Effects of paclobutrazol on growth and physiology of salad tomatoes (Lycopersicon esculentum Miller)

Abstract

Salad tomatoes represent an important vegetable crop within Australia. However, the costs of both materials and labour involved in providing this crop with artificial support, in the form of trellising or staking, is high. This project examined the feasibility of using the plant growth regulator, paclobutrazol, as an aid to crop grown without trellising or staking under coastal conditions.

Initially a rate of 4 mg plant-, applied as a soil drench 1, 15, 29, 47 or 57 days after transplanting were compared with non-treated plants under glasshouse conditions. The application of paclobutrazol 1 day after transplanting (DAT), and to a lesser extent 15 DAT. profoundly changed growth while later applications (29, 47 or 57 DAT) had little effect.

This sensitivity of young tomato plants to paclobutrazol was confirmed in a field trial where 5 rates (nil, 6.25, 12.50, 25.00 and 50.00 g a.i. ha-1) were applied at one of three application times (12, 40 or 60 DAT). Paclobutrazol only effected growth and physiology. when applied at the earliest time while later applications did not appreciably effect salad tomatoes regardless of rate. It was postulated that salad tomatoes remain sensitive to paclobutrazol up to the event of floral initiation. When applied early the highest rate tested produced the most profound changes and there was no evidence of residual effects on a gucceeding lettuce crop. though later applications did cause a slight stimulatory effect to lettuce dry matter accumulation. It was concluded that paclobutrazol was unlikely to cause residual effects to succeeding crops when applied to tomatoes during their sensitive stage and at rates within those tested.

A further field experiment tested in more detail the effects of this compound on growth and some aspects of physiology. Paclobutrazol was found to inhibit several important plant characters, viz: height, leaf and stem dry matter accumulation and leaf areas. Conversely it stimulated the partitioning of assimilate to leaves, specific leaf weights, net photosynthesis on a leaf basis, net assimilation rate and water use efficiency on a gas exchange basis. However the stimulatory and inhibitory effects appear to cancel each other out such that treated and untreated plants had similar crop growth rates and fruit yields.

Hence, these studies do not present evidence suggesting that this compound has a role to play as an aid to unsupported semi-determinate salad tomato crops, as no yield benefit was conferred.