SummaryThe relative turgidity technique consists in comparing the initial and turgid water contents, on a percentage basis, of disks punched from leaves, the turgid water content being obtained by floating the disks on water.Three main sources of error associated with flotation are recognized: (1) changes in dry weight of the disks; (2) continued increase in water content after the attain· ment of full turgidity; (3) injection of the intercellular spaces at the cut edges of the disks.An examination of these sources of error led to the following conclusions concerning them:(1) By regulating the light intensity approximately to the compensation point, dry weight changes can be reduced to unimportant proportions. This obviates the necessity of taking duplicate samples since the final dry weight can be used for calculating both the initial and turgid water content of the disks.(2) Water uptake by floating leaf disks can be divided into two phases, phase I in response to the initial water deficit, and phase II the continued uptake, due to growth. The aim of the technique is to measure phase I alone. Metabolic inhibitors eliminated phase II but their use in the technique is unpractical (anaerobiosis) or objectionable (potassium cyanide). Low temperature (3°C) eliminated phase II but reduced phase I which itself appears to be divisible into more than one phase.It was found that for Ricinus communis L. full turgidity was attained in 4 hr and in this period phase II increase did not occur. A similar period appears to be suitable for a wide range of material. This short flotation time also reduced any danger of dry weight changes.(3) Injection errors were measured and found to be negligible for Ricinus.For Sambucus nigra L. they were considerable. Evidently the magnitude of this error varies with the species concerned.Plastic flow or irreversible contraction of the cell walls of turgid or wilted disks respectively was not found to be of importance.

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Cyclic fluctuations in stomatal aperture, transpiration rate and leaf water potential under constant environmental conditions have been investigated in intact plants of cotton, pepper, and sunflower. Stomatal aperture and transpiration rate were least when leaf water potential was high and were greatest when leaf water potential was low. Lowest leaf water potential values lagged behind the occurrence of highest transpiration rates, and high overall resistance to water flow occurred in cycling plants. Both of these are considered essential for the occurrence of persistent cyclic behaviour. Hydropassive opening of stomates as the leaves wilted facilitated cycling in cotton and pepper, but not in sunflower, where hydropassive opening did not occur. The roots were identified as the site of the major resistance to water flow in the plant and further experiments directly showed the importance of this root resistance in initiating cycling by causing water stress in the leaves as the stomates opened. Root resistance varied diurnally, becoming increasingly important at night. Root resistance naturally rose to high levels in cotton. High levels were induced in pepper or sunflower by having the roots in deionized water for several days or by anoxia. Quantitative measurements of overall plant resistance were made from leaf water potential and transpiration rate data. The results are discussed and it suggested that plant resistance may indirectly be of importance in the movement of water from the plant to the air.

Siinultaneoiis cyclic variation in iate,s of both net photosynthesis and transpiration were induced in attached leaves of cotton and pe|)piM-plants under c()n,stanl environmontiil conditions. The cyclic variations in photosynthesis inut transpiration were toinid U> ho iti phase, and the ratio net i)hoto.synthotic iate/transj>iratii)n r:itireniaiiu'd constant over a wide range of gas exchange rates. A similar constancy of this ratio was also found as gas exchange rates declined following excision of a sunflower leaf, which was not initially cycling, in air. The.se results suggested ttiof change in sfoniatal aperture was the only controlling factor involved and that it was affecting lM)th processes proportionately. Visible loss of leaf turgor and measurable water stress developed in botli |)epper and cotton at peak exchange rates, hut (he gas exchange ratio remained constant.The failure of water stress and increa.sed stomatal aperture to lower the gas exchange ratio .suggested an absence of any significant leaf meso(ilnll resistance h',j to iiiwuid diffusif)u of CO^. The possibility that r',,, was low is discussed generally, and in relation to the use of chemical antitranspirants to raise the gas exchange ratio. Within the limits of the experiments, water stress apparently had no direel adverse effect on rates of net photosynthesis. The gas exchange ratio did not rise a.s exchange rates declined. Ultimately, at very low exchange rates, the ratio foil, declining to zero in cotton, hut not in pepper. This dectine was attrihuti'd to the onset of significant gas exchange through ttie cuticle, which was apparently less permeable to CO^ than to water vapour. Positive net cuticular photosynthesis therefore probably does not occur in cotton. Except at very low exchange rates, the gas exchange ratio was higher in cotton than in pepper; it was similar in sunflower and cotton. Phy.noL Plant.. 21. mS [918] RATIO OF TRANSPIRATION TO NET PHOTOSYNTHESIS 919