Factors affecting induction and differentiation of pistillate flowers on pecan trees

1. Factors affecting induction and differentiation of pistillate flowers on pecan trees Michael Smith Dept. of Horticulture & L.A. Oklahoma State University

2. Terminal mixed bud • Mixed bud – both vegetative (shoot & leaf) and reproductive parts (female flower) • Terminal mixed buds frequently abort, leaving a lateral primary compound bud as the distal bud. • Compound bud – contains multiple buds. • 2 catkin buds • Central mixed bud with 2 catkin groups, shoot, leaves and female flowers • Typically 1 to 3 distal primary buds develop shoots and catkins. The other primary buds initiate growth but the shoot aborts and the catkins continue to develop. • Secondary buds remain dormant unless the primary bud is killed. Primary compound bud Secondary compound bud

3. Pecan compound bud Outer bud scale Central bud scales Apex Leaf primordia Inner catkin bud scales

4. Shoot, leaves, and maybe female flower Catkin

5. Induction – Stimulus causing a bud to change from vegetative to reproductive • Differentiation – Visible evidence (microscopic) of reproductive tissue development

6. Budbreak Pollination Catkin differentiation for next year, ≈ 3 weeks after budbreak Water stage Pistillate flower induction Shuck split April May June July Aug Sept Oct Nov Dec Jan Feb Mar Defoliation Type 1 catkins nearly developed, Type 2 catkins incompletely developed Bud swell Pistillate flower differentiation Amling & Amling, 1983 Type 2 catkins resume development, Type 1 begins expansion

7. Effect of bud removal on female flower clusters All primary buds appear to have equal fruiting potential when growth is initiated. Wood and Payne, 1983

8. Return flowering of previous year’s shoot types Fruit development reduces return bloom. Early fruit maturation promotes return bloom.

9. Extending the postripening period during the “on” year of ‘Cheyenne’ on return bloomExtended by inducing early budbreak with Dormex Early fruit maturation, or extended leaf retention following fruit maturation promotes return bloom. Wood, 1995

10. Influence of cluster size on return bloom (%) of ‘Pawnee’All fruit on tree hand thinned at ½ kernel expansion When trees are overloaded, secondary shoot growth tends to increase return bloom, lateral fruiting shoots have less return bloom than terminal shoots.

11. Effect of defruiting date on return bloom of terminal and lateral shoots • Terminal shoots returned more bloom than lateral shoots • Return bloom of lateral shoots declined 2 – 4 weeks earlier than terminal shoots. Terminal shoots Lateral shoots Dough stage June July Aug Sept Oct Date of defruiting Wood, 1995

12. Avoid excessive crops by mechanical fruit thinning 50% kernel size

13. Defoliation date on return bloom of ‘Western’ Early defoliation will eliminate or reduce return bloom. Hinrichs, 1962

14. Effect of defoliation date on carbohydrate concentration and return bloom Worley’s data confirmed that of Hinrichs, and suggested that carbohydrate storage may be involved. Worley, 1979

15. Relationship of Jan. root starch to same-year yield • High Jan. root starch may reduce reversion of induced buds to a vegetative state, or abortion of flowers during differentiation. • Jan. starch conc. would have little impact on female flower induction. Wood, 1989

16. Foliage management for annual production • Maintain healthy foliage • Pests • Aphids, mites, walnut datana, other foliage feeders • Disease, particularly pecan scab • Balanced nutrition program • Deficient or excess N • Deficient K • Deficient Zn • Avoid either excess or deficient water • Excess water in spring is particularly detrimental • Reduces photosynthesis while flooded, plus recovery takes twice as long as flood duration. • Reduces leaf expansion – thus photosynthetic potential is reduced for the entire growing season. • Nut filling is the most critical time for drought

17. Effect of fruit development on leaf N and fruit N Fruit tends to act as a sink, first increasing leaf N on fruiting shoots, then depleting leaf N as it is transported to the rapidly developing nut.

18. 75 lb/a N applied in March and 50 lb/a applied in Oct • Most N absorbed while leaves rapidly expanding • Some N absorbed while trees are dormant • Little N absorbed at other times or loss similar to absorption • Leaves may act as a N storage reserve for reallocation during the • growing season 1997 1998 1999 2000 2001

19. Nitrogen application rate and time on yield of ‘Maramec’ No benefit from October applied N, all N treatments produced similar yield until 2002

20. N rate and application time on fruiting shoots of ‘Mohawk’ No benefit from Oct. application. No yield difference between N rates.

21. Nitrogen rate and application time on yield of ‘Mohawk’ No benefit from Oct. N, benefit 1 yr for Aug. N

22. Summary • Induction of catkins is within 3 weeks of budbreak, and female flowers in late July to early Aug. • The stimulus and hormonal/growth regulator changes associated with flower induction are unknown. • Winter stored carbohydrates are positively correlated with retention and development of female flowers. • Differentiation of catkins begins about 3 – 4 weeks after budbreak, and female flowers about bud swell. • Fruit development reduces return bloom. • Early fruit maturation promotes return bloom. • Fruit thinning and/or hedging to control crop load. • Premature defoliation or reduced leaf function reduces return bloom. • Follow a recommended pest management program. • Maintain a balanced nutrition program. • Avoid flooded or water saturated soils during leaf expansion. • Avoid late season drought stress.