THE INFLUENCE OF WATER-STRESS ON LEAF AND STEM PHOTOSYNTHESIS IN SPARTIUM-JUNCEUM L

Stem and leaf photosynthetic responses to environmental parameters were studied in Spartium junceum L., a legume with chlorophyllous stems. Stem net photosynthesis (P(n)) was consistently lower than leaf P(n). The low stem P(n) was due to lower quantum yield, lower mesophyll conductance and lower CO2-saturated P(n) than that of leaf P(n). Stomatal limitations to leaf and stem P(n) were similar (25%). Water stress caused a greater reduction in leaf P(n) than that of stems. Leaf P(n) was also reduced in water-stressed plants following rehydration. The reduced leaf P(n) was associated with a reduced photon saturated P(n) rate and a reduced CO2 saturated P(n) rate. Apparent quantum yield, mesophyll conductance and stomatal limitation of leaves were unaffected by water-stress. Stem P(n) following rehydration was not influenced by the water-stress treatment. In general, leaf P(n) was more responsive to environmental parameters and more sensitive to water stress than stem P(n). These data support the hypothesis that stem P(n) has greater tolerance of water stress, but is limited to low P(n) by biochemical means compared to leaves.