INTRODUCTION
The production of wheat needs to be doubled by the end of twentieth century -when India faces a target of 225 m illion tonnes of food grains. To attain this target the potential yield of the dwarf genotype, which has
brought wheat revolution in India, should be f u l l y exploited.
It is an uphill task as only 56 per cent of the area under wheat is irrigated (Rao, 1978) . With low river -water flows, and lim ited available ground water supplies during winter and summer months, a high yield potential and water use e ffic ie n c y (WUE) can only be attained i f the available irrigatio n water is optimally allocated based on actual water requirement, contribution of stored soil water, and su s c e p t ib ilit y characteristics of the crop. In practice, ir r ig a t io n water for wheat is scheduled by its c r it ic a l physiological stages of which the crown root in it ia t io n is the most v it a l one followed by jo in t in g , flowering and grain f i l l i n g (G-autam et a l , 1968 '$ Sekhon et a l , 1968) . The
production function can be optimised by applying fewer ir rig a tio n only at the highly susceptible stages. Frequent ir rig a tio n s at a ll phenological stages following a heavy presowing irrig a tio n may decrease the WUE by increasing the
i \
water expense and fewer irrigatio n a ft e r a lig ht pre-sowing
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irrig a tio n may lower the W E by decreasing growth and yield (Prihar et a l, 1976) .
Growth and yield ox crops are directly related to plant water use in dry land areas (Hanks et a l , 1 9 6 8 ) . When so il water is not lim itin g the crop water use remaining at or above potential evapotranspiration i s strongly influenced by micrometeorological condition. However, when soil water
is lim it in g , the water use remaining at or below potential evapotranspiration, is related more to so il water a v a ila b i l i t y tha'm to micrometerological parameters (Taylor, I 9 6 0 , Penman et a l, 1 9 6 7 ) . Water use by crop plants depends on their a b ilit y to extract water from soil overcoming the
resistances in so il-pl ant-at mo sphere continuum. The capacity of plant root system in their entirety to meet transpira- tional demand is in part determined by t h e ir gross morpho
lo g ic al nature and extent, and in nart by the specific impedence exhibited at the point of absorption (D en ielso n , 1967) .
The water use and growth of crop plants are manifesta
tions of le a f water relations that undergo diurnal and seasonal variations in response to meteorological and soil water conditions. Because of diurnal v a riatio n in evapora- t iv it y water extraction lags behind transpiration and l e a f water dependent parameters namely le a f water potential (
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l e a f d iffu sio n resistance and canopy temperature oscillate diurnally even in a managed agro-ecosystem. However, with lim itin g soil water, the lowering of midday increases further and induces stomatal closure in h ib itin g transpira
tion and carbon dioxide assim ilation. When flag loaf water potential fa l l s below “ 3 .3 MPa during panicle emergence stage of wheat, net photosynthesis and transpiration
decrease:, lin ea rly with decreasing Johnson et a l , 197 4 ) . The drought induced stomatal closure raises the canopy
temperature which thus monitors the plant water stress in response to soil water d e f ic it . As so il water d e fic it
in te n s ifie s further lowers to a threshold value at which stomata, close completely and photosynthesis ceases.
Likewise^the recovery of photosynthesis after rewatering is also related to recovery of .stomatal d iffu sio n r e s i s tance at t il l e r in g and heading stages of wheat (Kirkham and Eanernasu, 1 9 8 3 ) . Once suppressed the photosynthesis rate however, never recovers fu lly to its prestress level as does stomatal resistance.
A comprehensive knowledge on the patterns of l e a f water relations and photosynthesis in response to periodic soil water d e fic it and microclimate thus help in under
standing the.growth re latio n s of the crop under environmen
t a l stresses and in optimising the allo cation of irrigatio n wat e r .
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The changes in lea f water relations and photosynthesis in response to soil water deficit are greatly modified in tropical region w h e n temperature plays an added role in enhancing the evaporative demand and r a is in g the amplitude of the diurnal o scillations of le a f water dependent para
meters. Sustention of photosynthesis and water use due to the osm ^tically regulated pressure potential in lea f makes the phenomena s t ill more complex in tro p ical and subtropical regions. The adverse effects of soil water deficit on lea f water relations and photosynthesis of wheat plants in these regions are further accentuated by. low water supplying
capacity of the so ils and abrupt r is e in evaporative demand in late t il l e r in g and flowering stages. The latter type of situation prevails in the a llu v ia l so ils of Punjab. A l i t t l e is known about the modifying influences of so il water d e fic it on water relations and dry natter production of the pre
nonstressed wheat plants under changing conditions of climate in northern In d ia . The present investigation was therefore, conducted in lysim eters and f ie l d at Punjab A gricultural U niversity, Ludhiana, Punjab, India, during the crop growing season of 1981-82 and 1982-83 to assess the diurnal/daytim e changes in le a f water p o t e n t ia l, le a f
d iffu s io n resistance, canopy temperature, photosynthesis
and translo catio n, water use pattern as w ell as of growth and yield attributes of wheat in response to soil water d e fic it
imposed at late t i l l e r i n g and flowering stages.
