Source‐Sink Carbon Relations in Two Panicum Coloratum Ecotypes in Response to Herbivory

Abstract

Population samples of an African C4 grass, Panicum coloratum L., were collected from two locations in the Serengeti Grasslands varying in grazing intensity, one a high—grazing location (GA = grazing—adapted), the other a low—grazing location (NGA = nongrazing—adapted). Plants were cloned, put in controlled environments mimicking the natural photo—thermoperiod, and subjected to light grazing pressure by a generalist feeding North American grasshopper, Melanoplus sanguinipes. Carbon assimilation and redistribution were measured in the short term with an infrared gas analyzer and 11C—labelled CO2, coupled with a three—compartment analytical model, and by harvesting whole plants at the end of a 12—wk regrowth experiment. Results documented several significant differences between the GA and NGA samples, suggesting the evolution of physiological traits related to C assimilation, translocation, and storage in response to previous grazing history. Pregrazing net C—fixation rates, translocation rates, and C—storage pools were identical for the two ecotypes. After grazing, overall C—fixation rates were 39% higher for the GA plants, and the regrowth data suggest they remained higher than NGA rates throughout the experiment. Removal of < 10% of initial green—leaf biomass by grazing at each grazing period produced major differences in carbon flux between the two samples. Throughout the experiment GA plants produced and stored more C in leaves, stored less C in stem sinks, had higher phloem activity, and translocated more of the labile C to roots where it was stored in higher quantities. This suggests that storage of labile C reserves in sinks or pools readily accessible to the plant, which allows rapid mobilization after grazing, is an important element of adaptation to grazing. Major storage of labile reserves in stems, characteristic of the NGA plants, may be advantageous in ungrazed habitats where there is vertical growth due to canopy closure and competition for light, but such storage makes those reserves accessible to grazers. GA ecotype plants in the regrowth experiment compensated completely for the eight weekly defoliation events by the time of the 12—wk harvest; yield of the NGA ecotype was reduced 21% by the moderate level of grasshopper grazing. Increased total yield (biomass harvested plus compensation) by the GA ecotype was expressed in both above— and belowground biomass, suggesting that suppression of the latter does not contribute to compensation by the former.

Grasshopper crop and midgut extract effects on plants: an example of reward feedback.

“…Collectively, these act to redirect phloem transport of labile C and N compounds, cause shifts in C source-sink relationships (3,4), regulate C and N productivity (ref. 18; M.I.D., unpublished data), and may induce semiochemical release that functions as an intertrophic defense signal (2).…”

“…These include production of defense mechanisms (1,2) and changes in internal physiology (3,4) and productivity (5,6), providing overall a series of positive and negative feedback signals to the plant that can reorient its C and N distribution as well as its growth and development potential (7,8). The induction signal itself may emanate from endogenous biochemical pathways set in motion by physical damage to plant leaves (9)(10)(11) or, perhaps as likely, exogenously from biochemical messengers found in salivary and digestive systems of herbivores (12)(13)(14)(15)(16).…”

Why Does Herbivore Attack Reconfigure Primary Metabolism?

“…Carbon is allocated to roots in response to leaf damage or herbivory in several species, for example, after grasshopper damage to Zea mays (Holland et al, 1996) and Panicum coloratum (Dyer et al, 1991), after the defoliation of Lolium perenne (Bazot et al, 2005) and of two C 4 perennial grasses (Briske et al, 1996), and after methyl-JA treatment of Populus tremuloides (Babst et al, 2005). Recently, a SnRK kinase has been found Figure 1.…”

Carbon Fluxes in the Rhizosphere

“…It is also called the yield factor. Accumulated evidence suggests that a big proportion of root exudates is utilized and released as CO 2 in a very short period of time; only a small portion becomes microbial biomass (Dyer et al 1991;Harris and Paul 1991). The microbial assimilation efficiency of these exudates (6.5-15%; Helal and Sauerbeck 1989;Liljeroth et al 1990;Martin and Merckx 1992), is considerably lower than the theoretical maximum of 60 percent (Payne 1970) and of other sources of carbon in the soil.…”