Salinity Induced Limitations on Photosynthesis in Prunus salicina, a Deciduous Tree Species

Abstract

The response of photosynthetic CO2 assimilation to salinization in 19 year old Prunus salicina was evaluated under field conditions for a 3 year period. The observed decline in CO2 assimilation capacity was apparently related to increasing leaf chloride (Cl-) content, and independent of changes in leaf carbohydrate status. The response of net CO2 assimilation (A) to leaf intercellular CO2 partial pressure (C) indicated that the reduction in the capacity for A with Cl-was not the result of decreased stomatal conductance but a consequence of nonstomatal inhibition. The nonstomatal limitations to CO2 assimilation capacity, as determined by the response of A to C, and biochemical assay, were related to a decline in the activity of ribulose 1,5-bisphosphate carboxylase (Rubpcase) and the pool size of triose phosphate, ribulose 1,5-bisphosphate (Rubp) and phosphoglycerate with increasing salinity. Lack of agreement between the initial slope of the A to Ci response curve and Rubpcase activity suggests the occurrence of heterogeneous stomatal apertures with the high salinity treatment (28 millimolar). Prolonged exposure to chloride salts appeared to increase the Rubp or Pi regeneration limitation, decrease Rubpcase activity and reduce leaf chlorophyll content. Observed changes in the biochemical components of CO2 fixation may, in turn, affect total leaf carbohydrates, which also declined with time and salinity. The reduction in Rubpcase activity was apparently a consequence of a reduced Rubpcase protein level rather than either a regulatory or inhibitory effect.salinity reduces growth to a greater extent than photosynthesis, causing a subsequent carbohydrate buildup (either as starch and/or soluble sugars), which can be associated with a reduction in photosynthetic capacity (10, 11). Therefore, an investigation to effectively define limitations to the photosynthetic process as a result of increased salinity requires: (a) determination of the influence of salinity on leafcarbohydrate status and the potential for feedback inhibition, and (b) separation and quantification of the stomatal and nonstomatal factors which affect photosynthetic capacity. However, no study has examined both (a) and (b) for plants exposed to salinity under field conditions.The experiments reported here were designed to analyze limitations to CO2 assimilation in mature trees of Prunus salicina (L. cv Santa Rosa plum), acclimated to salinity under field conditions. Previous experiments with P. salicina in this location have shown that the decline in productivity associated with salinity was not related to a reduction in leaf water content or turgor potential (12, 30). Furthermore, measurements of the ion content of wood and leaf tissue, as well as cellular ion compartmentation, indicated a limited capacity of this species to regulate distribution of toxic ions (Cl-) (31). MATERIALS AND METHODS PlantsAs competition for available irrigation water intensifies, water quality is expected to decline with a subsequent reduction in crop productivity...

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