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MISCELLANEOUS PUBLICATION 1271, U.S. DEPARTMENT OF AGRICULTURE 



also suppress flowering, but stress after the in- 

 ductive period was without effect. It was of 

 interest that none of the treatments (down to 

 * = — 6 bars) suppressed vegetative growth, ex- 

 pressed either as node number or internode 

 length (figure 4). In X. strumarium, there was 

 little suppression of flowering when stress was 

 relieved before the inductive cycle. 



These studies suggest that floral induction can 

 be suppressed by water stress, at least in the 

 species under study and possibly in other species 

 if stress is sustained during the induction period. 



The effect of water stress on transmission of 

 the floral stimulus from the leaves to the apex 

 has also been studied by Aspinall and Husain. 

 Using L. temxdentum and X. strumarium they 

 established that, if stress was imposed at the end 

 of the inductive cycle, and the leaves removed 

 when stress was relieved, flowering was pre- 

 vented. If defoliation were delayed until 24 hours 

 after stress was relieved, a significant flowering 

 response was observed, and a progressive increase 

 in response occurred during this 24-hour period. 

 These results suggest that the floral stimulus is 

 not translocated out of the leaf while stress exists 

 but that, once formed, it is not destroyed by stress 

 (even though flower development is suppressed 

 compared with the controls), and when stress is 



O After induction 

 After induction 



Before induction 



-i 10 



8 

 6 

 4 

 2 

 



0) 

 £ 



C 

 0) 



■o 

 o 



z 



O Before induction 



O 



-// ' L - 



-4 -6 -12 -18 



Water potential (bars) 



■24 



Figure 4. — Flowering response and node production in 

 Pharbltis nil plants, subjected to osmotic stress during 

 a 16-hour dark inductive period. (After Aspinall and 

 Husain, (2.) 



relieved it is translocated to the apex. This is in 

 line with known effects of water stress on trans- 

 location of assimilates, even though translocation 

 of the floral stimulus may be independent of 

 assimilate transport (7). 



It must be appreciated that, in the field, photo- 

 inductive conditions continue for long periods, 

 but water stress is also likely to be protracted. 

 Also, variable degrees of stress may affect the 

 formation and translocation of the floral stimulus 

 differently than primordial initiation. [If 

 primordial initiation is the more sensitive, flow- 

 ering can be expected to occur at a lower node 

 number; if less sensitive, at a higher node 

 number. 



In either case, though, an associated effect of 

 water stress will almost always be a delay in 

 date of flowering, regardless of whether it occurs 

 at a younger ontogenetic stage. In the study of 

 Nicholls and May (22), stress did not affect 

 the number of vegetative primordia formed in 

 barley, before the first spikelet primordia ap- 

 peared, but the appearance of the first double 

 ridges was delayed several days by even mild 

 stress. Rate of initiation of floral primordia be- 

 came progressively slower in the stressed plants. 

 Upon rewatering, the initiation rate in the "mild" 

 (* = — 5 bars) stress treatments increased rap- 

 idly so that when stamens were initiated, total 

 spikelet number was almost the same as in the 

 controls. By comparison, in the "severe" (* = 

 — 15 bars) stress treatment, total spikelet number 

 was at a much lower level at the time when 

 development of the spike was concluded. 



Whiteman and Wilson (35) conducted experi- 

 ments with grain sorghum in which the timing 

 and duration of stress was varied around the 

 stage of floral initiation, but in which all treat- 

 ments constituted "severe" stress (wilted for 

 one week or more). They found that development 

 of the inflorescence could be suspended during 

 stress yet could be resumed on rewatering and 

 result in a flowering head not significantly dif- 

 ferent from that of control plants. 



In one experiment in which severe stress was 

 imposed for periods of about 1 week, on three 

 occasions during the normal time of floral initia- 

 tion, initiation was totally suspended but was re- 

 sumed upon rewatering. The time at which in- 

 florescence development was completed was de- 



