Theme 3 - Enhance N2-fixation capacity of grain legumes for annual and rotational crop production
Chickpea obtains N to grow and set seed using a combination of direct and indirect pathways. The direct pathway involves root uptake of inorganic N (NH4+, NO3-) and to a lesser extent organic N (amino acids and peptides) using root transport systems. The indirect pathway involves N2-fixation, which delivers fixed NH4+ when soil N concentrations are low enough to support root nodule development and rhizobial activity. Stored cotyledon N reserves are also important drivers of early seedling development and plant progression in the field. When faced with a choice in N supply, legumes generally favour the low C cost direct pathway, but how chickpea manages the N acquisition pathways is not understood. The sensitivity of chickpea to exogenous soil N requires investigation relative to more N tolerant legumes such as soy bean and pea. Likewise, we need basic information on the effect of soil conditions such as water and other nutrient availability, and the effect of competition from indigenous rhizobial populations, on nodule formation and activity. This information for Australian soils is not available at this time.
For growers, symbiotic N2 fixation is the keystone for the integration of legumes into continuous cropping systems. It provides an abundant and cheap source of N for legume protein production and provides an avenue for increasing soil N fertility and following crop production without N fertiliser use. When fixing N2, chickpea invests up to 25% of total plant N in its nodules, compared to 5-10% for other crop legumes (Unkovich & Pate, 2000). It also has a low seed N concentration compared to other grain legumes (Unkovich et al., 1997). These properties imply that the N2-fixation and N distribution machinery in chickpea is less efficient than for other grain legumes. Such potential N constraints may be one reason why chickpea crops have a lower leaf area index than other grain legumes (Heath et al., 1994). Improving nodule efficiencies and N redistribution pathways is key to identifying germplasm that makes N2-fixation more efficient and which enhances both indirect and direct N acquisition pathways. It is also key to delivering N to the grower either through elevated seed N and yields or improved soil N deposition.