THE EFFECT OF WIND ON PLANT GROWTH AND SOIL MOISTURE RELATIONS: A REPLY TO THE REASSESSMENT BY HUMPHRIES AND ROBERTS

Abstract

Humphries and Roberts (I965) have put forward an interesting re-assessment of our papers on the effect of wind on plant growth and soil moisture relations given in the New Phytologist in I962 and I963. They believe that the data presented can be accounted for in terms of a restricted development of roots. The essence of this re-assessment lies in the supposition that under experimental conditions of culture of plants at fractions of field capacity the soil was wetted in a restricted region only and that the remainder of the soil remained dry. Unfortunately, it was, perhaps, not made clear in the original papers that precautions were taken to avoid this situation. It seems that due to this omission they have made the assumption that the moisture content of the soil was maintained by surface addition of water. This was not the case since necessary amounts of water were added by means of a veterinary hypodermic syringe. The needle was plunged into the beakers in such a way that small amounts of water were added at different depths, etc., resulting in a much less uneven distribution than that produced by surface watering. It is not claimed that this led to a completely even distribution nor that the fractions of field capacity were maintained exactly in all parts of the soil. That it avoided any great reduction of the volume of soil available for root growth is clear from the fact that in all cases where plants were grown at less than field capacity there was proportionately less decrease in dry weight of root than that of shoot (see Whitehead, I963a, Fig. 4). To take some of the specific points raised by Humphries and Roberts in the section dealing with what they consider the more controversial conclusion. (i) and (2) 'Loss/area/time may be regarded as related to the leaf's efficiency to restrain water loss'. Humphries and Roberts do not agree with this conclusion. A more detailed examination of the results given in our papers shows: (a) These plants, as in all the experiments described, show a greater proportion of root to shoot which would be difficult to explain if there was a real restriction of soil available for root development. The main overall reduction of dry weight with increased moisture stress is mainly related to the reduction of internodes so that there is only a small reduction in root weight. (b) Although the plants in this particular experiment showed rather less reduction in leaf area with increased moisture stress than in other experiments reported, the leaves which were produced were much more xerophytic in character. As reported in Whitehead (i963b, p. 243) the leaf anatomy of wind and soil moisture treated plants is similar. Using data for Zea mays given in Whitehead and Luti (I962) it can be shown from the formula of Penman and Schofield (I95 I) that the theoretical flow resistance per unit area