On the importance of being co-transcriptional

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Abstract
At low water potential (ψw), dehydration reduces the symplast volume of leaf tissue. The effect of this reduction on photosynthetic capacity was investigated. The influence of osmotic adjustment on this relationship was also examined. To examine these relationships, comparative studies were undertaken on two wheat cultivars, one that osmotically adjusts in response to water deficits (`Condor9), and one that lacks this capacity (`Capelle Desprez9). During a 9-day stress cycle, when water was withheld from plants grown in a growth chamber, the relative water content of leaves declined by 30% in both cultivars. Leaf osmotic potential (ψs) declined to a greater degree in Condor plants. Measuring ψs at full turgor indicated that osmotic adjustment occurred in stressed Condor, but not in Capelle plants. Two methods were used to examine the degree of symplast (i.e. protoplast) volume reduction in tissue rapidly equilibrated to increasingly low ψw. Both techniques gave similar results. With well-watered plants, symplast volume reduction from the maximum (found at high ψw for each cultivar) was the same for Condor and Capelle. After a stress cycle, volume was maintained to a greater degree at low ψw in Condor leaf tissue than in Capelle. Nonstomatally controlled photosynthesis was inhibited to the same degree at low ψw in leaf tissue prepared from well-watered Condor and Capelle plants. However, photosynthetic capacity was maintained to a greater degree at low ψw in tissue prepared from stressed Condor plants than in tissue from stressed Capelle plants. Net CO2 uptake in attached leaves was monitored using an infrared gas analyzer. These studies indicated that in water stressed plants, photosynthesis was 106.5% higher in Condor than Capelle at ambient [CO2] and 21.8% higher at elevated external [CO2]. The results presented in this report were interpreted as consistent with the hypothesis that there is a causal association between protoplast (and presumably chloroplast) volume reduction at low ψw and low ψw inhibition of photosynthesis. Also, the data indicate that osmotic adjustment allows for maintenance of relatively greater volume at low ψw, thus reducing low ψw inhibition of chloroplast photosynthetic potential.