Sensitivity of Trifolium alexandrinum L. to salt stress is related to the lack of long-term stress-induced gene expression

In this study, the physiological and molecular defects leading to growth reduction under salt stress in Egyptian clover (Trifolium alexandrinum L.) were investigated. Time-course response of growth to salt stress with 200 mM Na+ and/or Cl− showed that significant reduction in the growth of the stressed plants was delayed until the sixth day of stress indicating that the plant is more resistant to the osmotic component of salt stress. The rate of photosynthesis started to decrease significantly from the third day of stress although the rate of transpiration decreased to 18.4% of the control after 2 h indicating that reduction in photosynthetic rate may have not been essentially due to CO2 limitation. The mutual relationships of photosynthesis and growth are discussed and evidence is provided that growth reduction is largely a consequence rather than a cause of inhibition of photosynthesis. Data also shows that inhibition of photosynthesis could have resulted from excessive accumulation of Na+ in the leaves along with downregulation of Na+-sequestering genes (NHX1, H+-PPase and H+-ATPase) that may have led to loss of the efficiency of Na+ sequestration into the vacuole and subsequent damage to photosynthesis. The role of HRD gene in regulating transpiration is discussed.