The quest for the function of ‘Hass’ avocado carbohydrates: clues from fruit and seed development as well as seed germination
S. Z. Tesfay A C , I. Bertling A , J. P. Bower A and Carol Lovatt BA Horticultural Science, University of KwaZulu-Natal, Pietermaritzburg, Private Bag X01, Scottsville 3209, South Africa.
B Department of Botany and Plant Sciences, University of California, Riverside, CA 92521-0124, USA.
C Corresponding author. Email: tesfay@ukzn.ac.za
Australian Journal of Botany 60(1) 79-86 https://doi.org/10.1071/BT11166
Submitted: 28 June 2011 Accepted: 7 December 2011 Published: 28 February 2012
Abstract
D-Mannoheptulose (D-manno-2-heptulose) and perseitol (D-glycero-D-manno-heptose) are unusual seven-carbon (C7) sugars that have been postulated to act as storage and transport sugars in avocado. However, thus far, there is no published evidence that satisfactorily explains the physiological functions of these carbohydrates. Various tissues at different stages of the avocado life-cycle were therefore analysed for C7 carbohydrates to derive clues on the function of the six-carbon (C6) and C7 sugars. Adult reproductive tissues (flower buds and fruit) contained significantly greater concentrations of C7 than C6 sugars, whereas in juvenile avocado tissue, the classic C6 storage carbohydrate, starch, was found only in developing and mature seeds, predominatly in the cotyledons. The dramatic increase in shoot and cotyledon C7-sugar concentrations, together with a reduction of starch reserves in the cotyledons as a result of lack of light during germination, suggests that the C6 storage carbohydrate (CHO) starch can be converted to fuel the C7 metabolism. Therefore, a changeover from C6 carbohydrate to C7 sugar metabolism characterises the progression from juvenility to maturity in avocado. Avocado seems to mirror its evolutionary development characterised by a switch-over from the common C6- to a specialised C7-sugar metabolism in the juvenile versus adult tissue. The dominance of C7 carbohydrates over C6 in adult tissues is, therefore, due to the fact that C7 sugars have multifunctional roles as a source of energy and anti-oxidants.
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