PLANT MORPHOGENESIS FOR SCIENTIFIC MANAGEMENT OF RANGE RESOURCES 



11 



layed for approximately 10 days, as was the time 

 to commencement of flowering. 



In another experiment (35), stresses were im- 

 posed at about the conclusion of floral initiation 

 and persisted for 14, 21, and 28 days. Floral 

 initiation occurred in all treatments at about 

 the same time as in the controls but there was 

 no elongation of the inflorescence in the stressed 

 plants until rewatering. From then on, develop- 

 ment proceeded more or less at the same rate as 

 in the controls and emergence of flowers took 

 place at intervals of 10, 24, and 30 days after 

 the controls, periods corresponding fairly closely 

 to the durations of stress. 



From the stage of spikelet initiation to fertili- 

 zation of the ovules, a number of other processes, 

 associated with the development of the inflor- 

 escence, are likely to be sensitive to water deficits, 

 and thus cause a reduction in the number of 

 fertile flowers formed {27). 



The effects have been attributed, in some cases, 

 to specific interference with the sexual develop- 

 ment of the spikelets, such as meiosis in the 

 gametes, but the availability of mineral nutrients 

 and carbohydrates during the preflowering phase 

 can also influence floral development and may be 

 an important means by which water stress effects 

 are mediated. 



If it is severe, stress at anthesis can reduce 

 fertilization and fruit set, but mild stresses do 

 not appear to have a marked effect. Since stress 

 is frequently associated with arid atmospheric 

 conditions, it is sometimes suggested that it de- 

 hydrates pollen grains. However, it also seems 

 possible that germination of the pollen, or growth 

 of the pollen tube from the stigma to the ovule, 

 may be impaired. 



The relative sensitivity of different species to 

 stress at the preflowering and flowering stages 

 appears to vary somewhat, but comparable stress 

 treatments have seldom been imposed and in- 

 ternal water deficits have seldom been measured, 

 so detailed comparisons are not possible. Clearly, 

 though, species which flower over an extended 

 period have an advantage in avoiding stress 

 effects on reproductive development. 



In woody perennial plants the switch from ini- 

 tiation of vegetation to floral primordia fre- 

 quently appears to be enhanced by water stress 



and the intensity of flowering may be substan- 

 tially increased (1,5,20,27). 



In other cases, particularly in those species 

 which form overwintering buds, high water status 

 during the period of bud development appears 

 to enhance subsequent post-winter flowering in- 

 tensity and may advance flowering dates slightly 

 tt). 



Evergreen xermorphic shrubs, characteristic 

 of arid regions, may flower at almost any time 

 of the year. Preece (24), established that Acacia 

 aneura F. Muell. can flower in each season of the 

 year in western New South Wales. Furthermore, 

 with supplementary irrigation, the same shrubs 

 may flower several times in one year. His studies 

 did not estimate the period between floral induc- 

 tion and flowering, but it appears that any 

 watering treatment which induces a marked vege- 

 tative growth flush leads to flowering in a period 

 of 1 to 2 months. This pattern may well be re- 

 flected in other arid species, particularly those 

 growing in areas with relatively warm winters 

 and without definite seasonal rainfall patterns. 



On the other hand, many species may have a 

 preferred flowering season, coinciding with the 

 preferred period of vegetative growth. In Acacia 

 aneura, for example, there appears to be a pre- 

 ferred period for growth and flowering in the 

 spring and again in late summer. 



The period from flowering to fruit maturation 

 may be as short as a few days for some ephem- 

 eral species and longer than a year' for some 

 woody perennials. In the former case, water stress 

 during fruit development is seldom a problem, 

 since the species are well adapted to mature fruit 

 during a drying cycle. However, in other species, 

 water stress may frequently affect seed size, via- 

 bility and some aspects of seed dormancy. Seed 

 size, in particular, appears to be positively cor- 

 related with successful seedling growth, so water 

 stress can have long term indirect effects through 

 this means. 



Conclusions 



The preceding discussion on the effects of water 

 stress on morphogenesis in three loosely grouped 

 types of plants — annuals, perennial grasses, and 

 woody perennials are summarized as follows: 



The effect of water stress on annuals is likely 

 to result in reduced leaf size and internode length, 



