Structural and functional variability within the canopyand its relevance for carbon gain and stress avoidance

The functional variability in leaf angle distribution within the canopy was analysed with respect to regulation of light interception and photoprotection. Leaf orientation strongly determined the maximum photochemical efficiency of PSII (F_v/F_m) during summer: horizontal leaves were highly photoinhibited whereas vertical leaf orientation protected the leaves from severe photoinhibition. The importance of leaf orientation within the canopy was analysed in two Mediterranean macchia species with distinct strategies for drought and photoinhibition avoidance during summer. The semi-deciduous species (Cistus monspeliensis) exhibited strong seasonal but minimal spatial variability in leaf orientation. Reversible structural regulation of light interception by vertical leaf orientation during summer protected the leaves from severe photoinhibition. The evergreen sclerophyll (Quercus coccifera) exhibited high spatial variability in leaf angle distribution throughout the year and was less susceptible to photoinhibition. The importance of both strategies for plant primary production was analysed with a three-dimensional canopy photoinhibition model (CANO-PI). Simulations indicated that high variability in leaf angle orientation in Q. coccifera resulted in whole-plant carbon gain during the summer, which was 94 % of the maximum rate achieved by theoretical homogeneous leaf orientations. The high spatial variability in leaf angle orientation may be an effective compromise between efficient light harvesting and avoidance of excessive radiation in evergreen plants and may optimize annual primary production. Whole plant photosynthesis was strongly reduced by water stress and photoinhibition in C. monspeliensis; however, the simulations indicated that growth-related structural regulation of light interception served as an important protection against photoinhibitory reduction in whole-plant carbon gain.     

Seed Bank and Understorey Species Composition in a Semi-arid Environment: The Effect of Shrub Age and Rainfall

Understorey vegetation in patches of Retama sphaerocarpa shrubsin semi-arid environments is dependent on the overstorey shrublife history. Community structure changes with shrub age asa result of physical amelioration of environmental conditionsby the canopy and organic matter accumulation in the soil. Weinvestigated the effect of the canopy on understorey speciesdiversity in the field and its relationships with the soil seedbank under 50 shrubs from 5 to 25+ years old, and compared speciescomposition in the field in a wet and a dry year. Species compositionof the soil seed bank under R. sphaerocarpa shrubs did not differsignificantly with shrub age, but seed density increased asthe shrubs aged. In the field, community composition changedwith shrub age, increasing species richness in a process thatdepended on the amount of spring rainfall. Our results suggestthat the soil seed bank is rather uniform and that the shrubcanopy strongly selects which species appear in the understorey.There were seeds of many species present under both young andold shrubs but which only established under old shrubs. Thisshowed dispersal was not limiting species abundance and suggestedthat the canopy was an important sorting factor for speciespresent in the understorey. Less frequent species contributedthe most to patch diversity, and rainfall effectively controlledspecies emergence. Understorey community composition dependedon multiple interspecific interactions, such as facilitationby the shrub and competition from neighbours, as well as ondispersal processes. Facilitation in this environment is a keyfeature in the structuring of plant communities and in governingecosystem functioning. Copyright 2000 Annals of Botany Company Community structure, competition, dispersal, facilitation, species composition, rainfall variability, Retama sphaerocarpa, seed bank, semi-arid environments     

Comparative Physiology and Growth of Two Perennial Tussock Grass Species in a Semi-Arid Environment

The physiology and growth ofLygeum spartumandStipa tenacissima,two perennial tussock grasses which dominate wide areas of semi-aridsouth-eastern Spain, were compared at times of high and lowavailability of water, in autumn and summer respectively, tostudy the adaptation of this growth form to arid environments.The two species differed in morphological and physiologicaltraits.Stipatussocks were larger and had opportunistic growth,andStipaleaves had a smaller specific area and lower diffusiveconductance to water vapour. The two grasses were similar inmaximum photosynthetic rate, leaf nitrogen concentration, andin the response to high light conditions but had different tissuewater relations.Stipawas better suited to cope with droughtand erratic rainfall, because of its more effective controlof water loss and its growth patterns.Lygeumseems to be adaptedto less dry conditions and more saline soils. The tussock growthform provides an adaptive advantage in these infertile environmentsby reducing radiation absorbance. leaf extension; Lygeum spartum; perennial grassland; semi-arid environments; Stipa tenacissima; tussock grass; water relations; Adaptation; growth; photosynthesis     

Light Distribution in Discontinuous Canopies: Calculation of Leaf Areas and Canopy Volumes Above Defined 'Irradiance Contours' for use in Productivity Modelling

Plant canopies can be considered as assemblages of leaves, stemsand fruits growing in zones of differing irradiance demarcatedby contours of mean irradiance as measured on a horizontal surface. The following general equations have been derived to calculatethe leaf area (L_I) and the canopy volume (CV_I) in zones externalto any chosen contour of mean irradiance: (1) L_I = ((1nl)/(–K)(I–T_f) or leaf area index (LAI) if this is less (2) CV_I = L_I/(leaf area density m² m^–2), where I is the specified value of irradiance (horizontal surface)expressed as a decimal fraction of that above the canopy, Kis the appropriate extinction coefficient and T_f is the proportionof the total of available radiation which, if the canopy isdiscontinuous, would reach the ground by passing through gapsbetween the discrete canopy units. Where the canopy is continuousT_f is zero so expression (1) simplifies to L₁ = 1n I/–K(or LAI if this is less). For a range of model hedgerow orchards of varying dimensions,spacings and LAIs, it has been shown that the use of these equationsgives very similar results to those obtained by detailed calculationof light penetration. They therefore seem to be of potentialuse in calculating both potential dry-matter production by discontinuouscanopies of any type and, in the case of orchard fruit crops,the potential effect of changes in tree size, leaf area density,spacing etc. on the canopy volume in which irradiation is adequatefor fruit bud initiation and fruit colour development. light distribution, discontinuous canopy, irradiance contours, leaf area index, orchards     

Crown architecture in sun and shade environments: assessing function and trade-offs with a three-dimensional simulation model

Sun and shade environments place markedly different constraints on the photosynthetic performance of plants. Leaf-level photosynthetic responses to sun and shade have been extensively investigated, whereas there has been much less research on the functional role of crown architecture in these environments. This paper focuses on the role of architecture in maximizing light capture and photosynthesis in shaded understories and in minimizing exposure to excess radiation in open high light environments. Understanding these contrasting roles of architecture is facilitated by application of a three-dimensional structural-functional model, Y-plant. Surveys of understory plants reveal a diversity of architectures but a strong convergence at only modest light-capture efficiencies because of significant self-shading. Simulations with Psychotria species revealed that increasing internode lengths would increase light-capture efficiencies and whole plant carbon gain. However, the costs of the additional required biomechanical support was high, which, in terms of relative growth rates, would override the advantage provided by higher light-capture efficiencies. In high light environments, leaf angles and self-shading provide structural photoprotection, minimizing potential damage from photoinhbition. Simulations reveal that without these structural protections photoinhibition of photosynthesis is likely to be much greater with daily carbon gain significantly reduced.     

UV-B-induced damage and photoreactivation in three species of Boeckella (Copepoda, Calanoida)

Solar ultraviolet (UV) radiation poses a threat to most livingorganisms. Aquatic organisms have evolved three basic mechanismsto cope with harmful levels of radiation. Two mechanisms, avoidance(e.g. vertical migration) and photoprotection (e.g. productionof photoprotective compounds that act as filters, antioxidants,etc.), serve to minimize the dose of UV radiation that reachesthe organism's vital structures (DNA, membranes, etc.). Thethird mechanism, repair (e.g. dark repair mechanisms, such asnucleotide excision repair; or photoreactivation mechanisms,such as photoenzymatic repair), serves to repair the damagefollowing UV exposure. Here, we compare the vulnerability toUV-B radiation of three copepod species (Boeckella brevicaudata,Boeckella gibbosa, and Boeckella gracilipes) that occur in lakesthat differ in UV-B penetration and depth. Our aim was to gaininsight into the significance of each of the three mechanismsin different UV-B environments. Results from a 3-day ‘insitu’ incubation in ultra-oligotrophic Lake Toncek showedthat B.gracilipes is highly vulnerable to UV-B and UV-A radiation.In contrast, virtually no mortality was observed in B.gibbosaand B.brevicaudata during the same period. In order to discriminatethe contribution of photoprotection and photoreactivation, thethree species were subsequently exposed in the laboratory toan artificial source of UV-B radiation, both in the presenceand absence of visible radiation (recovery radiation). The photoprotectionpotential (i.e. resistance to UV-B in the absence of recoveryradiation) of B.gracilipes and B.gibbosa was lower than thatof B.brevicaudata. On the other hand, photoreactivation (higherresistance to UV-B in the presence of recovery radiation) wasobserved in B.brevicaudata and B.gibbosa, but not in B.gracilipes.To cope with damaging UV-B levels in nature, B.gracilipes dependsexclusively on the attenuation by the external media (i.e. avoidance).Although B.gibbosa tends to avoid the surface waters of lakes,it also occurs in shallow transparent pools. Most likely itsability to survive in these shallow, high UV environments isdue to its photoreactivation potential. Finally, despite itsoccurrence in highly turbid lakes, B.brevicaudata seems extremelywell suited to cope with UV-B radiation thanks to a combinationof photoreactivation and photoprotection.     

Growth, Allocation and Water Relations of Shade-grownQuercus rubraL. Saplings Exposed to a Late-season Canopy Gap

For understory saplings to exploit canopy gaps successfully,carbon gain must increase in the gap environment. We predictedthat total biomass of shade-grown red oak saplings would increaseafter exposure to a late-season canopy gap, and that increasedwater and nutrient demand within the canopy gap would drivechanges in the allocation of this carbon. Shade-grown red oaksaplings acclimated to gaps by increasing biomass during theseason of gap formation and increasing the potential for carbongain in the following summer. Within-season carbon gain didnot result from greater production of leaf area, so it mostlikely arose from higher photosynthetic rates of existing shade-developedfoliage, which may be linked to accumulation of leaf nitrogen.During the season of gap formation, shade-gap plants increasedallocation to storage of total non-structural carbohydrates(TNC), and to root growth. The increase in TNC storage suggeststhat shade-developed saplings exposed to gaps were also primedfor fast growth and carbon gain in the following summer. Theincrease in root growth suggests that higher nutrient and waterdemand drove allocation shifts to enhance the capacity for nutrientand water uptake in the gap. Plant hydraulic conductivity (K_a)of shade-grown plants was limited upon exposure to the gap,possibly because of embolism formation resulting from the abruptincrease in water demand. Greater water potential gradientscompensated for limitations to K_a, allowing saplings to maintainhigh transpiration rates, suggesting that actual water uptakeof shade-gap plants was unaffected by gap exposure. Acclimation; canopy gaps; carbon allocation; hydraulic conductivity; nitrogen allocation; non-structural carbohydrates;Quercus rubra L.; red oak; stomatal conductance; transpiration; water-relations     

Soil fertility and herb facilitation mediated by Retama sphaerocarpa

Abstract. Woody legumes growing in dry climates can increase soil nutrient content and facilitate plant growth in their understorey. We investigated differences in soil fertility and herbaceous community in relation to the presence and absence of the legume Retama sphaerocarpa in a shrubland in a mediterranean type climate. The results showed a higher content of limiting nutrients for plant growth, such as phosphorus and nitrogen, under the R. sphaerocarpa canopy. Herbaceous biomass, cover and nitrogen content increased below the canopy. However, species richness and diversity were diminished in the presence of a dense canopy of R. sphaerocarpa. Nitrogen isotopic analysis did not indicate a clear relationship between symbiotic fixation in R. sphaerocarpa and nitrogen content of soil and plants under its canopy. Nevertheless, herbs growing in the understorey showed a linear correlation between foliar N content and ¹⁵N values. The existence of a dense shrub canopy induced a smaller monthly variation in herb richness, diversity and biomass, suggesting that it provides a stable microhabitat facilitating herbaceous vegetation establishment and growth. The ‘shelter’ effect was more important when environmental conditions became stressful for herbaceous plants.     

Partial compensation in Psychotria marginata (Rubiaceae) after simulated defoliation increases photon capture and photosynthesis

We used Y-plant, a computer-based model of plant crown architecture analysis, to simulate effects of defoliation on daily canopy carbon gain in Psychotria marginata (Rubiaceae) plants under two contrasting irradiances. Five levels of defoliation were simulated using two different types of leaf blade damage. Compensatory increases in photon-saturated photosynthetic capacity (P _max) of 25, 50, and 100 % defoliation were also simulated. In all simulations daily photon capture and CO₂ assimilation increased with defoliation. However, without a compensatory response, daily canopy carbon gain also decreased with defoliation. Under high irradiance, reduction in daily canopy carbon gain was less than what would be expected if the response was proportional to leaf area reduction. Thus, 25 and 50 % defoliation resulted in only 20 and 41 % of daily canopy carbon gain reduction, respectively. In the scenario where 25 % of the leaf area was removed, if the P _max value was increased by 25 %, the remaining leaves compensated for 94 % of the daily canopy carbon relative to an undamaged non-compensated plant. At the same defoliation level, incrementing P _max values by 50 and 100 % resulted in overcompensation. Hence, because the increment of daily photon capture and CO₂ assimilation after defoliation was more a passive consequence of the reduction in leaf area than an active response, under the conditions tested photosynthetic compensation could be only possible through an active mechanism such as the increment of P _max values.     

Structure, Production and Leaf Area Dynamics: A Comparison of Lodgepole Pine and Sunflower

Populations of lodgepole pine (Pinus contorta var latifoliaEngelm) and sunflower (Heltanthus annuus L) were examined tocompare the influence of stand density on canopy structure andthe association between canopy structure and the productionof stem volume The plastic responses of canopy structure tocompetitive interactions indicated that the structural dynamicsof lodgepole pine and sunflower populations were similar, thoughthe degree of plastic response differed The influence of canopystructure on the production of stem volume, however, was fundamentallydifferent efficiency decreased as the mean crown size of lodgepolepine populations increased, but this decrease did not occurin sunflower populations This difference was attributed to thelarge accumulation of branch biomass required to display foliageeffectively in the canopies of the perennial lodgepole pine,accumulation which does not occur in the annual sunflower Lodgepole pine, Pinus contorta var latifolia Engelm, sunflower, Helianthus annuus L, leaf area, canopy structure, stand production, carbon allocation     

A tropical epiphytic orchid uses a low‐light interception strategy in a spatially heterogeneous light environment

Light is considered a non‐limiting factor for vascular epiphytes. Nevertheless, an epiphyte's access to light may be limited by phorophyte shading and the spatio‐temporal environmental patchiness characteristic of epiphytic habitats. We assessed the extent to which potential light interception in Rodriguezia granadensis, an epiphytic orchid, is determined by individual factors (plant size traits and leaf traits), or environmental heterogeneity (light patchiness) within the crown of the phorophyte, or both. We studied 104 adult plants growing on Psidium guajava trees in two habitats with contrasting canopy cover: a dry tropical forest edge, and isolated trees in a pasture. We recorded the number of leaves and the leaf area, the leaf position angles, and the potential exposure of the leaf surface to direct irradiance (silhouette area of the leaf blade), and the potential irradiance incident on each plant. We found the epiphytes experience a highly heterogeneous light environment in the crowns of P. guajava. Nonetheless, R. granadensis plants displayed a common light interception strategy typical of low‐light environments, resembling terrestrial, forest understory plants. Potential exposure of the total leaf surface to direct irradiance correlated positively with plant size and within‐plant variation in leaf orientation. In many‐leaved individuals, within‐plant variation in leaf angles produced complementary leaf positions that enhanced potential light interception. This light interception strategy suggests that, in contrast to current wisdom, enhancing light capture is important for vascular epiphytes in canopies with high spatio‐temporal heterogeneity in light environments.     

Antioxidant enzyme activities in shoots from three mycorrhizal shrub species afforested in a degraded semi-arid soil

Mycorrhizae may help plants to thrive in Mediterranean semi-arid ecosystems by altering antioxidant enzyme activities. Our objective was to determine the influence of mycorrhizal inoculation with an allochthonous arbuscular mycorrhizal (AM) fungus, Glomus claroideum , Schenck & Smith, or with a mixture of native AM fungi, on the activity of antioxidant enzymes from shoots of Olea europaea L. ssp. sylvestris , Retama sphaerocarpa (L.) Boissier and Rhamnus lycioides L. seedlings afforested in a degraded Mediterranean semi-arid soil. One year after planting, shoot biomass of inoculated O. europaea seedlings was about 630%, of non-inoculated plants. Shoot biomass of G. claroideum -colonized R. sphaerocarpa was greater than that of seedlings inoculated with the mixed native AM fungi after 12 months. Inoculation with a mix of native AM fungi was the most effective treatment for increasing shoot biomass and N, P and K contents in shoot tissues of R. lycioides . Both mycorrhizal inoculation treatments increased the nutrient contents in shoots of O. europaea and R. lycioides . In O. europaea plants, the inoculation treatments increased catalase, ascorbate peroxidase and dehydroascorbate reductase activities, but not monodehydroascorbate reductase and glutathione reductase activities. Inoculation with G. claroideum increased the activities of all antioxidant enzymes in R. sphaerocarpa . Monodehydroascorbate reductase, glutathione reductase and superoxide dismutase activities in R. lycioides leaves were preferentially increased by inoculation with the mixture of native AM fungi. This work support the view that increased antioxidant enzyme activities could be involved, at least in part, in the beneficial effects of mycorrhizal colonization on the performance of shrub species grown under semi-arid Mediterranean conditions.     

Drought can be more critical in the shade than in the sun: a field study of carbon gain and photo-inhibition in a Californian shrub during a dry El Niño year

Diurnal courses of leaf water potential (Ψ_l), gas exchange and chlorophyll fluorescence were measured in natural sun and shade populations of Heteromeles arbutifolia throughout the seasons of an unusually dry El Niño year in Central California. The onset of drought resulted in decreased stomatal conductance and net photosynthesis in both sun and shade plants. However, the decline in Ψ_l was much greater and carbon gain was much more strongly limited by the development of drought stress in the shade than in the sun. Photorespiratory energy dissipation was significantly higher in the sun than in the shade in spring and autumn, but not during the summer. Pre‐dawn photochemical efficiency (F_v/F_m) was significantly higher in the shade than in the sun during the spring but the differences disappeared during the summer and autumn. The strong irradiance in the open field site studied led to a chronic but only mild reduction in F_v/F_m, with values around 0·79. Summer sunflecks led to a sustained photo‐inhibition in shade plants, which exhibited a significant reduction in pre‐dawn F_v/F_m of 10% with the onset of drought. Photo‐inhibition became relatively more important for carbon gain in the shade than in the sun due to the low photochemical efficiency under the low light that follows sunflecks. Sun plants of H. arbutifolia exhibited a rather efficient photoprotection against strong irradiance conferred by both the architecture of the crown and the physiology of the leaves. There is evidence that El Niño events and the associated droughts have become more frequent and severe. Counter‐intuitively, the effects on plant performance of such extreme droughts could be more critical in the shade than in the sun.     

Microenvironmental Heterogeneity and Space Utilization by Desert Vines within their Host Trees

The three-dimensional biomass distribution and the microenvironmentsexperienced by several desert vine species growing within thecanopy of host trees were studied at the Centro Ecológicode Sonora in México. The light environment within thecrown of the host tree Cercidium microphyllum showed a horizontaland vertical gradient from the base of the trunk to the edgesof the canopy. Within this gradient total daily photosyntheticallyactive radiation (PAR) varied from 47.8 mol m^-2outside the crownto 4.6 mol m^-2at the centre of the crown and close to the ground.Maximum air temperature was 3 °C lower beneath the crownthan outside. Within the canopy, most vines experienced lessthan 50% of the daily available PAR outside the canopy. Formost of the day, leaves of vines received 15% or less of themaximum available PAR. Our study shows that vines do not growtowards full sunlight but rather they exploit different habitatpossibilities within their host tree crown. Leaves along thestems of vines experienced a wide range of light environments,showing coefficients of variation (CV) in total daily PAR from36.4 to 94.6%. Daily courses of PAR also showed that leaveswithin the canopy experienced short-term temporal variationin the light environment. Differences in CV of daily PAR valuesand preferences in heterogeneous light microenvironments amongspecies suggested that different vine species might be spatiallyseparated in the canopy. We suggest that in desert habitats,conditions within the crown of host trees result in an importantmicrohabitat that vines can exploit, allowing them to avoidthe high light, temperature and water deficits found in thesurrounding environment. Copyright 1999 Annals of Botany Company Sonoran Desert, vines, host trees, canopy light environment.     

Mechanisms of interaction between a leguminous shrub and its understorey in a semi-arid environment

Retama sphaerocarpa shrubs in semi-and environments often have a dense understorey of annual and perennial herbs forming so-called "islands of fertility" The effect of the canopy on soil fertility and microclimate and the combined effect of canopy and litter on species diversity and productivity were assessed under Retama spliaerocarpa shrubs in a semi-and environment in southeast Spain Soil chemical properties differed significantly among three positions under the canopy, particularly between inner and outer positions The potential mineralization rate of organic matter was significantly higher in soils from an intermediate position under the canopy than in soils from either the centre and the edge Soil chemical fertility and estimated soil seed bank were highest also in soil at an intermediate position and lowest in soil from the edge The understorey flora was favoured by the lower temperature and irradiation and increased soil fertility under the shrubs canopies Species emerging from the soil seed bank separated clearly into two groups which occupied inner and outer positions under the canopy Different levels of addition of Retama litter significantly decreased species richness and the number of emerged seedlings In the field, pots placed near the centre, at an intermediate position and at the edge of the canopy of Retama shrubs differed significantly in species richness and biomass production Overall, micro-climatic conditions combined with increased chemical fertility of the soil and inhibitory effects of litter to produce a large biomass of herbs at intermediate positions between the centre and the edge of the canopy. The high proportion of litter from annual species in that position increases the mineralization rate and hence nutrient dynamics in a process which also could benefit the shrub     

Dynamics of Vertical Leaf Nitrogen Distribution in a Vegetative Wheat Canopy. Impact on Canopy Photosynthesis

The development of vertical canopy gradients of leaf N has beenregarded as an adaptation to the light gradient that helps tomaximize canopy photosynthesis. In this study we report thedynamics of vertical leaf N distribution during vegetative growthof wheat in response to changes in N availability and sowingdensity. The question of to what extent the observed verticalleaf N distribution maximized canopy photosynthesis was addressedwith a leaf layer model of canopy photosynthesis that integratesN-dependent leaf photosynthesis according to the canopy lightand leaf N distribution. Plants were grown hydroponically attwo amounts of N, supplied in proportion to calculated growthrates. Photosynthesis at light saturation correlated with leafN. The vertical leaf N distribution was associated with thegradient of absorbed light. The leaf N profile changed duringcrop development and was responsive to N availability. At highN supply, the leaf N profiles were constant during crop development.At low N supply, the leaf N profiles fluctuated between moreuniform and steep distributions. These changes were associatedwith reduced leaf area expansion and increasing N remobilizationfrom lower leaf layers. The distribution of leaf N with respectto the gradient of absorbed irradiance was close to the theoreticaloptimum maximizing canopy photosynthesis. Sensitivity analysisof the photosynthesis model suggested that plants maintain anoptimal vertical leaf N distribution by balancing the capacityfor photosynthesis at high and low light. Copyright 2000 Annalsof Botany Company Canopy photosynthesis, leaf nitrogen distribution, nitrogen, Triticum aestivum L, wheat     

Vascular Plants and Biocrusts Modulate How Abiotic Factors Affect Wetting and Drying Events in Drylands

Understanding how organisms control soil water dynamics is a major research goal in dryland ecology. Although previous studies have mostly focused on the role of vascular plants on the hydrological cycle of drylands, recent studies highlight the importance of biological soil crusts formed by lichens, mosses, and cyanobacteria (biocrusts) as a major player in this cycle. We used data from a 6.5-year study to evaluate how multiple abiotic (rainfall characteristics, temperature, and initial soil moisture) and biotic (vascular plants and biocrusts) factors interact to determine wetting and drying processes in a semi-arid grassland from Central Spain. We found that the shrub Retama sphaerocarpa and biocrusts with medium cover (25–75%) enhanced water gain and slowed drying compared with bare ground areas (BSCl). Well-developed biocrusts (>75% cover) gained more water, but lost it faster than BSCl microsites. The grass Stipa tenacissima reduced water gain due to rainfall interception, but increased soil moisture retention compared to BSCl microsites. Biotic modulation of water dynamics was the result of different mechanisms acting in tandem and often in opposite directions. For instance, biocrusts promoted an exponential behavior during the first stage of the drying curve, but reduced the importance of soil characteristics that accentuate drying rates. Biocrust-dominated microsites gained a similar amount of water than vascular plants, although they lost it faster than vascular plants during dry periods. Our results emphasize the importance of biocrusts for water dynamics in drylands, and illustrate the potential mechanisms behind their effects. They will help to further advance theoretical and modeling efforts on the hydrology of drylands and their response to ongoing climate change.     

Phenotypic plasticity in response to light in the coffee tree

Phenotypic plasticity to light availability was examined at the leaf level in field-grown coffee trees (Coffea arabica). This species has been traditionally considered as shade-demanding, although it performs well without shade and even out-yields shaded coffee. Specifically, we focused our attention on the morpho-anatomical plasticity, the balance between light capture and excess light energy dissipation, as well as on physiological traits associated with carbon gain. A wide natural light gradient, i.e., a diurnal intercepted photon irradiance differing by a factor of 25 between the deepest shade leaves and the more exposed leaves in the canopy, was explored. Responses of most traits to light were non-linear, revealing the classic leaf sun vs. leaf shade dichotomy (e.g., compared with sun leaves, shade leaves had a lower stomatal density, a thinner palisade mesophyll, a higher specific leaf area, an improved light capture, a lower respiration rate, a lower light compensating point and a limited capacity for photoprotection). The light-saturated rates of net photosynthesis were higher in sunlit than in shade leaves, although sun leaves were not efficient enough to use the extra light supply. However, sun leaves showed well-developed photoprotection mechanisms in comparison to shade leaves, which proved sufficient for avoiding photoinhibition. Specifically, a higher non-photochemical quenching coefficient was found in parallel to increases in: (i) zeaxanthin pools, (ii) de-epoxidation state of the xanthophyll cycle, and (iii) activities of some antioxidant enzymes. Intracanopy plasticity depended on the suite of traits considered, and was high for some physiological traits associated with photoprotection and maintenance of a positive carbon balance under low light, but low for most morpho-anatomical features. Our data largely explain the successful cultivation of the coffee tree in both exposed and shade environments, although with a poor resource-use efficiency in high light.     

Carbon gain and photosynthetic response of chrysanthemum to photosynthetic photon flux density cycles

Most models of carbon gain as a function of photosynthetic irradiance assume an instantaneous response to increases and decreases in irradiance. High- and low-light-grown plants differ, however, in the time required to adjust to increases and decreases in irradiance. In this study the response to a series of increases and decreases in irradiance was observed in Chrysanthemum × morifolium Ramat. “Fiesta” and compared with calculated values assuming an instantaneous response. There were significant differences between high- and low-light-grown plants in their photosynthetic response to four sequential photosynthetic photon flux density (PPFD) cycles consisting of 5-minute exposures to 200 and 400 micromoles per square meter per second (μmol m⁻²s⁻¹). The CO₂ assimilation rate of high-light-grown plants at the cycle peak increased throughout the PPFD sequence, but the rate of increase was similar to the increase in CO₂ assimilation rate observed under continuous high-light conditions. Low-light leaves showed more variability in their response to light cycles with no significant increase in CO₂ assimilation rate at the cycle peak during sequential cycles. Carbon gain and deviations from actual values (percentage carbon gain over- or underestimation) based on assumptions of instantaneous response were compared under continuous and cyclic light conditions. The percentage carbon gain overestimation depended on the PPFD step size and growth light level of the leaf. When leaves were exposed to a large PPFD increase, the carbon gain was overestimated by 16 to 26%. The photosynthetic response to 100 μmol m⁻² s⁻¹ PPFD increases and decreases was rapid, and the small overestimation of the predicted carbon gain, observed during photosynthetic induction, was almost entirely negated by the carbon gain underestimation observed after a decrease. If the PPFD cycle was 200 or 400 μmol m⁻² s⁻¹, high- and low-light leaves showed a carbon gain overestimation of 25% that was not negated by the underestimation observed after a light decrease. When leaves were exposed to sequential PPFD cycles (200-400 μmol m⁻² s⁻¹), carbon gain did not differ from leaves exposed to a single PPFD cycle of identical irradiance integral that had the same step size (200-400-200 μmol m⁻² s⁻¹) or mean irradiance (200-300-200 μmol m⁻² s⁻¹).     

Structural cost of shoot modules and its implications on plant potential fitness in a Mediterranean perennial shrub, Retama sphaerocarpa (L.) Boiss

This paper introduces a methodology to analyse the structural costs on plant potential fitness, empirically exemplified in the hierarchical shoot system of a Mediterranean perennial plant, Retama sphaerocarpa (L.) Boiss. During growing season every year (March-August), the terminal shoot (which is the basic unit of growth) develops inflorescences, flowers and fruits, as well as new shoots (first-, second- and third-order branching shoots) which have the potential to "behave" as terminal shoots in the following year. Different morphological and demographical aspects of the modules within the terminal shoot were measured in 100 terminal shoots selected from different plants of a natural population of R. sphaerocarpa. Complementary samples of 100 shoots of different branching orders were collected to obtain biomass estimations of the terminal shoots. We propose a simple procedure to estimate structural cost (biomass investment) on plant potential fitness (flowering buds) as a methodology for interpreting and comparing the consequences on fitness of different plant growth patterns. The results of this study exemplify how differential allocation patterns among plant structural modules, depending on their position within the shoot system, can be quantified to estimate their influence upon plant potential fitness.