Abstract:
Two spring-planted and one summer-planted crops of the potato var. Scbago have been grown under conditions of high solar input (daily averages throughout each experiment 23.1, 22.2, and 13.9 MJ m-2 respectively) in the Murrumbidgee Irrigation Areas of New South Wales. Treatments were a combination of three levels of solar input (0, 21, and 34% shade throughout growth) with two levels of irrigation (soil restored to field capacity at each estimated soil moisture deficit of 2.0 cm or 3.5 cm in two experiments, 3.5 cm or 5.0cm in the other). Growth analyses were made throughout each experiment. Environmental measurements showed little effect of shade covers on air temperature or relative humidity, and occasional measurements of leaf temperature and leaf water potential showed only small differences between shaded and unshaded plants. Differences in leaf and stem dry weights between plants in any treatments were small, but leaf area and stein length increased and specific leaf weight decreased with increasing shade. An increase in shade also decreased the numbers of tubers per stem which developed, and, while it did not influence the time of initiation, the time between onset of initiation and development of maximum bulking rate was greater as shade increased. Bulking rates with no shade were high (up to 1.4 t ha-l day-l) and only slightly less on shaded plots. In the drier of the two irrigation treatments in each experiment bulking rate was only slightly reduced, but the rate of leaf senescence was greater. The treatments did not affect the time of cessation of bulking. The results are consistent with the hypothesis that photosynthesis in the potato is controlled largely by the size of the 'sink' provided by the growing tubers, and it is suggested that the main effect of solar input was in controlling the 'sink' size which developed in the two or three weeks following the onset of initiation. The high growth rates and accelerated ontogeny were probably largely a result of the high temperatures of the region. Final yields are considered in the next paper.