Analysis of the relative increase in photosynthetic O-2 uptake when photosynthesis in grapevine leaves is inhibited following low night temperatures and/or water stress

We found similarities between the effects of low night temperatures (5 degr ees C-10 degrees C) and slowly imposed water stress on photosynthesis in gr apevine (Vitis vinifera L.) leaves. Exposure of plants growing outdoors to successive chilling nights caused light- and CO2 saturated photosynthetic O -2 evolution to decline to zero within 5 d. Plants recovered after four war m nights. These photosynthetic responses were confirmed in potted plants, e ven when roots were heated. The inhibitory effects of chilling were greater after a period of illumination, probably because transpiration induced hig her water deficit. Stomatal closure only accounted for part of the inhibiti on of photosynthesis. Fluorescence measurements showed no evidence of photo inhibition, but nonphotochemical quenching increased in stressed plants. Th e most characteristic response to both stresses was an increase in the rati o of electron transport to net O-2 evolution, even at high external CO2 con centrations. Oxygen isotope exchange revealed that this imbalance was due t o increased O-2 uptake, which probably has two components: photorespiration and the Mehler reaction. Chilling- and drought-induced water stress enhanc ed both O-2 uptake processes, and both processes maintained relatively high rates of electron flow as CO2 exchange approached zero in stressed leaves. Presumably, high electron transport associated with O-2 uptake processes a lso maintained a high Delta pH, thus affording photoprotection.