Carbon allocation in cassava is affected by water deficit and potassium application - A 13 C-CO2 pulse labelling assessment

Abstract

Rationale: Cassava production faces challenges in a changing climate. Pulse labelling cassava with ¹³ C-CO₂ has the potential to elucidate carbon allocation mechanisms of cassava under drought stress and with potassium application. Understanding these mechanisms could guide efforts to mitigate effects of drought in cassava cropping systems.

Methods: Forty-eight cassava plants received a nutrient solution high or low in potassium. Water deficit was imposed on half of the plants at bulk root initiation stage, after which they were labelled for 8 h with ¹³ C-CO₂ in a 15 m³ growth chamber. Plants were harvested 8 h, 9 days and 24 days after labelling, and separated into leaves, stems and roots. δ¹³ C values of the different parts were measured using an isotope ratio mass spectrometer, from which ¹³ C excess was calculated.

Results: Water deficit decreased transpiration (P < 0.001) and increased carbon respiration (P < 0.05). Potassium application increased assimilate distribution to the roots (P < 0.05) at 9 days after labelling, more strongly for plants under water deficit. The opposite was found at 24 days (P < 0.05) with the legacy of water deficit additionally increasing assimilate distribution to roots (P < 0.05). Youngest, fully expanded leaves contained up to 47% of initial ¹³ C excess at 24 days after labelling.

Conclusions: Pulse labelling proved to be successful in shedding light on carbon allocation in relation to water and potassium availability. This technique, once adapted to field conditions, could further be used to improve fertilizer recommendations or change agronomic practices to cope with plant stress.

FIGURE 1

Subdivision of plant parts based on Tag 1 and Tag 2 at H0 (before labelling), H1 (8 h after labelling), H2 (9 days after labelling) and H3 (24 days after labelling). Source: https://bit.ly/3KinU53 via vecteezy.com [Color figure can be viewed at wileyonlinelibrary.com]

FIGURE 2

A, Daily transpiration of cassava plants over the course of the experiment. Lines and dots are averages per treatment combination. Black colour represents plants at W+ (100% of field capacity), and grey colour represents plants at W– (50% of field capacity) during the period of imposed water deficit. Full lines are plants at optimal potassium nutrient solution (K+), whereas dotted lines represent plants at suboptimal potassium nutrient solution (K–). The vapour pressure deficit is given as the daily mean VPD and is represented by a black dashed line. H0, H1, H2 and H3 indicate harvest moments: H0 (before labelling), H1 (8 h after labelling), H2 (9 days after labelling) and H3 (24 days after labelling). B–E, Total transpiration for the different experimental periods. N per group is 30, 15, 8 and 4 for plot (B, C, D and E), respectively

FIGURE 3

Whole plant ¹³C excess at H1 (8 h after labelling), H2 (9 days after labelling) and H3 (24 days after labelling). Whole plant ¹³C excess is given in mg ¹³C. Black colour represents plants at W+ (100% of field capacity), and grey colour represents plants at W– (50% of field capacity) during the period of imposed water deficit. Full lines are plants at optimal potassium nutrient solution (K+), whereas dotted lines represent plants at suboptimal potassium nutrient solution (K–). Values for treatment combinations (W+K+, W+K–, …) per harvest are presented as estimated marginal means. Error bars represent estimated standard errors

FIGURE 4

Distribution of ¹³C excess in the three major plant parts as expressed by percentage of whole plant ¹³C excess for the three harvests. Letters show significant differences over time per treatment and part, based on a type‐III ANOVA (fixed effect of harvest, random intercept for growth chamber and table) followed by a Tukey's HSD. Shown differences over time are based on α = 0.05 (black letters) or α = 0.1 (grey letters)