GROWTH RESPONSE OF SUGAR BEET 353 



moisture content at the first sampling and the subsequent relative leaf 

 growth rate may indicate an after-effect of drought of the type already 

 discussed. The water suppHed before the start of the drought was probably 

 enough to reheve the water stress (without completely abohshing moisture 

 content differences) but the weather did not permit the stunted plants 

 to grow rapidly enough to overcome the check. 



Weather may influence the drought response even when changes in root 

 growth are not important. Bierhuizen (loc. cit.) showed that when trans- 

 piration is rapid, variation in soil moisture stress can influence transpiration 

 rate at stresses as low as pF 2, although under conditions giving slow 

 transpiration the rate is uninfluenced by soil conditions. According to 

 Philip (1957) the permeability of the system must change. Although 

 transpiration rate tends to a constant value independent of atmospheric 

 conditions, at high soil moisture tension the change in permeabihty, and 

 presumably the physiological change causing it, must increase when 

 atmospheric conditions favour increased transpiration rates. Growth is 

 usually more sensitive than transpiration to water stress so the growth of a 

 plant in dry soil should interact with atmospheric conditions. This was not 

 observed in my experiments; the effects of treatment C on L, net leaf 

 number and moisture content were not significantly greater in 1959 than 

 in 1958 despite a 50% greater transpiration potential as estimated from the 

 water consumption of the A plots. 



There was no evidence that net assimilation rate (E) was decreased by 

 drought; the C plots had almost the same dry matter yield as the A plots, 

 despite considerable differences in the leaf area index which imphes that 

 drought increased E. This does not require a change in the internal physio- 

 logy of the leaf Watson (1958) showed that the rate of dry matter produc- 

 tion (crop growth rate) of sugar beet increases with increasing L and is 

 greatest when L is between 6 and 9, considerably above the values observed 

 in my experiment. If the leaf area of the A plots was near or above the 

 optimum, small changes in L would have no effect on dry matter produc- 

 tion. Below optimal L, dry matter production is restricted because the crop 

 fails to intercept all available radiation and above the optimum dry matter 

 is lost by respiration of the leaves in heavy shade. At high levels of direc- 

 tional illumination (as on a clear summer day) dry matter production is also 

 restricted because the upper leaves intercept Ught energy more rapidly 

 than they can use it in carbon assimilation (the photosynthesis of sugar beet 

 leaves reaches saturation at 0-3 of full simhght, Gaastra, 1958). Watson and 

 Witts (1959) ascribe the high optimum L (6-9) of cultivated beet as com- 

 pared with wild beet or kale (optimum L 3-4) to its erect habit. Sunlight 



