Favorable effects of soil solarizarion on plant growth and crop yield were largely documented in many studies (Stapleton and Devay 1984; Davis 1991; Gamiel and Katan 1991). Increased growth response following solarization extended also to nursery seedlings and deciduous tree crops (Stapleton and DeVay 1982; Salerno et al. 2000), and resulted particularly evident under greenhouse conditions, where crop yield and quality was found to last for more than two crop cycles (Candido et al. 2008) (Fig. 9.6). However, solarization was also reported for a negative growth response, as Bendavid-Val et al. (1997) found a growth retardation of carrot and onion sown in solarized soil and Caussanel et al. (1998) related the reduced yield of corn salad, Valerianella locusta (L.) Laterrade, after the heat treatment to a suppressed mycorrhizal root infection. Earliness and shorter duration of vegetable crops were also documented by several authors in addition to the increased yield (Chen et al. 1991; Stapleton and DeVay 1995; Sinigaglia et al. 2001; Patricio et al. 2006).
Gruenzweig et al. (1993) related increased growth response of solarization to a number of physiological changes, as increased photosynthetic activity and protein levels, accelerated tissue development and delayed senescence occurring in the late developmental stages of plants grown in solarized soil. Higher concentrations of gibberellins, linearly related to leaf dry weight increase, reported by Grunzweig et al. (2000) in tomato plants from solarized soil, may suggest that also an alteration of normal plant hormonal balances was stimulated in heat-treated soil.
Most authors agreed that the increased growth response of solarization is not strictly disease-dependent, as occurring also in pathogen-free soils (Abd El-Megid et al. 1998), but rather the result of several effects on soil and plant previously described, including the increase of soluble mineral nutrients and mineralized organic matter (Chen and Katan 1980; Stapleton et al. 1984; Chen et al. 1991; Chen et al. 2000) and of growth regulator factors (Grünzweig et al. 2000), the increased soil biological activities and the control of minor pathogens (Gruenzweig et al. 1993; Gamliel and Stapleton 1995; Tjamos and Fravel 1995; Le Bihan et al. 1997).
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