Transpiration and the Ascent

Introduction 203
Table 7.3 Water Requirement or Transpiration Ratio in Grams of Water per Gram
Dry Matterfor the Years 1911-1917 at Akron, Colorado, and the
Evaporation from a Free Water Surface from April to September 1a
Plant 1911 1912 1913 1914 1915 1916 1917
Alfalfa 1068 657 834 890 695 1047 822
Oats, Burt 639 449 617 615 445 809 636
Badey, Hannchen 527 443 513 501 404 664 522
Wheat, Kubanka 468 394 496 518 405 639 471
Corn, N..W. Dent 368 280 399 368 253 495 346
Millet, Kursk 287 187 286 295 202 367 284
Sorghum, Red Amber 298 239 298 284 303 296 272
Evaporation: April 1 to 1239 957 1092 1061 848 1196 1084
September 1 (mm)
aFrom Miller (1938).
supplied by a single rain or irrigation, assuming that evaporation from the soil
was controlled by mulching.
It sometimes is argued that transpiration is beneficial because it cools leaves,
accelerates the ascent of sap, and increases the absorption of minerals (Clements,
1934; Gates, 1968). Rapidly transpiring leaves usually are cooler than
slowly transpiring leaves (Gardner et at., 1981), but leaves in the sun rarely are
seriously overheated even when transpiration is reduced by wilting. Water
moves to the tops of plants as they grow and transpiration merely increases the
quantity and speed of movement. Absorption of minerals probably is increased,
but some understory plants thrive in shady, humid habitats where the rate of
transpiration is relatively low. Although Winneberger (1958) reported that high
humidity reduced plant growth, Hoffman et at. (1971), O'Leary and Knecht
(1971), and others found that growth generally was better in high than in intermediate
or low humidity and Tanner and Beevers (1990) concluded that transpiration
is not essential. The numerous harmful effects of water stress caused
by rapid transpiration are discussed in tater chapters.
Transpiration can be regarded as an unavoidable evil, unavoidable because a
leaf structure favorable for uptake of the carbon dioxide necessary for photosynthesis
also is favorable for loss of water, and evil because it often causes
injury by dehydration. The evolution of a lear structure favorable for high rates
of photosynthesis apparently has had greater survival value in most habitats
than one conserving water, but reducing photosynthesis. Thus, the leaf anatomy
of most mesophytic plants causes them to live in danger of injury from excessive
transpiration. In general, plants adapted to dry environments cannot compete
effectively with plants adapted to moist environments when both are well watered
(Bunce, 1981; Orians and Solbrig, 1977). The relationship between tran-