The root system of guar: Spatial and temporal analysis of root and nodule development

Abstract

AbstractGuar (Cyamopsis tetragonoloba [L.] Taub.) is a leguminous crop plant produced in semi‐arid regions worldwide, primarily for industrial uses of guar gum. Little research has been conducted on critical belowground traits of the plant, particularly on spatial and temporal development of the root system, including nodules. Providing such analysis was the objective of this research. We observed root and nodule character traits at regular time intervals in 130‐cm‐tall soil columns sectioned by depth in 2 years. In this tap‐rooted plant, root weight was greatest at the soil surface (0–15 cm) and decreased with depth. Root length density (RLD) from 0 to 15 cm and 15 to 30 cm soil depths was similar. The RLD and root weight were generally minimal in deeper soil layers, although deeper roots seem to play an important role in water uptake for guar, especially in drought conditions. Compared to most crops, guar has fine rooting and a small root‐to‐shoot ratio. At all soil depths analysed, ≥86% of root length was comprised of roots <1 mm in diameter, although most were very fine roots <0.5 mm. The root‐to‐shoot ratio ranged from 0.055 to 0.17 g g−1 over the growing seasons. Significant accumulation of nodule weight occurred by 60 days, as did the first observations of nodule senescence, but nodule growth continued after this time. Most nodules and nodule weight were concentrated near the soil surface and the average weight per nodule decreased with depth, although smaller nodules were present at all depths. Because guar is typically grown in dryland conditions in hot semi‐arid regions, nodules near the soil surface may often be susceptible to shedding or loss of function due to dehydration and/or heat stress and the plant may rely on deeper nodules for N fixation. The basic biological information provided in this report is needed to hasten progress in guar genetic improvement and management.