The rates of foliar dark respiration and net photosynthesis in attached leaves of 25 C3, C4, and C3-C4 intermediate dicotyledonous weed species were determined using the infrared gas analyzer. The ratio of dark respiration to photosynthesis per unit leaf area in attached leaves of each species was inversely proportional to leaf age. Highly significant, positive linear correlation was observed between the rates of foliar dark respiration and net photosynthetic CO2 uptake in dicot weeds irrespective of the photosynthetic type. The higher foliar dark respiration rate found in some of the weed species can be attributed in part to the higher carbohydrate levels as generated by a rapid photosynthetic CO2 assimilation. The significance of higher dark respiration rate in relation to carbon and energy economy of weeds is discussed.

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Seed germination of four Litchi chinensis Sonn. cultivars (`Deshi', `Kasba', `Purbi', and `Early Bedana') was studied under various conditions, viz. in soil beds exposed to sunlight or in shade, in sand beds exposed to sunlight or in shade, and on moist filter paper. Among all, shaded, humid sand at 35 ± 2C gave the highest germination. Delaying sowing seeds after removal from the fruit significantly reduced germination. Litchi seeds held in polyethylene bags up to 4 days at 37 ± 2C at 90% relative humidity delayed loss of seed viability. Germination was improved by ethephon in `Deshi' and `Early Bedana', by IBA in `Deshi' and `Purbi', and by 100 mm GA3 in all litchi cultivars. Cultivars responded differently to growth regulators, with `Deshi' responding significantly better than `Purbi', `Kasba', or `Early Bedana'. These studies point to the recalcitrant nature of litchi seeds. Chemical names used: gibberellic acid (GA3); indole butyric acid (IBA); 2-chloroethylphosphonic acid (ethephon).

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Abstract:The accumulation of starch and total sugar, mainly as sucrose, is a feature of seeds of the litchi cultivars 'Deshi' and 'Kasba' up to the fully ripe stage of fruits. The precocious loss of food reserves, as evidenced by a decline in starch content in over-mature seeds of senescing fruits, may be detrimental to loss of seed viability. Kinetin application leads to low contents of starch, sugar and lactic acid. This indicates that starch synthesis is possibly slowed down due to non-availability of sugars resulting from the reversal of their translocation as a consequence of kinetin application, which also facilitates aerobic metabolism of pyruvate affecting the lactate level. By contrast, cycloheximide brings about initial decline but later accumulation of starch and sugars, through a rise in sucrose content indicating an enzymic pathway and that the enzymes need to be synthesised, but the synthesis is inhibited by cycloheximide application. Similarly, enzyme-mediated lactate production is also interfered by cycloheximide application.