Root Growth of Kiwifruit Vines and the Impact of Canopy Manipulations

Abstract

Root growth dynamics of kiwifruit vines and the impact of altering source (leaf) to sink (fruit) ratios were examined in field experiments. White root incidence, an index for root growth, increased during spring and early summer, peaking at about 100 mm m -1 . The peak relative growth rate of root length was estimated at 0.35 × 10 -2 day -1 . This compared with peak relative growth rates of 26.7 × 10 -2 day -1 for shoot length (recorded 14 days after bud burst) and 5.07 × 10 -2 day -1 for fruit diameter (recorded 14 days after fruit set). The results suggested that significant resource allocation to the root system occurred only after shoot and fruit growth were largely complete. Partial defoliation reduced white root incidence at the end of summer, by up to 70%. The effects of defoliation on root growth were generally greater than the effects on fruit yields (maximum reduction of 32%), suggesting that root growth is more sensitive than fruit growth to reducing the source (leaf) to sink (fruit) ratio within the canopy. Vines grown with a leaf to fruit ratio of 2 had much lower levels of white root from the time of fruit set (early summer) to fruit harvest (late autumn) compared to vines with leaf to fruit ratios of 5 or 10. Vines with the higher leaf to fruit ratios recorded white root incidences exceeding 100 mm m -1 during mid to late summer, while the white root incidence at this time for vines with a low leaf to fruit ratio never exceeded 50 mm m -1 . By the end of one growing season, the total root length density of vines with the low leaf to fruit ratio had declined to 63% of the density for vines with intermediate or high leaf to fruit ratios. This difference resulted principally from a reduction in root densities at deeper soil horizons rather than near the soil surface, although differences in white root incidence between the various leaf to fruit ratio treatments did not show this trend. This result suggests that altering the leaf to fruit ratio may affect root turnover as well as growth. The major influences of canopy management on root growth indicate the importance of managing the root system for optimum performance of the entire vine. The importance for root growth of the balance between assimilate supply and demand within the vine is discussed.