GENETIC-CONTROL OF STEM FORM IN PINUS-PINASTER AIT. SEEDLINGS EXPOSEDTO LATERAL LIGHT

In order to compare family differences in stem form determination, see dlings of eight half-sib progenies of Pinus pinaster were exposed to t hree different light treatments: fixed lateral light (T1), alternate l ateral light (T2), and direct sunlight (T3). From spatial coordinates of several points along the stem we computed four variables: global le aning (GL), apical leaning (APL), flexuosity index (FL) and length of the stem (LEN). Based on comparisons of the four variables, the photot ropic stimulus caused different responses in stem form at the familial level. Treatment had an important effect on stem form, although the f amilial response to treatments was quite variable. Heritability values (h(2)) of the four analyzed variables showed a different pattern of v ariation for the three treatments, being very stable for LEN and FL, w hereas hz of APL was much greater based on T1 and T2 compared to T3, a nd h(2) of GL based on T2 was much greater compared to T3 and T1. Diff erences in stem form were due not only to photomorphogenetic response to different light stimuli, but also to the straightening processes su ch as compression wood formation. The results suggest that lateral lig ht treatments may be useful in revealing differences in efficiency of the mechanisms involved in stem form determination.