Vertical Leaf Angles Research Articles

Temporal dynamics in vertical leaf angles can confound vegetation indices widely used in Earth observations

Earth observation data is key for monitoring vegetation dynamics across temporal and spatial scales. The most widely used method to estimate vegetation properties from Earth observation data is vegetation indices. However, temporal dynamics in vertical leaf angles can strongly alter reflectance signals and, hence, vegetation indices. Here, we derive leaf angles from plant photographs to simulate the effect on vegetation indices with radiative transfer models. We show that leaf angle dynamics systematically confound widely applied vegetation indices. Moreover, we demonstrate that these effects are not random but tightly linked to abiotic environmental conditions. These systematic effects of vertical leaf angles have implications for monitoring plant properties, biodiversity, and ecosystem functional properties. We discuss the related challenges and opportunities to assess spatio-temporal vegetation dynamics with remote sensing and vegetation indices.

Quantifying water-use efficiency in plant canopies with varying leaf angle and density distribution.

Variation in architectural traits related to the spatial and angular distribution of leaf area can have considerable impacts on canopy-scale fluxes contributing to water-use efficiency (WUE). These architectural traits are frequent targets for crop improvement and for improving the understanding and predictions of net ecosystem carbon and water fluxes. A three-dimensional, leaf-resolving model along with a range of virtually generated hypothetical canopies were used to quantify interactions between canopy structure and WUE by examining its response to variation of leaf inclination independent of leaf azimuth, canopy heterogeneity, vegetation density and physiological parameters. Overall, increasing leaf area index (LAI), increasing the daily-averaged fraction of leaf area projected in the sun direction (Gavg) via the leaf inclination or azimuth distribution and increasing homogeneity had a similar effect on canopy-scale daily fluxes contributing to WUE. Increasing any of these parameters tended to increase daily light interception, increase daily net photosynthesis at low LAI and decrease it at high LAI, increase daily transpiration and decrease WUE. Isolated spherical crowns could decrease photosynthesis by ~60 % but increase daily WUE ≤130 % relative to a homogeneous canopy with equivalent leaf area density. There was no observed optimum in daily canopy WUE as LAI, leaf angle distribution or heterogeneity was varied. However, when the canopy was dense, a more vertical leaf angle distribution could increase both photosynthesis and WUE simultaneously. Variation in leaf angle and density distributions can have a substantial impact on canopy-level carbon and water fluxes, with potential trade-offs between the two. These traits might therefore be viable target traits for increasing or maintaining crop productivity while using less water, and for improvement of simplified models. Increasing canopy density or decreasing canopy heterogeneity increases the impact of leaf angle on WUE and its dependent processes.

Regulation of Leaf Angle Protects Photosystem I under Fluctuating Light in Tobacco Young Leaves.

Fluctuating light is a typical light condition in nature and can cause selective photodamage to photosystem I (PSI). The sensitivity of PSI to fluctuating light is influenced by the amplitude of low/high light intensity. Tobacco mature leaves are tended to be horizontal to maximize the light absorption and photosynthesis, but young leaves are usually vertical to diminish the light absorption. Therefore, we tested the hypothesis that such regulation of the leaf angle in young leaves might protect PSI against photoinhibition under fluctuating light. We found that, upon a sudden increase in illumination, PSI was over-reduced in extreme young leaves but was oxidized in mature leaves. After fluctuating light treatment, such PSI over-reduction aggravated PSI photoinhibition in young leaves. Furthermore, the leaf angle was tightly correlated to the extent of PSI photoinhibition induced by fluctuating light. Therefore, vertical young leaves are more susceptible to PSI photoinhibition than horizontal mature leaves when exposed to the same fluctuating light. In young leaves, the vertical leaf angle decreased the light absorption and thus lowered the amplitude of low/high light intensity. Therefore, the regulation of the leaf angle was found for the first time as an important strategy used by young leaves to protect PSI against photoinhibition under fluctuating light. To our knowledge, we show here new insight into the photoprotection for PSI under fluctuating light in nature.

Drivers of spatial variability in greendown within an oak-hickory forest landscape

Declining near-infrared (NIR) surface reflectance between early and late summer, here termed greendown, is a common, yet poorly understood phenomena in remote sensing time series of temperate deciduous forests. As revealed by phenology analysis of Landsat satellite data, there are strong spatial patterns in the rate of greendown across temperate deciduous forest landscapes, and analyzing these patterns could help advance our understanding of surface reflectance drivers. Within an oak-hickory (Quercus spp. – Carya spp.) forest landscape in western Maryland, USA, we tested how spatial patterns in greendown related to potential drivers at the landscape-, tree crown- and leaf-levels. We found that 50% of the spatial variability in greendown was explained by landscape variables, with greendown particularly higher in locations with higher maximum greenness, more southerly aspects, or locations with greater abundance of white oak (Quercus alba). The importance of species composition as a driver of greendown was supported at the tree crown level, where, relative to three other tree species, white oak exhibited the most consistent trend toward more vertical leaf angles later in the season. At the leaf level, NIR reflectance decreased in productive sites where %N increased, and δ13C decreased, through the season. However, among all sites, there were no consistent seasonal trends in foliar NIR reflectance, and we found no correlation among leaf-level NIR reflectance and satellite-observed greendown. Collectively, these results suggest that the spatial variability of greendown in this oak-hickory forest is most strongly controlled by an interaction of topographic and species compositional drivers operating at the landscape and tree crown levels. We found spatial analysis of greendown to be a useful approach to explore landscape-, tree crown-, and leaf-level controls on surface reflectance, and thereby help translate land surface phenology data into predictions of ecosystem structure and functioning.

Manejo integrado de plantas daninhas na cultura do milho

Estratégia de manejo de plantas daninhas pode ser realizada de forma integrada com o método químico na redução da utilização de herbicidas na cultura do milho. O objetivo deste trabalho é avaliar o efeito do manejo integrado do espaçamento entre linhas, ou de híbridos de milho com características morfológicas distintas e redução dos níveis de herbicidas, para controlar plantas daninhas na cultura do milho. O experimento foi instalado em delineamento de blocos ao acaso, com quatro repetições, arranjado em um fatorial 2 x 2 x 5, em que: o primeiro fator foram os híbridos Penta (superprecoce, com folhas mais planas) e Flash (superprecoce com folhas mais eretas); o segundo fator foram os espaçamentos entre linhas de milho (0,45 e 0,9 m) e o terceiro fator os níveis de manejo de plantas daninhas (capinado, sem capina e atrazine + foramsulfurom + iodosulfuron nas doses de 3000+45+3, 2250+33,8+2,3 e 1500+22,5+1,5 g ha-1 i.a.). Foram avaliados a altura e o ângulo foliar vertical das plantas, o número de plantas daninhas e a matéria seca de plantas daninhas por área. O ângulo foliar de plantas de milho torna-se mais planófilo com a adoção de espaçamentos reduzidos entre linhas, mas outras características de plantas de milho estão envolvidas no desempenho dos espaçamentos. O uso do espaçamento de 0,45 m entre linhas não contribui para a redução dos níveis de herbicidas para a cultura, mas sua adoção beneficia o manejo de plantas daninhas e o rendimento de grãos do híbrido Flash, embora não seja vantajosa para o híbrido Penta.

Water relations of woody plants on contrasting soils during drought: does edaphic compensation account for dry rainforest distribution?

The occurrence of dry rainforest in climates considered drier than the recognised limit for rainforest has been explained by the edaphic compensation hypothesis, which proposed that finer-textured soils facilitate the occurrence of rainforest at climatic extremes. We tested this by examining the effect of soil type on the water relations and plant traits of four dry rainforest species, during a severe drought and subsequent non-drought periods. We predicted plants growing in sandy soils would exhibit higher levels of water stress (lower predawn water potential and stomatal conductance) and possess morphological and physiological traits that more typically reflect drought resistance (late leaf fall in deciduous species, low specific leaf area, vertical leaf angles and stomata that close at low water potential) than those growing in loam soils. During drought, levels of water stress were similar across soil types, while post-drought plants on sandy soils were less stressed. Soil type did not cause shifts in drought tolerance traits, suggesting there has been no ecotypic differentiation of dry rainforest species across soil types for these traits. Hence, we found no support for the edaphic compensation hypothesis in adult plants; future studies should consider other life-cycle stages, such as seedlings.

Changes in photosynthesis, xanthophyll cycle, and sugar accumulation in two North Australia tropical species differing in leaf angles

Two tropical species of North Australia, Acacia crassicarpa and Eucalyptus pellita, have similar leaf size and leaf structure but different leaf angles. A. crassicarpa with near vertical leaf angle directly reduced photon absorption and leaf temperature (T l) and had relatively high photosynthetic activity (P max) and low xanthophyll cycle activity. In contrast, E. pellita with a small leaf angle exhibited high T l, low P max, and high activity of xanthophyll cycle which was useful for the dissipation of excessive energy and reduction of photoinhibition. In the dry season, contents of soluble sugars including pinitol, sucrose, fructose, and glucose in A. crassicarpa increased whereas larger amounts of only fructose and glucose were accumulated in E. pellita. Different sugar accumulation may be involved in osmotic adjustment of leaves during water stress that makes photosynthesis more efficient. The leaf angle may be critical for developing different protective mechanisms in these two tropical tree species that ensure optimal growth in the high irradiance and drought stress environment in North Australia.

A comparison of two canopy radiative models in land surface processes

This paper compares the predictions by two radiative transfer models—the two-stream approximation model and the generalized layered model (developed by the authors) in land surface processes—for different canopies under direct or diffuse radiation conditions. The comparison indicates that there are significant differences between the two models, especially in the near infrared (NIR) band. Results of canopy reflectance from the two-stream model are larger than those from the generalized model. However, results of canopy absorptance from the two-stream model are larger in some cases and smaller in others compared to those from the generalized model, depending on the cases involved. In the visible (VIS) band, canopy reflectance is smaller and canopy absorptance larger from the two-stream model compared to the generalized model when the Leaf Area Index (LAI) is low and soil reflectance is high. In cases of canopies with vertical leaf angles, the differences of reflectance and absorptance in the VIS and NIR bands between the two models are especially large.

Characteristics in Diverse Wear Tolerant Genotypes of Kentucky Bluegrass

Evaluations of Kentucky bluegrass (Poa pratensis L.) wear tolerance have been conducted; however, studies investigating wear mechanisms within this species is limited. This information would be valuable in selecting wear tolerant genotypes. The objective of this study was to identify anatomical and morphological characteristics in diverse wear tolerant Kentucky bluegrasses. Wear treatments were applied to the 2000 National Turfgrass Evaluation Program (NTEP) Kentucky bluegrass field plots in the fall of 2002 at the University of Massachusetts Joseph Troll Turf Research Center at South Deerfield, MA. Wear treatments were applied to 173 genotypes with a differential slip‐wear apparatus, and plots were visually rated for wear injury. The 10 most wear tolerant (TOL) and intolerant (INTOL) genotypes were selected for further evaluation. Twelve characteristics were measured in 2003 and 2004 comparing TOL and INTOL genotypes in field plots and as greenhouse grown spaced‐plants. Characteristics included tiller density, shoot fresh weight and dry weight, shoot water content, leaf turgidity, number of leaves per shoot, leaf width, leaf strength, leaf angle, and leaf cell wall constituents [total cell wall content (TCW), hemicellulose, and lignocellulose]. Not all differences observed in the greenhouse were present in field plots. Significant differences were found between genotypes and TOL and INTOL groupings. No significant interaction with year was detected. Tolerant genotypes were associated with a more vertical leaf angle, greater TCW and lignocellulose content, and a lower shoot moisture content and leaf turgidity. Leaf angle was the single most important attribute separating wear TOL and INTOL genotypes. Biological explanation for the relationship between leaf orientation and wear tolerance is proposed. These characteristics may be useful as potential selection criteria for breeding to improve wear tolerance within this species.

Leaf gas exchange and cholorphyll fluorescence in relation to leaf angle, azimuth, and canopy position in the tropical pioneer tree, Macaranga conifera

We tested the hypothesis that, in tropical pioneer tree species, vertical leaf angle contributes to high carbon gain because it minimizes damage caused by high irradiances. Diurnal changes in leaf gas exchange and chlorophyll fluorescence were measured in east-facing (EL), west-facing (WL) leaves, and in leaves artificially held horizontal (HL) in the uppermost canopy of Macaranga conifera (Zoll.) Muell. Arg. Maximum values of net photosynthetic rate (P(n)) for EL and HL reached 12 &mgr;mol m(-2) s(-1), whereas maximum P(n) for WL was only 6 &mgr;mol m(-2) s(-1). Midday depressions of P(n) and stomatal conductance occurred at high photosynthetic photon flux densities (PPFD), especially for HL. Photosystem II quantum yield (DeltaF/F(m)') of HL for a given PPFD at the leaf surface was lower in the afternoon than in the morning. Values of DeltaF/F(m)' for HL measured at dusk were lower than those measured just before dawn, suggesting that HL suffered from high light and heat load. Variations in the morphology and physiology of the canopy leaves were associated with different light environments, and there was circumstantial evidence of a transitional point at a PPFD of about 20-30% of full sunlight. Maximum P(n) and nitrogen (N) content were higher in upper canopy leaves than in lower canopy leaves, and the differences were mainly associated with differences in lamina thickness. We conclude that the vertical leaf angle and thick lamina of the top canopy leaves contributed to enhance total carbon gain of the whole plant.

‘Photoinhibition’ of Heliconia under natural tropical conditions: the importance of leaf orientation for light interception and leaf temperature

ABSTRACTThe influence of irradiance on photosynthesis under natural conditions was studied in aseasonal Singapore using three Heliconia taxa: H. rostrata, H. psittacorum × H. spathocircinata cv. Golden Torch and H. psittacorum cv. Tay. When grown under full sunlight, all three heliconias exhibited reduced phatosynthetic capacities and lowered chlorophyll content per leaf area as compared with those grown under intermediate and deep shade. A marked decrease in the chlorophyll fluorescence Fv/Fm ratio and an increase in photochemical quenching (1‐ qp) and non‐photochemical quenching (qN) were observed in upper leaves of plants grown under full sunlight. Increases in qN suggest that ‘photoinhibition’ (decreases in Fv/Fm) in Heliconia grown under natural tropical conditions are probably due to photoprotective energy dissipation processes. The quantum yield, the maximum photosynthetic rate, Fv/Fm and the chlorophyll content of upper leaves were lower than those of lower leaves on the same plants grown under full sunlight. Similarly, lower values were obtained for the tip (sun) portion than for the base (shaded) portion of the leaves. The changes in Fv/Fm and in the levels of (1 –qp) in leaves grown under intermediate and deep shade were negligible in plants during the course of day. However, there was a steep decrease in Fv/Fm and an increase in the levels of (1 –qp), along with an increase in incident light in the sun leaves. The lowest Fv/Fm and the highest level of (1 –qp) indicated minimum PSII efficiency at midday in full sun. These results indicate that, in Heliconia, the top leaves (particularly leaf tips) experienced sustained decreases in PSII efficiency upon exposure to full sunlight. Although all three taxa exhibited sustained decreases in photosynthetic capacity in full sunlight, the sun leaves of ‘Tay’ showed higher photosynthetic capacity than those of the other two taxa. This could be due, at least in part, to the vertical leaf angle and smaller lamina area. When the upright leaves of ‘Tay’ were constrained to a horizontal angle, they exhibited lower PSII efficiency (FvIFm ratio), while horizontal leaves of ‘Rostrata’ and ‘Golden Torch’ inclined lo near‐vertical angles showed increased efficiency. Thus, an increase in leaf angle helps to achieve a reduction in the sustained decrease in PSII efficiency by decreasing the levels of incident sunlight and subsequently the leaf temperature.

RESPONSE OF TROPIC LEAF MOVEMENTS AND PHOTOSYNTHETIC GAS EXCHANGE TO WATER AVAILABILITY IN N2‐FIXING AND NO3‐FED SOYBEAN

Leaf movements, water status, and gas exchange were measured in soybean inoculated with Bradyrhizobium and grown under high and low soil water and nitrate availabilities. We hypothesized that paraheliotropism in low NO3‐grown plants (which have greater N2 fixation rates) would differ from that of high NO3‐grown plants (which have lower N2 fixation rates), such that carbon return on nitrogen investment into photosynthesis would be enhanced. Low NO3‐low water plants had more vertical leaf angles and received lower solar irradiances at midday than high NO3‐low water plants. Under constant, vertical illumination, low NO3‐low water plants had steeper leaf angles, increased rates of leaf movement, lower photosaturated photosynthetic rates, and lower stomatal conductances for a given leaf water potential than high NO3‐low water plants. Leaves of high NO3 plants had lower photosynthetic nitrogen use efficiencies than did low NO3 plants. Low water plants had lower leaf osmotic potentials and ratios of intercellular/ambient CO2 concentration than high water plants, but NO3 treatment did not affect these parameters. Results provided support for our initial hypothesis, and demonstrated a high degree of correspondence between gas exchange and heliotropic response to soil nitrogen and water availabilities in soybean.

Response of Tropic Leaf Movements and Photosynthetic Gas Exchange to Water Availability in N 2 -Fixing and NO 3 -Fed Soybean

Leaf movements, water status, and gas exchange were measured in soybean inoculated with Bradyrhizobium and grown under high and low soil water and nitrate availabilities. We hypothesized that paraheliotropism in low NO3-grown plants (which have greater N2 fixation rates) would differ from that of high NO3-grown plants (which have lower N2 fixation rates), such that carbon return on nitrogen investment into photosynthesis would be enhanced. Low NO3-low water plants had more vertical leaf angles and received lower solar irradiances at midday than high NO3-low water plants. Under constant, vertical illumination, low NO3-low water plants had steeper leaf angles, increased rates of leaf movement, lower photosaturated photosynthetic rates, and lower stomatal conductances for a given leaf water potential than high NO3-low water plants. Leaves of high NO3 plants had lower photosynthetic nitrogen use efficiencies than did low NO3 plants. Low water plants had lower leaf osmotic potentials and ratios of intercellular/ambient CO2 concentration than high water plants, but NO3 treatment did not affect these parameters. Results provided support for our initial hypothesis, and demonstrated a high degree of correspondence between gas exchange and heliotropic response to soil nitrogen and water availabilities in soybean.

Influence of solar radiation and leaf angle on leaf xanthophyll concentrations in mangroves.

Mangroves have similar xanthophyll cycle components/chlorophyll ratios [i.e. (V+A+Z)/chl] to other plant species. (V+A+Z)/chl ratios were sensitive to the light environment in which leaves grew, decreasing as light levels decreased over a vertical transect through a forest canopy. The (V+A+Z)/chl ratio also varied among species. However, in sun leaves over all species, the (V+A+Z)/chl ratios correlate with the proportion of leaf area displayed on a horizontal plane, which is determined by leaf angle. Thus, leaf angle and the xanthophyll cycle may both be important in providing protection from high light levels in mangrove species. A canopy survey assessed whether (V+A+Z)/chl ratios could be correlated with species dominance of exposed positions in forest canopies.Rhizophora mangroves, with near-vertical leaf angles, andBruguiera parviflora, with small, horizontal, xanthophyllrich leaves, dominated the canopy, whileB. gymnorrhiza, a species with large, horizontally arranged leaves, was less abundant at the top of the canopy. Thus, two different strategies for adapting to high solar radiation levels may exist in these species. The first strategy is avoidance through near vertical leaf angles, and the second is a large capacity to dissipate energy through zeaxanthin. The (V+A+Z)/chl ratio was also negatively correlated with the epoxidation state of the xanthophyll cycle pool (the proportion present as violaxanthin and half that present as antheraxanthin) at midday. This suggested that the requirement for dissipation of excess light (represented by the midday epoxidation state) may influence the (V+A+Z)/chl ratio.

Leaf Structures of Xerohalophytes from an East Jordanian Salt Pan

Leaves from 13 perennial halophytes growing in typical zonations along a vegetational transect were comparatively studied for their morphological-anatomical features. With SEM, curvatures of the outer epidermal cells walls, epicuticular waxes, trichomes and prickle hairs were depicted. Using light microscopy, mesophyll orientation, fortification of the outer periclinal epidermal cell walls, sunken stomata, C4-anatomy, enrolled leaves, salt glands and leaf succulence were investigated. The ultrastructure of the salt gland of Reaumuria alternifolia was analysed by TEM. On the basis of their leaf structures the species were subdivided into a xeromorphic and a mesomorphic leaf type group. In all species of the xeromorphic group marked curvatures of the outer epidermal cell walls in combination with a tight cover of waxes were observed, which did not occur in the mesomorphic group. In contrast, a tight layer of trichomes or bladders was, except for one species, only present in the mesomorphic group. Vertical leaf angles by flagging or parahelionasty were only observed amongst the mesomorphous species. Enrolled leaves only occurred in the xeromorphous Frankenia hirsuta. All leaves except the amplexicaulous ones of Tamarix passerinoides had an isobilateral mesophyll structure. In all xeromorphous species, except for one, the stomata were either deeply sunken or protected and combined with a pronounced fortification of the outer periclinal epidermal cell walls. In the mesomorphic leaf type group, the stomata are sunken in half of the species only and the outer periclinal epidermal cell walls are not at all or only slightly fortified. Three of the four C4-plants had a xeromorphous leaf structure. Seven species were crinohalophytes, three were leaf succulent euhalophytes and three could be termed as pseudohalophytes; they are all distributed among the mesomorphic as well as in the xeromorphic leaf type group. Projected on the transect, no continuous gradient of xeromorphy can be seen. In nine out of eleven zones species of the different leaf type groups coexist due to small heterogeneities of the sites.