Legumes are different:Leaf nitrogen, photosynthesis, and water use efficiency

Using robust, pairwise comparisons and a global dataset, we show that nitrogen concentration per unit leaf mass for nitrogen-fixing plants (N(2)FP; mainly legumes plus some actinorhizal species) in nonagricultural ecosystems is universally greater (43–100%) than that for other plants (OP). This difference is maintained across Koppen climate zones and growth forms and strongest in the wet tropics and within deciduous angiosperms. N(2)FP mostly show a similar advantage over OP in nitrogen per leaf area (N(area)), even in arid climates, despite diazotrophy being sensitive to drought. We also show that, for most N(2)FP, carbon fixation by photosynthesis (A(sat)) and stomatal conductance (g(s)) are not related to N(area)—in distinct challenge to current theories that place the leaf nitrogen–A(sat) relationship at the center of explanations of plant fitness and competitive ability. Among N(2)FP, only forbs displayed an N(area)–g(s) relationship similar to that for OP, whereas intrinsic water use efficiency (WUE(i); A(sat)/g(s)) was positively related to N(area) for woody N(2)FP. Enhanced foliar nitrogen (relative to OP) contributes strongly to other evolutionarily advantageous attributes of legumes, such as seed nitrogen and herbivore defense. These alternate explanations of clear differences in leaf N between N(2)FP and OP have significant implications (e.g., for global models of carbon fluxes based on relationships between leaf N and A(sat)). Combined, greater WUE and leaf nitrogen—in a variety of forms—enhance fitness and survival of genomes of N(2)FP, particularly in arid and semiarid climates

Rainfall drives variation in rates of change in intrinsic water use efficiency of tropical forests.

Rates of change in intrinsic water use efficiency (W) of trees relative to those in atmospheric [CO2] (ca) have been mostly assessed via short-term studies (e.g., leaf analysis, flux analysis) and/or step increases in ca (e.g., FACE studies). Here we use compiled data for abundances of carbon isotopes in tree stems to show that on decadal scales, rates of change (dW/dca) vary with location and rainfall within the global tropics. For the period 1915-1995, and including corrections for mesophyll conductance and photorespiration, dW/dca for drier tropical forests (receiving ~ 1000 mm rainfall) were at least twice that of the wettest (receiving ~ 4000 mm). The data also empirically confirm theorized roles of tropical forests in changes in atmospheric 13C/12C ratios (the 13C Suess Effect). Further formal analysis of geographic variation in decade-to-century scale dW/dca will be needed to refine current models that predict increases in carbon uptake by forests without hydrological cost

Rainfall drives variation in rates of change in intrinsic water use efficiency of tropical forests.

Rates of change in intrinsic water use efficiency (W) of trees relative to those in atmospheric [CO2] (ca) have been mostly assessed via short-term studies (e.g., leaf analysis, flux analysis) and/or step increases in ca (e.g., FACE studies). Here we use compiled data for abundances of carbon isotopes in tree stems to show that on decadal scales, rates of change (dW/dca) vary with location and rainfall within the global tropics. For the period 1915-1995, and including corrections for mesophyll conductance and photorespiration, dW/dca for drier tropical forests (receiving ~ 1000 mm rainfall) were at least twice that of the wettest (receiving ~ 4000 mm). The data also empirically confirm theorized roles of tropical forests in changes in atmospheric 13C/12C ratios (the 13C Suess Effect). Further formal analysis of geographic variation in decade-to-century scale dW/dca will be needed to refine current models that predict increases in carbon uptake by forests without hydrological cost

Stomatal structure and physiology do not explain differences in water use among montane eucalypts

Understanding the regulation of water use at the whole-tree scale is critical to advancing the utility of physiological ecology, for example in its role in predictive hydrology of forested catchments. For three eucalypt species that dominate high-elevation catchments in south-eastern Australia, we examined if whole-tree water use could be related to three widely discussed regulators of water use: stomatal anatomy, sensitivity of stomata [i.e. stomatal conductance (gs)] to environmental influences, and sapwood area. While daily tree water use varied sixfold among species, sap velocity and sapwood area varied in parallel. Combined, stomatal structure and physiology could not explain differences in species-specific water use. Species which exhibited the fastest (Eucalyptus delegatensis) and slowest (Eucalyptus pauciflora) rates of water use both exhibited greater capacity for physiological control of gs [indicated by sensitivity to vapour pressure deficit (VPD)] and a reduced capacity to limit gs anatomically [indicated by greater potential gs (gmax)]. Conversely, gs was insensitive to VPD and gmax was lowest for Eucalyptus radiata, the species showing intermediate rates of water use. Improved knowledge of stomatal anatomy will help us to understand the capacity of species to regulate leaf-level water loss, but seems likely to remain of limited use for explaining rates of whole-tree water use in montane eucalypts at the catchment scale

Crops, Nitrogen, Water: Are Legumes Friend, Foe, or Misunderstood Ally?

Biological nitrogen fixation (BNF) by crop legumes reduces demand for industrial nitrogen fixation (INF). Nonetheless, rates of BNF in agriculture remain low, with strong negative feedback to BNF from reactive soil nitrogen (N) and drought. We show that breeding for yield has resulted in strong relationships between photosynthesis and leaf N in non-leguminous crops, whereas grain legumes show strong relations between leaf N and water use efficiency (WUE). We contrast these understandings with other studies that draw attention to the water costs of grain legume crops, and their potential for polluting the biosphere with N. We propose that breeding grain legumes for reduced stomatal conductance can increase WUE without compromising production or BNF. Legume crops remain a better bet than relying on INF

Does triacylglycerol (TAG) serve a photoprotective function in plant leaves? An examination of leaf lipids under shading and drought

Plant survival in many ecosystems requires tolerance of large radiation loads, unreliable water supply and suboptimal soil fertility. We hypothesized that increased production of neutral lipids (triacylglycerols, TAGs) in plant leaves is a mechanism for dissipating excess radiation energy. In a greenhouse experiment, we combined drought and shade treatments and examined responses among four species differing in life form, habitat, and drought- and shade-tolerance. We also present a lipid extraction protocol suitable for sclerophyllous leaves of native Australian trees (e.g. Acacia, Eucalyptus). Fluorescence measurements indicated that plants exposed to full sunlight experienced mild photoinhibition during our experiment. Accumulation of TAGs did not follow photosynthetic capacity, but instead, TAG concentration increased with non-photochemical quenching. This suggests that plants under oxidative stress may increase biosynthesis of TAGs. Moderate drought stress resulted in a 60% reduction in TAG concentration in wheat (Triticum aestivum). Shading had no effect on TAGs, but increased concentrations of polar lipids in leaves; for example, acclimation to shade in Austrodanthonia spp., a native Australian grass, resulted in a 60% increase in associated polar lipids and higher foliar chlorophyll concentrations. Shading also reduced the digalactosyldiacylglycerol:monogalactosyldiacylglycerol (DGDG:MGDG) ratio in leaves, with a corresponding increase in the degree of unsaturation and thus fluidity of thylakoid membranes of chloroplasts. Our results suggest that prevention of photodamage may be coordinated with accumulation of TAGs, although further research is required to determine if TAGs serve a photoprotective function in plant leaves

CO2, nitrogen deposition and a discontinuous climate response drive water use efficiency in global forests

Reduced stomatal conductance is a common plant response to rising atmospheric CO2 and increases water use efficiency (W). At the leaf-scale, W depends on water and nitrogen availability in addition to atmospheric CO2. In hydroclimate models W is a key driver of rainfall, droughts, and streamflow extremes. We used global climate data to derive Aridity Indices (AI) for forests over the period 1965-2015 and synthesised those with data for nitrogen deposition and W derived from stable isotopes in tree rings. AI and atmospheric CO2 account for most of the variance in W of trees across the globe, while cumulative nitrogen deposition has a significant effect only in regions without strong legacies of atmospheric pollution. The relation of aridity and W displays a clear discontinuity. W and AI are strongly related below a threshold value of AI ≈ 1 but are not related where AI > 1. Tree ring data emphasise that effective demarcation of water-limited from non-water-limited behaviour of stomata is critical to improving hydrological models that operate at regional to global scales

Crops, Nitrogen, Water:Are Legumes Friend, Foe, or Misunderstood Ally?

Biological nitrogen fixation (BNF) by crop legumes reduces demand for industrial nitrogen fixation (INF). Nonetheless, rates of BNF in agriculture remain low, with strong negative feedback to BNF from reactive soil nitrogen (N) and drought. We show that breeding for yield has resulted in strong relationships between photosynthesis and leaf N in non-leguminous crops, whereas grain legumes show strong relations between leaf N and water use efficiency (WUE). We contrast these understandings with other studies that draw attention to the water costs of grain legume crops, and their potential for polluting the biosphere with N. We propose that breeding grain legumes for reduced stomatal conductance can increase WUE without compromising production or BNF. Legume crops remain a better bet than relying on INF

Differences in water use between mature and post-fire regrowth stands of subalpine Eucalyptus delegatensis R. Baker

We estimated plot level water use from sap flux measurements over a 9-month period, in post-fire regrowth and mature plots of Eucalyptus delegatensis R. Baker (Alpine ash) in high elevation catchments near Falls Creek, Victoria, Australia, seven years after a major stand-replacing bushfire. Water use was more than double (460 ± 100 mm year⁻¹ more) in regrowth as compared to mature plots, whereas sap flux was similar between age classes. This difference in water use reflected 72% greater sapwood area index and 35% greater leaf area index in regrowth than in mature plots. A small part of the difference in water use can be attributed to nocturnal transpiration, which was greater in regrowth than in mature plots (10.3 ± 0.8% vs 7.3 ± 0.8% of diel totals). As evaporative demand was 41% greater in mature than in regrowth plots, these data suggest mean transpiration rate and stomatal conductance per unit leaf area were approximately 1.6 and 2.3 times greater, respectively, in the regrowth. However, mid-day leaf water potential and photosynthetic capacity were similar in both age classes. Evaporative demand was the primary environmental driver of water use in all cases, whereas soil moisture was not a strong driver of either water use or canopy conductance (estimated as sap flux/evaporative demand). Together, our results suggest (a) stand water use rapidly recovers after fire in these high elevation forests and quickly surpasses rates in mature stands, confirming projections by Kuczera (1987) for lower-elevation Mountain Ash (E. regnans) forests and highlighting the potential impact of tree water use on water yield in the first decade of forest regeneration after fires, and (b) stomatal conductance and sapwood area/leaf area ratio are both less in tall, older Alpine ash trees, whereas leaf water status and photosynthetic capacity appear to be sustained – consistent with predictions from optimisation theory but not Pipe-Model Theory

Plant and soil P determine functional attributes of subalpine Australian plants

Replacement of phospholipids with phosphorus (P)-free lipids in cellular membranes has been identified as a mechanism facilitating fast rates of photosynthesis when phosphorus availability is limited. We measured photosynthetic rates, leaf and soil P fractions, and foliar membrane lipid compositions for five species (Geranium antrorsum, Ranunculus graniticola, Poa costiniana, Poa hiemata, and Veronica derwentiana) common to two Australian subalpine ecosystems of contrasting parent material to characterize the extent to which they have adapted to long-term P availability. Our results indicate limited tolerance to reduced P, albeit adaptation strategies differ among species. Under reduced P conditions, phospholipids were replaced in foliage by galactolipids and sulfolipids, but photosynthesis was still impaired owing to reduced stomatal conductance. Accumulation of antioxidants, including carotenoids and alpha-tocopherol, in leaves with limited P supply suggests oxidative stress. Our field study shows that while subalpine Australian plants of a variety of life forms adapt to P availability by replacing phospholipids with P-free lipids in foliar membranes, this adaptation is insufficient to fully mitigate the effects of reduced P on photosynthesis