Stable carbon isotopes in archaeobotanical remains and palaeoclimate

Es descriu una metodologia recent per a inferir la precipitació en el passat basada en l’anàlisi de la composició isotòpica del carboni (δ13C) en restes arqueobotàniques. Un cop descrita la base fisiològica de la tècnica, s’il·lustra l’aplicabilitat de δ13C mitjançant un exemple referent al NE peninsular. Hom pretén proporcionar una estimació quantitativa de l’evolució de la precipitació estacional (primavera) i anual al llarg dels darrers quatre mil anys basada en δ13C. Les mostres analitzades comprenen carbons (pi blanc) i llavors carbonitzades (blat i ordi), i s’obtenen estimes pluviomètriques superiors en el passat que actualment, amb una tendència gradual cap a condicions progressivament més àrides. No obstant això, aquesta tendència no esdevé uniforme, i es detecten dues fases de major precipitació (1800-900 aC; 300 aC - 300 dC) alternadament amb períodes relativament secs (900-300 aC; 900 dC - present). Dels resultats presentats també es desprèn que la importància relativa de la pluja primaveral en el passat fou variable. Des d’aproximadament el 300 aC en endavant, el període primaveral subministrà una major proporció de pluja anual que actualment. Contràriament, durant el període 1800-800 dC la seva contribució va esdevenir inferior, i va aparèixer una fase transitòria (800-300 aC) que mostra una recuperació sobtada en aportació primaveral. Posteriorment a aquesta fase la sincronia de canvis en δ13C en granes i carbons suggereix l’arribada del clima mediterrani a la regió.

Grain yield, carbon isotope discrimination and mineral content in mature kernels of barley under irrigated and rainfed conditions

The relationship between yield, carbon isotope discrimination and ash content in mature kernels was examined for a set of 13 barley (Hordeum vulgare) cultivars. Plants were grown under rainfed and well-irrigated conditions in a Mediterranean area. Water deficit caused a decrease in both grain yield and carbon isotope discrimination (Δ). The yield was positively related to Δ and negatively related to ash content, across genotypes within each treatment. However, whereas the correlation between yield and Δ was higher for the set of genotypes under well-irrigated (r=0.70, P<0.01) than under rainfed (r=0.42) conditions, the opposite occurred when yield and ash content were related, ie r=-0.38 under well-irrigated and r=-0.73, (P<0.01) under rainfed conditions. Carbon isotope discrimination and ash content together account for almost 60% of the variation in yield, in both conditions. There was no significant relationship (r=-0.15) between carbon isotope discrimination and ash content in well-irrigated plants, whereas in rainfed plants, this relationship, although significant (r=-0.54, P< 0.05), was weakly negative. The concentration of several mineral elements was measured in the same kernels. The mineral that correlated best with ash content, yield and A, was K. For yield and Δ, although the relationship with K followed the same pattern as the relationhip with ash content, the correlation coefficients were lower. Thus, mineral accumulation in mature kernels seems to be independent of transpiration efficiency. In fact, filling of grains takes place through the phloem pathway. The ash content in kernels is proposed as a complementary criterion, in addition to kernel Δ, to assess genotype differences in barley grain yield under rainfed conditions.

Genetic variation for carbon isotope composition in Juglans regia L.: relationships with growth, phenology and climate of origin

Among the traits of breeding interest for the common walnut tree Juglans regia L., characteristics such as timing of budbreak and leaf fall, water-use efficiency and growth performance are regarded as being of utmost relevance in Mediterranean conditions. The authors evaluated intraspecific variation in $\delta$13C (carbon isotope composition, surrogate of intrinsic water-use efficiency, WUE$_{\rm i}$) for 22 J. regia families grown in a progeny test under supplementary irrigation, and investigated whether such variation correlated with climatic indicators of native habitats. The genetic relationships between $\delta$13C, growth and phenology were also assessed during two consecutive years. Overall, the most water-use-efficient families (i.e. with higher $\delta$13C), which originated mainly from drought-prone provenance regions which have a high vapour pressure deficit and low rainfall, exhibited less height growth and smaller DBH. Using a stepwise regression procedure, $\delta$13C was included as the main explanatory variable of genotypic variation in growth traits, together with growing season duration (for DBH in both years) and flushing (for height in 2007). It was concluded that WUE$_{\rm i}$ is largely unconnected to phenology effects in the explanation of growth performance for J. regia, therefore suggesting the opportunity of simultaneously selecting for low WUE$_{\rm i}$ and extended growing period to maximise productivity in non-water-limited environments.