MORPHOGENESIS OF BUTTERHEAD LETTUCE (LACTUCA SATIVA VAR. CAPITATIS) UNDER DOUBLE DIRECT CROP COVER (DC)
This was ascribed to the fact that admittedly more leaves developed in 1988, but that these grew on a stalk the same size as in 1987. The hypothesis was forwarded that in this way in 1988 a looser head developed, which transpired more, and consequently lost more weight, than in 1987.
Hence, the fact that double DC in 1987 yielded heavier heads than in 1988 was interpreted as a consequence of a more compact head formation, in which fewer leaves developed on a smaller stalk.
As a matter of fact, the higher temperature and the brighter month of April in 1988 did stimulate the growth process, which, however, was impaired more under the darker double DC and the more ventilated PP-sheeting than under the single DC with its perforation intensity of only 4%.
The more thermpohile and translucent PE-EVA-IR-AC-sheeting consistently yielded the better results.
The moment of removal of the upper perforated sheet is determined by the hearting stage at which half the number of leaves (20 to 30) have developed.
During the 2nd half of the hearting phase the lettuce is thought to be even more light-sensitive than during the 1st half.