Grimes, Donald W. : Donald W. Grimes is Water Scientist, Department of Land, Air and Water Resources, University of California, Davis, stationed at the San Joaquin Valley Agricultural Research and Extension Center, Parlier

Yamada, Hidemi : Hidemi Yamada is Staff Research Associate, U.C. West Side Field Station, Five Points.

Cotton arowth related to plantâ??s water status Donald W . Grimes 0 Hidemi Yamada Leaf water potential may be a better guide to irrigation timing than soil water status . Cotton irrigation schedulingvariesgreatly in arid and semiarid regions , where large dif - ferences occur in soil water - retention and transmission properties . Compact soil zones may restrict root growth and extension and substantially reduce the plantâ??s ability to use stored soil water . On the other hand , a shal - low water table in or near the potential root zone may contribute water to the growing plant . Therefore , measurementsof soil water content may not always be useful in schedul - ing irrigations . Because the plant reflects its total environment , a measurement on the plant to determine its water status will some - timesbe a better indication of when irrigation is needed . The water status , or leaf water potential , of vascular plants can be measured with a pres - sure chamber . Water present in the conduc - ting tissue ( xylem ) of a transpiring plant is subjected to negative pressure , with the pres - sure becoming more negative as water stress increases . Equipment required for measuring Portable pressure chamber for measuring leaf water potential leaf water potential is commerciallyavailable is small enough to be hand - at a reasonable cost and is sufficiently por - carried to measurement site . table to be hand - carried to a measurement site . Primary features of the apparatus are the pressure chamber , gauge , and control valve , and a small nitrogen gas tank that provides the pressure . To avoid samplingleavesof dif - ferent physiologic age and exposure history , we select the youngest fully expanded leaves , usually on the fourth readily visible node down from the terminal . The petiole and at - tached leaf are cut from the plant , leaving about 7to 9 cm of petiole from the cut to the base of the leaf . Once the petiole is severed , water withdrawswithin the xylem vessels , be - cause the external pressure is several times are Leaf and intact petiole that inside the system . After the initial cut , inserted into underside of any further trimming of the petiole would in - to chamber lid and subjected negative pressure . troduce an error into the measurement and CALIFORNIA AGRICULTURE , NOVEMBER - DECEMBER 1982 13 at minimum leaf water potential , measure - - ' Acala SJ - 2 ' - - 2 . 3.0 - ment time is convenient , and the values are Panoche clay loam - 10 July - 4 . - Panoche clay loam . * - ' - 2,5 relatively stable in this climatic region for a . * 2 - 6 . . - - - - Wasco sandy lo - p - o - - - 8 - day - to - day comparison . m 2.0 . - E - 10 . O , " C Cotton mainstem growth is greatest from - 2 - 12 . 1.5 * . , Wet ( Max . ) ' - 45 about 20 June to 20 July . After this , the boll B - 14 ' 42 : . ' - 1.0 load becomes heavy , and vegetative growth slows . To eliminatevariation associated with - 0.5 of plant development , only main - the stage u d 01 ' stem elongation rates from the maximum - 24 - 22 - 20 - 18 . - 16 - 14 - 12 Midday leaf water potential ( bars ) growth period were used . Mainstem growth was most rapid after stress was alleviated by 2 . Cotton mainstem growth declined Fig . water potential uniformlywith declining leaf A linear reduction accompanied irrigation . Pacific Standard Time on contrasting soils . declining VI until little or no growth was 1 . After sunrise , leaf water potential Fig . declines rapidly until solar noon , then - 24 to - 25 bars ( fig . 2 ) . observed at about 2.5 to 3 hours . begins recovery after Contrasting soils gave the same results . The effect of decliningleaf water potential on fiber elongation and weight increase was 8June ( Wasco ) ' Elongation - 2 - determined by expressing the rate observed 16 June ( Panoche ) I t ? - 4 - as a ratio of the non - for stressed treatments Q A = ' P stressed treatment rate for bolls of the same UI 0.9 . - 7 - 6 - . - - age ( fig . t 0.8 ' 3 ) . Neither fiber elongation nor wall s - 8 - b 0.7 thickening was reduced until minimum Vl B 5 0.6 was lowered to about - 27 to - 28 bars . No 5 0.5 ' fiber growth was observed at greater stress levels . This response contrasts with the linear decline in vegetative growth as leaf water ; : I , , , I o , , , , , , , , , , , , potential declined and possibly indicates a ; 0.1 preferred photosynthate sink for fiber = o - 32 - 28 - 24 - 20 - 16 - 12 growth . bars ) , k ( I lidday leaf water potential Changes in leaf water potential over time stress plots for contrasting soils were examined to evalu - Fig . 3 . Fiber growth was not affected by of this measurement for ate the usefulness water stress until minimum leaf water po - scheduling irrigations . Figure 4 shows only tential decreased to - 27 to - 28 bars . slight fluctuations of Yl from a straight line function of time since irrigation . Variation from the line reflects normal day - to - day dif - should be avoided . Also , any leaf drying that clay loam ( 1975 through 1977 ) at the Univer - ferences in evaporative demand . Leaf water of California West Side Field Station in occurs before the measurement makes the sity potential declines much faster on the sandy western Fresno County and on Wasco sandy readings too low . loam , reflecting the lower water - retention loam We find that immediatemeasurementgives capacity . ( 1975 and 1976 ) at the U.S . Cotton Re - good results with cotton , but if this is not In this study treatments that varied timing search Station in Kern County . Both soils possible , leaves should be wrapped with of the first irrigation of the growing season allow deep root development , and water ex - moist cheeseclothor tightlysealed in a vapor - or traction is commonly observed in mid - showed best results when leaf water potential late - season at depths of 1.5 meter for the - 16 bars just proof plastic bag ( exhaling into the bag will was allowed to decline to about Wasco soil and 2 meters for the Panoche soil . before the irrigation . This degree of stress loss ) . Leaves are sealed help minimize vapor This study was done to establish the rela - slows growth slightly . After substantial root inside the pressure chamber with the petiole tionship of cotton vegetative and fiber extensionand plant conditioning , yields were extending above the chamber top through a growth to leaf water potential optimum when midseason irrigations were ( Vl ) and to rubber stopper that is pressure - sealedinto the assess this measurement's effectiveness for of scheduled at midday leaf water potentials undersideof the chamber lid . The chamber is scheduling irrigations . pressurized until water in the xylem is forced - 18 to - 20 bars . At peak bloom , results may back exactlyto the cut petiolesurface . At this be improved by irrigation at V [ during the day is - 18 bars , since Typical variation in point , pressurizationis stopped and the read - shown in figure 1 for contrasting soil water the plant is more sensitive to yield loss from conditions . Leaf water potential is greatest ing is taken from the gauge . The balance excessive stress at this stage . pressure now matches the negative pressure before sunrise , then it declinesrapidly , reach - a minimum at about solar noon . Values ing of the plant system . We find that the average Donald W Grimes is Water Scientist , Department are relatively constant for the next 2.5 to 3 of readings from three to five leaves is ade - of Land , Air and Water Resources , Universityof hours , after which recovery begins . We chose quate to characterize a sampling site . California , Davis , stationed at the San Joaquin Valley Agricultural Research and Extension Cen - We conducted to relate cotton growth to the minimum a three - year field study on ter , Parlier , and Hidemi Yamadais Staff Research values observed at and following solar noon , ( Gossypium hirsutum L . ) in the San cotton Associate , U.C . West Side Field Station , Five because treatment differences were greatest Joaquin Valley . The tests were on Panoche Points . 14 CALIFORNIA AGRICULTURE , NOVEMBER - DECEMBER 1982