Proline content of sugar beet storage roots: Response to water deficit and nitrogen fertilization at field conditions

Drought stress is one of the major factors causing profit loss of the sugar beet crop. The accumulation of proline, an indicator of water stress, has been reported in response to osmotic and salt stress in sugar beet leaves, but there is little information about its levels in storage roots. Proline in storage roots is potentially useful as indicator of situations that lead to decreased yield and quality of the root, such as drought stress and excess nitrogen. This study has been focused towards proline quantification in storage roots from field trials concerning irrigation, sugar beet variety and nitrogen fertilization. Water deficit is the main factor leading to proline accumulation in sugar beet roots. Excess N supply also increases proline levels, partially by increasing leaf area index (LAI) and exacerbating drought stress. The two varieties studied (Claudia and Ramona) had different responses to water shortage and to nitrogen. Maximum proline levels were measured in Claudia roots subjected to a combination of water shortage and excess N. A positive and significant correlation was found between proline and glucose levels in sugar beet roots, pointing to a relationship between stress responses, carbohydrate catabolism, and proline and glucose accumulation. This proposal was supported by the effect of treatments with di-1-p-menthene (anti-transpirant) and with DMDP (2,5-dihydroxymethyl-3,4-dihydroxypyrrolidina, a glycosidase inhibitor), which lead to decreased level of proline in non-irrigated Claudia sugar beet roots.