetic apparatus to seasonal environments.
CO2 response of photosynthesis in Polygonumcuspidatum in seasonal environment
Yusuke Onoda, Kouki Hikosaka, and Tadaki Hirose
Tohoku University, Japan
Most C3 plants increase photosynthesis with atmospheric CO2elevation. There are several steps that limit the photosynthetic rateacross CO2 concentration. At the present low CO2, photosynthesis isoften limited by the Rubisco activity (key enzyme of photosynthesis),while at high CO2 it is limited by the RuBP regeneration activity(and by triose-P utilization). Since the dependence on CO2 of theRubisco-limited rate is stronger than that of the RuBPregeneration-limited rate, the degree of photosynthetic enhancementwith CO2 elevation depends on whether the rate is limited by theRubisco capacity or the RuBP regeneration capacity. From literaturesurvey on 109 species, Wullschleger (1993) suggested that the balancebetween the Rubisco capacity and RuBP regeneration capacity isconserved irrespective of species and growth conditions. Recentstudies, however, have shown that the temperature regimesignificantly affects the balance between the Rubisco capacity andRuBP regeneration capacity. We hypothesized that seasonal change intemperature could alter the balance and thus the degree ofphotosynthetic enhancement with elevated CO2. We grew plants outdoorsin OTC at two CO2 levels (370 and 700 ppm). Photosyntheticmeasurements were conducted on young fully expanded leaves in summer(August) and autumn (October). Since we were primarily interested inthe capacity of photosynthesis at different seasons, we measuredphotosynthesis at a constant temperature, 25oC. Light-saturatedphotosynthetic rates at growth CO2 increased in plants grown atelevated CO2 by 34% in August, and by 50% in October. Comparisons ofphotosynthetic rates at common CO2 indicate that down-regulation ofthe photosynthetic capacity occurred in elevated CO2-grown plants,and that the degree of down-regulation was similar in the twoseasons. As was expected, the balance between the Rubisco capacityand RuBP regeneration capacity changed seasonally. In October leaves,the RuBP regeneration capacity was increased relative to the Rubiscocapacity. This ameliorated the limitation of RuBP regeneration, andthe Rubisco capacity determined the photosynthetic rate at elevatedCO2. Since CO2 dependence of the Rubisco-limited rate is strongerthan that of RuBP regeneration-limited rate, the autumn leaves showeda more pronounced enhancement of photosynthesis with elevated CO2. Weconclude that the balance between the Rubisco capacity and RuBPregeneration capacity changes seasonally and that it is important toconsider the seasonal changes in the balance when studying CO2stimulation of photosynthesis in C3 species.
Toshihiko Kinugasa, Kouki Hikosaka, and Tadaki Hirose
Tohoku University, Japan
As plants are an important sink of atmospheric CO2, plant growthunder elevated CO2 can provide an important implication for futureglobal environment. Although plant photosynthesis is stimulated byelevated CO2, the stimulation does not always enhance plant growth,because part of photosynthates is consumed by respiration and alsomatter allocation can change with CO2 enrichment. Dark respirationper unit mass is sometimes reduced by elevated CO2. However, as therespiration rate usually decrease with increase in plant mass,reduction in the rate of respiration with CO2 enrichment may simplybe a result of increased biomass accumulation. Because studies arestill limited in which effects of high CO2 on plant respiration wereinvestigated over a long period, it is unclear how the change inrespiration influences plant growth at high CO2. In this study, weanalyzed the effect of elevated CO2 on respiration in an annual(Xanthium canadense). Respiration rates of organs were determinedevery 2 weeks until seed production, and changes in respiration withCO2 enrichment and their effect on plant growth were quantified.Elevated CO2 slightly decreased respiration rates, but it increasedlifetime respiratory consumption by 7% due to increased biomass inleaves and stems. Gross production, sum of dry mass production andrespiratory consumption, showed 10% increase with CO2 enrichment. Asthe increase in gross production was larger than that in respiratoryconsumption, increase in total plant growth (net production; 16%)exceeded that in gross production. Our results showed that reductionin respiration rate with CO2 enrichment was small (on average 4%) atany point of the growth period in X. canadense. Enhanced plant growthat elevated CO2 primarily caused by increased photosynthesis and wasonly slightly influenced by the change in respiration. Effects ofelevated CO2 on two components of respiration, maintenance and growthrespiration, are also discussed.
Tohoku University, Japan
"Growth analysis" is a useful tool to study how plants respond toenvironmental change. For example, shading reduces plant growth.Shading primarily reduces NAR, but a simultaneous increase in LARpartly compensates for the lowered NAR and consequently reduction ofplant growth (RGR = NAR x LAR) is not as large as might have beenexpected. Increase in LAR often results from lowered LMA rather thanincrease in LMR (LAR = LMR x LMA). However, growth analysis does notanswer a question why plants responded to shading like that. Whydidnﾕt plants increase LMR? Does that response contribute toincreasing plant fitness? Or more specifically to what extent doesthe change e.g. in LMA ameliorate plant growth under shadeconditions? To answer these questions we need growth models. Hirose(1987, 1988) developed a simple, empirical model to predict plantgrowth at different N availabilities. Plant growth comprises twoprocesses: assimilation and partitioning of matter (dry mass and N).Net assimilation is a function of leaf N per area and partitioning ofdry mass and N is a function of plant N concentration. LMA iscontroled by leaf N concentration. The model gave RGR and theshoot/root (S/R) ratio as a function of SAR (specific N absorptionrate, rate of N absorption per unit root mass, which is expected torespresent N availability in soil). It predicted optimal S/R ratiosthat maximize RGR at a given N availability. On this model, we canevaluate adaptive significance of change in matter allocation inplants. Elevated CO2 influences plant growth through increasingphotosynthesis (Onoda), reducing respiration (Kinugasa) and changingmatter allocation (this paper). We studied the effect of CO2elevation on plant growth using a perennial herb Polygonum cuspidatum(Ishizaki, Hikosaka and Hirose 2002). Elevated CO2 (700 mmol mol-1)lowered LAR, which offset the increase in NAR and consequentlydecreased RGR, although plant growth was accelerated by CO2elevation. High LMA rather than low LMR caused low LAR at elevatedCO2. A steady-state growth model suggested that the increase in LMAcontributed to growth enhancement under elevated CO2. Real plants arenot as flexible as the optimal ones, probably because of otherphysiological, biomechanical, ecological, and evolutionaryconstraints.
Shimpei OIKAWA, Kouki HIKOSAKA, Yoshimichi HORI, MasaeSHIYOMI, Shigeo TAKAHASHI
The growing period of deciduous species is limited by the comingof winter in a temperate region. In plants with succeedingleaf-emergence pattern, leaves emerged later in the growing seasonmay have shorter leaf longevity. Previous studies have shown specieswith longer leaf longevity have lower photosynthetic capacity, lowernitrogen content and higher LMA. We address a question whether suchcorrelations are held among leaves emerged at different time in theseason. Leaf longevity, maximum photosynthetic rate (Pmax), nitrogencontent, leaf construction cost and leaf mass per unit leaf area(LMA) of leaves emerged at different times were examined in deciduousfern bracken Pteridium aquilinum (L.) Kuhn var. latiusculum whichproduced from late-April through early-October successively. Earlierleaves with longer longevity had higher LMA and leaf constructioncost, while they had higher Pmax and leaf nitrogen content, which isinconsistent with interspecific patterns. Earlier in the season,plants may invest more biomass and nitrogen to their leaves and theseleaves have enough time to pay back the cost. This indicates thatplants alter characteristics of leaves depending on the time ofappearance.
Yuko Yasumura, K. HIKOSAKA, K. MATSUI and T. HIROSE
In a forest stand, canopy and understorey species grow atcompletely different irradiances and consequently with different Cand N availability ratios. We studied how the difference in growthirradiance influenced N use of these species in a mature beech forestin Japan. Leaf-level N use efficiency (NUEL) was defined as theamount of the leaf dry mass produced per unit N taken up by leaves.NUEL was similar between the canopy species (Fagus crenata) and theunderstorey species (Lindera umbellata and Magnolia salicifolia).NUEL was analyzed further as the product of two components;leaf-level N productivity (NPL; growth rate per unit leaf nitrogen)and the mean residence time of leaf N (MRTL). Plants with higher NPLcan produce biomass more rapidly, and those with longer MRTL can usethe same nitrogen for a longer period of time. The canopy species hadsignificantly larger NPL and significantly shorter MRTL than theunderstorey species. As the photosynthetic capacity was similar amongthe species, different NPL between the species was attributablelargely to the difference in light conditions to which their leaveswere exposed. The difference in MRTL was not attributable toﾒpotentialﾓ efficiency of N resorption determined at leaf senescence,but to ﾒactualﾓ resorption efficiency, which depended on the amountof green leaf lost before full senescence. The canopy species had asignificantly shorter actual resorption efficiency because it lost aconsiderable amount of presenescent leaves. The canopy species wasmore susceptible to wind, which would blow leaves off the tree. Weconclude that the canopy and understorey species had similar NUEL,but its mechanism was different: the canopy species had larger NPLand shorter MRTL than the understorey species. Interplay of localenvironmental factors such as light and wind strongly influenced Nuse of plants in the beech forest.