IS CARBON STARVATION RATHER THAN EXCESSIVE NITROGEN SUPPLY THE CAUSE OF INFLORESCENCE NECROSIS IN VITIS-FINIFERA L

Five different levels of nitrogen (0, 1, 5, 10, 100 mM NH4NO3) were ap plied to potted Vitis vinifera L. plants grown in a controlled environ ment facility (phytotron) under 2 different levels of irradiance (30, 140 muEm-2s-1 PFD) during bloom. They were compared with potted plants in the field, exposed to differential leaf or inflorescence shading i n combination with 2 levels of N supply (0, 10 mM NH4NO3). Net CO2 ass imilation rate, transpiration rate and stomatal conductance were reduc ed, and intercellular CO2 partial pressure was increased under conditi ons of light limitation. The low-light treatments decreased net photos ynthesis 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 influe nced only transpiration rate and stomatal conductance. In the phytotro n, but not in the field, these parameters decreased with increasing N level. Severe symptoms of inflorescence necrosis appeared in the low-l ight treatment in the phytotron at the end of flowering. Necrotic symp toms also developed on shoot tips and tendrils, leading to abscission of these organs. Tissue necrosis was independent of N nutrition, and t here 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.