Leaf gas films,underwater photosynthesis and plant species distributions in a flood gradient

Traits for survival during flooding of terrestrial plants include stimulation or inhibition of shoot elongation, aerenchyma formation and efficient gas exchange. Leaf gas films form on superhydrophobic cuticles during submergence and enhance underwater gas exchange. The main hypothesis tested was that the presence of leaf gas films influences the distribution of plant species along a natural flood gradient. We conducted laboratory experiments and field observations on species distributed along a natural flood gradient. We measured presence or absence of leaf gas films and specific leaf area of 95 species. We also measured, gas film retention time during submergence and underwater net photosynthesis and dark respiration of 25 target species. The presence of a leaf gas film was inversely correlated to flood frequency and duration and reached a maximum value of 80% of the species in the rarely flooded locations. This relationship was primarily driven by grasses that all, independently of their field location along the flood gradient, possess gas films when submerged. Although the present study and earlier experiments have shown that leaf gas films enhance gas exchange of submerged plants, the ability of species to form leaf gas films did not show the hypothesized relationship with species composition along the flood gradient.     

Three-year field response of young olive trees (Olea europaea L., cv. Arbequina) to soil salinity: Trunk growth and leaf ion accumulation

Irrigated olive is rapidly increasing in arid and semiarid areas, many of which may be negatively affected by soil salinity. We evaluated changes in trunk growth and leaf Cl^–, Na⁺ and K⁺ concentrations in young Arbequina olives (Olea europaea L.) grown in a saline-sodic field over a three-year period. The trunk diameter was measured at the beginning and the end of the 1999 (70 trees), 2000 (59 trees) and 2001 (42 trees) growing periods. Leaves, sampled in August of each year, were analyzed for Cl^–, Na⁺ and K⁺ concentrations. Soil salinity (apparent electrical conductivity, ECa) of each monitored tree was measured 14 times during the 1999–2001 experimental period with an electromagnetic sensor and converted to root zone electrical conductivity of the soil saturation extract (ECe) based on ECa–ECe calibration curves. Salinity tolerance was determined using the Maas and Hoffman threshold–slope response model. Based on salinity thresholds (ECe_thr), the tolerance of olive in terms of trunk growth was high in 1999 (ECe_thr = 6.7 dS m^–1), but declined with age and time of exposure to salts by 30% in 2000 (ECe_thr = 4.7 dS m^–1) and by 55% in 2001 (ECe_thr = 3.0 dS m^–1). Based on the high absolute slopes obtained in all years (values between 16% and 23% dS^–1 m), olive was classified as very sensitive to ECe values above the threshold. Trunk growth thresholds based on leaf ion concentrations varied, depending on years, between 2.6 and 4.0 mg g^–1 (Cl_thr) and between 1.0 and 1.2 mg g^–1 (Na_thr), indicating that Arbequina olive was less sensitive to leaf Cl^– and much more sensitive to leaf Na⁺ than values reported as toxic in greenhouse studies. Leaf K⁺ slightly decreased with increasing salinity, whereas the K⁺/Na⁺ ratio sharply decreased with increasing salinity. We concluded that the initial salinity tolerance of olive was high, but declined sharply with time of exposure to salts and became quite sensitive due primarily to increasing toxic concentrations of Na⁺ in the leaves.     

Salinity and flooding level as determinants of soil solution composition and nutrient content inPanicum hemitomum

A greenhouse experiment was conducted to examine the effects of salinity and flooding level on interstitial solute speciation and solute uptake byPanicum hemitomum grown on intact marsh substrates. The experimental set-up consisted of a factorial arrangement of treatments (5 salinity levels×3 flooding levels) with 4 replications.Salinity treatments with the addition of salt (Instant Ocean^r) successfully increased interstitial pore water conductivities and resulted in significantly different treatment means. Redox potentials and proton activities were significantly higher in the drained treatment, with only minor differences between the two flooded treatments. There was not a significant pH effect due to salinity, although a significant interaction between salinity and flooding level was observed. Analysis of variance suggested that electro-chemical and interstitial solute behaviour could significantly be described by salinity and flooding treatments.GEOCHEM calculations were performed in order to relate leaf concentrations to ion activities in interstitial soil solutions. Leaf contents of Mg, Ca, K, Mn, and Cu were significantly correlated with the activities of corresponding ions in the interstitial pore water. However, most of the variabilitiy in leaf metal content could be accounted for by treatment effects. Regression analysis showed that the ion activities explained less than 25% of the variability in leaf metal content.     

龙眼叶片膜脂脂肪酸组分与龙眼耐寒性的关系

龙眼叶片膜脂不饱和脂肪酸含量和脂肪酸不饱和度与龙眼不同品种的耐寒性呈正相关;在年周期中,不饱和脂肪酸含量的变化与龙眼耐寒性的变化呈现平行关系;龙眼叶片膜脂脂肪酸组分含量反映着龙眼品种间耐寒性遗传上的差异,可作为耐寒性的鉴定指标。     

Mesostructure of the ginseng photosynthetic apparatus in relation to ecological strategy of the species

Mesostructure of the photosynthetic apparatus was investigated in the wild plants of ginseng (Panax ginseng C.A. Mey, Araliaceae) taken from various habitats. The revealed features of the structure of phototrophic tissues (large cells, small number of photosynthesizing cells and chloroplasts per leaf area unit, low values of membrane indices of the cells and chloroplasts) point to stress-tolerant type of ecological strategy ginseng pursues in Nature.     

Development and structure of drought-tolerant leaves of the Mediterranean shrub Capparis spinosa L

Capparis spinosa (caper), a winter-deciduous perennial shrub, is a consistent floristic element of Mediterranean ecosystems, growing from May to October, i.e. entirely during the prolonged summer drought. The internal architecture of young and fully expanded leaves was studied, along with certain physiological characteristics. Capparis spinosa possesses thick, amphistomatic and homobaric leaves with a multilayered mesophyll. The latter possesses an increased number of photosynthesizing cells per unit leaf surface, a large surface area of mesophyll cells facing intercellular spaces (Smes) and a low percentage of intercellular space per tissue volume. Smes and chlorophyll content attain their maximum values synchronously, slightly before full leaf expansion. Nitrogen investment is also completed before full leaf expansion. The structural features, in combination with the water status, could contribute to enhanced rates of transpiration and photosynthesis under field water shortage conditions.     

利用离体叶片鉴定杨树耐盐潜力

以受体杨树107和转基因杨树18-1的一年生枝条为材料,采用Hoagland营养液水培方法,添加不同浓度的NaCl,检测二者植株生长以及叶中Na＋和K＋含量的变化。结果表明,在0.6%NaCl处理下18-1生根率明显高于107,生物量较大,叶片中Na＋积累量是107的1.45倍左右。以107和18-1离体叶片为材料,NaCl处理下测定其生理活性指标,发现18-1叶盘的失绿速度和相对电导率都显著低于107叶盘。把离体叶片接种在改良MS培养基上,1.2%NaCl处理可使107叶盘几乎停止伸长,细胞大小不再增加;而18-1叶盘培养7天后伸长了近50%。以上结果表明利用离体叶片可以鉴定出不同基因型杨树的耐盐潜力。     

Ontogeny, understorey light interception and simulated carbon gain of juvenile rainforest evergreens differing in shade tolerance

Background and Aims

A long-running debate centres on whether shade tolerance of tree seedlings is mainly a function of traits maximizing net carbon gain in low light, or of traits minimizing carbon loss. To test these alternatives, leaf display, light-interception efficiency, and simulated net daily carbon gain of juvenile temperate evergreens of differing shade tolerance were measured, and how these variables are influenced by ontogeny was queried.

Methods

The biomass distribution of juveniles (17–740 mm tall) of seven temperate rainforest evergreens growing in low (approx. 4 %) light in the understorey of a second-growth stand was quantified. Daytime and night-time gas exchange rates of leaves were also determined, and crown architecture was recorded digitally. YPLANT was used to model light interception and carbon gain.

Results

An index of species shade tolerance correlated closely with photosynthetic capacities and respiration rates per unit mass of leaves, but only weakly with respiration per unit area. Accumulation of many leaf cohorts by shade-tolerant species meant that their ratios of foliage area to biomass (LAR) decreased more gradually with ontogeny than those of light-demanders, but also increased self-shading; this depressed the foliage silhouette-to-area ratio (STAR), which was used as an index of light-interception efficiency. As a result, displayed leaf area ratio (LAR_d = LAR × STAR) of large seedlings was not related to species shade tolerance. Self-shading also caused simulated net daily carbon assimilation rates of shade-tolerant species to decrease with ontogeny, leading to a negative correlation of shade tolerance with net daily carbon gain of large (500 mm tall) seedlings in the understorey.

Conclusions

The results suggest that efficiency of energy capture is not an important correlate of shade tolerance in temperate rainforest evergreens. Ontogenetic increases in self-shading largely nullify the potential carbon gain advantages expected to result from low respiration rates and long leaf lifespans in shade-tolerant evergreens. The main advantage of their long-lived leaves is probably in reducing the costs of crown maintenance.     

Submergence-induced leaf acclimation in terrestrial species varying in flooding tolerance

Earlier work on the submergence-tolerant species Rumex palustris revealed that leaf anatomical and morphological changes induced by submergence enhance underwater gas exchange considerably. Here, the hypothesis is tested that these plastic responses are typical properties of submergence-tolerant species. Submergence-induced plasticity in leaf mass area (LMA) and leaf, cell wall and cuticle thickness was investigated in nine plant species differing considerably in tolerance to complete submergence. The functionality of the responses for underwater gas exchange was evaluated by recording oxygen partial pressures inside the petioles when plants were submerged. Acclimation to submergence resulted in a decrease in all leaf parameters, including cuticle thickness, in all species irrespective of flooding tolerance. Consequently, internal oxygen partial pressures (pO(2)) increased significantly in all species until values were close to air saturation. Only in nonacclimated leaves in darkness did intolerant species have a significantly lower pO(2) than tolerant species. These results suggest that submergence-induced leaf plasticity, albeit a prerequisite for underwater survival, does not discriminate tolerant from intolerant species. It is hypothesized that these plastic leaf responses may be induced in all species by several signals present during submergence; for example, low LMA may be a response to low photosynthate concentrations and a thin cuticle may be a response to high relative humidity.     

Is leaf dry matter content a better predictor of soil fertility than specific leaf area?

Background and Aims

Specific leaf area (SLA), a key element of the ‘worldwide leaf economics spectrum’, is the preferred ‘soft’ plant trait for assessing soil fertility. SLA is a function of leaf dry matter content (LDMC) and leaf thickness (LT). The first, LDMC, defines leaf construction costs and can be used instead of SLA. However, LT identifies shade at its lowest extreme and succulence at its highest, and is not related to soil fertility. Why then is SLA more frequently used as a predictor of soil fertility than LDMC?

Methods

SLA, LDMC and LT were measured and leaf density (LD) estimated for almost 2000 species, and the capacity of LD to predict LDMC was examined, as was the relative contribution of LDMC and LT to the expression of SLA. Subsequently, the relationships between SLA, LDMC and LT with respect to soil fertility and shade were described.

Key Results

Although LD is strongly related to LDMC, and LDMC and LT each contribute equally to the expression of SLA, the exact relationships differ between ecological groupings. LDMC predicts leaf nitrogen content and soil fertility but, because LT primarily varies with light intensity, SLA increases in response to both increased shade and increased fertility.

Conclusions

Gradients of soil fertility are frequently also gradients of biomass accumulation with reduced irradiance lower in the canopy. Therefore, SLA, which includes both fertility and shade components, may often discriminate better between communities or treatments than LDMC. However, LDMC should always be the preferred trait for assessing gradients of soil fertility uncoupled from shade. Nevertheless, because leaves multitask, individual leaf traits do not necessarily exhibit exact functional equivalence between species. In consequence, rather than using a single stand-alone predictor, multivariate analyses using several leaf traits is recommended.     

Role of leaf apoplast in silicon-mediated manganese tolerance of Cucumis sativus L.

Silicon (Si) supplied as sodium silicate (1·8 mm ) clearly decreased symptoms of manganese (Mn) toxicity in Cucumis sativus L. (cv. Chinesische Schlange) grown in nutrient solution with low to elevated Mn concentrations (0·5–1000 µm ). Despite approximately the same total Mn content in the leaves, plants not treated with Si had higher Mn concentrations in the intercellular washing fluid (IWF) compared with plants treated with Si, especially in the BaCl₂‐ and DTPA‐exchangeable fraction of the leaf apoplast. The Mn concentration of the IWF correlated positively with the severity of Mn‐toxicity symptoms and negatively with the Si supply. Furthermore, in Si‐treated plants less Mn was located in the symplast (< 10%) and more Mn was bound to the cell wall (> 90%) compared with non‐Si‐treated plants (about 50% in each compartment). Manganese present in Si‐treated plants is therefore less available and for this reason less toxic than in plants not treated with Si. It is concluded that Si‐mediated tolerance of Mn in C. sativus is a consequence of stronger binding of Mn to cell walls and a lowering of Mn concentration within the symplast. These results support the role of Si as an important beneficial element in plant nutrition.