Respiration and Phloem Translocation in the Roots of Chickpea (Cicer arietinum)

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Abstract
Root growth in chickpea (Cicer arietinum) has been studied from the early vegetative phase to the reproductive stage in order to elucidate its growth and maintenance respiration and to quantify the translocation of assimilates from shoot to root. A carbon balance has been drawn for this purpose using the growth and respiration data. The increase in the sieve tube cross-sectional area was also followed simultaneously. Plants growing in a nutrient culture medium were studied to determine the relative growth rate (RGR) 5–60 d after germination. RGR declined from 113 to 41 mg d−1 g−1 during the measurement period. Simultaneous with the RGR analysis, respiration rate was also measured using an oxygen electrode. The respiration rate declined as the plants aged and a drastic reduction was recorded following anthesis. The relationship between RGR and respiration rate was used to extrapolate the maintenance respiration (m) and growth respiration (1/YEG). The respiration quotient (r.q.) of the roots was 1.2 and the Q10 in the range 20–25 °C was 2·2. A carbon balance for the roots was constructed by subtracting the carbon lost during respiration from that gained during growth. The roots were found to respire no less than 80% of the carbon translocated. The increase in the cross-sectional area composed of sieve tubes was measured near the root-shoot junction as the plants grew. Chickpea has storied sieve plates which simplifies these measurements. Their cross-sectional area increased during growth mainly because of an increase in sieve tube number. The diameter of individual sieve tubes remained constant. Specific mass transfer (SMT) values for seive tubes into the roots have been computed during various stages of growth. SMT values were relatively constant before anthesis (approx. 6·5 g h−1 cm−2), but decreased following anthesis. We did not evaluate possible retranslocation from roots: any such retranslocation would have the effect of increasing our SMT values.