Ecogeographic and evolutionary approaches to improving adaptation of autumn-sown chickpea ( Cicer arietinum L.) to terminal drought: The search for reproductive chilling tolerance

Abstract

Terminal drought is the most important abiotic stress of dryland chickpea. The principal adaptive strategy of the crop is drought escape through early phenology. In regions where average temperatures at flowering <14–16 °C, such as southern Australia or northern South Asia, the lack of reproductive chilling tolerance forces chickpea to delay podset. This delay compromises drought escape by exposing chickpea to terminal drought during much of the pod filling phase, reducing yield potential and stability, depending on seasonal climatic fluctuations. This paper defines chickpea growing seasons in time throughout the crop's global production regions using published data and feedback from local breeders. This information is used to calculate and map regionally specific chickpea bioclimatic variables such as flowering phase temperatures and the peak rate of growing season temperature change. Flowering phase temperatures <14 °C are uncommon. Mediterranean climates in southern Australia, Chile, California and Portugal tend to be cool during flowering (<15.4 °C), while much of WANA is moderate to warm (15.4–24.4 °C), with exceptions in coastal N Africa, the Nile valley and higher elevations in Morocco, the Balkans, central Afghanistan, NE and SW Iran. Ethiopia contains a wide range of flowering phase temperatures depending on altitude. In South Asia flowering phase temperatures tend to decrease with increasing latitude, with large cool regions in the Punjab, N Haryana, N Uttar Pradesh and the Nepali terai. Superimposing the peak rates of temperature change within the growing season identifies habitats which are likely to be consistently cool throughout the reproductive phase by eliminating regions with transient chilling stress, such as much of N South Asia. It is suggested that reproductive chilling tolerance is rare in chickpea because of the late Neolithic shift from autumn to spring sowing in WANA, and subsequent dissemination into predominantly moderate to warm flowering habitats with only transient low temperature stress. The search for reproductive chilling tolerance should concentrate on these relatively uncommon consistently cool habitats. These are not well represented in the world chickpea collection, a shortcoming which is exacerbated by the lack of passport data in regions of potential interest from Iran to Central Asia. An alternative strategy is to search for reproductive chilling tolerance in the wild Cicer species, which have maintained a winter annual lifecycle and appear to be more cold tolerant than the cultigen. This approach is limited by the lack of germplasm. Currently, there are <30 original wild accessions in the primary genepool of chickpea ( C. reticulatum, n = 18; C. echinospermum, n = 10) in the world's genebanks, so more collection is required.