Carbon and nitrogen translocation in response to shading of the seagrass Posidonia sinuosa

Translocation of carbon (C) and nitrogen (N) was investigated in response to shading of the seagrass Posidonia sinuosa in control (ambient light) and shade (below minimum light requirement) treatments after 10 d shading. A mature leaf was incubated in situ in C-13- and N-15-enriched seawater for 2 h and the appearance of the isotopes in the young leaf and adjacent rhizome monitored over 29 d. C and N isotopes gradually reduced in the mature leaf: of N-15 contained in the entire shoot (mature leaf, young leaf and 4 cm rhizome), 95% (control) and 97% (shade) was found in the mature leaf after 2 h incubation and only 75% and 60% remained in the mature leaf after 29 d; 98% and 94% of C-13 was found in the mature leaf after 2 h, and it had reduced to 36% and 44% after 29 d. This corresponded to an equal increase in the young leaf + rhizome indicating that the mature leaf is a source of these nutrients to the young leaf and rhizome. C translocation from mature leaves was not significantly affected by the shade treatment. In contrast, there was an increase in N-15 taken up by the mature leaves (1.9x higher in the shade), the percent of N-15 translocated to the young leaf and rhizome (24% in control and 40% in shade) and N concentration in the young leaf (1.24% control and 1.41% shade) and rhizome (0.86% control and 0.99% shade). Resorption of C and N was also estimated from changes in the total C and N content of the mature leaf over 29 d. N resorption from the mature leaf contributed up to 63% of young leaf N requirements in the control treatment but only 41% in the shade treatment. We conclude that uptake and translocation of N by mature leaves is a response to shading in P. sinuosa and would provide additional N to growing leaves, enhancing light harvesting efficiency. (C) 2010 Elsevier B.V. All rights reserved.