Substantial differences occur between canopy and ambient climate: Quantification of interactions in a greenhouse-canopy system.

Organ temperature and variation therein plays a key role in plant functioning and its responses to e.g. climate change. There is a strong feedback between organ, especially leaf, temperature and the climate within the canopy (canopy climate), which in turn interacts with the climate outside the canopy (ambient climate). For greenhouses, the determinants of this interplay and how they drive differences between canopy and ambient climate are poorly understood. Yet, as many experiments on both regular greenhouse crops and field crops are done in greenhouses, this is crucial to know. Therefore, we designed an experiment to quantify the differences between ambient and canopy climate and leaf temperature. A path analysis was performed to quantify the interactions between components of the greenhouse canopy-climate system. We found that with high radiation the canopy climate can be up to 5°C cooler than the ambient climate, while for cloudy days this was only 2°C. Canopy relative humidity (RH) was up to 25% higher compared to ambient RH. We showed that radiation is very important for these climate differences, but that this effect could be partly counteracted by turning off supplementary light (i.e. due to its indirect effects e.g. changing light distribution). Leaf temperature was substantially different, both higher and lower, from the canopy air temperature. This difference was determined by leaf area index (LAI), temperature of the heating pipe and the use of supplementary light, which all strongly influence radiation, either shortwave or thermal radiation. The difference between leaf and ambient air temperature could be decreased by decreasing the LAI or increasing the temperature of the heating pipe.

A high throughput method for quantifying number and size distribution of Arabidopsis seeds using large particle flow cytometry.

BACKGROUND: Seed size and number are important plant traits from an ecological and horticultural/agronomic perspective. However, in small-seeded species such as Arabidopsis thaliana, research on seed size and number is limited by the absence of suitable high throughput phenotyping methods. RESULTS: We report on the development of a high throughput method for counting seeds and measuring individual seed sizes. The method uses a large-particle flow cytometer to count individual seeds and sort them according to size, allowing an average of 12,000 seeds/hour to be processed. To achieve this high throughput, post harvested seeds are first separated from remaining plant material (dust and chaff) using a rapid sedimentation-based method. Then, classification algorithms are used to refine the separation process in silico. Accurate identification of all seeds in the samples was achieved, with relative errors below 2%. CONCLUSION: The tests performed reveal that there is no single classification algorithm that performs best for all samples, so the recommended strategy is to train and use multiple algorithms and use the median predictions of seed size and number across all algorithms. To facilitate the use of this method, an R package (SeedSorter) that implements the methodology has been developed and made freely available. The method was validated with seed samples from several natural accessions of Arabidopsis thaliana, but our analysis pipeline is applicable to any species with seed sizes smaller than 1.5 mm.

Drought responses, phenotypic plasticity and survival of Mediterranean species in two different microclimatic sites.

Climate models predict a further drying of the Mediterranean summer. One way for plant species to persist during such climate changes is through acclimation. Here, we determine the extent to which trait plasticity in response to drought differs between species and between sites, and address the question whether there is a trade-off between drought survival and phenotypic plasticity. Throughout the summer we measured physiological traits (photosynthesis - Amax , stomatal conductance - gs , transpiration - E, leaf water potential - ψl) and structural traits (specific leaf area - SLA, leaf density - LD, leaf dry matter content - LDMC, leaf relative water content - LRWC) of leaves of eight woody species in two sites with slightly different microclimate (north- versus south-facing slopes) in southern Spain. Plant recovery and survival was estimated after the summer drought period. We found high trait variability between species. In most variables, phenotypic plasticity was lower in the drier site. Phenotypic plasticity of SLA and LDMC correlated negatively with drought survival, which suggests a trade-off between them. On the other hand, high phenotypic plasticity of SLA and LDMC was positively related to traits associated with rapid recovery and growth after the drought period. Although phenotypic plasticity is generally seen as favourable during stress conditions, here it seemed beneficial for favourable conditions. We propose that in environments with fluctuating drought periods there can be a trade-off between drought survival and growth during favourable conditions. When climate become drier, species with high drought survival but low phenotypic plasticity might be selected for.

Elevated CO2 and nitrogen availability have interactive effects on canopy carbon gain in rice.

• Here we analysed the effects of CO2 (Ca ) elevation and nitrogen availability on canopy structure, leaf area index (LAI) and canopy photosynthesis of rice (Oryza sativa). • Rice was grown at ambient and elevated Ca (c. 200 µmol mol-1 above ambient, using the free-air CO2 enrichment, FACE) and at two N availabilities. We measured leaf area, area-based leaf N contents and leaf photosynthesis, and calculated net daily canopy photosynthesis. • FACE plants had higher light-saturated rates of photosynthesis (Pmax ) and apparent quantum yields than ambient plants, when measured at their own growth CO2 . Ca elevation reduced the total leaf N in the canopy (Nleaf ) but had no effect on LAI, and the average leaf N content (Nleaf /LAI) was therefore reduced by 8%. This reduction corresponded well with our model predictions. Leaf area index increased strongly with N availability, which was also consistent with our model. • Calculated canopy photosynthesis increased more strongly with Nleaf under elevated than under ambient Ca . This indicates that there is an N × Ca interactive effect on canopy carbon gain. This interaction was caused by the increase in LAI with N availability, which enhanced the positive effect of the higher quantum yield under Ca elevation.

Canopy structure and nitrogen distribution in dominant and subordinate plants in a dense stand of Amaranthus dubius L. with a size hierarchy of individuals.

The objective was to investigate how nitrogen allocation patterns in plants are affected by their vertical position in the vegetation (i.e. being either dominant or subordinate). A garden experiment was carried out with Amaranthus dubius L., grown from seed, in dense stands in which a size hierarchy of nearly equally aged individuals had developed. A small number of dominant plants had most of their leaf area in the highest layers of the canopy while a larger number of subordinate plants grew in the shade of their dominant neighbours. Canopy structure, vertical patterns of leaf nitrogen distribution and leaf photosynthetic characteristics were determined in both dominant and subordinate plants. The light distribution in the stands was also measured. Average N contents per unit leaf area (total canopy nitrogen divided by the total leaf area) were higher in the dominant than in the subordinate plants and this was explained by the higher average MPA (leaf dry mass per unit area) of the dominant plants. However, when expressed on a weight basis, average N contents (LNCav; total canopy N divided by the total dry weight of leaves) were higher in the subordinate plants. It is possible that these higher LNCav values reflect an imbalance between carbon and nitrogen assimilation with N uptake exceeding its metabolic requirement. Leaf N content per unit area decreased more strongly with decreasing relative photon flux density in the dominant than in the subordinate plants showing that this distribution pattern can be different for plants which occupy different positions in the light gradient in the canopy. The amount of N which is reallocated from the oldest to the younger, more illuminated leaves higher up in the vegetation may depend on the sink strength of the younger leaves for nitrogen. In the subordinate plants, constrained photosynthetic activity caused by shading might have reduced the sink intensity of these leaves.

Patterns of light and nitrogen distribution in relation to whole canopy carbon gain in C3 and C4 mono- and dicotyledonous species.

An analytical model was used to describe the optimal nitrogen distribution. From this model, it was hypothesized that the non-uniformity of the nitrogen distribution increases with the canopy extinction rate for light and the total amount of free nitrogen in the canopy, and that it is independent of the slope of the relation between light saturated photosynthesis (Pm) and leaf nitrogen content (nL). These hypotheses were tested experimentally for plants with inherently different architectures and different photosynthetic modes. A garden experiment was carried out with a C3 monocot [rice, Oryza sativa (L.)], a C3 dicot [soybean, Glycine max (L.) Merr] a C4 monocot [sorghum, Sorghum bicolor (L.) Moensch] and a C4 dicot [amarantus, Amaranthus cruentus (L.)]. Leaf photosynthetic characteristics as well as light and nitrogen distribution in the canopies of dense stands of these species were measured. The dicot stands were found to have higher extinction coefficients for light than the monocot stands. Dicots also had more non-uniform N distribution patterns. The main difference between the C3 and C4 species was that the C4 species were found to have a greater slope value of the leaf-level Pm-nL relation. Patterns of N distribution were similar in stands of the C3 and C4 species. In general, these experimental results were in accordance with the model predictions, in that the pattern of nitrogen allocation in the canopy is mainly determined by the extinction coefficient for light and the total amount of free nitrogen.

Leaf nitrogen distribution in relation to leaf age and photon flux density in dominant and subordinate plants in dense stands of a dicotyledonous herb.

We studied the effects of photon flux density (PFD) and leaf position, a measure of developmental age, on the distribution of nitrogen content per unit leaf area (N area) in plants of different heights, in dense stands grown at two nitrogen availabilities and in solitary plants of the erect dicotyledonous herb Xanthium canadense. Taller more dominant plants received higher PFD levels and experienced a larger difference in relative PFD between their youngest and oldest leaves than shorter subordinate plants in the stands. Differences in PFD between leaves of solitary plants were assumed to be minimal and differences in leaf traits, found for these plants, could thus be mainly attributed to an effect of leaf position. In the solitary plants, N area decreased with leaf position while in the plants from the stands it decreased with decreasing relative PFD, indicating both factors to be important in determining the distribution of N area. Due to the effect of leaf position on N area, leaves of subordinate plants had a higher N area than older leaves of dominant plants which were at the same height or slightly higher in the canopy. Consequently, the N area distribution patterns of individual plants plotted as a function of relative PFD were steeper, and probably closer to the optimal distribution which maximizes photosynthesis, than the average distribution in the stand. Leaves of subordinate plants had a lower mass per unit area (LMA) than those of dominant plants. In the dominant plants, LMA decreased with decreasing relative PFD (and with leaf position) while in the subordinate plants it increased. This surprising result for the subordinate plants can be explained by the fact that, during the course of a growing season, these plants became increasingly shaded and newer leaves were thus formed at progressively lower light availability. This indicates that LMA was strongly determined by the relative PFD at leaf formation and to a lesser extent by the current PFD. Leaf N content per unit mass (N mass) was strongly determined by leaf position independent of relative PFD. This indicates that N mass is strongly ontogenetically related to the leaf-aging process while changes in N area, in response to PFD, were regulated through changes in LMA.

Interactive effects of mechanical stress, sand burial and defoliation on growth and mechanical properties in Cynanchum komarovii.

In drylands, wind, sand burial and grazing are three important factors affecting growth and mechanical properties of plants, but their interactive effects have not yet been investigated. Plants of the semi-shrub Cynanchum komarovii, common in semi-arid parts of NE Asia, were subjected to brushing, burial and defoliation. We measured biomass allocation and relative increment rates of dry mass (RGR(m)), height (RGR(h)) and basal diameter (RGR(d)). We also measured the stem mechanical properties, Young's modulus (E), second moment of area (I), flexural stiffness (EI) and breaking stress (σ(b)), and scaled these traits to the whole-plant level to determine the maximum lateral force (F(lateral)) and the buckling safety factor (BSF). Brushing increased RGR(m); neither burial nor defoliation independently affected RGR(m), but together they reduced it. Among buried plants, brushing positively affected stem rigidity and strength through increasing RGR(d), E, I and EI, and at whole plant level this resulted in a larger BSF and F(lateral). However, among unburied plants this pattern was not observed. Our results thus show that effects of mechanical stress and grazing on plants can be strongly modified by burial, and these interactions should be taken into account when considering adaptive significance of plant mechanical traits in drylands.

The limited importance of size-asymmetric light competition and growth of pioneer species in early secondary forest succession in Vietnam.

It is generally believed that asymmetric competition for light plays a predominant role in determining the course of succession by increasing size inequalities between plants. Size-related growth is the product of size-related light capture and light-use efficiency (LUE). We have used a canopy model to calculate light capture and photosynthetic rates of pioneer species in sequential vegetation stages of a young secondary forest stand. Growth of the same saplings was followed in time as succession proceeded. Photosynthetic rate per unit plant mass (P(mass): mol C g(-1) day(-1)), a proxy for plant growth, was calculated as the product of light capture efficiency [Phi(mass): mol photosynthetic photon flux density (PPFD) g(-1) day(-1)] and LUE (mol C mol PPFD(-1)). Species showed different morphologies and photosynthetic characteristics, but their light-capturing and light-use efficiencies, and thus P (mass), did not differ much. This was also observed in the field: plant growth was not size-asymmetric. The size hierarchy that was present from the very early beginning of succession remained for at least the first 5 years. We conclude, therefore, that in slow-growing regenerating vegetation stands, the importance of asymmetric competition for light and growth can be much less than is often assumed.

Above-ground biomass investments and light interception of tropical forest trees and lianas early in succession.

BACKGROUND AND AIMS: Crown structure and above-ground biomass investment was studied in relation to light interception of trees and lianas growing in a 6-month-old regenerating forest. METHODS: The vertical distribution of total above-ground biomass, height, diameter, stem density, leaf angles and crown depth were measured for individual plants of three short-lived pioneers (SLPs), four long-lived pioneers (LLPs) and three lianas. Daily light interception per individual Phi(d) was calculated with a canopy model. The model was then used to estimate light interception per unit of leaf mass (Phi(leaf mass)), total above-ground mass (Phi(mass)) and crown structure efficiency (E(a), the ratio of absorbed vs. available light). KEY RESULTS: The SLPs Trema and Ochroma intercepted higher amounts of light per unit leaf mass (Phi(leaf mass)) because they had shallower crowns, resulting in higher crown use efficiency (E(a)) than the other species. These SLPs (but not Cecropia) were also taller and intercepted more light per unit leaf area (Phi(area)). LLPs and lianas had considerably higher amounts of leaf mass and area per unit above-ground mass (LMR and LAR, respectively) and thus attained Phi(mass) values similar to the SLPs (Phi(mass)=Phi(area)xLAR). Lianas, which were mostly self-supporting, had light interception efficiencies similar to those of the trees. CONCLUSIONS: These results show how, due to the trade-off between crown structure and biomass allocation, SLPs, and LLPs and lianas intercept similar amount of light per unit mass which may contribute to the ability of the latter two groups to persist.