Effect of water shortage on photosynthesis, growth and storage carbohydrate accumulation in walnut (Juglans regia L.).

Two year old seedlings of J. regia L. were subjected to water shortage from August to the end of the growing season. Photosynthesis, growth and storage carbohydrate accumulation were monitored during the growing season. Cumulated water transpired, stem radial growth and total plant dry matter were significantly reduced by drought treatment. Although stomatal conductance was greatly reduced by drought, a clear non-stomatal component of the reduction in photosynthesis was evident from the response curves of assimilation rate to changes in substomatal partial pressure of CO2. The drought-induced reduction in photosynthesis was associated with a decrease in the amount of storage carbohydrates. Phenology and spring shoot growth rate were strongly affected by starch content at the beginning of the season. Especially, plants subjected to drought during the previous year were characterised by earlier sprouting and by reduced spring growth rate compared to control plants. Carbon isotope discrimination was used to evaluate the contribution of the photosynthetic products assimilated during the drought period on starch accumulation in different plant organs and on spring growth during the following season.