Dry matter partitioning in Zantedeschia K. Spreng, as influenced by temperature and photosynthetic photon flux : a thesis presented in partial fulfilment of the requirements for the degree of Doctorate of Philosophy in Horticultural Science at Massey University
The in vivo and in vitro dry matter accumulation and partitioning in plants of the Zantedeschia pentlandii-like (Watson) Wittm. selection 'Best Gold' were described under a range of either temperature and photosynthetic photon flux (PPF) regimes, or sucrose concentrations, using plant growth analysis. The initiation of tuber growth, as denoted by increases in both structural and starch dry weights, did not require an obligative environmental trigger. Relative rates of dry matter accumulation (RGRW) increased linearly with increasing temperature up to a maximum of 28 C, with maximum final total and tuber dry weight occurring between 21 and 26 C both in vivo and in vitro. The linear relationship between the relative rate of dry matter accumulation of the tuber (RGRT) and temperature, indicated a PPF dependent base temperature for tuber growth between 4.8 and 6.1 C. By principally altering dry matter partitioning, total dry matter accumulation was highly adaptive to PPF regime. The ability to alter the photosynthetic rate and the partitioning of the daily increment of dry matter into leaf area (LWP), resulted in greater values of the estimated final total plant dry matter under the low PPF regime (348 µmol m-2s-1), at temperatures less than 22 C. At temperatures greater than 19 C the estimated maximum total plant dry weight was either not influenced by PPF or was slightly greater under the high PPF regime (694 µmol m-2s-1). This ability to effectively utilize a low PPF regime indicates that this selection is shade tolerant. The optimum PPF for growth was found to be temperature dependent: estimated maximum total plant dry weight occurred under high PPF at 25 C, whereas the estimated maximum tuber dry weight occurred at 24.5 C under low PPF. RGRW was highly correlated with LWP. In contrast, only a poor correlation was determined between RGRW, and either the efficiency of these leaves to produce additional dry matter, i.e., net assimilation rate (NAR), or starch concentration or soluble carbohydrate concentration. Photosynthetic rate was correlated with RGRW, but not with RGRT. While the photosynthetic process must be involved in contributing photoassimilates for tuber growth, it was suggested that the plant's response to dry matter partitioning into the leaf, i.e., LWP, and the tuber, i.e., TWP, had a greater influence in determining tuber growth than could be accounted for by the photosynthetic rate. Mechanisms of acclimation under both PPF regime suggested that tuber growth was principally source limited. Source limitation was expressed either in terms of: 1) enhanced intersink competition for assimilates, as occurred under the low PPF regime, where enhanced leaf area development (LWP) was in direct competition with enhanced tuber growth (RGRT). This was also confirmed in vitro where dry matter partitioning to the tuber was reduced under limited source strength. 2) efficiency of dry matter accumulation of leaf area present, as occurred under the high PPF regime, where large increases in RGRT were correlated with increased NAR. This was also confirmed in vitro where increased source strength increased tuber dry weight. However, in vitro experiments where source strength was controlled, illustrated that tuber growth was also potentially sink limited at temperatures both lower and higher than the optimum. At 31 C the sink limitation of tuber growth arose from more than the temperature-induced limitation on growth and respiration found at other sink limiting temperatures. At this temperature an additional form of sink limitation was evident where partitioning of dry matter towards the tuber was also restricted. It was suggested that this additional form of sink limitation may have arisen from high temperature inactivation of starch metabolising or sucrose unloading enzymes. Application of the dry matter partitioning term TWP, provided a more sensitive measure of short term changes in partitioning than the conventionally used term, harvest index. The optimum temperature range for growth was close to the average daily air temperature during the season for the sites of natural habitat of the suggested parent specie, Zantedeschia pentlandii. Similarly the shade tolerance status of this selection was paralleled by the diversity of PPF habitats it naturally occupies, as created by open grassland and forest margins. It was therefore suggested that Zantedeschia 'Best Gold' is well adapted to optimise growth under the temperature and PPF regimes of its natural habitat. This study suggests that improvements in commercial yield of Zantedeschia tubers can be achieved in all regions of New Zealand through the use of protected cultivation with supplemental heating. However, unless using protected cultivation, the potential improvements in commercial tuber yields, through the application of shading, are only likely to be evident in warmer regions of New Zealand where growers utilize extended periods of cultivation and optimise leaf area duration.