Respones Of C And N Allocation Of Maize Seedlings Under Elevated Co₂

Rises in ambient CO₂are expected to cause global climate changes, includingincreases in air temperature and shifts of regional scale rainfall patterns, which leadto decreased soil water availability in some areas of the world. Elevated CO₂affectplant physiological and ecosystem processes and probably lead to the suppression ofplant N availability that limits the effect of CO₂fertilization. Previous study haveshown that C₃plants under elevated CO₂often maintain growth during short termdrought due to improved water use efficiency, however, reduce long-term adaptionand result in down-regulation of photosynthesis. The maintenance of rapid growthunder conditions of CO₂enrichment is directly related to the capacity ofphotosynthesis and carbon and nitrogen transport in plants and its contribution to thenew foliar formation. Less is known about the phorosynthesis and carbon andnitrogen transport in C4plants in response to drought, N limitation, precipitationfrequency and increasing CO₂.To test the effects of water and nitrogen limitation on plants under elevated CO₂,maize （Zea mays）, the world’s most important C4crop, was planted to experiencecombined elevated CO₂(380or750μmolmol^-1, climate chamber), water stress （15%PEG-6000） and nitrogen limitation （N deficiency treated since the144th droughthour） and rewatered at three intensities （300mL,600mL,900mL of distilled water）.During the growing period, the performance of PSII and electron transport,stomatallimitation, non-stomatal limitation, photosynthetic potential parameters, leafnitrogen use efficiency, patterns of carbon and nitrogen delivery, and new leafproductivities of the maize plants were investigated using chlorophyll-a fluorescenceOJIP induction curves, A/Cicurves and¹³C and¹⁵N as tracers.（1）. Compared to water-stressed maize under atmospheric CO₂, the elevated CO₂treatment interacted with water stress decreased number of active reactioncenters but increased antenna size and energy flux （absorb photon flux, trapping fluxand electron transport flux） of per reaction center in PSII. So the electron transportrate （J） was increased, despite of the indistinctively changed quantum yield forelectron transport and energy dissipation. In carbon reaction, the combination ofelevated CO₂and water stress treatment had the robust saturated photosynthetic rate(A_sat). This study demonstrates that maize at doubled CO₂was capable oftransporting more electron flow into carbon reaction.（2）. Elevated CO₂could alleviate drought-induced photosynthetic limitationthrough increasing capacity of PEPC carboxylation （Vpmax） and decreasing stomatallimitations （SL）. The N deficiency exacerbated drought-induced photosynthesislimitations in ambient CO₂. Elevated CO₂partially alleviated the limitation inducedby drought and N deficiency through improving the capacity of Rubiscocarboxylation （Vmax） and decreasing SL. Plants with N deficiency transported moreN to their leaves at elevated CO₂, leading to a high photosynthetic nitrogen-useefficiency but low whole-plant nitrogen-use efficiency. The stress mitigation byelevated CO₂under N deficiency conditions was not enough to improving plant Nuse efficiency and biomass accumulation. The study demonstrated that elevated CO₂could alleviate drought-induced photosynthesis limitation, but the alleviation variedwith N supplies.（3）. Compared to water-stressed maize under atmospheric CO₂, the treatmentcombining elevated CO₂with water stress increased the accumulation of biomassand partitioned more carbon and nitrogen to the formation of new leaves. Maizeseedlings enhanced the carbon resource in aging leaves and the carbon pool in newleaves but decreased the carbon counterflow capability of roots. The seedlings alsohad increased residence times of new nitrogen in roots and then delivered morenitrogen to new leaves. Thus maize supported the development of new leaves atelevated levels of CO₂by altering the transport and remobilization of carbon andnitrogen. In drought presence condition, increased activity of new leaves to storecarbon and nitrogen sustains enhanced growth under elevated CO₂in maize.（4）. Elevated CO₂significently increased the accumulation of nitrogen inwhole plant and new leaves. With nitrogen starvation, elevated CO₂retard the newnitrogen transport in functional leaves, accelerate the new nitrogen transport in new leaves to sustains growth under drought.（5）. After they were rewatered, pre-drought stressed and N limited plants withambient CO₂increased their water content higher than that of elevated CO₂, whilethe enhancement of growth rate were negatively proportional to the increasing plantwater content. Elevated CO₂could help rewatered seedlings to have higherphotosynthetic capacity (F_v/F_m, Φ_PSII, P_n, P_n/T_rand P_n/G_s) and new leaf recoveryability under low water content, no matter the seedlings suffered nitrogen deficiencyor not. The study demonstrated that elevated CO₂could help drought stressedseedlings to have higher carbon assimilation rates under low water uptakes, as aresult to improve leaf water use efficiency, which allows the plants to have muchbetter performance under drought following being re-watered.