Genetic diversity and potential high temperature tolerance in Brassica rapa L.

[Truncated abstract] Brassica rapa is the most widely distributed and diverse agricultural Brassica species. Different morphotypes (oilseed, root vegetable and leaf vegetable) and flowering types (winter, spring or semi-winter) occur throughout its range from northern and southern Europe to south and east Asia. B. rapa can be readily intercrossed with most agricultural Brassica species. Therefore, B. rapa is an important source of new genetic diversity, including genes for heat stress tolerance, for agricultural Brassica species in warming climates. A genetic diversity study was carried out on a global collection of 187 accessions of putative oilseed-type B. rapa subsp. oleifera, based on simple sequence repeat (SSR) molecular markers. From 164 confirmed oilseed-types B. rapa, three SSR groups were found which were related to the geographic origins of accessions: SSR group 1 (south Asia), SSR group 2 (southern Europe), and SSR group 3 (northern Europe). The reproductive traits of flowering habit (winter, spring or semi-winter) and self-compatibility or incompatibility were distributed across all three SSR groups. Among 74 oilseeds B. rapa accessions from India, the yellowseeded self-compatible types (most likely yellow sarson) were restricted to one subgroup, which suggested regional selection of the major oilseed types in India. Some accessions from European sources were in SSR group 1, and probably were introduced to Europe from south Asia. SSR allelic diversity in this global collection of B. rapa was high, suggesting that B. rapa could be a valuable source of genes for heat tolerance. High temperature stress often occurs during the reproductive stage of crops, and may cause major losses in seed production. Accessions of B. rapa were selected for heat tolerance screening from regions where heat stress is known to occur during flowering. Plants were grown in pots in controlled environment rooms with constant replacement of water lost through transpiration. One room was used for the high temperature treatment (daily maximum 35 °C, minimum 25 °C) and one room for the "control" temperature treatment (daily maximum 23 °C, minimum 15 °C) for one week from first flowering on the main stem. Leaf temperature and leaf conductance observation confirmed there was no water stress in the plants during the heat screening process. One leaf vegetable-type of B. rapa from Indonesia set seed equally well in the high temperature or normal treatment, whereas pod set and seed yield was severely restricted in the high temperature treatment in several oilseed B. rapa types from south Asia or Europe. There was a small decrease in pollen viability from 100% at control temperature to less than 75% at high temperature. Bud number, bud length and pod number produced during high temperature were correlated with and were useful predictors of seed yield under high temperatures in B. rapa...