PLANT MORPHOGENESIS FOR SCIENTIFIC MANAGEMENT OF RANGE RESOURCES 



photosynthesisers. Continued growth clearly re- 

 quires a flow of assimilates, so this tendency for 

 reduction in photosynthesis leads to a negative 

 feedback, and the overall metabolism tends to 

 slow down. 



In species with endogenously controlled and 

 well defined seasonal growth patterns, such as 

 many woody perennials, bud formation may be 

 the primary control for the next season's growth. 

 Water stress at this stage is clearly of profound 

 importance and the dominance of this control 

 mechanism has been clearly described by Clem- 

 ents (6). In a study of the shoot responses of 

 young red pine (P. resinosa), Clements imposed 

 three watering treatments during the bud forma- 

 tion stage, from late summer until early autumn, 

 and two watering treatments in the subsequent 

 spring. Size of apical buds, date of bud swell 

 and bud burst in the spring, number of needle 

 fasicles on new shoots, shoot length, and the spac- 

 ing of the needle fasicles were closely related 

 to the late summer-autumn treatments and rela- 

 tively unaffected by the spring treatments (table 

 1). Most of these responses were also closely 

 related to bud size. 



Woody species without such strong control 

 from prior bud formation, and other perennial 

 species, do not appear to show much dependence 

 of subsequent growth on stress during the pre- 

 vious season, except perhaps for the first spring 

 growth flush. In such cases the pattern of shoot 

 development is largely controlled by contempo- 

 rary stress conditions. 



Table 1. — Length and number of fasicles on 

 shoots of P. resinosa exposed to differential 

 watering treatments' 



Response 



Treatment in Treatment in 1964 2 



1963 Dry 



Wet 





Wet 10.7 (±1.2) 



9.8 (±1.5) 



Mean shoot 



Intermediate 11.0 (±0.9) 



8.0 (±1.2) 



length (cm.) 



Dry 6.0 (±0.7) 



6.0 (±0.8) 



Number of 



Wet 105.3 (±8.0) 



105.2 (±8.9) 



fasicles 



Intermediate 94.6 (±2.8) 



93.6 (±7.5) 



on shoots 



Dry (±7.6) 



88.9 (±6.0) 



'From Clements (6). 



2 Figures in parentheses are standard error of mean. 



The effects of water deficits on root develop- 

 ment are less well known that on shoots. Al- 

 though it is sometimes stated that root develop- 

 ment is enhanced relative to shoot development 

 during stress, there is little quantitative informa- 

 tion on this point (25, 33, 31i). 



Two types of effects can be expected: (1) a 

 reduction in rates of meristematic activity, and 

 of root elongation, directly associated with the 

 level of internal water deficit; and (2) an effect 

 of suberization on the water and nutrient uptake 

 properties of the root system as a whole. 



Most research has shown a progressive reduc- 

 tion in rate of root elongation as water stress is 

 imposed (21, 25). In some cases, root elongation 

 ceases before shoot growth. In this regard, New- 

 man's (21) work with flax shows a high depend- 

 ence of rate of root extension on local soil water 

 potential. Thus, roots in relatively moist soil 

 may continue to elongate' even though the plant 

 as a whole is subject to severe internal water 

 stress. In consequence continued exploration of 

 the soil mass by root systems might be expected 

 until the root zone is reduced to the same general 

 level of water potential. 



The effect of water stress on suberization can 

 be expected to vary, depending upon the local 

 rates of root extension. In rapidly growing roots 

 a nonsuberized zone, several cm. long, may occur 

 in some species, constituting a highly active and 

 extensive absorbing surface. As rates of root 

 elongation are reduced the rate of suberization 

 exceeds J:he rate of elongation, and the nonsuber- 

 ized zone is reduced, until it is effectively elim- 

 inated in nonelongating roots. This phenomenon, 

 common under conditions of severe water stress, 

 substantially reduces the effective surface area of 

 the roots and their activity as absorbing organs. 



On rewatering, the shoot and root responses 

 differ. Shoot growth, as mentioned previously, 

 tends to resume — sometimes more rapidly than 

 before — with only a short recovery lag being 

 observed. Photosynthesis in the shoot may be 

 depressed for longer periods than cell division 

 and enlargement but this may be due to delayed 

 recovery of stomatal opening capacity rather than 

 to lesions in the photosynthetic apparatus itself. 

 The important point, though, is that as long as 

 the stress does not cause permanent injury, shoot 

 response is relatively rapid, and recurrent stress 



