Salt stress differentially regulates mobilisation of carbon and nitrogen reserves during seedling establishment of Pityrocarpa moniliformis.

Seedling establishment is a critical step in environment colonisation by higher plants that frequently occurs under adverse conditions. Thus, we carried out an integrated analysis of seedling growth, water status, ion accumulation, reserve mobilisation, metabolite partitioning and hydrolase activity during seedling establishment of the native Caatinga species Piptadenia moniliformis (Benth.) Luckow & R.W. Jobson under salinity. Two-day-old seedlings were cultivated in vitro for 4 days in water agar (control) or supplemented with 50 or 100 mm NaCl. Biochemical determinations were performed according to standard spectrophotometric protocols. We found that 100 mm NaCl stimulated starch degradation, amylase activity and soluble sugar accumulation, but limited storage protein hydrolysis in the cotyledons of P. moniliformis seedlings. Although Na+ accumulation in the seedling affected K+ partitioning between different organs, it was not possible to associate the salt-induced changes in reserve mobilisation with Na+ toxicity, or water status, in the cotyledons. Remarkably, we found that starch content increased in the roots of P. moniliformis seedlings under 100 mm NaCl, probably in response to the toxic effects of Na+ . The mobilisation of carbon and nitrogen reserves is independently regulated in P. moniliformis seedlings under salt stress. The salt-induced delay in seedling establishment and the resulting changes in the source-sink relationship may lead to storage protein retention in the cotyledons. Possibly, the intensification of starch mobilisation in the cotyledons supported starch accumulation in the root as a potential mechanism to mitigate Na+ toxicity.

Regulation of reserve mobilisation in sunflower during late seedling establishment in continuous darkness.

Reserve mobilisation, metabolite partitioning and reserve-degrading enzyme activity were studied in sunflower seedlings cultivated in vitro under a 12-h photoperiod or in the dark to investigate the involvement of source-sink relation and carbon starvation in the regulation of reserve mobilisation under continuous darkness. Reserves, metabolites and enzyme activity were determined with standard spectrophotometric methods. At the first 24 h of treatment (acclimation phase), darkness did not affect growth, but restricted carbon and nitrogen use, as indicated by sugar and amino acid accumulation in the different seedling parts. After 5 days of treatment (survival phase), extended darkness limited growth and retarded storage lipid mobilisation due to carbon starvation, as evidenced by the depletion of carbohydrates in cotyledons and hypocotyl, as well as the consumption of amino acids in hypocotyls and roots. Alterations in the source-sink relationship might have been a response to prolonged darkness, instead of a mechanism used to regulate reserve mobilisation, as these alterations cannot be associated with negative feedback mediated by metabolite accumulation. Storage lipid degradation depends, at least in part, on mechanisms that co-ordinately regulate the activities of lipases and isocitrate lyase. Taking these results together, it is possible that reserve mobilisation in sunflower seedlings cultivated in the dark might be regulated by mechanisms that perceive the absence of light and predict carbon starvation, adjusting reserve use according to future energy demands to allow, at least in the short term, seedling survival.