Analysis of fruit growth in grapefruit as affected by irrigation intervals

Abstract

A study of the daily changes in fruit volume was carried out in an attempt to analyse the suitability of the apparent fruit growth (AFG) rate as an indicator for timing of irrigation in grapefruit. Three distinct fruit growth phases could be distinguished within an irrigation cycle. Phase 1, lasting about 3 days, was characterized by a relatively rapid volume increment rate owing to both turgor-attributed physical swelling of the fruit and its normal growth; Phase 2, with a moderate volume increment rate, defined as normal growth; Phase 3, in which fruit volume growth stopped altogether, sometimes even showing some fruit shrinkage. Even though no changes in fruit volume were discernible during Phase 3, it is proposed that some growth did take place, being counterbalanced by an equivalent amount of shrinkage. Comparison of the growth patterns of fruits from moisture stressed (S) and regularly irrigated (RI) trees showed that, after irrigation was renewed, the fruits from S trees swelled and grew faster for longer periods. This more rapid growth could compensate for most or all of the growth lost during the stress period. Girdling of branches was effective in prolonging the period of faster fruit growth rate in S trees. These observations argue against using the AFG rate as an indicator for timing of irrigation. They also lead to the question as to the nature of the factor(s) which promote the faster growth of fruits from S trees after irrigation renewal.