Predicting shifts in the functional composition of tropical forests under increased drought and CO2 from trade‐offs among plant hydraulic traits

Tropical forest responses are an important feedback on global change, but changes in forest composition with projected increases in CO₂ and drought are highly uncertain. Here we determine shifts in the most competitive plant hydraulic strategy (the evolutionary stable strategy or ESS) from changes in CO₂ and drought frequency and intensity. Hydraulic strategies were defined along a spectrum from drought avoidance to tolerance by physiology traits. Drought impacted competition more than CO₂, with elevated CO₂ reducing but not reversing drought‐induced shifts in the ESS towards more tolerant strategies. Trait plasticity and/or adaptation intensified these shifts by increasing the competitive ability of the drought tolerant relative to the avoidant strategies. These findings predict losses of drought avoidant evergreens from tropical forests under global change, and point to the importance of changes in precipitation during the dry season and constraints on plasticity and adaptation in xylem traits to forest responses.     

拟南芥微丝加帽蛋白β亚基参与壳梭孢素诱导的气孔开放过程

气孔是植物响应外源信号,与环境进行水分和气体交换的门户。由外源信号引起的保卫细胞微丝骨架动态变化在气孔运动中发挥重要作用,但是具体的精确调节机制仍不清楚。微丝结合蛋白家族(ABPs) 是微丝动态组装最直接的调控者,它们的作用不容忽视。本文运用反向遗传学,以微丝结合蛋白—加帽蛋白 (CP) β-亚基 (CPB) 突变体cpb-3为实验材料,探究其在壳梭孢素 (FC)诱导气孔开放中的作用。结果发现:离体叶片干燥3 h,cpb-3突变体的叶片失水率为63.45%,明显高于野生型的48.99%。气孔开度测量及激光共聚焦显微镜观察发现,cpb-3突变体的气孔开放程度以及微丝动态重排对FC分子更敏感。气孔开度相比野生型增大了20% (P<0.05),含辐射状微丝排布的保卫细胞数量比例增幅达到58.3%,比对照组高出18.5%。此外,非损伤微测技术记录保卫细胞Ca²⁺、K⁺等跨膜运输动态,FC处理下,cpb-3突变体保卫细胞中Ca²⁺外流速度升至212.86 pmol cm^-2s^-1,野生型仅为68.76 pmol cm^-2s^-1,明显快于野生型。且K⁺内流也有相同表现。综上表明,微丝加帽蛋白CP的β亚基CPB可能通过调节保卫细胞微丝骨架动态重排以及离子流动,在FC诱导的气孔运动中发挥重要的作用。     

Using unmanned aerial vehicle‐based multispectral,RGB and thermal imagery for phenotyping of forest genetic trials: A case study in Pinus halepensis

The assessment of genetic differentiation in functional traits is fundamental towards understanding the adaptive characteristics of forest species. While traditional phenotyping techniques are costly and time‐consuming, remote sensing data derived from cameras mounted on unmanned aerial vehicles (UAVs) provide potentially valid high‐throughput information for assessing morphophysiological differences among tree populations. In this work, we test for genetic variation in vegetation indices (VIs) and canopy temperature among populations of Pinus halepensis as proxies for canopy architecture, leaf area, photosynthetic pigments, photosynthetic efficiency and water use. The interpopulation associations between vegetation properties and above‐ground growth (stem volume) were also assessed. Three flights (July 2016, November 2016 and May 2017) were performed in a genetic trial consisting of 56 populations covering a large part of the species range. Multispectral (visible and near infrared wavelengths), RGB (red, green, blue) and thermal images were used to estimate canopy temperature and vegetation cover (VC) and derive several VIs. Differences among populations emerged consistently across flights for VC and VIs related to leaf area, indicating genetic divergence in crown architecture. Population differences in indices related to photosynthetic pigments emerged only in May 2017 and were probably related to a contrasting phenology of needle development. Conversely, the low population differentiation for the same indices in July 2016 and November 2016 suggested weak interpopulation variation in the photosynthetic machinery of mature needles of P. halepensis. Population differences in canopy temperature found in July 2016 were indicative of variation in stomatal regulation under drought stress. Stem volume correlated with indices related to leaf area (positively) and with canopy temperature (negatively), indicating a strong influence of canopy properties and stomatal conductance on above‐ground growth at the population level. Specifically, a combination of VIs and canopy temperature accounted for about 60% of population variability in stem volume of adult trees. This is the first study to propose UAV remote sensing as an effective tool for screening genetic variation in morphophysiological traits of adult forest trees.     

Modeling leaf CO2 assimilation and Photosystem II photochemistry from chlorophyll fluorescence and the photochemical reflectance index

We present a simple model to assess the quantum yield of photochemistry (Φ_P) and CO₂ assimilation rate from two parameters that are detectable by remote sensing: chlorophyll (chl) fluorescence and the photochemical reflectance index (PRI). Φ_P is expressed as a simple function of the chl fluorescence yield (Φ_F) and nonphotochemical quenching (NPQ): Φ_P = 1–bΦ_F(1 + NPQ). Because NPQ is known to be related with PRI, Φ_P can be remotely assessed from solar‐induced fluorescence and the PRI. The CO₂ assimilation rate can be assessed from the estimated Φ_P value with either the maximum carboxylation rate (V_cmax), the intercellular CO₂ concentration (C_i), or parameters of the stomatal conductance model. The model was applied to experimental data obtained for Chenopodium album leaves under various environmental conditions and was able to successfully predict Φ_F values and the CO₂ assimilation rate. The present model will improve the accuracy of assessments of gas exchange rates and primary productivity by remote sensing.     

Exceptional Photosynthetic Performance of Capparis spinosa L. Under Adverse Conditions of Mediterranean summer

Diurnal and seasonal fluctuations in water potential (), stomatal conductance (g _s), transpiration rate (E), and net photosynthetic rate (P _N) were monitored in Capparis spinosa L., a Mediterranean plant growing during summer, i.e. at the period considered the most stressful for local plant life. In spite of the complete absence of rain, exhibited a modest drop at midday (–2.7 MPa), but was fully recovered overnight, indicating sufficient access to water sources. The stomata remained open throughout the day and season and the high E resulted in leaf temperatures up to 3.9 °C below air temperature. Additionally, P _N of the fully exposed leaves was higher than 25 mol m^–2 s^–1 for more than 10 h per day throughout the summer growth period. No symptoms of photooxidative stress were shown, as judged by maximum photosystem 2 photochemical efficiency (F_v/F_m) and the function of xanthophyll cycle. Indeed, diurnal inter-conversions of the xanthophyll cycle components were modest during the summer and a more intensive function of the cycle was only evident during leaf senescence in autumn. In comparison with a semi-deciduous and an evergreen sclerophyll co-existing in the same ecosystem, C. spinosa assimilated up to 3.4 times more CO₂ per m² during its growth period (May to October) and up to 1.8 times more on an annual basis.     

Hydraulic responses to height growth in maritime pine trees

As trees grow taller, decreased xylem path conductance imposes a major constraint on plant water and carbon balance, and is thus a key factor underlying forest productivity decline with age. The responses of stomatal conductance, leaf area: sapwood area ratio (A_L : A_S) and soil–leaf water potential gradient (ΔΨ_S–L) to height growth were investigated in maritime pine trees. Extensive measurements of in situ sap flow, stomatal conductance and (non‐gravitational) needle water potential (_L = Ψ_L ? ρ_wgh) were made during 2 years in a chronosequence of four even‐aged stands, under both wet and dry soil conditions. Under wet soil conditions, _L was systematically lower in taller trees on account of differences in gravitational potential. In contrast, under dry soil conditions, our measurements clearly showed that _L was maintained above a minimum threshold value of ?2.0 MPa independently of tree height, thus limiting the range of compensatory change in ΔΨ_S–L. Although a decrease in the A_L : A_S ratio occurred with tree height, this compensation was not sufficient to prevent a decline in leaf‐specific hydraulic conductance, K_L (50% lower in 30 m trees than in 10 m trees). An associated decline in stomatal conductance with tree height thus occurred to maintain a balance between water supply and demand. Both the increased investment in non‐productive versus productive tissues (A_S : A_L) and stomatal closure may have contributed to the observed decrease in tree growth efficiency with increasing tree height (by a factor of three from smallest to tallest trees), although other growth‐limiting responses (e.g. soil nutrient sequestration, increased respiratory costs) cannot be excluded.     

分根区施保水剂对玉米气孔导度和单叶WUE的影响

盆栽条件下,研究了陕单9号玉米(zea mays L．)在根区不施保水剂(对照)、分根区施保水剂和根区全施保水剂3种处理下,叶片气孔导度、CO2吸收和H2O蒸腾的变化。结果表明,在75％土壤饱和持水量下,各指标在3种处理之间没有明显差别;在50％土壤饱和持水量下,分根区施保水剂显著降低了叶片气孔导度,叶片CO2吸收量和H2O蒸腾量也同时降低,但H2O蒸腾量下降幅度更大;在两种水分条件下,分根区施保水剂均能提高玉米单叶水分利用效率(water use efficiency,WUE)。     

Photosynthetic responses of the tropical spiny shrub Lycium nodosum (Solanaceae) to drought, soil salinity and saline spray

Water relations and photosynthetic characteristics of plants of Lycium nodosum grown under increasing water deficit (WD), saline spray (SS) or saline irrigation (SI) were studied. Plants of this perennial, deciduous shrub growing in the coastal thorn scrubs of Venezuela show succulent leaves which persist for approx. 1 month after the beginning of the dry season; leaf succulence is higher in populations closer to the sea. These observations suggested that L. nodosum is tolerant both to WD and salinity. In the glasshouse, WD caused a marked decrease in the xylem water potential (psi), leaf osmotic potential (psi(s)) and relative water content (RWC) after 21 d; additionally, photosynthetic rate (A), carboxylation efficiency (CE) and stomatal conductance (gs) decreased by more than 90 %. In contrast, in plants treated for 21 d with a foliar spray with 35 per thousand NaCl or irrigation with a 10 % NaCl solution, psi and RWC remained nearly constant, while psi(s) decreased by 30 %, and A, CE and gs decreased by more than 80 %. An osmotic adjustment of 0.60 (SS) and 0.94 MPa (SI) was measured. Relative stomatal and mesophyll limitations to A increased with both WD and SS, but were not determined for SI-treated plants. No evidence of chronic photoinhibition due to any treatment was observed, since maximum quantum yield of PSII, Fv/Fm, did not change with either drought in the field or water or salinity stress in the glasshouse. Nevertheless, WD and SI treatments caused a decrease in the photochemical (qP) and an increase in the non-photochemical (qN) quenching coefficients relative to controls; qN was unaffected by the SS treatment. The occurrence of co-limitation of A by stomatal and non-stomatal factors in plants of L. nodosum may be associated with the extended leaf duration under water or saline stress. Additionally, osmotic adjustment may partly explain the relative maintenance of A and gs in the SS and SI treatments and the tolerance to salinity of plants of this species in coastal habitats.