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Stable Isotpes. Example of Deuterium (H) and Carbon (C). Competition among plant species in diverse communities

Carbon

Discrimination against 13C during photosynthesis is a well characterized phenomenon. Plant matter produced during photosynthesis is depleted in 13C (lower 13C/12C ratio), compared to atmospheric CO2. The magnitude of this depletion mainly depends on the photosynthetic pathway of CO2 fixation.

Plants endowed with C3 pathway have a lower 13C/12C ratio than plants exhibiting C4 (dicarboxylic acid) pathway. The difference in 13C/12C ratio between C3 and C4 plants is related to difference in the isotopic fractionation existing between Rubisco activity in C3 plants and the phosphoenolpyruvate carboxylase (PEPC) activity in C4 plants. For example, maize is a C4 plant using PEP carboxylase and Rubisco as CO2 acceptors. Rubisco discriminates more against 13C (around 29%) than PEPC (around 2.2%). As a consequence, average carbon isotope discrimination value is less in C4 plants (around 4%) than in C3 plants (around 20%).

The use of carbon isotope discrimination appears consequently accurate for a first screening of lines or hybrids highly contrasting for drought tolerance, but not for a more advanced selection among tolerant hybrids (Monneveux et al. 2007).

 

Deuterium

Water is a critical resource that limits the distribution and abundance of vegetation in arid and semiarid regions of the world. Because of water deficiency competition between plant species occurs.

The stable isotope method can be used to evaluate weather two species are competing for the same pool of water. The hydrogen stable isotope as tracer is used to evaluate the competition of two species. Because no isotopic fractionation occurs during the movement of water from the roots to the shoot base the stable isotope ratio (deuterium/hydrogen) contained in xylem sap water at the shoot base reflects the water sources of a plant.

1. Example: Pearl Millet vs. Cowpea under Drought and Soil Compaction Stress
Compared with monocropped pearl millet intercropped pearl millet hat significantly higher deuterium concentrations in xylem sap under drought and soil compaction. Cowpea does not show a significantly difference between monocropping and intercropping. The enriched deuterium values in xylem sap of intercropped pearl millet show a higher dependence of pearl millet on recently supplied water (rainfall or irrigation) under drought. The water sources of cowpea are not modified by the competition of pearl millet under any circumstances indicating the higher ability to extract existing soil water.

2nd Example: Wheat vs. Rice in a Direct-sawn Sequential Cropping System
The growth period of rice and wheat must be overlapped for about two months because the growing season of rice is too short when the rice is seeded directly into the field after harvest of wheat. The direct sawn treatment significantly increased the deuterium concentration in xylem sap of wheat. This indicates that the water source of wheat is shifted to the deuterium-labeled simulated rainfall by the practice of no tillage. Under no-tillage conditions wheat has a higher water-uptake dependency on rainfall than on stroed soil water. Rice seedling roots mainly accumulate in the surface layer. With this higher competition for sotred soil water may occur between wheat and rice under no-tillage conditions. That is why the growing of rice is sometimes restricted during the dry years due to water competition

Hydraulic lift
The introduction of deep rooted plants into intercropping systems can provide an opportunity for the associated crops to utilize water from deep in the soil layers through hydraulic lift. Hydraulic lift is the passive transport of water through the root axis from wetter to drier soil layers. It is driven by the water potential gradient between plant roots and the soil and is a result of the increase in water potential due to stomatal closure. Hydraulic lift has been proposed as a mechanism that can buffer plants against water stress during water deficit serving as a drought-tolerance mechanism for some plants.

References
Corbin et al. (2005): Summer water use by California coastal prairie grasses: fog, drought, and community composition. In: Oecologia 145, p. 511-521

Zegada-Lizarazu et al. (2006): Water competition of Intercropped Pearl Millet with Cowpea under Drought and Soil Compaction Stresses. In: Plant Prod. Sci. 9, p. 123-132

Iijima et al. (2005): Productivity and Water Source of Intercropped Wheat and Rice ina Direct-sown Sequential Cropping System: The Effects of No-tillage and Drought. In: Plant Prod. Sci 8, p. 368-374

Zegada-Lizarazu, Iijima (2004): Hydrogen Stable Isotope Analysis of Water Acquisition Ability of Deep Roots and Hydraulic Lift in Sixteen Food Crop Species. In: Plant Prod. Sci. 7, p. 427-434

Sekiya, Yano (2003): Do pigeon pea and sesbania supply groundwater to intercropped maize through hydraulic lift? – Hydrogen stable isotope investigation of xylem waters. In: Field Crops Research 86, p. 167-173

Dawson (1993): Hydraulic lift and water use by plants: implications for water balance, performance and plan-plant interactions

Monneveux et al. (2007): Using carbon isotope discrimination to select maize (Zea mays L.) inbred lines and hybrids for drought tolerance. Plant Science 173 (390-396)

January 31st, 2008
Topic: Crop Science, Plant biodiversity Tags: None

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