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RELATION BETWEEN CANOPY MINUS AIR TEMPERATURE AND LEAF WATER POTENTIAL IN FIELD GROWN WHEAT

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RELATION BETWEEN CANOPY MINUS AIR TEMPERATURE AND LEAF WATER POTENTIAL IN FIELD GROWN WHEAT

V.P. SINGH AND M.

SINGH-Department or Agronomy, HarYaDa Agricultural University, Hissar-I2S004, India

(Revised : December 12, 1983)

SUMMARY

his study was undertaken to demonstrate that crop canopy temperature responds to changes in leaf water potential (-JlL),and therefore can be used for monitoring irrigation. The canopy minus air temperature difference (6T) was related to -JlL in field grown wheat under two diffe­ rential water supplies at 50% rnd 65% depletion of available soil moisture in tbe root zone soil. -JlL ranged from 14 bars when valumetric soil water content(QV) was 0.25 to -JlL-33 bars at the wilting point(QV =0.14). At 1400 bours, 6 Twas - 3.7°C at -JI L=-IO bars. Increasing water stress decreased

-JIL markdely and increased 6 T. At -JIL=-17 bars, t:..T was zero; and at -JlL=-45 bars, T increased to S·C. Tbese results support the validity of the temperatnre difference method for sensing plant response to water stress conditions.

INTRODUCTION

Several researchers (Idso and Ehrler, 1976; Blad and Rosenberg, J976) have suggested that the difference between leaf or canopy temperature and air temperature (L':':,T) can be used to indicate plant water stress. The hypothesis is that the crop canopy temperature data can be correlated with the soil moisture status to ascertain the suitability of remote sensing for irrigation scheduling and yield prediction. However, the nature of the L':':,T response can be correlated with a change in leaf water potential. and thus be an indicator of plant water status. The present study is, therefore, planned to establish wheather or nol changes in L':':,T can be related to changes in Jeaf water potentiaJ and to establish these relationships for wheat.

MATERIALS AND METHODS

An aestivum wheat variety WH-157 was sown on November 26. 1978 in rows 23cm apart on sandy loam soils at Haryana Agricultural University Farm,

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CANOPY TEMPERATURE AND LEAF WATER POTENTIAL IN WHEAT 21

Hisar. Two differential moisture conditions were maintained in the field by scheduling irrigations at 50% (11) and 65% (12) depletion of water from the root zone. The experiment was laid out in a randomized block design with six replications. The irrigation treatments were applied after a common irrigation of 6cm depth at crown root initiation stage. II treatment of irrigation received 19.9 cm water through three irrigations and [2 treatment received 16.9 cm water through two irrigation~. Total rainfall received during the crop season was 8.3 cm. The gress plot size was 5m x 2.76m. A basal dose 60 kg N/ha and 60 kg P20s/ha was applied in the form of urea and single super phosphate respectively. Another dose of 60 kg N/ha was top-dressed after first irrigation at crown root initiation stage. A mechanical weeding was done to the crop at 45-50 days aftar sowing. No insecticide application was needed. The crop was harvested on April 21, 1979.

Soil moisture was recorded with the Neutron Moisture Meter (Troxler International Ltd. Cornvallis Road, R.T.P. Northern California-27709. U.S.A.), three to four times each week in each plot at 30 cm intervals to 120 cm depth from 23 to 128 days after sowing. Soil moisture for each depth were smoothed statistically using a sliding cubic technique, which allowed water contents to be interpolated for each day of the experiment. Daily water depletion rates were calculated.

The air temperature was measured at 1 m above the crop canpoy using Psychrometer Unit type WVU (Delta-T Devices, Cambridge England). The Infrared Thermometer Ranger II (Raytek Inc. California, 68043, U.S.A.) was used to measure the canopy temperature. The air and canopy temperatures were recorded daily at 1400-1500 hours except on rainy days. The I/IL measurements were. made with pressure chamber apparatus (PMS Instrument Corporation, Corvallis, Oregon, U.~.A.) utilizing compressed N gas. .pL was measured near 1400 hours beginning on day 46 until day 118. Three to four leaves were randomly sampled from each plot for .pL readings.

RESULTS AND DISCUSS[ON

The changes in soil moisture content in different soil depths with time · revealed that the effective rooting zone for the aestivum wheat was 0.9 m, regar­

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Fig. I. Values of leaf water potential at 1400 hours as affected by the volu­ metric soil water content (QV) in the root zone (l.2m) of wheat during

- ; " ~ 1978-9. witb each pata point beiag (1) the midpoint of a water content elass with a range of ±O.O:Z and (2) the mean of from 14 to 28 leaf water potential values in a given class.

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--CANOPY TEMPERATURE AND LEAF WATER POTENTIAL IN WHEAT 23

apparently <due to the< prevailing high saturation deficit (S.D.) under II level of irrigation for a ,larger time of the growing season. Similar effects of S.D. were obtained by Yang and de Jong (1973) and Ehrler et. al. (1978). ",L decreased gradually from ~14 to -17 bars, as QV decreased from 0.25 to 0.19. The decrease in ",L was somewhat linear at QV values below 0~19, indicating higher dehydra­ tion_of plant tissues as ",L became more negative. These determinations show that the relationship between "'L and QV in wheat exists only at values less than

0.19 QV. .

To find out whether the temperature difference method (.6.T) for sensing plant response to 'water stress is based on physiological responses,56 pairs of data points taken at 1400 hours were used to plot .6.T against",L, derived from data. for 27 clear days (Fig.2). Data from both II and 12 treatments of irrigation are included. A single curve was drawn through the data since the degree of scatter did not seem to be significantly different among irrigation treatments.

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Fig. 2. Canopy temperature (l;. T) minus the air temperature (c) at 1m above the crop at 1400 hours as affected by the leaf water potential, the data consis­ ting of 56 points from measurements taken on 27 clear days, with each point being the mean of 8 to i1 pressure chamber readings on separate leaf, and from 3 to 1 l;.T readings .. The S6 points were derived from 193 individual sets of data taken from wheat planted 26 Nov., 1978. l;., II and

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The curve (Fig. 2) shows if;L to be about -17 bars when D.T is zero; i.e. canopy temperature is equal to air' temperature. When the plant was well dehydrated if;L was greater than -17 bars and D.T was either zero or negative (Canopy temperature was the same as or lower than air temperature). As if;L decreased from -17 bars to -35 bars, D.T increased from zero to about 4°C, after which the rate of increase showed to an inperceptible value. When if;L reached -45 bars, D.T was about 5°C.

The if;L value of about -17 bars, where D.T is equal to zero(Fig. 2)corres­ ponds to QV=0.19 in Fig. I, the water content below which the rate of decrease in if;L was more. Moreover, the difference in QV between 0.25 at "field capacity" and 0.19 represents a 55% depl~tion of the available water in the root zone soil (using QV =0.14 as the wilting point). On the similar type ofsoils, 55% depletion has been used for the proper time of irrigation to obtain maxi­ mum production consistent with efficient water use for wheat (Singh and Narang, 1971; Lal and Sharma, 1976). In Australia, Miller and Denmead (1976) reported that if;L of field grown whert decreased acropetally, with the uppermost leaf, No.4, having the most negative value. The critical value for leaf No.4 at which turgor became low enough for stomatal closure to begin; was - 19 bars. These results are applicable to our data for leaf water potential. Other variables affecting if;L are temperature and growth stage (Frank et. 01., 1973). Neverthe­ less, it appears that at QV levels below 0.19, transpiration began to be decreased significantly resulting in rapid increases in D.T as if;L decreased below -17 bars, consequently, for our data -17 bars seems to be a critical value of if;L.

In conclusion, canopy minus air temperature appears to respond to chan­ ges in leaf water potential which are elicited by changes in volumetric water content in the root zone soil. Therefore, the temperature difference method for monitoring plant stress in wheat should be reliable and may be used in monitor­ ing large areas of wheat grown in semi-arid regions. The applicability of the method for other crops and for other climatic areas need to be studied.

REFERENCES

Blad, B.L. and Rasenberg, N.J. (1976). Measurement of crop temperature by lear ther­ mocouple, infrared thermometry, and remotely sensed thermal imagery. Agron. J.,

68 : 635-41.

Ehrler, W.L., Idso, S.B., Jackson, R.D. and Reginato, R.J. (1978). Wheat canopy temperature Relation to plant water potential. Agron. J., 70 ; 251·56.

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CANOPY TEMPERATURE AND LEAF WATER POTENTIAL IN WHEAT 25

Hsiao, T.C. (1973). Plant responses to water stress, p. 519-70. In W.L. Briggs (ed.) Ann. Rev.

PI. Physiol., 24 : Annual Review Inc. Palo Alto, CalifornIa.

Idso, S.B. and Ehrler, W.L. (1976). Eastimating soil moisture in the root zone of crops: A technique adoptable to remote sensing. Geophysics Res. Letters, 3 : 23-5.

Lal, P. and Sharma, K.C. (1976). Water use studies in two dwarf varieties of wheat grown at different levels of soil moisture and .nitrogen under shallow water table conditions.

Indian J. Agron., 21 : 453-59.

Singh, K. and Narang, R.S. (1971). Influence of timings of first irrigation on the performance of high yielding wheat strains and their water use pattern. Indian J. Agron, 16 :

366-67.

Miller, B.D. and Denmead, O.T. (1976). Water relations of wheat leaves in the field. Argon J., 68: 303-7.

Figure

Fig. 2.   Canopy temperature (l;.T) minus the air temperature (c) at 1m above the crop at 1400 hours as affected by the leaf water potential, the data consis­

References

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