The inscrutable mandarin

Under the climatic conditions of Northern New Zealand, low temperatures limit fruit maturation of early-season satsuma mandarins. Yet within the Kerikeri district, consistent differences in juice quality can be found between similarly managed orchards subject to the same synoptic weather patterns and inputs of photosynthetic radiation. This study examines possible causes for such differences, using comparative plant and simple climatological measurements taken over two seasons at three orchards located within a 2.5 km radius. Microclimatic differences were largely confined to windspeed and root zone soil temperatures, with the latter having a profound influence on the timing of budburst. In both seasons, delays in budbreak of around 1 month were observed at the orchard with the highest juice quality and this was attributable to a 2°C reduction in spring root zone temperatures. These lags in budhurst resulted in comparable delays in flowering, with the consequence that the season for fruit growth at the tardy site was substantially reduced. Despite these shortened seasons, subsequent juice quality (ratio of total soluble solids to titratable acids) at harvest was significantly higher at this orchard in both years. A number of possible mechanisms for this unexpected result are discussed, and lead to the following conclusion: because of the delay in budbreak, both flowering and the early stages of fruit development took place during periods of higher air temperatures, thereby encouraging higher initial rates of sugar accumulation and improvements in final fruit quality. The importance of early-season flower or fruit temperatures on final fruit quality also emerges from published statistical studies relating fruit quality to climate variables, but the biochemical basis for this influence has never been explored. Future work should concentrate on elucidating the physiological processes occurring during this critical early period of fruit development.