Ammonium assimilation in Pinus radiata seedlings: effects of storage treatments, transplanting stress and water regimes after planting under simulated field conditions

Abstract

This study is concerned with the effects of storage conditions, planting shock and water availability at planting sites on nitrogen assimilation in Pinus radiata. Seedlings were stored for 1, 8 or 15 days at 4 or 10 °C with or without soil around the roots and afterwards planted in well-irrigated or dry soils. Our analysis of protein content and the activities of glutamine synthetase (GS; EC 6.3.1.2) and glutamate dehydrogenase (GDH; EC 1.4.1.2) showed that, as storage duration increased, there was a gradual reduction in soluble protein content and an inhibition of GS, whereas GDH activity increased. Thus there appears to be a close relationship between desiccation and changes in nitrogen assimilation. These effects were more pronounced in bare-root seedlings than in seedlings with soil around the roots, indicating that the rooting-plug medium provides protection against desiccation. Seedlings were susceptible to planting shock even under well-watered conditions. Drought enhanced the severity of planting shock and both protein content (80%) and GS activity (75%) were drastically reduced when water was withheld, whereas GDH activity increased by more than 170%. Thus, the decline in needle relative water content (RWC) appeared closely related to a decrease in protein content and GS activity, and on the other hand, to an increase in GDH. There was a remarkable recovery of these parameters after rewatering, depending however, on stock quality at the moment of planting.