MISCELLANEOUS PUBLICATION 1271, U.S. DEPARTMENT OF AGRICULTURE 



olites and structural materials, and the transport 

 of substances within the plant body. 



The manner in which water stress affects nu- 

 cleic acid metabolism and protein synthesis is ob- 

 scure but several generalizations can be made. It 

 is known that water stress reduces the incorpora- 

 tion of amino acids into proteins, but amino acid 

 synthesis does not appear to be unduly inhibited 

 since amino acid pools tend to accumulate during 

 stress periods (3). The synthesis of proteins from 

 amino acids requires several steps, all of which 

 are interwoven with nucleic acid metabolism and 

 function. In the first place, transfer ribonucleic 

 acid (RNA) is required to bring the amino acids 

 to the ribosomes which are the sites of protein 

 synthesis. Messenger RNA is then required to 

 transcribe the amino acid sequences from the 

 deoxynucleic acid (DNA) to the forming protein 

 molecule. This leads to the accumulation of mono- 

 ribosomes into polyribosomes specific for each pro- 

 tein. Water stress can, therefore, influence protein 

 synthesis at one or more points in the overall se- 

 quence. Recent research has shown that polysome 

 formation is markedly retarded by water stress 

 (9). This may be a direct effect but there is 



evidence that messenger RNA is itself inactivated 

 by prolonged stress and that DNA may be 

 affected. 



Clearly, effects on these fundamental cellular 

 processes must have profound effects on cellular 

 development, even though the nature of the pri- 

 mary trigger for the sequence of events is still 

 obscure. The key requirements for growth, cell 

 division and cell enlargement are certainly af- 

 fected by quite small water deficits. 



At the roof and shoot apex, both cell division 

 and cell enlargement, and the initiation of new 

 primordia appear to be completely suspended at 

 water potentials of the order of * = — 1 to — 5 

 bars. (9, 25). 



The effect on primordial initiation appears to 

 be superficially similar to that of dormancy, in the 

 sense that initiation can be suspended without the 

 potential for subsequent development being im- 

 paired, as long as the stress is not too severe or 

 too protracted. Figure 1, from Gates {12), il- 

 lustrates this point, hy using apical development 

 of lupine as experimental material. The virtual 

 cessation of appearance of new primordia, when 

 stress is imposed, followed by renewed develop- 



1 2 



Days after water deprived 



1 2 3 



Days after water reapplied 



Figube 1. — Numbers of foliar primordia in lupine apices during water deprival and upon rewatering (shaded col- 

 umns), compared with well watered controls (solid columns) (after Gates (12)). 



