Temperature and irradiance effects on tuber growth of Sandersonia aurantiaca

The effects of temperature and irradiance on tuber growth and the development of secondary tubers in Sandersonia aurantiaca (Hook.) were investigated. Plants were grown to flowering during summer in a glasshouse. At flowering, plants were trimmed to leave nine leaves on the stem, and transferred to controlled environment rooms set at constant temperatures of 15, 18, 21, 24 and 27 degrees C. Within each room, three irradiance treatments were imposed using neutral density shade cloth: 210, 460 and 700 mu mol.m(-2).s(-1) photosynthetic photon flux density (PPFD). Plants were sampled at intervals during tuber growth and at maturity to assess tuber fresh weight, tuber dimensions and the development of secondary tubers. The date of leaf senescence was also recorded. Tuber growth rates were significantly affected by both temperature and irradiance, responding positively to increasing irradiance at all temperatures. Senescence date was similar amongst treatments. Across all treatments, mean tuber fresh weights at maturity ranged from 8.1 to 18.0g with maximum weights in the 21 degrees C/700 mu mol treatment. The incidence of secondary tuber formation was strongly influenced by both temperature and irradiance. Very few secondary tubers were formed at 15 or 18 degrees C, irrespective of irradiance. At temperatures greater than or equal to 21 degrees C, the incidence of secondary tubers increased with both increasing temperature and irradiance, reaching a maximum of 88% in the 27 degrees C/700 mu mol treatment. Treatment conditions which maximized tuber fresh weight and minimized secondary tuber incidence were those with cool temperatures and high irradiance i.e., 15 or 18 degrees C and 700 mu mol. Across all treatments, there was no correlation between the presence of a secondary tuber and the fresh weight of the primary tuber. Taken together with the relatively small effect of temperature on tuber growth rate prior to the onset of secondary tuber formation, this suggests that assimilate supply is not the primary factor involved in the onset of secondary tuber formation. It is proposed that the primary effect of temperature on secondary tuber development is mediated via the hormonal status of the growing point on the primary tuber.