The responses of stomata and leaf gas exchange to vapour pressure deficits and soil water content

The responses of leaf conductance, leaf water potential and rates of transpiration and net photosynthesis at different vapour pressure deficits ranging from 10 to 30 Pa kPa^-1 were followed in the sclerophyllous woody shrub Nerium oleander L. as the extractable soil water content decreased. When the vapour pressure deficit around a plant was kept constant at 25 Pa kPa^-1 as the soil water content decreased, the leaf conductance and transpiration rate showed a marked closing response to leaf water potential at-1.1 to-1.2 MPa, whereas when the vapour pressure deficit around the plant was kept constant at 10 Pa kPa^-1, leaf conductance decreased almost linearly from-0.4 to-1.1 MPa. Increasing the vapour pressure deficit from 10 to 30 Pa kPa^-1 in 5 Pa kPa^-1 steps, decreased leaf conductance at all exchangeable soil water contents. Changing the leaf water potential in a single leaf by exposing the remainder of the plant to a high rate of transpiration decreased the water potential of that leaf, but did not influence leaf conductance when the soil water content was high. As the soil water content was decreased, leaf conductances and photosynthetic rates were higher at equal levels of water potential when the decrease in potential was caused by short-term increases in transpiration than when the potential was decreased by soil drying.As the soil dried and the stomata closed, the rate of photosynthesis decreased with a decrease in the internal carbon dioxide partial pressure, but neither the net photosynthetic rate nor the internal CO₂ partial pressure were affected by low water potentials resulting from short-term increases in the rate of transpiration. Leaf conductance, transpiration rate and net photosynthetic rate showed no unique relationship to leaf water potential, but in all experiments the leaf gas exchange decreased when about one half of the extractable soil water had been utilized. We conclude that soil water status rather than leaf water status controls leaf gas exchange in N. oleander.     

Oxygation Enhances Growth, Gas Exchange and Salt Tolerance of Vegetable Soybean and Cotton in a Saline Vertisol

Impacts of salinity become severe when the soil is deficient in oxygen. Oxygation (using aerated water for subsurface drip irrigation of crop) could minimize the impact of salinity on plants under oxygen-limiting soil environments. Pot experiments were conducted to evaluate the effects of oxygation (12% air volume/volume of water) on vegetable soybean (moderately salt tolerant) and cotton (salt tolerant) in a salinized vertisol at 2, 8, 14, 20 dS/m ECe. In vegetable soybean, oxygation increased above ground biomass yield and water use efficiency (WUE) by 13% and 22%, respectively, compared with the control. Higher yield with oxygation was accompanied by greater plant height and stem diameter and reduced specific leaf area and leaf Na+ and Cl-concentrations. In cotton, oxygation increased lint yield and WUE by 18% and 16%, respectively, compared with the control, and was accompanied by greater canopy light interception, plant height and stem diameter. Oxygation also led to a greater rate of photosynthesis, higher relative water content in the leaf, reduced crop water stress index and lower leaf water potential. It did not, however, affect leaf Na+ or Cl- concentration. Oxygation invariably increased, whereas salinity reduced the K+ : Na+ ratio in the leaves of both species. Oxygation improved yield and WUE performance of salt tolerant and moderately tolerant crops under saline soil environments, and this may have a significant impact for irrigated agriculture where saline soils pose constraints to crop production.     

Differences in the responses of stem diameter and pod thickness to drought stress during the grain filling stage in soybean plants

This study investigated the factor of the physiological characteristics causing the reduction of yield of soybean plants (Glycine max (L.) Merr.) by drought stress, by monitoring changes in stem diameter and pod thickness, and photosynthetic activity, partitioning of ¹³C-labeled photosynthate. Drought stress reduced the whole plant dry weight due to the decrease in leaf and pod dry matter accumulation; however, this stress did not have a significant effect on stem growth. Leaf photosynthesis was also severely decreased by drought stress in the early stage of stress treatment as leaf water potential decreased. Imposition of stress decreased pod thickness, but stem diameter increased. The adverse effect of drought stress on pod thickness was more evident at night than during the day. The stem diameter also shrank during the day and expanded at night, but the nocturnal increase in stem diameter during drought stress treatment was greater for stressed plants compared with well-watered controls. Drought stress significantly promoted ¹³C partitioning from the fed leaf to other parts of the plant; the stem was the largest beneficiary. Soluble carbohydrates accumulated in various plant parts under the influence of the stress, but starch concentration declined in all organs except the stem. These results indicated that stem growth was promoted by drought stress compared to pod growth at the early grain-filling stage.     

短期干旱对水稻叶水势、光合作用及干物质分配的影响

采用盆栽水分试验,研究了不同生育期短期干旱处理对水稻叶水势、光合作用和干物质分配的影响．结果表明,干旱胁迫后,水稻叶水势低于对照,午后叶水势回升缓慢。凌晨叶水势随土壤含水量的降低而降低,表现为阈值反应。叶片净光合速率与凌晨叶水势密切相关,低于凌晨叶水势临界值,水稻叶片净光合速率急剧下降在水稻抽穗期和灌浆期叶片净光合速率显著下降的凌晨叶水势临界值为-1.04和-1．13MPa,对应的土壤含水量阈值分别为饱和含水量的61.0％和50．9％,土壤水势分别为-0．133和-0．240MPa干旱胁迫下单叶净光合速率的日变化规律表现为：胁迫较轻时,单叶净光合速率在正午附近出现低谷;胁迫严重时,净光合速率全天低于对照,且不及对照的一半。短期干旱后,水稻叶、根、穗的分配指数均降低,茎鞘的分配指数升高。本研究可为水稻节水灌溉管理和水分限制下水稻的生长模拟提供生理基础和理论依据。     

Variations among woody plants in stomatal conductance and photosynthesis during and after drought

Acer saccharum, Fraxinus americana, Juglans nigra, Acer rubrum, Cornus amomum, and Ulmus americana seedlings were subjected to a soil drying cycle and then rewatered. At frequent intervals during the drying cycle and following rewatering, determinations were made of equilibrium photosynthesis rates, leaf conductances and leaf water potentials. As the drying cycle progressed, leaf water potentials decreased, stomata closed, and rates of transpiration and photosynthesis were reduced. Stomata of the two Acer species initially were more sensitive to water stress than were those of the other species. At low leaf water potentials, stomata of Juglans and Cornus were more open than those of the other species. Photosynthesis of Acer saccharum, Fraxinus and Juglans was significantly reduced by plant water stress, while photosynthetic water use efficiency of Cornus and Juglans was most unfavourable. Photosynthesis/leaf conductance ratios in water stressed leaves were higher in Fraxinus than in the other species. Immediately after rewatering, only limited stomatal opening occurred in Acer saccharum and Cornus with recovery of stomatal opening most protracted in Fraxinus and Ulmus. There was extended reduction of photosynthesis of all species as a result of the soil drying treatment. This effect was most significant in Acer saccharum and Juglans. Survival of plants on moist and dry sites is discussed in relation to stomatal control of transpiration and metabolic responses to water stress. Research supported by the College of Agricultural and Life Sciences, University of Wisconsin-Madison and the International Shade Tree Conference. The cooperation of the Wisconsin Department of Natural Resources is acknowledged. Research supported by the College of Agricultural and Life Sciences, University of Wisconsin-Madison and the International Shade Tree Conference. The cooperation of the Wisconsin Department of Natural Resources is acknowledged.     

Effect of drought and rewatering on photosynthetic physioecological characteristics of soybean

Field experiments were conducted on soybean Glycine max, yudou29, a major cultivated variety in the Henan Province of China to study the relationship between photosynthetic characteristics and other physioecological parameters of its leaves under soil drying and rewatering treatments. The study showed that the dawn water potential of soybean leaves under the drying treatment was very close to that of soybean leaves under well-watered treatments (CK) when soil water content was higher than 47% of field water capacity (FWC). But when soil water content dropped below 47% of FWC, the leaf water potential decreased rapidly, indicating a significant threshold reaction. The dawn water potential threshold of soybean leaves was about ?1.02 MPa. Below this, the leaf water potential and net photosynthesis ratio dropped rapidly. When the soil water content was 47%, the leaf water potential and net photosynthesis ratio were nearly as high as those in CK, but the transpiration ratio was 67% lower, indicating that transpiration was more sensitive to drought than photosynthesis. After rewatering, the water status of soybean leaves improved, the net photosynthesis ratio and transpiration ratio increased linearly, and leaf stomata conductance (Gs) also recovered quickly. These results showed that after stress removal, soybean had fast-growing characteristics.     

温室茄子茎直径微变化与作物水分状况的关系

在温室条件下,采用盆栽土培和小区试验相结合的方法,以茄子(Solanummelongena,品种新乡糙青茄)为材料进行了植株茎直径微变化(膨胀或收缩)与作物体内水分状况的关系试验研究,旨在为利用茎直径微变化无损快速诊断作物水分状况提供理论依据。盆栽和小区试验均采用两因素(土壤水分梯度和作物不同生育阶段)随机区组设计,土壤水分控制下限分别取田间持水量的80%FC(Fieldwatercapacity),70%FC,60%FC和50%FC;生育阶段分别为苗期、花果期和采收期;共有4×3=12个处理组合,重复3次。结果表明:无论是在较高土壤含水量或在较低土壤含水量条件下,在晴好的天气里,茄子茎直径都是在白天收缩,傍晚、夜间复原或膨胀,而且这种微变化动态与植株体内的水分状况密切相关,不同土壤含水量条件下植株茎胀缩的幅度存在明显差异。高水分条件下,植株茎收缩幅度小,复原能力强;低水分条件下,植株茎收缩幅度大,恢复能力差。茎直径变化对环境因子水汽压差(VPD)的响应比较敏感,二者呈正相关关系,相关系数R2为0·8938。茎直径变化量(ΔSd)与叶水势(ψL)、叶片相对含水量(LRWC)呈极显著正相关关系,相关系数R2分别为0·867和0·965。这些结果显示,茎直径变化量能灵敏、实时、准确地反映植株体内的水分状况;与其它作物水分诊断方法(叶水势法,叶片相对含水量法,细胞液浓度法等)相比,茎直径微变化法可能具有简便、稳定、无损、连续监测和自动记录的优势。     

Precipitation pulse use by an invasive woody legume: the role of soil texture and pulse size

Plant metabolic activity in arid and semi-arid environments is largely tied to episodic precipitation events or “pulses”. The ability of plants to take up and utilize rain pulses during the growing season in these water-limited ecosystems is determined in part by pulse timing, intensity and amount, and by hydrological properties of the soil that translate precipitation into plant-available soil moisture. We assessed the sensitivity of an invasive woody plant, velvet mesquite (Prosopis velutina Woot.), to large (35 mm) and small (10 mm) isotopically labeled irrigation pulses on two contrasting soil textures (sandy-loam vs. loamy-clay) in semi-desert grassland in southeastern Arizona, USA. Predawn leaf water potential (Ψ_pd), the isotopic abundance of deuterium in stem water (δD), the abundance of ¹³C in soluble leaf sugar (δ¹³C), and percent volumetric soil water content (θ_v) were measured prior to irrigation and repeatedly for 2 weeks following irrigation. Plant water potential and the percent of pulse water present in the stem xylem indicated that although mesquite trees on both coarse- and fine-textured soils quickly responded to the large irrigation pulse, the magnitude and duration of this response substantially differed between soil textures. After reaching a maximum 4 days after the irrigation, the fraction of pulse water in stem xylem decreased more rapidly on the loamy-clay soil than the sandy-loam soil. Similarly, on both soil textures mesquite significantly responded to the 10-mm pulse. However, the magnitude of this response was substantially greater for mesquite on the sandy-loam soil compared to loamy-clay soil. The relationship between Ψ_pd and δ¹³C of leaf-soluble carbohydrates over the pulse period did not differ between plants at the two sites, indicating that differences in photosynthetic response of mesquite trees to the moisture pulses was a function of soil water availability within the rooting zone rather than differences in plant biochemical or physiological constraints. Patterns of resource acquisition by mesquite during the dynamic wetting–drying cycle following rainfall pulses is controlled by a complex interaction between pulse size and soil hydraulic properties. A better understanding of how this interaction affects plant water availability and photosynthetic response is needed to predict how grassland structure and function will respond to climate change.     

Fog interception by Sequoia sempervirens (D. Don) crowns decouples physiology from soil water deficit

Although crown wetting events can increase plant water status, leaf wetting is thought to negatively affect plant carbon balance by depressing photosynthesis and growth. We investigated the influence of crown fog interception on the water and carbon relations of juvenile and mature Sequoia sempervirens trees. Field observations of mature trees indicated that fog interception increased leaf water potential above that of leaves sheltered from fog. Furthermore, observed increases in leaf water potential exceeded the maximum water potential predicted if soil water was the only available water source. Because field observations were limited to two mature trees, we conducted a greenhouse experiment to investigate how fog interception influences plant water status and photosynthesis. Pre-dawn and midday branchlet water potential, leaf gas exchange and chlorophyll fluorescence were measured on S. sempervirens saplings exposed to increasing soil water deficit, with and without overnight canopy fog interception. Sapling fog interception increased leaf water potential and photosynthesis above the control and soil water deficit treatments despite similar dark-acclimated leaf chlorophyll fluorescence. The field observations and greenhouse experiment show that fog interception represents an overlooked flux into the soil–plant–atmosphere continuum that temporarily, but significantly, decouples leaf-level water and carbon relations from soil water availability.     

Influence of soil water stress on evaporation, root absorption, and internal water status of cotton

Diurnal variations in leaf water potential, diffusion resistance, relative water content, stem diameter, leaf temperature, and energy balance components were measured in cotton (Gossypium hirsutum L. var. Lankart 57) during drought stress under field conditions. A plot of leaf water potential against either relative water content or stem diameter during the 24-hour period yielded a closed hysteresis loop. The relation between cell hydration and evaporation is discussed.     

The relations of stomatal closure and reopening to xylem ABA concentration and leaf water potential during soil drying and rewatering

Two tropical tree species, Acacia confusa and Leucaena leucocephala, were used to study the relationships among stomatal conductance, xylem ABA concentration and leaf water potential during a soil drying and rewatering cycle. Stomatal conductance of both A. confusa and L. leucocephala steadily decreased with the decreases in soil water content and pre-dawn leaf water potential. Upon rewatering, soil water content and pre-dawn leaf water potential rapidly returned to the control levels, whereas the reopening of stomata showed an obvious lag time. The length of this lag time was highly dependent not only upon the degree of water stress but also on plant species. The more severe the water stress, the longer the lag time. When A. confusa and L. leucocephala plants were exposed to the same degree of water stress (around –2.0 MPa in pre-dawn leaf water potential), the stomata of A. confusa reopened to the control level 6 days after rewatering. However, it took L. leucocephala about 14 days to reopen fully. A very similar response of leaf photosynthesis to soil water deficit was also observed for both species. Soil drying resulted in a significant increase in leaf and xylem ABA concentrations in both species. The more severe the water stress, the higher the leaf and xylem ABA concentrations. Both leaf ABA and xylem ABA returned to the control level following relief from water deficit and preceded the full recovery of stomata, suggesting that the lag phase of stomatal reopening was not controlled by leaf and/or xylem ABA. In contrast to drying the whole root system, drying half of the root system did not change the leaf water relations, but caused a significant increase in xylem ABA concentration, which could fully explain the decrease of stomatal conductance. After rewatering, the stomatal conductance of plants in which half of the roots were dried recovered more rapidly than those of whole-root dried plants, indicating that the leaf water deficit that occurred during the drying period was related to the post-stress stomatal inhibition. These results indicated that the decrease in stomatal conductance caused by water deficit was closely related to the increase in xylem ABA, but xylem ABA could not fully explain the reopening of stomata after relief of water stress, neither did the leaf ABA. Some unknown physiological and/or morphological processes in the guard cells may be related to the recovery process.     

干旱-复水处理对杠柳幼苗光合作用及活性氧代谢的影响

采用人工控制土壤水分试验,以80%田间持水量处理为对照,研究了3次干旱-复水处理对2年生杠柳幼苗叶片光合作用,根、茎、叶膜脂过氧化和抗氧化系统的影响.结果表明: 在干旱条件下, 杠柳叶片相对含水量和光合速率(P_n)显著降低,光合色素含量升高. 干旱复水后,叶片相对含水量完全恢复,反映了杠柳较强的旱后修复能力;叶绿素含量和P_n均明显高于对照,表现出补偿效应,适度干旱诱导了杠柳的抗旱适应性.干旱胁迫使P_n下降,以气孔限制为主,但在中午发生了非气孔限制;幼叶、新茎和细根中的超氧阴离子产生速率升高,丙二醛含量则降低,说明这些幼嫩组织未受到氧化伤害.不同器官中SOD、CAT和POD活性及变化趋势不同,以细根的3种保护酶的反应最为灵敏,说明细根是杠柳适应干旱环境的重要器官.器官间的相互合作与协调使杠柳能有效地适应干湿交替的干旱环境.     

Photosynthesis, Transpiration, Leaf Temperature, and Stomatal Activity of Cotton Plants under Varying Water Potentials

Cotton plants, Gossypium hirsutum L. were grown in a growth room under incident radiation levels of 65, 35, and 17 Langleys per hour to determine the effects of vapor pressure deficits (VPD's) of 2, 9, and 17 mm Hg at high soil water potential, and the effects of decreasing soil water potential and reirrigation on transpiration, leaf temperature, stomatal activity, photosynthesis, and respiration at a VPD of 9 mm Hg.

Transpiration was positively correlated with radiation level, air VPD and soil water potential. Reirrigation following stress led to slow recovery, which may be related to root damage occurring during stress. Leaf water potential decreased with, but not as fast as, soil water potential.

Leaf temperature was usually positively correlated with light intensity and negatively correlated with transpiration, air VPD, and soil water. At high soil water, leaf temperatures ranged from a fraction of 1 to a few degrees above ambient, except at medium and low light and a VPD of 19 mm Hg when they were slightly below ambient, probably because of increased transpirational cooling. During low soil water leaf temperatures as high as 3.4° above ambient were recorded. Reirrigation reduced leaf temperature before appreciably increasing transpiration. The upper leaf surface tended to be warmer than the lower at the beginning of the day and when soil water was adequate; otherwise there was little difference or the lower surface was warmer. This pattern seemed to reflect transpiration cooling and leaf position effects.

Although stomata were more numerous in the lower than the upper epidermis, most of the time a greater percentage of the upper were open. With sufficient soil water present, stomata opened with light and closed with darkness. Fewer stomata opened under low than high light intensity and under even moderate, as compared with high soil water. It required several days following reirrigation for stomata to regain original activity levels.

Apparent photosynthesis of cotton leaves occasionally oscillated with variable amplitude and frequency. When soil water was adequate, photosynthesis was nearly proportional to light intensity, with some indication of higher rates at higher VPD's. As soil water decreased, photosynthesis first increased and then markedly decreased. Following reirrigation, photosynthesis rapidly recovered.

Respiration was slowed moderately by decreasing soil water but increased before watering. Respiration slowed with increasing leaf age only on leaves that were previously under high light intensity.

     

Biomass,anatomical changes and osmotic potential in Atriplex nummularia Lindl. cultivated in sodic saline soil under water stress

Atriplex nummularia exhibits excellent adaptability to environments with high salinity and low water availability. Accordingly, many studies have been conducted to identify the tolerance of the plant. We cultivated Atriplex in sodic saline soil under conditions of water stress in Northeast Brazil. The purpose of the study was to evaluate the growth characteristics and production of leaves, stems and roots of Atriplex under these conditions in order to identify anatomical changes in vesicular cells in leaf epidermis as well as to assess the osmotic potential of the soil solution and the leaves. The experiment was performed in a greenhouse where Atriplex was cultivated for 134 days in pots with sodic saline soil. The treatments comprised four moisture levels (35%, 55%, 75% and 95% of field capacity – FC). The height, diameter and dry mass of leaf, stem and root exhibited their highest values at levels of soil moisture that were 75% and 95% of FC. The high yields of dry biomass indicate the potential use of this halophyte for restoration of salt-affected soils. The vesicular cells were influenced by the soil moisture. The osmotic potential can serve as a good index for evaluating plant responses to water stress and salinity.     

Stem diameter in relation to plant water status

An instrument containing a linear variable differential transformer was constructed to obtain continuous, nondestructive measurements of both short term changes in stem diameter and long term growth. In cotton plants, stem diameter, leaf water potential, and leaf relative water content are all closely related to net radiation at the top of the canopy. Leaves from the east and west sides of a plant show slight, but consistent differences in diurnal water potential patterns.     

Effects of tussock size and soil water content on whole plant gas exchange in Stipa tenacissima L.: Extrapolating from the leaf versus modelling crown architecture

In this study we evaluated daily whole plant transpiration and net photosynthetic rates in Stipa tenacissima L. (Poaceae) tussocks of different sizes subjected to three levels of soil moisture. The crown architecture of 12 tussocks was reconstructed with the 3D computer model Yplant taking into account the morphology and physiology of the leaves determined at different soil moisture levels. We also calculated whole plant transpiration by extrapolating leaf transpiration in different senescence conditions measured with a diffusion porometer. This extrapolated transpiration overestimated transpiration, particularly when the soil moisture level was high (>15% of volumetric soil water content). At this high level of soil moisture, large tussocks (>60 cm in diameter), which were sexually mature and had a large leaf surface area, were the most efficient with regard to daily water use efficiency (whole plant net photosynthesis/whole plant transpiration). Whole plant water use efficiency decreased with tussock size primarily because small tussocks exhibited high transpiration rates. Small tussocks were more sensitive to soil drying than large and intermediate ones, presenting a faster rate of leaf senescence as water deficit increased. Leaf acclimation to irradiance, which was significantly influenced by the degree of mutual shading among neighbouring leaves, along with the ontogeny of the tussock and its effect upon leaf senescence were found to be the main mechanisms involved in the different responses to water limitations found in whole plant gas exchange variables. Our results show that the size of each individual plant must be taken into account in processes of scaling-up of carbon gain and transpiration from leaf to stand, as this is a particularly relevant aspect in estimating water use by semiarid vegetation.     

咸水浇灌后持续干旱条件下棉花生长及光合作用的变化

用咸水(不同浓度的NaCl溶液)浇灌盆栽棉花植株,随后进行持续干旱处理.测定干旱处理期间棉花的生长情况、光合速率、叶绿素荧光等参数的变化,并对植株的相对含水量、水势、渗透势等水分状况和Na⁺、K⁺含量进行分析,探索环境Na⁺在棉花适应干旱胁迫中的作用.结果表明: 干旱可以明显抑制植株的生长,降低叶片的净光合速率;用25～100 mmol·L^-1NaCl溶液浇灌后进行持续干旱处理的棉花植株,其株高、生物量、净光合速率和F_v/F_m值均明显高于用水浇灌后进行持续干旱处理的植株.同时,前者的土壤和叶片相对含水量、细胞膨压、Na⁺含量也明显高于后者,但植株水势和组织渗透势则显著低于后者,且组织渗透势的降低与Na⁺含量具有显著相关性.上述结果说明,土壤适量Na⁺的存在能够提高土壤和植株的保水力、增加棉花对Na⁺的吸收和积累、降低组织渗透势,从而增强植株吸水力、保持较高的细胞膨压,维持相对较高的光合速率和生长速度.土壤中存在一定浓度的NaCl可以有效缓解干旱对棉花的不利影响.     

Influence of Rate of Development of Leaf Water Deficits upon Photosynthesis, Leaf Conductance, Water Use Efficiency, and Osmotic Potential in Sorghum

This study reports the effect of rate of development of leaf water deficits in soil-grown sorghum (Sorghum bicolor) on the relationship of net photosynthesis, leaf conductance, and water use efficiency to leaf water potential, and on the degree of solute accumulation (osmotic adjustment). Recovery of these processes on rewatering, and responses during a second stress cycle were also studied. The most rapid rate of stress (1.2 MPa day^?1) resulted in no solute accumulation and the lowest rate of net photosynthesis and leaf conductance for any given leaf water potential during stress. Stress at 0.7 and 0.15 MPa day^?1 led to equal solute accumulations of approximately 0.6 MPa, but net photosynthesis, leaf conductance, and water use efficiency at a given leaf water potential were lower with the faster rate of stress (0.7 MPa day^?1). Additionally, leaf conductance at a given leaf turgor potential was lowest at the 1.2 MPa day^?1 stress rate, slightly higher at the intermediate rate of stress, and clearly highest at the slowest rate of stress. Recovery of both net photosynthesis and leaf conductance upon rewatering was rapid, taking less than 3 days, but full recovery of osmotic potential took between 6 and 11 days. One slow stress cycle had no influence on relationships during a second cycle. The concept of a threshold leaf water potential for stomatal closure is discussed and the conclusion reached that stomatal closure occurs slowly over a wide range of leaf water potential (> 1.0 MPa), the range being greater for slower rates of stress.     

Water relations, gas exchange and abscisic acid content of Lupinus cosentinii leaves in response to drying different proportions of the root system

Soil columns in which the root system was divided into threeequal layers, each 24 cm in diameter and 33 cm high were usedto examine the influence of drying different proportions ofthe root system on the water relations, gas exchange and abscisicacid (ABA) concentration of lupin (Lupinus cosentinii Guss.cv. Eregulla) leaves. The treatments imposed were (i) all threelayers adequately watered (control), (ii) the upper layer unwateredwith the remaining layers kept adequately watered, (iii) thetwo upper layers unwatered with the basal layer kept adequatelywatered, (iv) all three layers unwatered. The treatments wereapplied at 56 d after sowing (DAS), and continued for 21 d inthe treatment in which the three layers were dried and for 36d in the other three treatments. After 21 d, the soil matricpotential in the layers that were unwatered had decreased toemdash 1.3MPa, compared to - 0.03 MPa in the adequately-wateredlayers. Within 8 d of cessation of watering, plants with the entireroot system in drying soil had significantly lower stomatalconductances, lower rates of net photosynthesis, and higherleaf ABA contents than did adequately-watered plants. Whilethe leaf osmotic potential decreased within 8 d of cessationof watering, the leaf water potential did not change for thefirst 15 d after water was withheld. After withholding waterfrom all layers, the shoot dry matter was 63% lower than thatin the adequately-watered plants. In the two partially-droughtedtreatments, 17% and 48% of the root length was subjected todrying. Compared to the adequately-watered plants, drying upto 50% of the root system for 36 d, in the two partially-droughtedtreatments, did not reduce stomatal conductance, net photosynthesis,or plant growth. Similarly, there was no significant effecton leaf water potential or osmotic potential. When either theupper or upper and middle layers of soil were dried, the ABAcontent of the leaves for most of the drying period was slightly,but not significantly, higher than in leaves of the adequately-wateredplants. The results suggest that lupins with a well-established rootsystem can utilize localized supplies of available soil waterto maintain leaf gas exchange despite appreciable portions ofthe root system being in dry soil. In contrast to other studies,the results also suggest that when only a portion of the soilvolume is dry and adequate water is available in the wet zone,root signals do not influence stomatal conductance and leafgas exchange of lupin. Key words: Abscisic acid, gas exchange, lupins, split-roots, water deficit     

Plant leaf area expansion and assimilate production in wheat (Triticum aestivum L.) growing under low phosphorus conditions

Under phosphorus deficiency reductions in plant leaf area have been attributed to both direct effects of P on the individual leaf expansion rate and to a reduced availability of assimilates for leaf growth. In this work we use experimental and simulation techniques to identify and quantify these processes in wheat plants growing under P-deficient conditions. In a glasshouse experiment we studied the effects of soil P addition (0–138 kg P₂O₅ ha^-1) on tillering, leaf emergence, leaf expansion, plant growth, and leaf photosynthesis of wheat plants (cv. INTA Oasis) that were not water stressed. Plants were grown in pots containing a P-deficient (3 mg P g^-1 soil) sandy soil. Sowing and pots were arranged to simulate a crop stand of 173 plants m^-2. Experimental results were integrated in a simulation model to study the relative importance of each process in determining the plant leaf area during vegetative stages of wheat. Phosphorus deficiency significantly reduced plant leaf area and dry weight production. Under P-deficient conditions the phyllochron (PHY) was increased up to a 32%, compared to that of high-P plants. In low-P plants the rate of individual leaf area expansion during the quasi-linear phase of leaf expansion (LER) was significantly reduced. The effect of P deficiency on LER was the main determinant of the final size of the individual leaves. In recently expanded leaves phosphorus deficiency reduced the photosynthesis rate per unit leaf area at high radiation (AMAX), up to 57%. Relative values of AMAX showed an hyperbolic relationship with leaf P% saturating at 0.27%. Relative values of the tillering rate showed an hyperbolic relationship with the shoot P% saturating at values above 0.38%. The value of LER was not related to the concentration of P in leaves or shoots. A morphogenetic model of leaf area development and growth was developed to quantify the effect of assimilate supply at canopy level on total leaf area expansion, and to study the sensitivity of different model variables to changes in model parameters. Simulation results indicated that under mild P stress conditions up to 80% of the observed reduction in plant leaf area was due to the effects of P deficiency on leaf emergence and tillering. Under extreme P-deficient conditions the simulation model failed to explain the experimental results indicating that other factors not taken into account by the model, i.e. direct effects of P on leaf expansion, must have been active. Possible mechanisms of action of the direct effects of P on individual leaf expansion are discussed in this work.