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  • Author or Editor: Ken-ichiro Yasuba x
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When we grow lettuce in the hot season in Japan, lettuce heads sometimes grow abnormally. In early autumn, we frequently find strange shapes of lettuce because of a projecting leaf midrib. Poorly shaped lettuce is unmarketable. We found that lettuce grew abnormally under the high temperature and low solar radiation conditions at the time of 1200 °C accumulative temperature from seeding (average temperature was >20 °C and daily total solar radiation was under 18 MJ/m2). Midribs of both good-shaped (resistance of projecting midrib) and poorly shaped cultivars projected this condition, but the symptom of the former was milder than that of the latter. So, we investigated the difference of leaf growth between good and poorly shaped cultivars using growth chambers. We set up six patterns of environmental conditions, which consisted of three patterns of temperature (30/22 °C, 24/16 °C, and 18/10 °C) and two patterns of light (4 and 2 MJ/m2). The two- to three-leaf seedlings, which were grown in the greenhouse for 27 days after sowing, were transplanted in the 250-mL pots and were carried to growth chambers. We measured width and length of each leaf 9 days after planting. Consequently, the ratio of width to length (w:l) of new leaves became low when we grew lettuce in high temperature or low light conditions. The w:l of good-shaped cultivars were higher than that of poorly shaped cultivars. Good-shaped cultivars did not grow spindly with ease on high temperature and low light conditions, like an early autumn environment. Now we will try to investigate the relationship between leaf shape and head shape on the poorly shaped conditions of some lettuce heads.

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The yield of greenhouse tomatoes in Japan has not increased since the 1980s and remains much less than 30 kg·m−2 per year. To investigate the cause of this low yield, we compared six Japanese tomato cultivars that were commonly grown or released during the past 80 years to see whether fruit yield (fruit fresh weight per area) and dry matter (DM) content per fruit improved under current cultivation conditions. Fruit yield in ‘Momotaro’ (released in 1985) was lower than that in older cultivars. Total DM was determined mainly by light use efficiency and photosynthetic rate, and light use efficiency was correlated with maximum photosynthetic rate. The more modern cultivars did not show improved DM content per fruit. The DM content per fruit was strongly correlated with the soluble solids content in fruits except in ‘Momotaro’ and ‘Momotaro colt’, but soluble solids in fruits of the ‘Momotaro’-type cultivars were higher than in other cultivars for a given DM content per fruit. Thus, tomato breeding in Japan appears to have focused on fruit soluble solids content per unit DM rather than fruit yield or DM content; as a result, only the former parameter has improved greatly.

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