Heartwood formation is a unique phenomenon of tree species but the mechanisms by which the substances involved accumulate are unclear. Kuroda et al. use time-of-flight secondary ion mass spectrometry (TOF-SIMS) in conjunction with quantitative analyses to study the distribution of ferruginol in a 30-year tree of Cryptomeria japonica (Taxodiaceae). They find that accumulation begins in the middle of the intermediate wood, initially in the earlywood near the annual ring boundary, then throughout the entire earlywood, and finally across to the whole annual ring in the heartwood. They conclude that the heterogeneous timing of ferruginol accumulation could be related to the distribution of ray parenchyma cells and/or water in the heartwood-forming xylem.
The continuous formation of earlywood vessels is crucial for the growth of ring-porous hardwoods. Kudo et al. study Quercus serrate seedlings and find that a combination of localized heating and disbudding of dormant stems results in earlier cambial reactivation and differentiation of first vessel elements than in non-heated seedlings. A few narrow vessel elements are formed during heating after disbudding, while many large earlywood vessel elements are formed in heated seedlings with buds. The results suggest that elevated temperature is a direct trigger for differentiation of first vessel elements, and that whilst bud growth is not essential for differentiation of first vessel elements, it might be important for the continuous formation of wide vessel elements.
Increased homozygosity caused by population fragmentation can directly affect individual plant fitness through the expression of deleterious alleles, and drought stress induced by climate change may exacerbate these effects. Vranckx et al. investigate various transpiration and growth traits of seedlings of pedunculate oak, Quercus robur, correlate them with their multilocus heterozygosity (MLH), and then study the effects of drought stress on these relationships. They find significant heterozygosity–fitness correlations for most fitness traits, and high atmospheric stress increases the strength of these correlations for the transpiration variables. They conclude that that ongoing climate change may strengthen the negative fitness responses to low MLH, highlighting the need to maximize individual multilocus heterozygosity in forest tree breeding programs.
The Lathyrus genus (Fabaceae) includes 160 species, some of which have economic importance as food, fodder and ornamental crops (mainly L. sativus, L. cicera and L. odoratus, respectively) and are cultivated in over 1.5 million ha worldwide. Vaz Patto and Rubiales review the current status and future prospects of Lathyrus diversity conservation and characterization, highlighting their use in L. sativus and L. cicera breeding. They conclude that efforts for improvement of these species should concentrate on the development of publicly available joint core collections, and on high-resolution genotyping. This should result in more efficient and faster breeding approaches, which are especially needed for these neglected, under-utilized Lathyrus species.
Using a thermodynamic flow–force interpretation of nitrate uptake isotherms, Malagoli and Le Deunff develop a functional– structural model to predict N uptake in winter oilseed rape, Brassica napus. The structural component of the model, the active root biomass, is derived from a combination of root mapping in the field, the relationship between specific root length and external nitrate concentration, and the assignment of an absorption capacity related to integrated root system age. They find that model simulations are well matched to measured data for N uptake under field conditions at three different levels of fertilizer application. Model ouputs indicate that the topsoil layers contain about 80 % of the total root system and account for 90–95 % of N taken up at harvest.
Difficulties in linking the various regulations of nitrate transport acting at different levels of time and on different spatial scales have hindered the development of models for nitrogen uptake. Le Deunff and Malagoli substitute the more usual enzyme–substrate interpretation for a ﬂow–force approach of nitrogen uptake isotherms and combine it with experimentally determined regulation in order to model nitrate in winter oilseed rape, Brassica napus. This approach avoids the use of unique nitrate uptake reference kinetics and allows root plasticity in response to environmental and in planta factors to be taken into account. Furthermore, it allows the regulation of nitrate uptake by roots to be scaled up relatively easily in time from hours to months.
Maintenance of tension in the transpiration stream requires coherence of the water molecules, but also their adherence to vessel walls (i.e. the lumen-facing surface must be wettable). As lignin is generally considered hydrophobic this presents a conundrum. McCully et al. observe wall contact with oil perfused into emptied vessels of maize (Zea mays) roots, contact angles of refilling sap, and wall substantivity to histochemical probes. Their observations reveal fine-scale heterogeneity of surface wettability, with lumen-facing bordered pits having hydrophilic aperture rims that repel oil, while oil adheres strongly to pit borders in empty vessels. As sap refills vessels through pits their borders become increasingly wettable by sap, which then spreads along the walls with low contact angles.
Oaks can provide a model to study hybridization as they often maintain species integrity and distinct environmental adaptations despite recurrent gene flow. Lind-Riehl et al. identify candidate genes potentially involved in local adaptation in the interfertile species Quercus rubra and Q. ellipsoidalis, which are characterized by contrasting adaptations to drought. They find that a CONSTANS-like (COL) gene is nearly fixed on alternative alleles in both species, as reflected by high FST values (55–80 %) between neighbouring Q. rubra and Q. ellipsoidalis populations. All other markers show low interspecific differentiation (approx. 5 %). They conclude that this COL gene may play a role in adaptive divergence and reproductive isolation (via flowering time), making it a promising candidate speciation gene.
Recently radiated groups pose a taxonomic challenge even with extensive molecular data, as they may be genetically differentiated only at small and/or patchy regions of the genome. Griffin and Hoffmann investigate species’ structure among the Poa that dominate the Australian alpine zone, which have radiated in the last 0.5–1.2 million years. Using a Bayesian approach to co-estimate nuclear and chloroplast gene trees with an overall dated tree, they find that most species are not genetically distinct, despite distinguishable phenotypes, which suggests recent adaptive divergence with ongoing inter-taxon gene flow.
Several species of Agave were cultivated prehistorically for food and fibre in Arizona, USA, and relict populations that remain in the landscape today can provide an insight into ancient cultivation practices. Parker et al. test whether formerly cultivated Agave parryi populations still bear a predictable genetic and morphological signature by comparing populations of uncertain origin with known wild and cultivated populations. They find less genetic and morphological variation and stronger population differentiation in populations actively cultivated in the past than in wild counterparts, and conclude that where archaeological information is lacking, the genetic signature of Agave populations can thus be used to infer their evolutionary history and to identify fruitful sites for archaeological investigation of pre-Columbian cultivation practices. The same approach can clearly be adopted for other species in similar situations.