The effect of nitrogen fertilization and irrigation frequency on photosynthesis of potatoes (Solanum tuberosum)

Gas exchange parameters for the potato (Solanum tuberosum L., cv. 'Desiree') crop were measured under field conditions. Nitrogen nutrition was found to have a positive effect on photosynthesis throughout the growing season. The gas exchange data and the chlorophyll content in the leaves indicated that this effect was essentially in the mesophyll. Stomatal resistance increased as photosynthetic rate decreased, but substomatal CO₂ concentration (Ci) was not affected by nitrogen treatments.Long term effects of water deficit on photosynthetic capacity were also mostly mesophyllic in origin. A reduction in photosynthetic rate was observed after a period of moisture stress and also after relief of drought by irrigation, indicating a limited capacity for recovery. A short term afternoon increase in stomatal resistance, associated with lower Ci values, occurred after long periods without irrigation. Leaf water potential was affected by irrigation frequency, but less strongly than the gas exchange parameters. Mesophyll activity and stomatal behaviour were both affected by water stress and by nitrogen deficiency, whereas Ci values were not affected in most cases.