254 J- F- BIERHUIZEN 



of the rather low energy consumption in growth compared with the other 

 terms in the heat balance, the energy consumption is not included in such 

 calculations. In photosynthesis experiments with algae and leaves of higher 

 plants, conversion yields between lo and 20% are obtained between COg- 

 consumption and absorbed visible radiation. In the field, energy conversion 

 of growth is only i to 2% (Wassink, 1948). In our greenhouse experiments 

 a value of 3-3% was calculated (Bierhuizen, i960). The 1-5 times larger 

 value in the greenhouse, still does not effect the individual terms of the 

 energy balance to a large extent. It is possible, therefore, that total water 

 use per kg dry matter produced, is 1-5 times less in the greenhouse. In our 

 case, for y = o-7ix, 100 mm water use produces 71 gm per 0-2 m^ or 36 kg 

 dry matter per 100 m^. The same amount of grass yield would necessitate 

 170 mm of water (Visser, 1959), which is approximately 1-5 times more 

 than in the greenhouse. 



GROWTH AND WATER USE OF LETTUCE 



IN DIFFERENT SEASONS 



From the results discussed in the preceding sections, the length of growing 

 period and the total water use necessary for a given yield in different 

 seasons can be predicted. In Table i calculations are given for lettuce 

 planted at different dates (column i). It was assumed that harvest took place 

 with a head-weight of 150 gm. In our case 4 heads per container yielded, 

 therefore, 600 gm. From daily radiation (De Vries, 1955) and temperature 

 data in the greenhouse (ranging between 5° and 23°C for winter and 

 summer) the date of harvest (column 2) was calculated, using formula i. 

 Radiation data alone can be used without a large error, in order to facilitate 

 the calculations. In that case the total radiation is approximately 10,000 cal 

 cm-2 (Fig. 3). The harvest dates, calculated in this way, are confirmed by 

 those in horticultural practice. Total water use (column 4) could be read 

 from Fig. 4. The average daily radiation for each growth period could be 

 calculated and is approximately 10,000 cal cm^^ divided by the number of 

 days of growth. From this value, pan-evaporation in mm per day (colunm 

 6) could be read from Fig. i . The ratio water-use pan-evaporation approxi- 

 mates closely unity and tends to decrease in the summer months and to 

 rise in winter. The variation in this ratio agrees with that obtained in water- 

 balance studies in the field (Bloemen, i960). The steeper decrease in 

 summer in Bloemen's results, might be due to the fact that deeper ground- 

 water levels existed. 



