Is carbon starvation rather than excessive nitrogen supply the cause of inflorescence necrosis in Vitis vinifera L.?

Five different levels of nitrogen (0, 1, 5, 10, 100 mM NH4NO3) were applied to potted Vitis vinifera L. plants grown in a controlled environment facility (phytotron) under 2 different levels of irradiance (30, 140 µE m-2 s-1 PFD) during bloom. They were compared with potted plants in the field, exposed to differential leaf or inflorescence shading in combination with 2 levels of N supply (0, 10 mM NH4NO3). Net CO2 assimilation rate, transpiration rate and stomatal conductance were reduced, and intercellular CO2 partial pressure was increased under conditions of light limitation. The low-light treatments decreased net photosynthesis to zero in either experiment. Separate shading of the flower clusters failed to affect gas exchange of adjacent leaves, indicating low sink strength of the inflorescences. Nitrogen fertilization influenced only transpiration rate and stomatal conductance. In the phytotron, but not in the field, these parameters decreased with increasing N level. Severe symptoms of inflorescence necrosis appeared in the low-light treatment in the phytotron at the end of flowering. Necrotic symptoms also developed on shoot tips and tendrils, leading to abscission of these organs. Tissue necrosis was independent of N nutrition, and there were no necrotic manifestations in the field study. These results provide evidence that a stress-induced limitation of photoassimilate supply, along with competitive interactions among sinks, are involved in triggering senescence processes in grapevines