Tree-ring isotopic imprints on time series of reproductive effort indicate warming-induced co-limitation by sink and source processes in stone pine.

Increasing evidence indicates that tree growth processes, including reproduction, can be either sink- or source-limited, or simultaneously co-limited by sink and source, depending on the interplay between internal and environmental factors. We tested the hypothesis that the relative strengths of photosynthate supply and demand by stem growth and reproduction create variable competition for substrate that is imprinted in the tree-ring isotopes (C and O) of stone pine (Pinus pinea L.), a masting gymnosperm with large costs of reproduction, under warming-induced drought. Across five representative stands of the Spanish Northern Plateau, we also identified reproductive phases where weather drivers of cone yield (CY) have varied over a 60-year period (1960-2016). We found that these drivers gradually shifted from winter-spring conditions 3 years before seed rain (cone setting) to a combination of 3- and 1-year lagged effects (kernel filling). Additionally, we observed positive regional associations between carbon isotope discrimination (Δ13C) of the year of kernel filling and CY arising at the turn of this century, which progressively offset similarly positive relationships between Δ13C of the year of cone setting and CY found during the first half of the study period. Altogether, these results pinpoint the increasing dependence of reproduction on fresh assimilates and suggest sink and source co-limitation superseding the sink-limited functioning of reproduction dominant before 2000. Under climate warming, it could be expected that drier conditions reinforce the role of source limitation on reproduction and, hence, on regeneration, forest structure and economic profit of the nutlike seeds of the species.

Direct and correlated responses to artificial selection for growth and water-use efficiency in a Mediterranean pine.

PREMISE: Persistence of tree populations in the face of global change relies on their capacity to respond to biotic and abiotic stressors through plastic or adaptive changes. Genetic adaptation will depend on the additive genetic variation within populations and the heritability of traits related to stress tolerance. Because traits can be genetically linked, selective pressure acting on one trait may lead to correlated responses in other traits. METHODS: To test direct and correlated responses to selection for growth and drought tolerance in Pinus halepensis, we selected trees in a parental population for higher growth and greater water-use efficiency (WUE) and compared their offspring with the offspring of random trees from the parental population in two contrasting common gardens. We estimated direct responses to selection for growth and WUE and correlated responses for growth and tolerance to abiotic and biotic stressors. RESULTS: We found a strong response to selection and high realized heritability for WUE, but no response to selection for growth. Correlated responses to selection in other life-history traits were not significant, except for concentration of some chemical defenses, which was greater in the offspring of mother trees selected for growth than in the offspring of unselected control trees. CONCLUSIONS: The empirical evidence of direct responses to selection for high WUE suggests that P. halepensis has the potential to evolve in response to increasing drought stress. Contrary to expectations, the results are not conclusive of a potential negative impact of WUE and growth selection on other key life-history traits.

Host-specific proteomic and growth analysis of maize and tomato seedlings inoculated with Azospirillum brasilense Sp7.

Azospirillum brasilense Sp7 (Sp7) is a diazotrophic, free-living plant growth-promoting rhizobacterium (PGPR) that is increasingly used for its ability to reduce stress and improve nutrient uptake by plants. To test the hypothesis that Sp7 interacts differently with the primary metabolism in C3 and C4 plants, differential proteomics were employed to study weekly protein expression in Sp7-treated maize (Zea mays cv. B73) and tomato (Solanum lycopersicum cv. Boludo) seedlings. Plant and root growth parameters were also monitored. Protein changes were most striking at the four-leaf stage (T1) for both species. Proteins related to metabolism and redox homeostasis were most abundant in tomato at T1, but later, plants experienced inhibited Calvin-Benson (CB) cycle and chloroplast development, indicating that photosynthetic proteins were damaged by reactive oxygen species (ROS). In maize, Sp7 first increased ROS-scavenging enzymes and decreased those related to metabolism, which ultimately reduced photoinhibition at later sampling times. Overall, the early interaction with maize is more complex and beneficial because the photosynthetic aparatus is protected by the C4 mechanism, thereby improving the interaction of the PGPR with maize. Better seedling emergence and vigor were observed in inoculated maize compared to tomato. This study provides an integrated perspective on the Sp7 strain-specific interactions with young C3 and C4 plants to modulate primary metabolism and photosynthesis.