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South African Journal of Botany 83 (2012) 165 – 171 www.elsevier.com/locate/sajb The effect of temperature and relative humidity on Acacia mearnsii polyad viability and pollen tube development
S.L. Beck-Pay *
Institute for Commercial Forestry Research, P.O. Box 100281, Scottsville, Pietermaritzburg 3209, South Africa
Received 31 January 2012; received in revised form 27 August 2012; accepted 29 August 2012
Available online 26 September 2012

Abstract
Acacia mearnsii (black wattle) is a commercially important forestry species in South Africa, grown for its timber and bark. Due to its invasiveness, it is also considered be an alien invader species and for this reason the production of a sterile triploid variety would be highly desirable for South African commercial forestry. Previous research on crosses between diploid and tetraploid parent plants to produce triploid progeny has resulted in poor seed set. One possible barrier preventing seed set could be the effect of temperature and relative humidity, within the isolation bags used during cross-pollination operations. For this reason in 2011 diploid polyads were subjected to various temperature and relative humidity combinations, to simulate conditions recorded within the isolation bags being used in the 2010 flowering season in order to see if the conditions were detrimental to polyad viability. The results showed that when polyads were exposed to extreme temperatures (N 30 °C) and low relative humidities (RH's) (10%), polyad viability and pollen tube development, decreased significantly. In contrast the effect of high RH's in combination with low temperatures for long periods appeared to be beneficial to polyad viability and pollen tube growth. The results also indicated that the Australian Centre for International Agricultural Research (ACIAR) agar germination medium was superior to the Brewbaker and Kwack
(BK) 30% agar germination medium for determining polyad viability as it resulted in greater number of pollen tubes per polyad, which were healthier in appearance. The Sigma® DAB peroxidase vital stain test overestimated polyad viability and showed no significant differences between the various treatments, highlighting its unreliability as a test. Polyad viability and pollen tube development were compared across three flowering seasons (2009, 2010 and 2011) and similar trends were apparent with some seasonal differences.
© 2012 SAAB. Published by Elsevier B.V. All rights reserved.
Keywords: Isolation bags; Polyads; Relative humidity; Temperature

1. Introduction
Black wattle (Acacia mearnsii) is grown as a commercial forestry species in South Africa with approximately 7.6% of the total area under commercial forestry plantations (Forestry South
Africa, 2009). It is grown primarily for the tannins present in the bark and pulp. Native to Australia black wattle is a prolific seed producer in South Africa. Seed can remain dormant in the soil for many years and still retain viability, and for this reason it has been

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identified as an invasive species in unmanaged stands. The production of seedless or sterile wattle would be beneficial both to the wattle industry and the indigenous vegetation. A sterile variety of black wattle can be produced through the production a triploid, which is created by crossing tetraploids and diploids
(Beck et al., 2003).
A. mearnsii pollen is grouped together in structures called polyads, with each polyad typically containing 16 pollen grains.
The polyads are designed to protect the pollen and ensure that maximum pollen germination is possible when the polyad attaches to the stigma (Kenrick and Knox, 1982). One of the reproductive barriers limiting cross-pollinated seed production could be the viability of the pollen used in cross pollinations. Stiehl-Alves and