Jan. ia, 1924 
Water Requirement of Rust-Infected Wheat 
109 
Reynolds (75) had previously described the effect of infection of apple 
leaves by Gymnosporangium, and of Xanthium by Puccinia xanthii , upon 
the tissue and cell morphology of the host. In general, the rusts were 
found to produce hypertrophic changes, but the separation from the 
mesophyll and the rupture of the epidermis at pustule formation exposed 
uncutinized cells to the drying effects of the atmosphere, resulting in 
atrophy. In Xanthium the loose organization of the mesophyll in 
healthy leaves is altered in rusted ones to a compact tissue composed of 
mycelium and empty host cells. The work of Ward, Evans, and others 
showed that in the grass rusts atrophic changes in the host cells do not 
appear until extensive development of the rust mycelium prior to pustule 
formation. In the case of resistant hosts, Stakman (77, 18) found that 
disorganization of the chloroplasts followed by death and disintegration 
of the cells occurs in proximity to the invading germ tube. 
Weaver rightly considered that rupture of the epidermis and death 
of the chlorenchyma cells in the infected area produce the marked 
acceleration in transpiration evident at pustule formation. He believed 
that toxic excretory products of the parasite also might stimulate trans¬ 
piration, but are of slight importance in the cereal rusts. It was pointed 
out that transpiration may not always be an index of growth, for water 
loss from infected, plants is excessive and bears no relation to the produc¬ 
tion of new substance. He did not determine the correlative transpira¬ 
tion, or amount of water transpired per unit of weight. The question of a 
sustained accelerating effect of rust infection on transpiration was raised, 
and the statement made that after the first pustules erupted no further 
change in rate was evident, the accelerated rate being maintained by the 
appearance of new pustules. 
Dufrenoy (8) has given an interesting summary of the effects produced 
by a number of parasites and epiphytes on the transpiration of their 
hosts. Chlorotic areas of variegated leaves, and such as surround points 
of infection, transpire less rapidly than the green portions; the red- 
pigmented areas surrounding lesions caused by Polystigma rubra on plum 
leaves transpire more rapidly, contrary to the behavior of anthocyanin- 
containing cells generally. A number of parasites accelerate transpira¬ 
tion, owing to localized necrosis, rupture of the epidermis by fruiting 
bodies, or the production of overgrowths, although some witches’ brooms 
appear to be xerophytic modifications. As the same effect sometimes is 
shown by epiphytes, changes in permeability of the host cells are involved 
also. A table is given which shows that among diverse hosts and para¬ 
sites the transpiration of infected, as compared with healthy, leaves and 
parts of organs varies in a ratio of 0.5 to 1 to 100 to 1. The results were 
obtained by the use of hygroscopic paper. 
It is to be noted that in none of the experimental demonstrations of 
acceleration of transpiration rate produced by parasitic infection which 
are reviewed above has the ultimate effect on the water economy of the 
host been determined. This would seem to be the most logical criterion 
of the general effect on the plant’s water relations caused by parasitic 
attack. 
Burkholder (7) has taken this into account in his study of the reduc¬ 
tion in yield of beans resulting from infection by Fusarium rootrot. 
Infected plants gave only about half the dry weight of seed yielded by 
controls and transpired less water, the reduction amounting to about 
50 per cent at soil temperatures of 18 0 and 26° C. Transpiration thus 
appeared to be an approximate index of growth. 
