Exogenously supplied glycine betaine in spinach and rapeseed leaf discs: Compatibility or non-compatibility?

When leaf discs of spinach (Spinacia oleracea cv. Junius) and rapeseed (Brassica napus var. oleifera cv. Samourai) were incubated in the light in the presence of glycine betaine (GB), they accumulated GB at a very high level. In comparison with the spinach leaf explants, the uptake of GB by rapeseed tissues was restricted, probably by the destabilizing effects exerted by GB in this plant material. In contrast, the viability of spinach leaf discs, as assessed by their capacity to reduce 2,3,5-triphenyltetrazolium chloride (TTC), was not affected, suggesting that the GB taken up was compatible in the leaf tissues of the GB accumulator. In rapeseed leaf discs treated with GB, chlorophyll loss as well as significant changes in polyamine content were induced, leading to a dramatic increase of the putrescine/(spermidine + spermine) ratio. In contrast, this ratio remained constant in the GB treated spinach explants, suggesting that spinach has the capacity to stabilize polyamine metabolism in the presence of high amounts of GB. The treatment of spinach leaf discs with GB prior to application of osmotic or salt shocks provided protection from stress. A weak capacity to accumulate proline under stress conditions was partially suppressed. The protein content decreased while the free amino acid level increased independently of the presence of GB. It is concluded that GB behaves as a true compatible solute in spinach, which is a typical GB accumulator, and that GB is damaging when loaded into the leaf tissues of rapeseed, which do not normally accumulate GB.