Proline accumulation in cultivated citrus and its relationship with salt tolerance

Abstract

SummaryThe constitutive content of free proline occurring in the leaves of a number of citrus and related genera, as well as in those of other tropical trees growing under the same environmental conditions, has been determined. Species belonging to the Aurantioideae were found to be conspicuous by the high level of free proline in their leaves. In grafted trees, the origin of the rootstock did not affect the proline content of the scions. Taking into account the fact that the proline amount is generally greater in higher plants subjected to environmental stresses, the relevance of this trait as a biochemical marker in breeding citrus plants for salt tolerance has been investigated. This has been addressed by comparing five citrus varieties exhibiting contrasted tolerance to NaCl, for their capacity to accumulate proline in response to water stress. For this purpose, excised leaf discs of 4–6 month old non-fruiting shoots from plants grown under the same environmental conditions were treated in vitro in presence of NaCl or PEG 6000 solutions of osmotic potential close to –2.MPa. Assuming that under these conditions the metabolic traits involved in proline accumulation were fully expressed, it was found that the amount of proline accumulated in the leaf discs reflected the relative sensitivity rather than the salt tolerance of the five genotypes. The suitability of such leaf disc test for screening citrus varieties on the basis of their salt tolerance is discussed.