Experimental and simulated light responses of photosynthesis in leaves of three tree species under different soil water conditions

Water deficit is one of the major limiting factors in vegetation recovery and restoration in loess, hilly-gully regions of China. The light responses of photosynthesis in leaves of two-year old Prunus sibirica L., Hippophae rhamnoides L., and Pinus tabulaeformis Carr. under various soil water contents were studied using the CIRAS-2 portable photosynthesis system. Light-response curves and photosynthetic parameters were analyzed and fitted using the rectangular hyperbola model, the exponential model, the nonrectangular hyperbola model, and the modified rectangular hyperbola model. Under high light, photosynthetic rate (P _N) and stomatal conductance (g _s) were steady and photoinhibition was not significant, when the relative soil water content (RWC) varied from 56.3–80.9%, 47.9–82.9%, and 33.4–92.6% for P. sibirica, H. rhamnoides, and P. tabulaeformis, respectively. The light-response curves of P _N, the light compensation point (LCP), and the dark respiration rate (R _D) were well fitted using the above four models. The nonrectangular hyperbola was the best model in fitting the data; the modified rectangular hyperbola model was the second, and the rectangular hyperbola model was the poorest one. When RWC was higher or lower than the optimal range, the obvious photoinhibition and significant decrease in P _N with increasing photosynthetic photon flux density (PPFD) were observed in all three species under high light. The light saturation point (LSP) and apparent quantum yield also decreased significantly, when the upper limit of PPFD was 200 μmol m^?2 s^?1. Under these circumstances, only the modified rectangular hyperbola model was able to fit well the curves of the light response, LCP, LSP, R _D, and light-saturated P _N.     

A mathematical model for describing light-response curves in <Emphasis Type="Italic">Nicotiana tabacum</Emphasis> L.

A modified exponential model was used to describe light-response curves of Nicotiana tabacum L. The accuracies of an exponential model, a nonrectangular hyperbola model, a rectangular hyperbola model, a modified rectangular hyperbola model and the modified exponential model were evaluated by Mean square error (MSE) and Mean absolute error (MAE). The tests MSE and MAE of the modified exponential model were the lowest among the five models. The light saturation point (LSP) obtained by the exponential model, the nonrectangular hyperbola model and the rectangular hyperbola model were much lower than the measured values, and the maximum net photosynthetic rates (P _max) calculated from these models, were greater than the measured values. P _max at LSP of 1,077 μmol m⁻² s⁻¹ calculated by the modified exponential model was 12.34 μmol(CO₂) m⁻² s⁻¹, which was more accurate than the values obtained from the modified rectangular hyperbola model. The results show that the modified exponential model is superior to other models for describing light-response curves.     

不同土壤水分下山杏光合作用CO2响应过程及其模拟

在半干旱黄土丘陵区,以2年生盆栽山杏为材料,应用CIRAS-2型光合作用系统,测定了8个土壤水分梯度下山杏光合作用的CO₂响应过程,并采用直角双曲线模型、指数方程和直角双曲线修正模型对其CO₂响应数据进行拟合,分析了山杏光合作用与土壤水分的定量关系.结果表明: 山杏CO₂响应过程对土壤水分有明显的阈值响应特征.维持山杏叶片较高的光合速率(P_n)和羧化效率(CE)的土壤相对含水量(RWC)在46.3%～81.9%,在此水分范围内,光合作用没有发生明显的CO₂饱和抑制现象;当RWC超出此范围,土壤水分升高或降低均明显降低山杏叶片的光合能力(P_{n max})、CE和CO₂饱和点(CSP).在不同土壤水分条件下,3个模型对山杏CO₂响应数据的模拟效果有明显差别.在46.3%～81.9%土壤水分范围内,3个模型均能较好地拟合山杏CO₂响应过程及其特征参数CE、CO₂补偿点(Γ)和光呼吸速率(R_p),其拟合精度均表现为直角双曲线修正模型＞指数方程＞直角双曲线模型;当土壤水分含量过高(RWC＞81.9%)或过低(RWC＜46.3%)时,只有直角双曲线修正模型能较好地拟合山杏CO₂响应过程及其特征参数.RWC在46.3%～81.9%范围内,山杏具有较高的光合作用效率;与传统直角双曲线模型和指数方程相比,直角双曲线修正模型具有更好的适用性.     

Response of stem sap flow and leaf photosynthesis in <Emphasis Type="Italic">Tamarix chinensis</Emphasis> to soil moisture in the Yellow River Delta,China

Soil moisture is the main limiting factor for vegetation growth at shell ridges in the Yellow River Delta of China. The objective of this study was to explore the soil moisture response of photosynthetic parameters and transpiration in Tamarix chinensis Lour., a dominant species of shell ridges. Leaf photosynthetic light-response parameters and sap flow were measured across a gradient of relative soil water content (RWC), from drought (23%) to waterlogging (92%) conditions. Leaf photosynthetic efficiency and stem sap flow of T. chinensis showed a clear threshold response to soil moisture changes. Leaf net photosynthetic rate, water-use efficiency (WUE), light-saturation point, apparent quantum yield, maximum net photosynthetic rate, and dark respiration rate peaked at moderately high RWC, decreasing towards high and low values of RWC. However, peak or bottom RWC values substantially differed for various parameters. Excessively high or low RWC caused a significant reduction in the leaf photosynthetic capacity and WUE, while the high photosynthetic capacity and high WUE was obtained at RWC of 73%. With increasing waterlogging or drought stress, T. chinensis delayed the starting time for stem sap flow in the early morning and ended sap flow activity earlier during the day time in order to shorten a daily transpiration period and reduce the daily water consumption. The leaf photosynthetic capacity and WUE of T. chinensis were higher under drought stress than under waterlogging stress. Nevertheless, drought stress caused a larger reduction of daily water consumption compared to waterlogging, which was consistent with a higher drought tolerance and a poor tolerance to waterlogging in this species. This species was characterized by the low photosynthetic capacity and low WUE in the range of RWC between 44 and 92%. The RWC of 49–63% was the appropriate range of soil moisture for plant growth and efficient physiological water use of T. chinensis seedlings.     

黄河小浪底栓皮栎、刺槐叶片电子传递速率-光响应的模拟

为了比较直角双曲线模型、非直角双曲线模型与叶子飘模型的优缺点, 研究阴生叶和阳生叶电子传递速率的差异, 探讨环境/生物因素对电子传递速率等参数的影响, 该文采用LI-6400XT荧光测定系统对黄河小浪底栓皮栎(Quercus variabilis)、刺槐(Robinia pseudoacacia)叶片电子传递速率-光响应(J-I)曲线进行了测定, 利用直角双曲线模型、非直角双曲线模型和叶子飘模型对J-I曲线进行了拟合。结果表明, 3种模型对叶片J-I曲线拟合的决定系数(R ²)在0.96以上, 叶子飘模型的R ²最高(> 0.99)。直角双曲线模型和非直角双曲线模型无法模拟植物叶片光系统II动力学下调现象, 且不能得出饱和光强(I_sat); 直角双曲线模型对最大电子传递速率(J_max)的模拟明显大于实测值; 叶子飘模型能很好地模拟光系统II动力学下调现象, 得出的J_max和I_sat均最接近实测值。对阴生叶和阳生叶J-I曲线研究发现, 栓皮栎、刺槐阴生叶的J_max分别低于阳生叶25.0%和18.0%, 阳生叶的I_sat分别高于阴生叶26.0%和10.1%。栓皮栎和刺槐J_max与气温显著正相关; 刺槐I_sat与气温、土壤水分含量和净光合速率具有显著的正相关关系; 栓皮栎和刺槐J-I曲线初始斜率α均与净光合速率呈显著负相关关系。     

Simulation on the light-response curves of electron transport rate of Quercus variabilis and Robinia pseudoacacia leaves in the Xiaolangdi area,China

为了比较直角双曲线模型、非直角双曲线模型与叶子飘模型的优缺点, 研究阴生叶和阳生叶电子传递速率的差异, 探讨环境/生物因素对电子传递速率等参数的影响, 该文采用LI-6400XT荧光测定系统对黄河小浪底栓皮栎(Quercus variabilis)、刺槐(Robinia pseudoacacia)叶片电子传递速率-光响应(J-I)曲线进行了测定, 利用直角双曲线模型、非直角双曲线模型和叶子飘模型对J-I曲线进行了拟合。结果表明, 3种模型对叶片J-I曲线拟合的决定系数(R ²)在0.96以上, 叶子飘模型的R ²最高(> 0.99)。直角双曲线模型和非直角双曲线模型无法模拟植物叶片光系统II动力学下调现象, 且不能得出饱和光强(I_sat); 直角双曲线模型对最大电子传递速率(J_max)的模拟明显大于实测值; 叶子飘模型能很好地模拟光系统II动力学下调现象, 得出的J_max和I_sat均最接近实测值。对阴生叶和阳生叶J-I曲线研究发现, 栓皮栎、刺槐阴生叶的J_max分别低于阳生叶25.0%和18.0%, 阳生叶的I_sat分别高于阴生叶26.0%和10.1%。栓皮栎和刺槐J_max与气温显著正相关; 刺槐I_sat与气温、土壤水分含量和净光合速率具有显著的正相关关系; 栓皮栎和刺槐J-I曲线初始斜率α均与净光合速率呈显著负相关关系。     

The responses of photosynthetic rate and stomatal conductance of Fraxinus rhynchophylla to differences in CO2 concentration and soil moisture

The photosynthetic parameters in leaves of three-year-old seedlings of Fraxinus rhynchophylla L. were studied under different soil water conditions and CO₂ concentrations ([CO₂]) with a LI-COR 6400 portable photosynthesis system. The objective was to investigate the response of photosynthesis and stomatal conductance (g _s) to various [CO₂] and soil water conditions, and to understand the adaptability of F. rhynchophylla to such conditions. The results showed that the soil water content (RWC) required to maintain high photosynthetic productivity in F. rhynchophylla was 49.5–84.3%; in this range, net photosynthetic rate (P _N) rose with [CO₂] increasing from 500 to 1,400 μmol mol^?1. Outside this RWC range, P _N decreased significantly. The apparent maximum photosynthetic rate (P _max,c) and carboxylation velocity (V _c) increased with increasing RWC and remained relatively high, when RWC was between 49.5 and 96.2%. CO₂ compensation points and photorespiration rate exhibited a trend opposite to that of P _max,c and V _c, indicating that moderate water stress was beneficial for increasing plant assimilation, decreasing photorespiration, and increasing production of photosynthates. g _s declined significantly with increasing [CO₂] under different water supplies, but the RWC range maintaining high g _s increased. g _s reached its maximum, when RWC was approximately 73% and then decreased with declining RWC. The maximal g _s was found with increasing RWC. Thus, based on photosynthetic characteristics in artificial, vegetation construction in semiarid loess hill and gully area, F. rhynchophylla could be planted in habitats of low soil water content.     

干旱胁迫下胡杨光合光响应过程模拟与模型比较

以塔里木干旱荒漠区2年生胡杨幼苗为试材,盆栽模拟荒漠生境5种水分梯度,利用Li-6400便携式光合作用系统测定胡杨在干旱胁迫下光合作用的光响应过程,并采用4种光响应模型对其进行拟合与比较,以期优选出适用于干旱荒漠环境的光响应模型,阐明胡杨光合作用对干旱胁迫的响应规律与适应机制。结果表明:胡杨净光合速率(P_n)随干旱胁迫加剧呈下降趋势,同一光强(PAR)下P_n降幅增大。中度干旱胁迫以下(土壤相对含水量,RSWC45%)胡杨在高PAR下仍能维持相对较高P_n,光抑制程度轻;直角双曲线、非直角双曲线和指数模型均可较好地模拟P_n-PAR响应过程,但最大净光合速率(P_(nmax))、光饱和点(LSP)拟合值与实测值差异极显著(P0.01)。中度干旱胁迫以上(RSWC45%)胡杨P_n随PAR升高而显著下降,LSP与P_(nmax)极显著降低,光抑制现象明显;仅直角双曲线修正模型拟合的胡杨光响应过程、光响应参数与实际情况较吻合。4种模型模拟效果顺序:直角双曲线修正模型指数模型非直角双曲线模型直角双曲线模型。4种光响应模型对干旱胁迫具有不同的适应性,直角双曲线修正模型适用于各种水分条件,尤其适用于干旱荒漠生境,其它3种模型适用于水分条件较好的生境。光响应特征参数对干旱胁迫的响应阈值不同。随干旱胁迫加剧,胡杨表观量子效率(AQY)、P_n、LSP与P_(nmax)持续降低,严重干旱胁迫下暗呼吸速率(R_d)、LCP反而明显增大。RSWC45%胡杨仍能保持较高的AQY、P_(nmax)、LSP,RSWC45%其P_(nmax)、LSP显著降低,干旱胁迫显著抑制了胡杨光合进程和光强耐受范围,降低了光合效率,严重干旱胁迫严重影响胡杨苗木的正常生长和光合作用。干旱荒漠环境下,胡杨采取缩窄光照生态幅、降低光能利用率和减少呼吸消耗来积极抵御荒漠干旱逆境伤害的生态对策。因此,从极端干旱荒漠区种群保护与植被恢复角度来看,胡杨林土壤水分应维持在RSWC 50%左右,符合干旱缺水地区植物生长和高效用水的管理原则。     

贝壳砂生境干旱胁迫下杠柳叶片光合光响应模型比较

以黄河三角洲贝壳堤岛3年生杠柳(Periploca sepium)苗木为试验材料, 模拟设置贝壳砂生境下的4种水分梯度, 利用CIRAS-2型光合作用系统测定杠柳叶片在不同干旱胁迫下的光合作用光响应过程, 采用4种光响应模型进行拟合分析, 以比较贝壳砂生境干旱胁迫下适宜的光响应模型, 探讨最佳光响应模型参数对干旱胁迫的适应规律。结果表明: 4种模型对杠柳叶片光合作用光响应过程拟合效果的优劣顺序为: 直角双曲线修正模型>非直角双曲线模型>指数模型>直角双曲线模型, 后3种模型均为没有极值的函数, 故不能很好地拟合光响应曲线光抑制过程, 并不能直接求解最大净光合速率(P_nmax)和光饱和点(LSP)。光响应参数拟合效果最佳表现为: 非直角双曲线模型的暗呼吸速率(R_d), 直角双曲线模型的光补偿点(LCP), 直角双曲线修正模型的P_nmax和LSP。4种光响应模型对干旱胁迫具有不同的适应性, 直角双曲线修正模型适应各种水分条件, 直角双曲线模型和指数模型较适合轻度干旱胁迫条件, 非直角双曲线模型较适合重度干旱胁迫条件。随干旱胁迫的加剧, 光响应参数表观量子效率(AQY)、R_d和LCP先升高后下降, 净光合速率(P_n)、P_nmax和LSP逐渐下降。轻度、中度和重度干旱胁迫下, LSP分别比对照下降5.2%、16.3%和34.5%, P_nmax分别比对照下降17.8%、39.0%和59.0%。水分充足条件下, 杠柳叶片光能利用最强, 光照生态幅最宽; 重度干旱胁迫下, 杠柳叶片表现出明显的光饱和、光抑制现象, 光能利用减弱, 光合能力受到较大限制。     

光响应曲线的指数改进模型与常用模型比较

光响应曲线的参数是研究植物生理状态的重要指标, 常用的光响应曲线模型无法准确地计算出光饱和点和最大净光合速率。该文利用光响应曲线新模型——指数改进模型、直角双曲线模型、直角双曲线修正模型、非直角双曲线模型和指数模型, 拟合高粱(Sorghum bicolor)、苋(Amaranthus tricolor)、大麦(Hordeum vulgare)和半夏(Pinellia ternata)的光响应曲线, 并随机选取部分数据进行检验, 得到了各模型计算出的主要生理参数, 并对这些数据进行了比较分析, 讨论了各模型之间的优缺点和准确性, 描述了C₃、C₄植物光响应的适宜性。结果表明, 基于C₃植物得到的指数改进模型和直角双曲线修正模型能较准确地计算出C₃、C₄植物饱和光强和最大净光合速率, 并在描述光响应曲线时比另外3个模型具有更高的精确性和适宜性。实验结果可为光响应曲线模型在C₃和C₄光合途径植物中的应用提供参考。     

Simulation on the light-response curves of electron transport rate of Quercus variabilis and Robinia pseudoacacia leaves in the Xiaolangdi area,China北大核心CSCD

Aims The objectives are to compare the merits and demerits of rectangular hyperbola, nonrectangular hyperbola and the electron transport rate light-response Ye model, to investigate the difference of electron transport rate between the shaded and sunlit leaves, and to discuss the influence of bioenvironmental factors on the characteristic parameters of electron transport rate. Methods The light-response (J-I) curves of electron transport rate were measured by the LI-6400XT fluorescence measurement system in Quercus variabilis and Robinia pseudoacacia plantations in north China. The rectangular hyperbola, nonrectangular hyperbola and the Ye model were used to simulate electron transport rate of the light-response curves. Important findings The results showed that the determination coefficient of the J-I curves fitted by three models were more than 0.96. Compared with the rectangular hyperbola and nonrectangular hyperbola, the determination coefficient of the Ye model was the highest (>0.99). The dynamic downregulation of photosystem II and the saturated light intensity (I sat ) cannot be simulated and obtained by the rectangular hyperbola model and the nonrectangular hyperbola model. The maximum electron transport rate (J max ) obtained by the rectangular hyperbola model was obviously higher than the measured one. The dynamic downregulation of photosystem II was well simulated by the Ye model. The J max and I sat values obtained by the Ye model were close to the measured ones. © 2018 Editorial Office of Chinese Journal of Plant Ecology. All Rights Reserved.     

生产力调控对翌年苹果园土壤水分和苹果叶片光合特性的影响

以盛果期‘长枝富士’为试验材料, 以生产力调控翌年苹果(Malus pumila)树坐果期为主要观测期, 对不同生产力水平下果园土壤水分状况及苹果叶片光合特性进行了研究。结果表明: 在生产力调控范围内, 土壤剖面各个层次土壤含水量均随着生产力水平的减小而增加, 其中在60 cm处达到最大增量, 为31.02%; 而在600 cm范围内土壤贮水量最大能够提高15.41%。随着生产力水平的降低, “光合午休”现象减弱, 净光合速率(P_n)增加, 最大增幅为25.71%, 下午时段的蒸腾速率(T_r)下降迅速, 水分利用效率(WUE)最大提高34.12%。通过相关分析表明, 土壤贮水量(WSC)与P_n、T_r、WUE之间均达到显著相关, 其相关系数分别为: 0.973^**、-0.543^*和0.992^**。土壤贮水量(x)与水分利用效率(y)之间符合y = 0.002 3x - 1.480 6, R²= 0.984 4^**的回归模型。通过生产力调控可以改善土壤水分状况和果树光合能力, 提高WUE。     

不同水分条件下胡杨光响应曲线拟合模型比较

本研究通过测量不同水分条件下胡杨（Populus euphratica Oliv.）叶片的光响应曲线,并采用4种光响应模型对其光合特征参数拟合值与实测值进行比较,分析了不同水分条件下光响应曲线模型对胡杨适用性的影响机制。结果表明,当水分供应充足时,胡杨非直角双曲线模型对暗呼吸速率（R_d）的拟合效果最优,直角双曲线修正模型拟合光饱和点（LSP）、最大净光合速率（P_nmax）、光补偿点（LCP）的结果与实测值较接近;但当胡杨受到水分亏缺后,直角双曲线修正模型对P_nmax和光饱和点（LSP）的拟合效果最优,直角双曲线模型对R_d和LCP的拟合效果最佳。因此,水分条件有利时胡杨应用直角双曲线修正模型、非直角双曲线模型较好;水分亏缺条件下采用直角双曲线修正模型、直角双曲线模型更为适合。     

Sequence of physiological responses in groundnut (Arachis hypogaea L.) subjected to soil moisture deficit

Responses of drought-tolerant (DT) and drought-susceptible (DS) pot-grown groundnut (Arachis hypogaea L.) varieties to changes in leaf relative water content (RWC) were studied. Water stress (WS) was imposed on 30-day-old plants for 2 weeks. Leaf RWC decreased significantly under WS conditions with simultaneous decrease in net photosynthetic rate (P _N) and stomatal conductance (g _s). Even though no significant difference was observed between DT and DS varieties with regard to RWC, DT varieties were able to maintain significantly higher P _N than DS varieties. Higher values of water use efficiency (WUE) were also observed in DT varieties during WS conditions. The decline in P _N due to WS could be attributed to both reduction in g _s (i.e. stomatal limitation) and to reduction in chlorophyll content (Chl). No significant difference in leaf area index (LAI) was found between DT and DS types and LAI was not reduced by WS. Significant differences were found among the studied groundnut varieties, but not between DT and DS types, in terms of root, aboveground, and total dry mass. These growth parameters significantly decreased under WS conditions. Based on the results, a sequence of physiological responses in groundnut crop subjected to WS was postulated.     

A new model for relationship between irradiance and the rate of photosynthesis in <Emphasis Type="Italic">Oryza sativa</Emphasis>

The calculated maximum net photosynthetic rate (P _N) at saturation irradiance (I _m) of 1 314.13 μmol m⁻² s⁻¹ was 25.49 μmol(CO₂) m⁻² s⁻¹, and intrinsic quantum yield at zero irradiance was 0.103. The results fitted by nonrectangular hyperbolic model, rectangular hyperbolic method, binomial regression method, and the new model were compared. The maximum P _N values calculated by nonrectangular hyperbolic model and rectangular hyperbolic model were higher than the measured values, and the I _m calculated by nonrectangular hyperbolic model and rectangular hyperbolic model were less than measured values. Results fitted by new model showed that the response curve of P _N to I was nonlinear at low I for Oryza sativa, P _N increased nonlinearly with I below saturation value. Above this value, P _N decreased nonlinearly with I.     

Arbuscular Mycorrhizal Fungi Alter Fractal Dimension Characteristics of Robinia pseudoacacia L. Seedlings Through Regulating Plant Growth,Leaf Water Status,Photosynthesis, and Nutrient Concentration Under Drought Stress

The influence of arbuscular mycorrhizal fungi (AMF), Funneliformis mosseae and Rhizophagus intraradices, on plant growth, leaf water status, chlorophyll concentration, photosynthesis, nutrient concentration, and fractal dimension (FD) characteristics of black locust (Robinia pseudoacacia L.) seedlings was studied in pot culture under well-watered, moderate drought stress, and severe drought stress treatments. Mycorrhizal seedlings had higher dry biomass, leaf relative water content (RWC), and water use efficiency (WUE) compared with non-mycorrhizal seedlings. Under all treatments, AMF colonization notably enhanced net photosynthetic rate, stomatal conductance, and transpiration rate, but decreased intercellular CO₂ concentration. Leaf chlorophyll a and total chlorophyll concentrations were higher in AM seedlings than those in non-AM seedlings although there was no significant difference between AMF species. AMF colonization improved leaf C, N, and P concentrations, but decreased C:N, C:P, and N:P ratios. Mycorrhizal seedlings had a larger FD value than non-mycorrhizal seedlings. The FD value was positively and significantly correlated to the plant growth parameters, photosynthesis, RWC, WUE, and nutrient concentration but negatively correlated to leaf/stem ratio, C:N and C:P ratios, and intercellular CO₂ concentration. We conclude that AMF lead to an improvement of growth performance of black locust seedlings under all growth conditions, including drought stress via improving leaf water status, chlorophyll concentration, photosynthesis, and nutrient uptake. Moreover, FD technology proved to be a powerful non-destructive method to characterize the effect of AMF on the physiology of host plants during drought stress.     

Growth, gas exchange, water-use efficiency, and carbon isotope composition of ‘Gale Gala’ apple trees grafted onto 9 wild Chinese rootstocks in response to drought stress

To determine the effects of rootstock choice on the scion response to drought stress, we compared the vegetative growth, biomass accumulation, gas exchange, and water-use efficiency (WUE) of ??Gale Gala?? apple (Malus domestica Borkh.) trees grafted onto nine wild Chinese Malus rootstocks. Compared with the well-watered control, drought treatment limited growth, as manifested by smaller increments in plant height (PH), trunk diameter (TD), total fresh biomass (TB), total dry biomass (TDB), total leaf area (LA), and relative growth rate (RGR). The extent of this effect differed among rootstocks. Stress conditions led to increases in the root/shoot ratio (RSR), leaf thickness (LT), water-holding capacity (WHC), carbon isotope composition (??¹³C), and WUE. Decreases were noted in stomatal density (SD), leaf relative water content (RWC), chlorophyll content (Chl), net photosynthetic rate (P _N), transpiration rate (E), and stomatal conductance (g _s), again varying by rootstock. Those that are generally considered more drought-tolerant, e.g., M. sieversii, M. prunifolia, and M. toringoides, had smaller declines in PH, TD, TB, TDB, LA, RGR, SD, RWC, Chl, P _N, E, and g _s and proportionally greater increases in RSR, LT, WHC, ??¹³C, and WUE compared with the droughtsensitive M. hupehensis and M. sieboldii. These results suggest that moisture stress has a significant dwarfing effect in the latter two species. Based on WUE calculations, trees on drought-tolerant rootstocks showed higher tolerance when stressed, whereas those on drought-sensitive rootstocks were less tolerant, as indicated by their lower WUE values.     

Effects of water deficit stress on growth,water relations and osmolyte accumulation in Medicago truncatula and M. laciniata populations

The effects of water stress were investigated in two Tunisian Medicago truncatula populations collected from arid (Mt-173) and sub-humid (Mt-664) climates and two Tunisian M. laciniata populations originating from arid (Ml-173) and semi-arid (Ml-345) regions. After a pre-treatment phase (24 days after sowing, DAS) of watering at 100% of field capacity (FC), the plants were either irrigated at 100% FC or at only 33% FC. After 12 days of treatment (36 DAS), one lot of dehydrated plants was rewatered at 100% FC. A final harvest was carried out after 24 days of treatment (48 DAS). Measured parameters were total dry weight (TDW), root shoot ratio (RSR), leaf relative water content (RWC), osmotic potential (OP), photosynthetic parameters (CO₂ net assimilation A, stomatal conductance g_s and transpiration E), malondialdehyde (MDA) concentration and leaf contents in inorganic (Na⁺ and K⁺) and organic solutes (proline and soluble sugars). Under water deficit conditions, compared to M. laciniata, M. truncatula populations showed a higher reduction in TDW, A, g_s and RWC associated with a higher increase in MDA concentration. Thus, the relative tolerance of M. laciniata populations to water shortage would be related to their lower intrinsic growth rate and stomatal control of gas exchange. TDW, A, g_s, E and RWC were more decreased by water deficit in Ml-345 than in Ml-173. Drought tolerance of Ml-173 was found to be associated with a more pronounced decrease of OP and a lower reduction in RWC due to the accumulation of solutes such as proline, soluble sugars and K⁺. In addition, Ml-173 showed the highest water use efficiency values (WUE) and the lowest MDA concentrations under water deficit stress.     

光合作用光响应模型的比较

 用美国Li-Cor公司生产的Li-6400光合作用测定仪控制CO₂浓度和温度, 测量了华北平原冬小麦(Triticum aestivum)的光响应数据。分别用C₃植物光响应新模型、直角双曲线模型、非直角双曲线模型和Prado-Moraes模型拟合这些实测数据, 分析了由直角双曲线模型、非直角双曲线模型和Prado-Moraes模型拟合这些数据得到的最大净光合速率(The maximum net photosynthetic rate)远大于实测值, 而光饱和点(Light saturation point)远小于实测值的原因。结果表明, 由C₃植物光响应新模型拟合的结果与实测数据符合程度最高(R²=0.999 4和R²=0.998 7); 表观量子效率(Apparent quantum yield)不是一个理想的表示植物利用光能的指标, 建议用植物光响应曲线在光补偿点处的量子效率作为表示植物光能利用的指标。     

Importance of Aerodynamic Resistance to Water Use Efficiency in Three Conifers under Field Conditions

The quantitative importance of aerodynamic resistance to H₂O vapor and CO₂ exchange was determined for shoots from saplings of three conifers (Abies lasiocarpa [Hook] Nutt., Pinus contorta Dougl., Juniperus communis L.) under natural conditions in the field. A combination of relatively low stomatal resistances (<300 seconds per centimeter) and low wind speeds (<30 centimeters per second) led to substantial contributions of the aerodynamic resistance (R_wv^a) to water use efficiency (WUE = photosynthesis/transpiration) for all three species. For A. lasiocarpa, transpiration was calculated to be 44% less and photosynthesis 17% less due to the presence of R_wv^a, which led to a predicted increase in WUE of 57% compared to the calculated WUE when R_wv^a was assumed negligible. Similar increases in WUE were computed for P. contorta (48%) with somewhat smaller values for J. communis (34%). These results are discussed in terms of the estimated importance of R_wv^a on water and photosynthetic relations of plants that have relatively low stomatal resistances and grow in microhabitats with low winds.