Root-zone sodium chloride influences photosynthesis, water relations, and mineral content of sapodilla foliage

Sapodilla [Manilkara zapota (L.) Royen], reportedly tolerant of saline conditions relative to other tropical fruit species, was studied in sand culture under greenhouse conditions to examine the physiology of sapodilla trees exposed to NaCl and to aid in determining the basis for this apparent tolerance, Treatments, consisting of a complete nutrient solution of 1 dS . m(-1) (control) or this solution amended to 12 or 20 dS . m(-1) with NaCl, were administered from 16 Nov, 1991 until 29 Jan, 1992, Net CO2 assimilation (A) of plants receiving NaCl gradually decreased relative to that of the control plants, At the end of 8 weeks of salinity, A of plants receiving 12 or 20 dS . m(-1) was 72% or 31% of control plants, respectively, Substrate NaCl reduced apparent quantum yield, photosynthetic CO2-use efficiency, leaf osmotic potential, and predawn xylem potential of sapodilla leaves, Dark respiration and the variable : maximal chlorophyll fluorescence ratio were not influenced by NaCl. Exposure to NaCl also increased leaf tissue Na+ and Cl- concentrations and the Na+: K+ ratio, These results indicate that gas exchange of sapodilla is relatively low for woody evergreen species. Moreover, sapodilla may not be as tolerant of salt stress as previously reported, The responses of sapodilla to root zone NaCl were consistent with other woody perennial glycophyte species. Photochemical efficiency of leaves on plants receiving NaCl was not different from that of leaves on control plants for >8 weeks after NaCl reduced gas exchange.