Carbon and nitrogen partitioning in Vitis vinifera L.: Responses to nitrogen supply and limiting irradiance

Potted Vitis vinifera L. plants were grown under controlled environmental conditions at five different levels of nitrogen (0, 1, 5, 10, 100 mM NH4NO3) in combination with two different levels of irradiance (photon flux densities: 30 and 140 µ mol · m-2 s-1 PAR) during bloom. Elevated N supply increased available N (particularly NO3-), K, Ca and Mg, and reduced P in the soil. Soil-NO3- and K were higher in the lower light regime, but NH4+ and other nutrients were not influenced by irradiance. The concentration of total N in the xylem sap increased as N supply was increased, although there was no further rise above intermediate soil-N levels. NO3- was the principal xylem solute, in particular under severe light restriction and high N availability. In the lower light regime, only traces of organic N could be detected in the xylem sap, whereas in the higher light treatment, glutamine and glutamate increased with increasing N application level. Light limitation reduced the concentrations of P, K and Mg in the xylem sap by about 50 %, but no response to N supply could be observed. There was a strong positive relationship between N availability and N concentration in all plant parts, while the effect on C content was minor and depended on the type and physiological age of the tissue. The amounts of total N per vine were not affected by the light treatments, although low-light stress increased N concentrations in the dry matter of the annual organs by 34-86 %. By contrast, low light led to a slight decrease of the C concentration in the annual plant parts. In the higher light regime, non-structural carbohydrates in the permanent parts of the vine declined as N availability increased. Under severe light restriction, however, the C reserve fraction was depleted and was not altered by N supply, indicating that reserves had been remobilized to support maintenance and growth processes, in order to guarantee survival of the vine