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Reducing Transpiration to Conserve Water in Soil and Plants

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Reducing Transpiration to Conserve Water in Soil and Plants major pathway of water loss from availability to the plant; and (2) conserves Reducing A plants is by transpiration, which ac- water in the plant itself, improving counts for 99 percent of the water taken growth by keeping water losses through transpiration to up by plant roots. This water is lost from the leaves in better balance with rate of the immediate area where the plant is uptake through the roots- i.e., the plant Consewe water in growing, since it passes via the atmos- is kept turgid. phere to some other point in the hydro- Plant growth depends basically on soil and plants logic cycle. Because the water that is two factors: (1) accumulating raw ma- transpired is essentially pure, salts in the terials for cell production, particularly soil water system become more concen- through photosynthesis, and (2) keeping trated. the plant turgid so cells can enlarge. The An antitranspirant (AT) applied to stomata1 pore lets in the carbon dioxide David C. Davenport transpiring plant surfaces to reduce needed for photosynthesis and allows Robert M. Hagan transpiration water losses (1) conserves water vapor to escape, influencing both Kay Uriu soil water in the root zone, extending its factors. An AT, which either plugs sto- Both pots of rugor beet were watered on the some day. Three days later, the unsprayed plant had exhausted Its sol1 molsture and wllted, whereas the plant prevlously sprayed wlth a fllm anMransplr ont stlll had sufflclent soil molsturo avallobie to prevent wlmng. 40~FORNlAAGRCULTURE.MAV1977 mata (“film forming”) or prevents com- ed for leaching. This wastage may be been improved. After spraying AT on plete opening (‘‘stomata1 inhibitor”), also minimized by an AT to slow soil water the stomatal-bearing leaf surfaces of fruit affects both growth factors. depletion, reducing the number of irriga- trees in commercial orchards one or two The net effect of applying an AT tions. In Israel, Dr. J. Gale applied a film weeks before harvest (when much of the may be beneficial in some cases and de- antitranspirant to field-grown banana carbohydrate accumulation in the fruits trimental in others. Crops will use contin- plants and reduced soil water extraction had already occurred, and fruit sizing de- ually available soil moisture at a “luxur- by 21 to 44 percent during seven irriga- pended chiefly on maintaining cell tur- ious” rate through wide-open stomates, tion cycles, without affecting growth or gidity), fruit volumes were increased by with accompanying rapid photosynthesis. fruit quality. The irrigation intervals 5 to 15 percent for olives, peaches, and On extremely hot and dry days, however, were extended from the normal seven cherries, and more fruit fell into larger these plants will tend to close their sto- days to about ten days. In California, we size grades. Fruit dry weight was not re- mates, because transpiration exceeds up- showed that a film-forming antitrans- duced significantly unless the AT was take of water, and growth factors 1 and 2 pirant effectively reduced soil water de- sprayed too early and at an excessive will be inhibited. This effect will be less pletion (measured by a neutron moisture- rate. severe in plants sprayed with AT. meter) by field-grown ornamental olean- We also found that preharvest In another case - initially moist soil der plantings, with the result that irriga- sprays of ATs on fruit trees curtailed followed by a dry period -photosynthesis tion intervals could be extended by at daytime shrinkage of peaches and pre- and transpiration will be rapid at first but least two weeks, and costly, hazardous ir- vented irreversible olive fruit shrivel. will slow as the soil water becomes de- rigations by tanker trucks along highways Furthermore, preharvest spray of an ac- pleted, especially if evaporative demand could be minimized. ceptable food-grade film AT reduced post- is high. Treatment with AT -preferably In many cases, the primary purpose harvest fruit desiccation, thereby extend- after most of the new foliar growth has in using an AT is improvement of plant ing shipping and shelf life. occurred but while there is still water in performance by increasing plant turgid- Dipping seedling tops in an AT solu- the soil profile - will initially retard trans- ity. Of course, soil water is also conserved tion just before transplanting can help piration and photosynthesis. However, in the process, but the grower’s main transplants survive the critical moisture- the time at which those functions start to benefit lies in greater monetary returns stress period; once the transplant is es- drop off severely because of water stress from the crop by minimizing plant water tablished, new foliar growth should per- will be delayed, and, throughout the in- stress-particularly at sensitive stages mit efficient photosynthesis. A pre-trans- terim period, the treated plants will be of crop growth. plant AT spray on seven-year-old citrus more turgid, will photosynthesize more It is unlikely that presently avail- trees reduced the decline in leaf turgidity rapidly, and will have higher growth rates able ATs would increase yield of an an- after transplanting, thereby reducing (see photo). nual field crop (which is highly dependent transplant shock. If virtually no soil moisture is avail- on current photosynthesis for growth and able, AT will be of little benefit, because final yield), unless it became stressed costs the plant’s own protective mechanisms from inadequate water and/or a very high will have come into full operation. Thus, evaporative demand. We found in Cali- At present prices, commercially a- an AT is not a cure for a plant that has al- fornia that a film AT on snap beans re- vailable ATs, usually of the film type ready wilted, but it can be a preventive duced leaf expansion, plant height, and (wax-based emulsions or polyterpenes), or delaying measure if applied while some yield of green beans when soil moisture are not economically feasible to reduce ir- soil moisture is still available. was adequate and evaporative demand rigation frequency, unless water is ex- was moderate. In Israel, however, Dr. tremely expensive, or to improve crop AT usefulness Gale showed that periodic AT sprays on yield, unless crops are high value, such as beans could increase dry-matter produc- cherries and olives. For instance, one of AT can be used to conserve water. tion, and there was only a slight dif- the wax-based ATs, at $4.50 per gallon, For instance, vast quantities of water are ference in plant dry weights between un- diluted to 3 percent (vlv) and sprayed at lost by phreatophytes (plants that get treated well-irrigated plots and anti- 300 gallons per acre in a cherry orchard most of their water from the water table), transpirant-treated inadequately irriga- to increase fruit size, would cost $40.50 such as saltcedar (Tamarkc sp.) and cotton- ted plots. When Dr. Fuehring sprayed AT per acre plus the cost of application. The wood (Populus sp.), especially if they grow on sorghum with limited irrigation in New increased fruit yield, along with the high- in riparian areas where ground water is Mexico, grain yield increased 5 to 17 per- er returns resulting from more fruit in available throughout the summer. Salt- cent. larger grade sizes, could result in a net cedar can transpire 60 to 70 inches (150 to Established perennial plants norm- gain - depending on current cherry pri- 180 cm) per year and, in the western ally have more food reserves in storage ces. United States, it annually uses more than tissues than do annual plants, so growth Most ATs have been produced for 5 million acre-feet of water. Our experi- is expected to be affected less severely the relatively small nursery ornamentals ments have shown that AT spraying can from photosynthetic reduction by an AT. market. It is hoped that when demand reduce this high rate of water loss by 30 There is some justification for expecting develops for an agricultural-scale market, percent without damaging the plants. The positive yield responses by perennials, prices will decrease as competition and accompanying temporary photosynthesis especially if the AT is applied at a stage sales volume increase. reduction is unimportant for these non- when growth of a particular plant part agricultural plants. This approach to im- depends more on maintaining high plant proving ground-water storage and in- water potential than on photosynthate David C. Davenport is Associate Re- creasing the supply of water for agricul- accumulation. search Water Scientist, and Robert M. tural and other demands may be more We observed that daytime shrink- Hagan is Professor of Water Science, De- environmentally acceptable than mechan- age of fruit tree trunks was significantly partment of Land Air, and Water Re- ical or chemical eradication of the plants. reduced by AT treatment, indicating sources (Water Science and Engineering On the farm, much water is wasted that the lag between water uptake and Section), U.C. Davis; Kay Uriu is Pomol- by inefficient irrigation - although some transpiration had been minimized and ogist, Department of Pomology, U. C. of the excess water applied may be need- that tree water status as a whole had Davis. CALIFORNIAAGR(CULWRE,MAY 1977 41 .
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