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In the present study we investigated variations in leaf respiration in darkness (RD) and light (RL), and associated traits in response to season, and along a gradient of soil moisture, in Mediterranean woodland dominated by holm oak (Quercus ilex L.) in central and north-eastern Spain respectively. On seven occasions during the year in the central Spain site, and along the soil moisture gradient in north-eastern Spain, we measured rates of leaf RD, RL (using the Kok method), light-saturated photosynthesis (A) and related light response characteristics, leaf mass per unit area (MA) and leaf nitrogen (N) content. At the central Spain site, significant seasonal changes in soil water content and ambient temperature (T) were associated with changes in MA, foliar N, A and stomatal conductance. RD measured at the prevailing daily T and in instantaneous R-T responses, displayed signs of partial acclimation and was not significantly affected by time of year. RL was always less than, and strongly related to, RD, and RL/RD did not vary significantly or systematically with seasonal changes in T or soil water content. Averaged over the year, RL/RD was 0.66±0.05s.e. (n=14) at the central Spain site. At the north-eastern Spain site, the soil moisture gradient was characterised by increasing MA and RD, and reduced foliar N, A, and stomatal conductance as soil water availability decreased. Light inhibition of R occurred across all sites (mean RL/RD=0.69±0.01s.e. (n=18)), resulting in ratios of RL/A being lower than for RD/A. Importantly, the degree of light inhibition was largely insensitive to changes in soil water content. Our findings provide evidence for a relatively constrained degree of light inhibition of R (RL/RD ~ 0.7, or inhibition of ~30%) across gradients of water availability, although the combined impacts of seasonal changes in both T and soil water content increase the range of values expressed. The findings thus have implications in terms of the assumptions made by predictive models that seek to account for light inhibition of R, and for our understanding of how environmental gradients impact on leaf trait relationships in Mediterranean plant communities.

Root system architecture is the spatial arrangement of roots that impacts the capacity of plants to access nutrients and water. We employed pharmacologically generated morphological and molecular phenotypes and used in situ 15N isotope labelling, to investigate whether contrasting root traits are of functional interest in relation to nitrate acquisition. Brassica napus L. were grown in solidified phytogel culture media containing 1mM KNO3 and treated with the cytokinin, 6-benzylaminopurine, the cytokinin antagonist, PI-55, or both in combination. The pharmacological treatments inhibited root elongation relative to the control. The contrasting root traits induced by PI-55 and 6-benzylaminopurine were strongly related to 15N uptake rate. Large root proliferation led to greater 15N cumulative uptake rather than greater 15N uptake efficiency per unit root length, due to a systemic response in the plant. This relationship was associated with changes in C and N resource distribution between the shoot and root, and in expression of BnNRT2.1, a nitrate transporter. The root:shoot biomass ratio was positively correlated with 15N cumulative uptake, suggesting the functional utility of root investment for nutrient acquisition. These results demonstrate that root proliferation in response to external nitrate is a behaviour which integrates local N availability and the systemic N status of the plant.

We measured a diverse range of foliar characteristics in shrub and tree species in temperate rainforest communities along a soil chronosequence (six sites from 8 to 120000 years) and used multilevel model analysis to attribute the proportion of variance for each trait into genetic (G, here meaning species-level), environmental (E) and residual error components. We hypothesised that differences in leaf traits would be driven primarily by changes in soil nutrient availability during ecosystem progression and retrogression. Several leaf structural, chemical and gas-exchange traits were more strongly driven by G than E effects. For leaf mass per unit area (MA), foliar [N], net CO2 assimilation and dark respiration rates and foliar carbohydrate concentration, the G component accounted for 60-87% of the total variance, with the variability associated with plot, the E effect, much less important. Other traits, such as foliar [P] and N:P, displayed strong E and residual effects. Analyses revealed significant reductions in the slopes of G-only bivariate relationships when compared with raw relationships, indicating that a large proportion of trait-trait relationships is species based, and not a response to environment per se. This should be accounted for when assessing the mechanistic basis for using such relationships in order to make predictions of responses of plants to short-term environmental change.

Seedlings of the shrub kānuka (Kunzea ericoides var. ericoides (A. Rich) J. Thompson) and the pasture grass brown top (Agrostis capillarus L.) were grown in intact soil cores in climate-controlled cabinets to analyse the thermal response of leaf-level carbon exchange at four levels of volumetric soil water content (θ). The objective was to resolve the combined effects of relatively rapid and short-term changes in θ and temperature on the thermal responses of both photosynthesis and respiration in these two contrasting plant types. Results showed that θ had a greater effect on the short-term temperature response of photosynthesis than the temperature response of respiration. The optimum value of θ for net photosynthesis was around 30% for both plants. The photosynthetic capacity of kānuka and the grass declined significantly when θ fell below 20%. The temperature sensitivity of photosynthesis was low at low soil water content and increased at moderate to high soil water content in both plant types. Statistical analysis showed that the temperature sensitivity of photosynthetic parameters was similar for both plant types, but the sensitivity of respiratory parameters differed. Respiratory capacity increased with increasing soil water content in kānuka but declined significantly when θ fell below 15%. There was no significant influence of soil water content on respiratory capacity in the grass. Collectively, our results indicate that θ influenced the temperature sensitivity of photosynthesis and respiration, and altered the balance between foliar respiration and photosynthetic capacity in both plant types.

Direct measurements of foliar carbon exchange through the growing season in Arctic species are limited, despite the need for accurate estimates of photosynthesis and respiration to characterise carbon cycling in the tundra. We examined seasonal variation in foliar photosynthesis and respiration (measured at 20°C) in two field-grown tundra species, Betula nana L. and Eriophorum vaginatum L., under ambient and long-term warming (LTW) conditions (+5°C), and the relationship of these fluxes to intraseasonal temperature variability. Species and seasonal timing drove most of the variation in photosynthetic parameters (e.g. gross photosynthesis (Agross)), respiration in the dark (Rdark) and light (Rlight), and foliar nitrogen concentration. LTW did not consistently influence fluxes through the season but reduced respiration in both species. Alongside the flatter respiratory response to measurement temperature in LTW leaves, this provided evidence of thermal acclimation. The inhibition of respiration by light increased by ~40%, with Rlight : Rdark values of ~0.8 at leaf out decreasing to ~0.4 after 8 weeks. Though LTW had no effect on inhibition, the cross-taxa seasonal decline in Rlight : Rdark greatly reduced respiratory carbon loss. Values of Rlight : Agross decreased from ~0.3 in both species to ~0.15 (B. nana) and ~0.05 (E. vaginatum), driven by decreases in respiratory rates, as photosynthetic rates remained stable. The influence of short-term temperature variability did not exhibit predictive trends for leaf gas exchange at a common temperature. These results underscore the influence of temperature on foliar carbon cycling, and the importance of respiration in controlling seasonal carbon exchange.

Snow tussocks (Chionochloa spp.) in New Zealand exhibit extreme mast (episodic) seeding which has important implications for plant ecology and plant-insect interactions. Heavy flowering appears to be triggered by very warm/dry summers in the preceding year. In order to investigate the physiological basis for mast flowering, mature snow tussock plants in the field and younger plants in a glasshouse and shadehouse were subjected to a range of manipulative treatments. Field treatments included combinations of warming, root pruning and applications of two native gibberellins (GAs) GA3, which is known to be highly floral inductive and GA4, which is associated with continued floral apex development in another long-day grass. Warming, GA3 alone and especially warming+GA3, significantly promoted flowering, as did applications of GA4 alone and GA4+CCC (2-chloroethyltrimethylammonium chloride, which is a known synergist of GA3-induced flowering in the annual grass, Lolium temulentum L.). Our results provide support for the concept that mast flowering events in tussock species are causally related to high temperature-induced increases in endogenous gibberellin levels. It is likely that GAs (endogenous or applied) promote the continued development of a previously long-day induced floral apex. In addition to the promotion of flowering, applied GA3 also disturbed the plant's innate resource threshold requirements, as shown by the death, over winter, of many non-flowering tillers. Applied GA4 did not show this effect, likely due to its rapid catabolic metabolism to an inactive form. High temperature-induced flowering mediated by elevated levels of endogenous floral-promotive GAs could have important implications for regulating the evolutionary interaction between these masting plants and their seed predators.

The influence of defoliation on nitrogen (N) re-translocation and the source for N remobilisation by the invasive shrub, Buddleia davidii Franch. (buddleia) was determined. Eighty plants were grown over two growing seasons, and half were repeatedly defoliated by removing 66% of their leaf area. During the second season, the N supply was labelled with 15 N (10 atom% enrichment), to distinguish the use of stored N (unlabelled) from N taken up by roots (labelled) for growth. Defoliation significantly decreased root (39%) and total biomass (26%). Old leaves were the main source of N for remobilisation which was accelerated and increased (by 50% in the second season) in response to defoliation. In spring, root uptake of N increased by 57% in defoliated plants. Thus, defoliation induced changes in N remobilisation and uptake as compensatory growth increased the demand for N. Continued leaf removal decreased the pool of stored N and caused a significant decline in biomass production, especially in roots (39%) and flowers (31%). This has important implications for the efficacy of defoliation as a control measure, as smaller roots suggest a reduced capacity for uptake of nutrients from the soil and reduced flower production may assist in reducing the invasive spread of the species. These findings clearly show that, although the success of B. davidii is associated, in part, with efficient remobilisation of N from storage, this advantage can be overcome by continued defoliation.

Pinus radiata L. were grown in climate-controlled cabinets under three night/day temperature treatments, and transferred between treatments to mimic changes in growth temperature. The objective was to determine the extent to which dark respiration and photosynthesis in pre-existing and new needles acclimate to changes in growth temperatures. We also assessed whether needle nitrogen influenced the potential for photosynthetic and respiratory acclimation, and further assessed if short-term (instantaneous, measured over a few hours) respiratory responses are accurate predictors of long-term (acclimated, achieved in days-weeks) responses of respiration to changing temperature. Results show that respiration displayed considerable potential for acclimation. Cold and warm transfers resulted in some acclimation of respiration in pre-existing needles, but full acclimation was displayed only in new needles formed at the new growth temperature. Short-term respiratory responses were poor predictors of the long-term response of respiration due to acclimation. There was no evidence that photosynthesis in pre-existing or new needles acclimated to changes in growth temperature. N status of leaves had little impact on the extent of acclimation. Collectively, our results indicate that there is little likelihood that respiration would be significantly stimulated in this species as night temperatures increase over the range of 10-20°C, but that inclusion of temperature acclimation of respiration would in fact lead to a shift in the balance between photosynthesis and respiration in favour of carbon uptake.

Variation in the oxygen isotope composition of within-canopy CO 2 has potential to allow partitioning of the ecosystem respiratory flux into above- and below-ground components. Recent theoretical work has highlighted the sensitivity of the oxygen isotope composition of leaf-respired CO 2 (δ Rl ) to nocturnal stomatal conductance. When the one-way flux model was tested on Ricinus communis L. large enrichments in δ Rl were observed. However, most species for which the isotope flux partitioning technique has been or would be applied (i.e. temperate tree species) are much more conservative users of water than R. communis. So, high stomatal conductance and very high enrichment of δ Rl observed may not be typical for temperate tree species. Using existing gas-exchange measurements on six temperate tree species, we demonstrate significant water loss through stomata for all species (i.e. statistically significantly greater than cuticular loss alone) at some time for some leaves during the night. δ Rl values predicted by the one-way flux model revealed that δ Rl might be very much more enriched than when the net flux alone is considered, particularly close to sunrise and sunset. Incorporation of the one-way flux model into ecosystem respiration partitioning studies will affect model outputs and interpretation of variation in the oxygen isotope composition of atmospheric CO 2 .

Many arid and saline habitat species possess sparse canopies with cylindrical foliage that is considered relatively invariable along environmental gradients. However, even in sparse canopies strong gradients of light develop between the canopy top and bottom. We studied structural and photosynthetic acclimation to within-canopy light gradient in Casuarina glauca Sieb. ex Spreng., the photosynthetic organs of which are cylindrical cladodes. Seasonal average integrated quantum flux density (Q int ) varied 25-fold between the canopy top and the canopy bottom. Cladode cross-sectional shape was unaffected by irradiance, but cladode dry mass per unit total area (M A ) varied 2-fold within the canopy light gradient. This resulted primarily from light-dependent changes in cladode thickness (volume to total area ratio,V / A T ) and to a lesser extent from changes in cladode density (D, M A = DV / A T ). Nitrogen content, and the volume of mesophyll per unit surface area increased with increasing Q int and V / A T , resulting in positive scaling of foliage photosynthetic potential (capacity of photosynthetic electron transport and maximum Rubisco carboxylase activity per unit area) with light. However, nitrogen content per unit dry mass and the volume fraction of mesophyll decreased with increasing irradiance. This was explained by greater fractional investment in mechanical tissues in cladodes with greater volume to surface area ratio. This trade-off between photosynthetic and support investments reduced the cladode photosynthetic plasticity. Our study demonstrates a significant acclimation potential of species with cylindrical foliage that should be included in larger-scale carbon balance estimations of arid and saline communities.

The aim of this study was to investigate the roles of carbon partitioning and sucrose metabolism in regulating cultivar differences in spear yield in asparagus (Asparagus officinalis L.). In the two cultivars studied, maximum net photosynthetic rate (Amax) was positively correlated with sucrose phosphate synthase (SPS) activity (r 2 =0.86), which was in turn linked to increases in sucrose content in cladophyll tissue. The high-yielding cultivar ASP-69 exhibited greater SPS activity and sucrose content than the low-yielding cultivar ASP-03, in fully-expanded and mature cladophyll tissue. ASP-69 also displayed a higher percentage of soluble solids in stem cell sap than did ASP-03. Sucrose synthase (SS) activity in storage roots in ASP-69 was significantly greater than in ASP-03 during fern growth season. Total non-structural carbohydrate (TNC) in storage roots did not differ in the two cultivars. Biomass analysis revealed that ASP-69 had a greater root/shoot ratio than ASP-03, suggesting that the total carbohydrate storage pool, rather than carbohydrate concentration, is an important determinant of asparagus yield. The overall results substantiate the conclusion that carbohydrate partitioning in the two asparagus cultivars studied is a property of the entire plant, and is influenced by both source and sink properties. This is highlighted by greater Amax, SPS activity and sucrose concentrations in cladophyll tissue in ASP-69, and greater SS activity and total carbohydrate content in storage root tissue in ASP-69.