Gas exchange in a 20-year-old stand of Scots pine

The rates of net photosynthesis and transpiration of one-year-old shoots were measured in situ in five different positions within the crown of a young Scots pine ( Pinus sylvestris L.). Measurements were carried out on south- and north-facing shoots on the third and sixth whorls, respectively, and on an east-facing shoot on the ninth whorl. In another investigation the rates of gas exchange of one-year-old shoots on the third whorl of eight different trees were studied. The measurements were made during June and July, 1977, under non-limiting conditions of soil water. The daily rates of net photosynthesis in whorls three and six followed the light conditions closely, with higher rates for the south side of each whorl and higher for whorl three than six. On whorl nine the shoot had a higher light compensation point and a low rate of photosynthesis at light saturation compared to the other shoot positions. The quantum yield for the shoot on the lowest whorl, as estimated from the linear part of the light response curve, was 50% lower than for shoots on whorl three and six.
The variation in transpiration rates was pronounced within the crown as an effect of differences in the absolute value and diurnal course of stomatal conductance. The variation in net photosynthesis was small between different trees while the variation in transpiration was much higher. Thus the variation in water use efficiency was great. It is concluded that it is possible to extrapolate measurements of net photosynthesis from individual trees up to a stand level without introducing large errors in the estimate. More caution must be paid before extrapolating tree transpiration up to stand transpiration. However, before an extrapolation of gas exchange is made from tree to stand level the variation in net photosynthesis and transpiration rate within the crown must be known.     

Drought stress influences leaf water content,photosynthesis, and water-use efficiency of <Emphasis Type="Italic">Hibiscus rosa-sinensis</Emphasis> at three potassium concentrations

The influence of drought stress (DS) upon whole-plant water content, water relations, photosynthesis, and water-use efficiency of Hibiscus rosa-sinensis cv. Leprechaun (Hibiscus) plants at three levels of potassium (K) nutritional status were determined after a 21-d gradually imposed DS treatment. Compared to K-deficient plants, adequate K supply improved the leaf water content (LWC) and leaf water relations of Hibiscus by decreasing the , and generally sustained rates of net photosynthesis (P _N) and transpiration (E), and stomatal conductance (g _s), both in DS and non-DS plants. In K-deficient Hibiscus, LWC, turgor potential ( _P), and P _N, E, and g _s as well as instantaneous water-use efficiency, WUE (P _N/E) were consistently lower, compared to K-sufficient plants. Carbon isotope discrimination () was lower (i.e. longterm WUE was greatest) in DS than non-DS plants, but K had no effect on during the 21-d drought treatment period under glasshouse conditions. However, the trend in the value of DS plants suggests that could be a useful index of the response of Hibiscus to DS under glasshouse growing conditions. Thus the incorporation of a properly controlled fertilization regime involving sufficient levels of K can improve the acclimation of P _N to low _leaf, increase P _N/E of Hibiscus, and may have potential benefit for other woody plants species.     

幼龄胡杨气孔行为对土壤质地和地下水埋深变化的响应

气孔调节是植物适应水分条件变化的关键途径,研究多变生境中植物气孔行为对认识植物的适应具有重要意义。洪水漫溢新形成的河漫滩是胡杨更新的自然生境,其土壤质地和地下水埋深具高度时空异质性。已有研究主要集中于胡杨对地下水埋深变化的生理生态响应,而对土壤质地与地下水变化交互作用影响植物水分关系的认识不足。通过设置土壤质地(砂土(S₁)、砂壤土(S₂)、黏壤土(S₃)与地下水埋深(W₁(30 cm)、W₂(60 cm)、W₃(90 cm))交互试验模拟幼龄胡杨自然生境,观测分析了不同条件下胡杨气孔导度(G_s)、气孔导度斜率(g₁)、光合的气孔限制(L_s)的变化。研究结果表明：(1)胡杨气孔行为对地下水变化的响应受土壤质地影响;(2)相同地下水埋深时不同土质间G_s具显著差异,W₁时S₂与S₃的G_s...     

Photosynthesis affects following night leaf conductance in Vicia faba

Night-time stomatal opening in C₃ plants may result in significant water loss when no carbon gain is possible. The objective of this study was to determine if endogenous patterns of night-time stomatal opening, as reflected in leaf conductance, in Vicia faba are affected by photosynthetic conditions the previous day. Reducing photosynthesis with low light or low CO₂ resulted in reduced night-time stomatal opening the following night, irrespective of the effects on daytime stomatal conductance. Likewise, increasing photosynthesis with enriched CO₂ levels resulted in increased night-time stomatal opening the following night. Reduced night-time stomatal opening was not the result of an inability to regulate stomatal aperture as leaves with reduced night-time stomatal opening were capable of greater night-time opening when exposed to low CO₂. After acclimating plants to long or short days, it was found that night-time leaf conductance was greater in plants acclimated to short days, and associated with greater leaf starch and nitrate accumulation, both of which may affect night-time guard cell osmotic potential. Direct measurement of guard cell contents during endogenous night-time stomatal opening will help identify the mechanism of the effect of daytime photosynthesis on subsequent night-time stomatal regulation.     

Flooding, gas exchange and hydraulic root conductivity of highbush blueberry

Highbush blueberry plants ( Vaccinium corymbosum L. cv. Bluecrop) growing in containers were flooded in the laboratory for various durations to determine the effect of flooding on carbon assimilation, photosynthetic response to varying CO₂ and O₂ concentrations and apparent quantum yield as measured in an open flow gas analysis system. Hydraulic conductivity of the root was also measured using a pressure chamber. Root conductivity was lower and the effect of increasing CO₂ levels on carbon assimilation less for flooded than unflooded plants after short-(i-2 days), intermediate-(10–14 days) and long-term (35–40 days) flooding. A reduction in O₂ levels surrounding the leaves from 21 to 2% for unflooded plants increased carbon assimilation by 33% and carboxylation efficiency from 0.012 to 0.021 mol CO₂ fixed (mol CO₂)⁻¹. Carboxylation efficiency of flooded plants, however, was unaffected by a decrease in percentage O₂, averaging 0.005 mol CO₂ fixed (mol CO₂)⁻¹. Apparent quantum yield decreased from 2.2 × 10⁻¹ mol of CO₂ fixed (mol light)⁻¹ for unflooded plants to 2.0 × 10⁻³ and 9.0 × 10⁻⁴ for intermediate- and long-term flooding durations, respectively. Shortterm flooding reduced carbon assimilation via a decrease in stomatal conductance, while longer flooding durations also decreased the carboxylation efficiency of the leaf.     

麦田冠层气孔导度的分层研究

小麦灌浆期和乳熟期冠层各层叶片上、下表面的气孔导度之间呈正相关关系;冠层不同层的叶片气孔导度从早到傍晚有平行变化的趋势,数值上存在较大的差异,一般从冠层上到下递减。经分析,这主要与冠层叶片接受的光强自上而下递减有关,且这时所对应的叶片水势自冠层上到下递增的幅度大。测算结果表明,冠层气孔导度白天亦呈明显的日变化,灌浆期的值大于乳熟期的值。     

几个气孔模型在自然条件下的适用性

在自然条件下,用气孔下腔与叶面间的水汽压差（ＶＰＤｓ）取代原有气孔模型中的大气湿度因子,可以明显提高气孔模型在自然条件下的适用性。理论分析指出,在气孔模型中,用ＶＰＤｓ表达气孔导度对湿度的响应与用蒸腾速率表达气孔导度对蒸腾失水的响应是等价的。     

Photosynthetic response of Eragrostis tef to temperature

Photosynthetic characteristics of leaves of tef, Eragrostis tef (Zucc.) Trotter, plants, grown at 25/15°C (day/night), were measured at temperatures from 18 to 48°C. The highest carbon exchange rates (CER) occurred between 36 and 42°C. and averaged 27 μmol m⁻² s⁻¹. At lower or higher temperatures, CER was reduced, but the availability of CO₂ to the mesophyll, measured as internal CO₂ concentration, was highest when temperatures were above or below the optimum for CER. In addition, CER and stomatal conductance were not correlated, but residual conductance was highly correlated with CER (r = 0.98). In additional experiments, relative ¹³C composition for leaf tissue grown at 25, 35 and 45°C averaged -14.4 per mille, confirming that tef is a C₄ grass species. Dry matter accumulation was higher at 35 than at 25, and lowest at 45°C. Leaf CER rates increased hyperbolically with increased light when measured from 0 to 2000 μmol m⁻² s⁻¹ PPFD. The highest CER, 31.8 μ-mol m_-2 s⁻¹, occurred at 35°C and 2000 μmol m⁻² s⁻¹ PPFR. At high light, CER at 25 and 35°C were nearly equal because of higher stomatal conductance at 25°C. Residual conductance was, however, clearly highest at 35°C compared to 25 and 45°C treatments. Stomatal conductance and residual conductance were not correlated in either set of experiments, yet residual conductance was always highest when temperatures were between 35 and 42°C across experiments, suggesting that internal leaf photosynthetic potential was highest across that temperature range.     

黄土高原苹果园蒸腾导度大气驱动规律比较

植物蒸腾导度是表征土壤-植物-大气连续体（SPAC）中植物-大气间水汽传导过程、反映植物水分调控能力的一类重要变量,常见有冠层导度（G_c）、冠层气孔导度（G_s）与叶片气孔导度（g_s）,明确三者在反映冠层蒸腾过程时的异同或关联性对于理解植物水分利用机制具有重要意义。本研究基于对黄土高原果园苹果树生长季内树干液流（J_s）及环境因子的连续观测,计算了G_c、G_s及脱耦联系数（Ω）等变量,并与短期连续观测的叶片气孔导度（g_s）比较,分析了G_c、G_s和g_s在反映冠层蒸腾特征方面的异同及其关系。结果表明,日变化过程中G_s、g_s呈\"单峰\"型曲线,而G_c则呈\"先增后减,午后抬升\"的\"双峰\"型曲线。g_s与G_s存在较紧密的线性关系（R²=0.80）,但与G_c的线性关系较弱（R²=0.02）。G_c、G_s均随大气水汽压亏缺（VPD）的变化呈现确定的规律,其中,上边界函数呈递减的对数函数关系,平均值则符合先增后减的Log-Normal函数关系（R²>0.95）,拐点对应的VPD值分别为1.33和1.16 kPa。在一日内,G_s对VPD变化的响应过程与g_s对VPDL （基于叶片温度计算的水汽压亏缺）变化的响应过程总体一致,其一致性高于G_c对VPD变化的响应。整个生长季（4-10月）中果树的Ω平均值为0.12,随着Ω递减,G_c与G_s的线性相关性愈趋紧密,其斜率呈递增趋势,G_c越来越趋近于G_s。研究结果表明,在北方地区,基于树干液流的监测能较准确的推导整株并估算林分的冠层蒸腾导度。与实测g_s的变化过程比较,G_s比G_c具有更高的一致性,G_s可以作为描述苹果树水分利用过程响应大气驱动的更为恰当的变量。     

Ontogeny and leaf gas exchange mediate the carbon isotopic signature of herbaceous plants

Values (Δ(i)) predicted by a simplified photosynthetic discrimination model, based only on diffusion through air followed by carboxylation, are often used to infer ecological conditions from the 13C signature of plant organs (δ13C(p)). Recent studies showed that additional isotope discrimination (d that includes mesophyll conductance, photorespiration and day respiration, and post-carboxylation discrimination) can strongly affect δ13C(p); however, little is known about its variability during plant ontogeny for different species. Effect of ontogeny on leaf gas exchange rates, Δ(i) , observed discrimination (Δ(p)) and d in leaf, phloem and root of seven herbaceous species at three ontogenetic stages were investigated under controlled conditions. Functional group identity and ontogeny significantly affected Δ(i) and Δ(p). However, predicted Δ(i) did not match Δ(p). d, strongly affected by functional group identity and ontogeny, varied by up to 14 ‰. d scaled tightly with stomatal conductance, suggesting complex controls including changes in mesophyll conductance. The magnitude of the changes in δ13C(p) due to ontogeny was similar to that due to environmental factors reported in other studies. d and ontogeny should, therefore, be considered in ecosystem studies, integrated in ecosystem models using δ13C(p) and limit the applicability of δ13C(leaf) as a proxy for water-use efficiency in herbaceous plants.     

羊草气孔导度的Jarvis-类模型

在干旱半干旱气候条件下,土壤水分状况通常是决定植物气孔导度的重要因素,现有气孔导度模型Jarvis-类和耦合模型(或光合-导度模型)未充分考虑这一因素对气孔导度的影响。本文以Jarvis气孔导度模型为基础,提出一个充分考虑土壤水分状况因素的气孔导度模型。该模型对羊草连续两年(1998~1999)野外实地观测结果拟合良好(R2=0.603),预测能力较线性回归方程(R2=0.361)有明显提高。     

Concurrent measurements of stem density, leaf and stem water potential, stomatal conductance and cavitation on a spaling of Thuja occidentalis L.

Abstract Concurrent estimates of stem density, leaf and stem water potential, stomatal conductance and ultrasonic acoustic emissions (cavitations) in an excised sapling of Thuja occidentalis L. were made. As the sapling dehydrated in air, the decline in leaf water potential to about - 2.0 MPa was followed by apparent rehydration of the foliage while the stem showed no sign of rehydration. The rate of acoustic emissions peaked prior to the onset of rehydration which coincided with virtual stomatal closure. There was a significant decline in stem density until maximum foliage rehydration level was reached. From this point, leaf water potential, stem water potential and stem density continued a relatively slow decline while acoustic emission rate and stomatal conductance remained low. Removal of the bark and majority of foliage from the sapling resulted in increased cavitation and more rapid deelines in leaf and stem water potential and stem density.     

Soil water deficits decrease the internal conductance to CO2 transfer but atmospheric water deficits do not

The internal conductance to CO₂ supply from substomatal cavitiesto sites of carboxylation poses a large limitation to photosynthesis.It is known that internal conductance is decreased by soil waterdeficits, but it is not known if it is affected by atmosphericwater deficits (i.e. leaf to air vapour pressure deficit, VPD).The aim of this paper was to examine the responses of internalconductance to atmospheric and soil water deficits in seedlingsof the evergreen perennial Eucalyptus regnans F. Muell and theherbaceous plants Solanum lycopersicum (formerly Lycopersiconesculentum) Mill. and Phaseolus vulgaris L. Internal conductancewas estimated with the variable J method from concurrent measurementsof gas exchange and fluorescence. In all three species steady-statestomatal conductance decreased by 30% as VPD increased from1 kPa to 2 kPa. In no species was internal conductance affectedby VPD despite large effects on stomatal conductance. In contrast,soil water deficits decreased stomatal conductance and internalconductance of all three species. Decreases in stomatal andinternal conductance under water deficit were proportional,but this proportionality differed among species, and thus therelationship between stomatal and internal conductance differedamong species. These findings indicate that soil water deficitsaffect internal conductance while atmospheric water deficitsdo not. The reasons for this distinction are unknown but areconsistent with soil and atmospheric water deficits having differingeffects on leaf physiology and/or root–shoot communication. Key words: Carbon dioxide, drought, internal conductance, mesophyll conductance, photosynthesis, stomatal conductance, transfer conductance, vapour pressure deficit, water deficit Received 11 October 2007; Revised 9 November 2007 Accepted 15 November 2007