PLANT MORPHOGENESIS FOR SCIENTIFIC MANAGEMENT OF RANGE RESOURCES 



85 



In a field study of A. pectinata 5 at two sites in 

 the summer rainfall region of northern Australia 

 (rainfall was 267 mm. and 371 mm.), it was 

 shown that previous seasons' tillers sprouted new 

 shoots a few days after the first effective rain. 

 The old vertical culms had from 5 to 9 nodes 

 from which new shoots grew, with the lower 

 nodes providing the stronger shoots. The maxi- 

 mum number of new tillers was reached some 20 

 to 30 days after the rain and then declined, per- 

 haps as a result of competition. The underground 

 rhizomes generally gave rise somewhat later to 

 strong, sparingty branched, vertical shoots which 

 eventually became the tallest tillers of all. In- 

 florescences were produced between 28 and 32 

 days after the start of growth. 



The great value of this study as a contribution 

 toward determining management policy is the 

 finding that the base of previous season's vertical 

 shoots are the sites for new tiller production and 

 the bulk of the current season's growth. Grazing 

 of A. pectinata, both during the short growth 

 period and the ensuing dormant period, should 

 aim to leave the lowermost part of the new 

 vertical tillers from which the bulk of forage will 

 be produced in the next growth period. 



Controlled environment studies of the four 

 Astrebla species (7) are the most significant of 

 their type to have been undertaken on Australian 

 arid zone perennial grasses. The studies showed 

 that tiller number was not influenced significantly 

 by photoperiods between 9 and 15 hours, al- 

 though plants in the shorter photoperiods fre- 

 quently had more tillers after 5 weeks of growth. 

 Temperature generally had significant effects, 

 with the optimum about 28°/23° C, although 

 the differences tended to decline after 8 weeks. 

 Species also differed from one another in their 

 tillering potential. A. lappacea had the most 

 tillers, followed by A. pectinata, A. elymoides, 

 and A. squarosa. Different strains of A. pectinata 

 varied significantly in tillering potentials. 



Elongation of tillers was not significantly af- 

 fected by photoperiod but, in general, elongation 

 increased with temperature. Not all species re- 

 acted in the same way to temperature, and the 

 time of rapid elongation was different in the 

 different species. A. squarosa was generally short 

 while A. elymoides was tall. 



Axillary tiller production was not influenced 



by photoperiod, but, again, temperature had a 

 marked effect on the number of such tillers pro- 

 duced — the regime of 28°/23° C. stimulating 

 the greatest number of tillers. Higher tempera- 

 ture also produced the greatest number of leaves 

 per tiller. 



The investigations on Astrebla, here reported 

 in some detail, have provided some basic descrip- 

 tive data on this very important genus. How- 

 ever, they raise a number of questions and point 

 to the need for further physiological and pheno- 

 logical research with Astrebla and other range 

 grasses. The three areas of research which require 

 particular attention are (1), studies on dry 

 weight accumulation and distribution; (2), the 

 reaction of plants to defoliation; and (3), carbo- 

 hydrate reserves and their effect on regrowth. 



Williams (14), in an account of the physiology 

 of wheat plant growth, described total dry weight 

 changes with time as well as the dry weight 

 changes of the component parts viz roots, stem, 

 leaf sheath and leaves. With these data he was 

 able to determine distribution indices, derived by 

 expressing the increment in dry weight of the 

 component part (root, stem, and so forth) for 

 any harvest interval as a percentage of the total 

 dry weight gain for the same interval. In Wil- 

 liams' paper, figure 8 shows that about 50 percent 

 of plant assimilate was used in root growth over 

 the first few days of germination. Most of this 

 was, in fact, derived from the seed. By the 10th 

 day, 75 percent of assimilate, an increasing pro- 

 portion of which came from current photosyn- 

 thesis, was used for leaf growth, and then this 

 proportion gradually fell. The methods used by 

 Williams have great potential for helping us to 

 understand the nature of adaptation of range 

 grasses. For instance, it would be valuable to 

 determine the distribution indices of these grasses 

 when grown at different temperatures or when 

 subjected to different degrees of water stress. 

 Information of this sort would help us determine 

 the extent to which dry weight distribution is 

 under developmental and environmental control. 

 One might expect that the distribution index to 

 root would be higher for a longer period in 

 range grasses when compared with wheat, and 

 leaves may never reach as high as 75 percent. 

 An investigation along these lines would indi- 

 cate the basic growth characteristics of the spe- 



