MISCELLANEOUS PUBLICATION 1271, U.S. DEPARTMENT OF AGRICULTURE 



This may be due in part to the availability of 

 cells, already differentiated, which can rapidly ex- 

 pand, or it may be associated with a renewed sup- 

 ply of growth substances, or with other factors. 

 However, higher relative growth rates are seldom 

 sustained for long, so that compensation is sel- 

 dom complete unless the stress period has been 

 mild and brief. When it is severe, and total pri- 

 mordial number is permanently suppressed, com- 

 plete compensation is clearly out of the question. 



Cell enlargement in non-meristematic tissue 

 may also proceed at stress levels which would 

 stop enlargement at the root or shoot apex; but, 

 in general, cell enlargement appears to be more 

 sensitive to stress than cell division. In fact, re- 

 duced leaf and shoot elongation is frequently the 

 first observable symptom of water deficits, and is 

 a main cause of stunting which is perhaps the 

 most common sign of water stress under field 

 conditions. 



The main difference between the long-term 

 effect of reduced rates of cell enlargement and 

 cell division is that cell enlargement is a plastic 

 phenomenon, closely linked with the deposi- 

 tion of fairly rigid cell wall materials (23). In 

 consequence, slower rates of enlargement are 

 generally associated with smaller final cell size. 



In some instances, leaf and stem elongation 

 is rapidly resumed when stress is relieved. Hsiao 

 et al. (14-) suggested that this may be due to an 

 effect of stress in increasing cell extensibility, 

 although it may be frequently associated with 

 both division and enlargement, rather than with 

 enlargement alone. This is probably the explana- 

 tion for Miller's (18) data with loblolly pine (P. 

 taeda L. ) , which showed that in the 60 days fol- 

 lowing a 30-day stress period, needle elongation 

 reached the same level as in the irrigated controls. 



In some species, cell enlargement is so sensitive 

 to water deficits that stem elongation or leaf 

 enlargement can be inhibited by small diurnal 

 water deficits that occur even with well watered 

 plants on days of high radiation incidence or 

 similar high evaporation-demand conditions (14, 

 32). A more general response is that of a pro- 

 gressive decline in rates of cell enlargement as 

 water deficits develop, and enlargement generally 

 ceases when turgor pressure levels are still at the 

 level of several bars. 



The relative effects on leaf and stem enlarge- 



ment appear to differ with different species. In 

 most cases stem elongation appears the more 

 sensitive; the results of Lotan and Zahner (17) 

 are typical. In a study of stem and needles elon- 

 gation in red pine (P. resinosa Ait.), they found 

 that elongation of internodes ceased several weeks 

 earlier than that of needles, as stress was imposed. 

 Also needles at the tops of shoots and on the 

 uppermost branches were affected to a greater 

 extent than needles lower on shoots or on lower 

 branches. By comparison, in tomato, Slatyer (28) 

 noted continued stem elongation at water poten- 

 tials sufficient to cause pronounced leaf wilting. 



The effect of water stress on the continued 

 development of tissues which are already mature, 

 or approaching maturity, resembles that of has- 

 tened senescence. One of the earliest signs of 

 stress is the migration of phosphorus from older 

 leaves to the stems and meristematic tissue (11, 

 36). This is closely followed by the movement of 

 nitrogen, suggesting progressive protein hydroly- 

 sis and breakdown of normal cell function. 



In summary, it seems that the effect of stress 

 tends to be most pronounced in those tissues 

 which are in rapid stages of development, a 

 conclusion already reached by other workers (11, 

 31). The developing tissues appear to enter a 

 rejuvenating phase on relief of stress and rela- 

 tive growth rates of such plants may be more 

 rapid than those of controls. This rapid growth 

 is aided by the continued slow cell division which 

 has occurred during stress and perhaps by the 

 availability of nutrients released from older 

 tissues. 



Relative Effects On Shoot And Root 

 Development 



Perhaps the most obvious effects of prolonged 

 water stress on shoot development are those of 

 reduced internode length and reduced leaf size. 

 The effects on leaf size, on rate of leaf expansion, 

 and on rate of appearance of new leaves have 

 profound effects on total dry matter production, 

 since the photosynthetic surface increases less 

 rapidly and stomata tend to become nonfunc- 

 tional in older leaves. Associated with this ten- 

 dency for stomata to be less responsive and, in 

 particular, to open less widely, is a progressive 

 atrophication of the photosynthetic apparatus, 

 so that older leaves tend to become less efficient 



