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The levels of invertase (E.C. 3.2.1.26) activity were measured throughout the development of the fourth leaf of Lolium temulentum. No neutral invertase activity was detected. Soluble acid invertase activity fell during leaf extension but rose again after ligule formation. This rise continued into senescence and was accompanied by the appearance of activity in the insoluble fraction. Evidence is presented that the insoluble activity was not an artefact of preparation, and that it represented an extracellular acid invertase. Fractionation of soluble invertase by gel filtration showed the appearance of a high molecular weight form at the time when insoluble activity was rising. The relationships between the different forms of the enzyme are discussed, together with their roles in leaf development.

Vegetative plants of Lolium temulentum L, grown at 20°C with an 8 h photoperiod were transferred to either 5 or 2°C (8 h photoperiod) at 4th leaf maturity. Measurement of dry weight gain indicated a marked decline in relative growth rate below 5°C, with growth being reduced as much between 5 and 2°C as between 20 and 5°C. This reduction was not associated with increased mortality and was reversible if plants were returned to 20°C. Tissue explants from cold‐treated plants retained the ability to extend if returned to 20°C Rates of extension in explants were less temperature sensitive than the relative growth rates observed in intact plants. Measurements of photosynthetic capacity, and of the patterns of accumulation of reserve carbohydrate in plants exposed to different temperatures, suggested that the inhibition of growth caused by chilling is not caused by an inability of the plants to fix sufficient carbon.

High-resolution growth measurements were conducted using a linear variable displacement transformer in conjunction with a temperature-programmed meristem-cooling collar. Chilling and rewarming profiles were determined for a range of Gramineae, in the presence and absence of varying concentrations of gibberellic acid (GA3). In wheat (Triticum aestivum L.) seedlings, the growth-constraining temperature (Pe) was progressively lowered by increasing GA3 concentration, with a difference of-4.8°C between controls and material treated with 10(-4) M GA3. Dwarf-5 maize (Zea mays L.) seedlings had a higher Pe than tall segregates and the difference was markedly reduced by exposure to a saturating concentration of GA3. A similar effect was observed with Tanginbozu dwarf rice (Oryza sativa L.). The growth ratetemperature responses of Rht3 gibberellin-insensitive dwarf wheat seedlings were unaffected by GA3 and the Pe values for these segregates were around 5° C higher than for normals. Slender (s1) barley (Hordeum vulgare L.) genotypes had Pe values of-7° C, compared with +4° C for wild-type material, and did not show positive hysteresis for growth rate during the rewarming phase. These studies indicate that GA3 modifies the thermal sensitivity of meristem function in Gramineae in a manner which enhances low-temperature growth.

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