PHOTOPERIODIC CONTROL OF GROWTH, DEVELOPMENT AND PHYTOHORMONE BALANCEIN SOLANUM-TUBEROSUM

Some lines of Solanum tuberosum ssp. andigena are strictly photoperiod ic, Forming tubers only in short days and flowers only in long days. W e used this advantageous phenomenon to study phytohormone involvement in the development of the plants, mainly that of tuber formation. Plan ts grown for 2 months under short days (SD) of 14 h darkness, night br eak (1 h white light in the middle of the 14-h dark period) and contin uous light (LD) were compared. Short day-grown plants formed tubers, w hile plants in LD flowered. Night break prevented tuber formation, but caused flowering, although it was weaker than in LD. Plants grown und er night break displayed many growth characteristics intermediate betw een SD and LD. Under LD and night break regimes ABA levels in all orga ns were about one-fourth of those under SD. An opposite trend was foun d for gibberellin content: it was very low in SD-grown plants and 4-10 times higher under both ether conditions with the exception of roots and stolons in night break. Thus, the ratio of ABA;GA, known to be imp ortant for tuber formation, was high in SD and low in night break and LD. The level of free indoleacetic acid (IAA) was high in LD- and nigh t break-grown plants and it was much lower in SD-grown plants, includi ng tubers. Cytokinin (CK) levels were also high in LD- and night break -grown plants. In SD, leaves had slightly decreased and stems and root s + stolons had more pronouncedly decreased cytokinin levels. The CK/I AA ratio was thus higher in SD in stems and root + stolons; it was als o high in tubers and no difference was found in leaves. These results indicate that the effect of photoperiod on tuber formation and develop ment may be mediated by photo-period-induced changes in hormone levers .