No abstract

Grafting is an ancient cloning method that has been used widely for thousands of years in agricultural practices. Graft-union development is also an intricate process that involves substantial changes such as organ regeneration and genetic material exchange. However, the molecular mechanisms for graft-union development are still largely unknown. Here, a micrografting method that has been used widely in Arabidopsis was improved to adapt it a smooth procedure to facilitate sample analysis and to allow it to easily be applied to various dicotyledonous plants. The developmental stage of the graft union was characterized based on this method. Histological analysis suggested that the transport activities of vasculature were recovered at 3 days after grafting (dag) and that auxin modulated the vascular reconnection at 2 dag. Microarray data revealed a signal-exchange process between cells of the scion and stock at 1 dag, which re-established the communication network in the graft union. This process was concomitant with the clearing of cell debris, and both processes were initiated by a wound-induced programme. The results demonstrate the feasibility and potential power of investigating various plant developmental processes by this method, and represent a primary and significant step in interpretation of the molecular mechanisms underlying graft-union development.

Despite the perceived importance of exudation to forest ecosystem function, few studies have attempted to examine the effects of elevated temperature and nutrition availability on the rates of root exudation and associated microbial processes. In this study, we performed an experiment in which in situ exudates were collected from Picea asperata seedlings that were transplanted in disturbed soils exposed to two levels of temperature (ambient temperature and infrared heater warming) and two nitrogen levels (unfertilized and 25 g N m À2 a À1). Here, we show that the trees exposed to an elevated temperature increased their exudation rates I (lg C g À1 root biomass h) in the unfertilized plots. The altered morphological and physiological traits of the roots exposed to experimental warming could be responsible for this variation in root exudation. Moreover, these increases in root-derived C were positively correlated with the microbial release of extracellular enzymes involved in the breakdown of organic N (R 2 = 0.790; P = 0.038), which was coupled with stimulated microbial activity and accelerated N transformations in the unfertilized soils. In contrast, the trees exposed to both experimental warming and N fertilization did not show increased exudation rates or soil enzyme activity, indicating that the stimulatory effects of experimental warming on root exudation depend on soil fertility. Collectively, our results provide preliminary evidence that an increase in the release of root exudates into the soil may be an important physiological adjustment by which the sustained growth responses of plants to experimental warming may be maintained via enhanced soil microbial activity and soil N transformation. Accordingly, the underlying mechanisms by which plant root-microbe interactions influence soil organic matter decomposition and N cycling should be incorporated into climate-carbon cycle models to determine reliable estimates of long-term C storage in forests.

In the North China Plain, the grain yield of irrigated wheat-maize cropping system has been steadily increasing in the past decades under a significant warming climate. This paper combined regional and field data with modeling to analyze the changes in the climate in the last 40 years, and to investigate the influence of changes in crop varieties and management options to crop yield. In particular, we examined the impact of a planned adaptation strategy to climate change -"Double-Delay" technology, i.e., delay both the sowing time of wheat and the harvesting time of maize, on both wheat and maize yield. The results show that improved crop varieties and management options not only compensated some negative impact of reduced crop growth period on crop yield due to the increase in temperature, they have contributed significantly to crop yield increase. The increase in temperature before over-wintering stage enabled late sowing of winter wheat and late harvesting of maize, leading to overall 4-6% increase in total grain yield of the wheat-maize system. Increased use of farming machines and minimum tillage technology also shortened the time for field preparation from harvest time of summer maize to sowing time of winter wheat, which facilitated the later harvest of summer maize.

a b s t r a c t(1) While it is well-known that trees release carbon (C) to soils as root exudates, the factors that control the magnitude and biogeochemical impacts of this flux are poorly understood.(2) We quantified root exudation and microbially-mediated nutrient fluxes in the rhizosphere for four 80 year-old tree species in a deciduous hardwood forest, Indiana, USA. We hypothesized that trees that exuded the most carbon (C) would induce the strongest rhizosphere effects (i.e., the relative difference in nutrient fluxes between rhizosphere and bulk soil). Further, we hypothesized that tree species that associate with ectomycorrhizal (ECM) fungi would exude more C than tree species that associate with arbuscular mycorrhizal (AM) fungi, resulting in a greater enhancement of nutrient cycling in ECM rhizospheres.(3) Mass-specific exudation rates and rhizosphere effects on C, N and P cycling were nearly two-fold greater for the two ECM tree species compared to the two AM tree species (P < 0.05). Moreover, across all species, exudation rates were positively correlated with multiple indices of nutrient cycling and organic matter decomposition in the rhizosphere (P < 0.05). Annually, we estimate that root exudation represents 2.5% of NPP in this forest, and that the exudate-induced changes in microbial N cycling may contribute~18% of total net N mineralization.(4) Collectively, our results indicate that the effects of roots on nutrient cycling are consequential, particularly in forests where the C cost of mining nutrients from decomposing soil organic matter may be greatest (e.g., ECM-dominated stands). Further, our results suggest that small C fluxes from exudation may have disproportionate impacts on ecosystem N cycling in nutrient-limited forests.