Effects of water stress on Haloxylon ammodendron seedlings in the desert region of Heihe inland river watershed, Gansu Province, China

The water relation and leaf gas exchange of saxoul (Haloxylon Ammodendron Bge, a C4 shrub) seedlings were studied under water stress in 2001. Saxoul seedlings maintained high transpiration when the soil moisture was above 11%. The seedlings were able to take up water from soil with above 6 % soil water content, which was the threshold level of soil moisture for seedlings. The relationship between transpiration and potential evaporation was linear for well-watered seedlings. The de-crease of soil water availability led to different degrees of down-regulation of stomatal conductance, leaf transpiration and net CO2 assimilation rate. The stomata played a relatively small part in determining the net CO2 assimilation rate for the same seedling. The relationship between net CO2 assimilation rate and transpiration was linear diurnally, and reduction scale of leaf transpiration was much bigger than that of net CO2 assimilation rate by waters tress treatments, therefore intrinsic wa-ter-use-efficiency increased. High evaporative demand increased the leaf transpiration but inhibited net CO2 assimilation rate. Because of the effect of VPD on transpiration in this region, the transpiration of well-watered and mild water stress seedlings becomes responsive to change in stomatal conductance over a wider range.

Precipitation regulates plant gas exchange and its long-term response to climate change in a temperate grassland

Aims Climate change largely impacts ecosystem carbon and water cycles by changing plant gas exchange,which may further cause positive or negative feedback to global climate change.However,long-term global change manipulative experiments are seldom conducted to reveal plant ecophysiological responses to climatic warming and altered precipitation regimes.Methods An 8-year field experiment with both warming and increased precipitation was conducted in a temperate grassland in northern China.We measured leaf gas exchange rates(including plant photosynthesis,transpiration and instantaneous water use efficiency[WUE])of two dominant plant species(Stipa sareptana var.krylovii and Agropyron cristatum)from 2005 to 2012(except 2006 and 2010)and those of other six species from 2011 to 2012.Important Findings Increased precipitation significantly stimulated plant photosynthetic rates(A)by 29.5%and 19.9%and transpiration rates(E)by 42.2%and 51.2%for both dominant species S.sareptana var.krylovii and A.cristatum,respectively,across the 8 years.Similarly,A and E of the six plant functional types were all stimulated by increased precipitation in 2011 and 2012.As the balance of A and E,the instantaneous WUEs of different plant species had species-specific responses to increased precipitation.In contrast,neither warming nor its interaction with increased precipitation significantly affected plant leaf gas exchange rates.Furthermore,A and E of the two dominant species and their response magnitudes to water treatments positively correlated with rainfall amount in July across years.We did not find any significant difference between the short-term versus long-term responses of plant photosynthesis,suggesting the flexibility of leaf gas exchange under climate change.The results suggest that changing precipitation rather than global warming plays a prominent role in determining production of this grassland in the context of climate change.

Changes in foliar carbon isotope composition and seasonal stomatal conductance reveal adaptive traits in Mediterranean coppices affected by drought

We estimated water-use efficiency and potential photosyn- thetic assimilation of Holm oak （Quercus ilex L.） on slopes of NW and SW aspects in a replicated field test examining the effects of intensifying drought in two Mediterranean coppice forests. We used standard tech- niques for quantifying gas exchange and carbon isotopes in leaves and analyzed total chlorophyll, carotenoids and nitrogen in leaves collected from Mediterranean forests managed under the coppice system. We pos- tulated that responses to drought of coppiced trees would lead to differ- ential responses in physiological traits and that these traits could be used by foresters to adapt to predicted warming and drying in the Mediterra- nean area. We observed physiological responses of the eoppiced trees that suggested acclimation in photosynthetic potential and water-use effi- ciency： （1） a significant reduction in stomatal conductance （p〈0.01） wasrecorded as the drought increased at the SW site; （2） foliar 813C increased as drought increased at the SW site （p〈0.01）; （3） variations in levels of carotenoids and foliar nitrogen, and differences in foliar morphology were recorded, and were tentatively attributed to variation in photosyn- thetic assimilation between sites. These findings increase knowledge of the capacity for acclimation of managed forests in the Mediterranean region of Europe.

Chlorophyll Fluorescence, Photosynthetic and Morphological Characteristics of Waterlogged Sesame Seedlings

Two sesame accessions, ZZM2541 and Ezhi-2, with different tolerance to waterlogging were selected, and the seedlings at the 4-true leaf stage were treated by waterlogging for 48 h. Growth parameters were evaluated 1 d before the beginning of waterlogging and on the 3^(rd)day after the removal of waterlogging(DARW). Morphological characteristics of root and chlorophyll fuorescence were measured on the 3^(rd)DARW and leaf gas exchange was measured on the zero, 3^(rd)and 15^(th) DARW. Results showed that sesame accessions ZZM2541 and Ezhi-2 responded to waterlogging in considerably different performance. The stress induced leaf chlorosis and abscission and slowed growth of plant height in both accessions, but symptom occurred seriously in the susceptive Ezhi-2. In the more tolerant ZZM2541, plentiful of adventitious roots formed above the level.All of the average values of maximum fuorescence yield(F_m), quantum efficiency of open PSII centres(F_v/F_m), initial fluorescence(F_v/F_0) and chlorophyll content(Chl) decreased at the 4-true leaf stage in both accessions after suffering to the stress.The decreases of F_m, F_v/F_0 and Chl were more pronounced in Ezhi-2 than in ZZM2541. Less reductions of mean photosynthetic rate(Pn), transpiration rate(Tr), and stomatal conductance(gs) were observed in the leaves of waterlogged ZZM2541 than in waterlogged Ezhi-2(compared to controls), and the leaves of Ezhi-2 showed a higher water use efficiency(WUE) after the removal of waterlogging. Based on the results, it was concluded that the tolerance to waterlogging of ZZM2541 appears to depend on a combination of photosynthetic characteristics responses and morphological adaption.

Drought tolerance in three hybrid poplar clones submitted to different watering regimes

Aims Poplars grownin North Chinamay experience water-deficient periods in their life cycle.The aim of the present paper was to quantify the response of three clones to different watering regimes and to determine whichcloneamongthe three is the best adapted to drought conditions.Methods Three hybrid poplar clones(clone DN-34,R-247 and OP-367)were used in the present experiment.The seedlings of the three clones were grown under four watering regimes:control(well watered,100%field water capacity(FC))and three drought treatments(drought stress I,50%FC;drought stress II,40%FC;drought stress III,30%FC).Changes in morphological,physical and biochemical indicators of the three hybrid poplar clones were investigated.Important Findings Drought treatment(50%,40%and 30%FC)decreased net photosynthetic rate(Pn),transpiration rate(Tr),stomatal conductance(gs),shoot height,total biomass and chlorophyll(Chl)content in all the three clones and it increased activities of antioxidant enzymes and free proline content.The highest values of the above-mentioned morphological and physiological parameters were recorded in clone OP-367 under 30%FC,followed by clone DN-34 and R-247.Relative leaf water content(RWC)and stem diameter(sd)markedly declined in clone R-247 and DN-34 under drought stress I,II and III,whereas RWC and sd declined in clone OP-367 only under drought stress II and III.Clone OP-367 had more RWC and sd than DN-34 and R-247.Only the 30%FC induced an increase in the root-to-shoot ratio(rs)and water use efficiency(WUE)in all the three clones.OP-367 was the most efficient clone in water absorption and use,for plants of the clone had the highest values of rs and WUE.Our data demonstrate that among the three clones,OP-367 was better able to maintain photosynthesis and growth and lower the damage caused by drought.

Effect of a new antitranspirant on the physiology and water use efficiency of soybean under different irrigation rates in an arid region

Antitranspirants are exogenous substances applied to leaves to reduce luxury transpiration by regulating stomatal conductance to increase water use efficiency(WUE). A cheap and environmentally-friendly antitranspirant, FZ, was newly developed, extracted mainly from Alhagi sparsifolia. Its effects on soybean water use were investigated in a field experiment using the locally-used irrigation rate and a low irrigation rate(The lower and upper limit of irrigation is 40%–70% of field capacity). Foliar application of FZ and measurement of leaf physiological characteristics, final biomass, seed yield and water use efficiency were carried out during the pod bearing and pod filling stages of drip-irrigated soybean with film-mulching. Under the low irrigation rate, leaf stomatal conductance(gs) and transpiration rate(Tr)decreased significantly by 7 d after spraying, but photosynthesis(Pn) and instantaneous water use efficiency(WUE_(in)) were not significantly affec ted. The stomatal frequency, stomatal aperture, gs, Tr and Pn decreased by 1 d after spraying, without significantly increasing WUE_(in).However, applying FZ during the pod bearing and pod filling stages did not significantly affect the final biomass,water consumption, seed yield and WUE of soybean.Under the locally-used irrigation rate, applying FZ increased the activities of superoxide dismutase and peroxidase in the leaves by 38% and 33%, respectively,but did not significantly affect gs, Tr, Pn, stomatal aperture and stomatal frequency. Applying FZ three times during pod bearing and pod filling stages enhanced seed yield and WUE by 24% and 21%, respectively, but did not significantly affect the final biomass and water consumption. Therefore, seed yield and WUE of soybean were significantly increased by foliar application of FZ during the pod bearing and pod filling stages under the locally-used irrigation rate in arid region, but applying FZ did not have a positive effect on water use efficiency of soybean under a low irrigation rate.

Adaptive responses to drought of two Retama raetam subspecies from Tunisia

Aims The survival and ecological distribution of plants in arid habitats are mainly conditioned by water availability and physiological adaptations to withstand drought.In the present study,we have compared the physiological responses to drought of two Retama raetam(retama)subspecies from Tunisia,one of them living under the desert climate(subsp.raetam)and the other one growing on the coast(subsp.bovei).Methods To physiologically characterize the two R.raetam subspecies,and to elucidate their main mechanisms underlying their tolerance to drought stress,parameters related to seed germination,growth,photosynthesis(net photosynthetic rate,intracellular CO_(2) concentration,transpiration rate,stomatal conductance and water-use efficiency)and accumulation of osmolytes(proline,glycine betaine[GB]and soluble sugars)were determined in 4-month-old plants subjected to stress for up to 1 month.Important Findings Drought significantly inhibited germination,growth and all the evaluated photosynthetic parameters.Plants of R.raetam subsp.bovei were severely affected by drought after 3 weeks of treatment when photosynthesis rates were up to 7-fold lower than in the controls.At the same time,proline and GB significantly accumulated compared with the irrigated controls,but much less than in R.raetam subsp.raetam;in the latter subspecies,proline and GB increased to levels 24-and 6-fold higher,respectively,than in the corresponding controls.In summary,the population living in the desert region exhibited stronger tolerance to drought stress than that adapted to the semiarid littoral climate,suggesting that tolerance in R.raetam is dependent on accumulation of osmolytes.