Development of Fertigation Control System Based on Measuring Plant Weight Using Load Cell for High-brix Tomato Hydroponics.

Abstract

緒 言 近年の トマ ト生産では，消費者の良食味嗜好に対応した高 糖度トマトの養液栽培が全国的に拡大している．高糖度トマ ト 生産を目的に大玉トマト品種の果実糖度を有利販売可能な Brix 7 % 以上 1) に上昇させるためには，給液制限や高濃度 培養液の給液によって根からの吸水を意図的に抑制し，果実 の肥大過程を通じて継続的な水分ス ト レスを与える必要があ る． しかし，数ヶ月に及ぶその栽培過程において，一時的で はあっても過大な水分ス ト レスが発生すると，着果不良はもち ろん，尻腐れ果や果頂褐変果 2) といった生理障害が多発し 大幅な減収をもたらす可能性がある 3) ． したがって，年間を通 じた高糖度トマ ト栽培の安定化を図るためには，過大な水分ス ト レスを未然に回避できる給液管理が不可欠である． さらに， 水分ス ト レスを自在にコン トロールできる給液管理が可能となれ ─ 16 ─ Abstract A fertigation control system based on plant weight using a load cell was developed to improve the stability of high-brix tomato fruit production with restricted root-zone hydro-ponics. In this control system, plant weight (Wx) was measured by hanging a tomato plant and a coconut fiber in a pot with nutrient solution dripping from a load cell. The maximum plant weight (Wmax) as Wx without plant water stress was measured after the first sufficient fertigation of the day. Relative plant weight (Rw) was used as an index of fertigation timing, which is calculated from a percentage of Wx towards Wmax. After the second fertigation, the fertigation pump starts when the Rw is less than the value of the start point for fertigation (SP) during the control period. Relative stem diameter (RSD) was used as an index of plant water stress by measuring tomato stem diameter using a laser sensor and was more closely correlated with Rw than with substrate water content and amount of evapotranspilation. The monitoring of Rw can be used to non-disruptively evaluate plant water stress. These results suggest that the fertigation control system based on measuring plant weight using a load cell can be used to avoid excessive plant water stress in tomato hydroponics producing high-brix fruit.