Transpiration, Water Absorption, and Internal Water Balance of Cotton Plants as Affected by Light and Changes in Saturation Deficit

Abstract

SmYnumarv. Tin controlled environnrent .ttudies of cottoni plants (Gossypium barbadenisc L.) a light-induced acceleration of transpiration tupset the water balance established in the (lark because of a lag in water absorption. A plant-water deficit could be generated either by sudden illumination at a given saturation deficit (sd) of the air. or bv raising the sd in conjunction with illumination, without different effects. Direct water balance measurements were confirmed in every experiment by beta ray gauge detection of changes in leaf-w%ater content resulting from unequal gain and loss of water by the whole plant. Recovery froml the initial loss of turgidity always was faster and more complete at the higher than at the lower values of sd. Recovery occurred even in the light at the higher values of sd, but was enhanced by return to darkness and a lower sd, which at times resulted in superhydration. Rehydration in the light could he attributed to at least 2 processes: A) a diminished transpiration rate if earlier water loss was sufficient to induce stomatal closure, and B) an increased rate of water absorption. The data suggest that a water deficit, temporary or persisting, does not cause a significantly lowered transpiration rate; thus, recovery miust depend on increased absorption. The comn'unicative link between the 2 processes appears weak, transmitting strong signals onily. A better uiulderstand(inig of plant-water relationis may be obtained from precise, siiiiultanieous measurements of transpirationi and water absorption by varying only one environmental factor at a timiie. Lachen-meier (2) used this approach, but tunider enivironl-mental conditions conducive to onlly a rather low transpiration rate. Kramer (1) studied the water balance of several types of plants, growilng in both soil and nutrient solution under greenlhouse condi-tiolns, using experimental periods of 2 hours or miiore. Since changes in plant-wvater balance can occtur rapidly, the uise of measurement p)eriods shorter thani 2 houirs seemiis desirable. The present investigation was tiun(lertakell with cottoin plants in a precisely colitrolled environmnent to determline lo, tlhe illterlial water balance is affected by sudden illumiiination or darkenin,g at constant amilbielnt conlditionls, alid bv a rapicl increase or decrease of the temiperatuire an(l! 71 hence, the saturation deficit (sd) at a constant vapor pressure. As used in this paper, sd is synonymous with vapor pressure deficit, and is not related to the plant-water deficit. Materials and Methods Aerial Environment. All experiments were carried out in a controlled environment room iwith an illumination of about 24,000 lux and a radiant energy fltux of 0.2 cal cmI2 imin-1. The sd of the air was maintained at a series of controlled valtues to indultice (liffereint transpiration rates. Both dry-and(wet-bulb temperatures were controlled to + 0.5' an(d recorded bv a Leeds and Northrup2 strip-chart recorder providing a contintuous record of temperature aii(l vapor pressure. Fronm these data, average valuies of the sd for each half-hotur measuiremenit perio(d \ere calculated. Root Environncitt. The air temperature was Illaintained sufficiently high that the root temperature in equilibriumn with it wotuld not limit water absorption. The nutrient solution, a mlodified one-half strength Hoagland solution, 10 meq liter-,. was such that pH, mineral balance, and electrolyte concentration were not limiting wvater absorption. Aerated xater