A root-shoot interaction hypothesis for high productivity of root crops

Abstract

Abstract Root crops such as potato, beet, and sweet potato produced high yields with application of coated fertilizers, which corresponded to the maximum yields reported in Hokkaido (northern part of Japan). Based on the following results, a root-shoot interaction model was proposed to explain the high productivity of root crops. 1. The highest biological yields (Y b, g m−2 based on dry matter) were 2,130 in potato, 2,120 in beet, and 1,660 in sweet potato. The highest economic yields ( Y e, g m−2 based on dry matter) were 1,710 in potato, 1,540 in beet, and 1,050 in sweet potato. In Hokkaido the growth duration of sweet potato was limited to only 105 d due to the cold climate, while, sweet potato achieved a high productivity during the short growth duration in comparison with cereal and legume crops.2. The k-value, which is an indicator of plant type, was not related to productivity within root crops, and the k-value of root crops was higher than that of cereal crops. Therefore, plant type is not the fundamental factor to explain the difference in crop productivity.3. The net assimilation rate of beet was lower than that of potato and sweet potato, but beet could obtain a high Y b because a high constant leaf area index was retained over the long period of growth.4. Accumulation of nitrogen, phosphorus, and potassium continued until harvest in the high-yielding varieties (hereafter abbreviates as HYVs) of potato and beet, while in sweet potato it almost ceased at the early swelling stage of tuberous root. The harvest index of N, P, and K of sweet potato was lower than that of potato and beet.5. Specific absorption rates of N, P, and K by roots were higher in the HYVs than in the varieties with standard yield (hereafter abbreviates as SYVs) at the maximum shoot growth stage. Consequently, plant type-theory was not sufficient to account for the high-yield of crops, especially root crops. Since the photosynthetic rate (dry matter increase) and root activity (nutrient absorption) remained constant during maturation in the HYVs, it is suggested that in these varieties, root and shoot mutually interact so that a high photosynthetic rate secures a high root activity by supplying a sufficient amount of photosynthates to roots and a high root activity secures a high photosynthetic rate by supplying a sufficient amount of nutrients to shoots. Thus in achieving high productivity root crops are able to maintain a balance between root and shoot activity because since in root crops the main sink is underground, photosynthates are actively distributed also to roots. Therefore, inferior plant type of root crops does not prevent a high productivity.