132 MINERAL SALTS ABSORPTION IN PLANTS 



Most investigators have found that, other things being equal, 

 nutrients penetrate more rapidly into young than into mature leaves. 

 Structural differences, for example, in the cuticle and epidermis may 

 be important in this connexion, but Oland and Opland (1956) 

 proposed that a difference in metabolic activity may also be involved. 

 They suggested that magnesium ions are taken up by leaves mainly 

 in exchange for metabolically-produced hydrogen ions, and a 

 diurnal variation in organic acid production may account for the 

 more effective penetration of magnesium ions when salts are sprayed 

 on to leaves in the evening than in the early afternoon. 



The rapid initial absorption of cations by leaves and the ease of 

 its reversal suggest that at least a part of uptake is by adsorption- 

 exchange with the cuticle and cell walls behaving as a cation exchange 

 membrane (cf. p. 104). Anion absorption on the other hand is almost 

 entirely irreversible, and judging by its sensitivity to metabolic 

 inhibitors, seems to be more completely dependent on active 

 absorption than is the uptake of cations. 



Some of the salt absorbed by leaves after foliar application, is 

 retained, especially when the leaf is young, and the remainder is 

 exported in the phloem. The mechanism of transport of salts from 

 the leaf has been studied particularly by Biddulph. He showed 

 (Biddulph, 1941) that when phosphate labelled with ^^p was injected 

 into the phloem of a bean {Phaseolus vulgaris) leaf, movement into 

 the stem occurred more rapidly during the hours of daylight than 

 at night with a maximum rate at about 10 a.m. and a minimum 

 about 12 hr later. Transport in the stem was at first predominantly 

 in the downward direction, but some of the tracer eventually 

 migrated across into the xylem and then began to move upwards. 

 Using a technique similar to that employed by Stout and Hoagland 

 (see p. 122), Biddulph and Markle (1944) were able to distinguish 

 more exactly between phloem and xylem transport, and rates of 

 transport in phloem exceeding 20 cm/hr were observed. Biddulph 

 (1959) showed by radioautography of stem and petiole sections that 

 sulphate and phosphate injected into a vein or sprayed on to a leaf 

 moves out in the phloem. The phloem of a vascular bundle did not 

 seem to behave as a single unit, since some sieve tubes became 

 radioactive whereas neighbouring ones did not. 



A discussion of the mechanism of phloem transport is beyond the 

 scope of this monograph. For accounts of tliis problem you are 



