Root growth and solute accumulation in pea in response to water stress and powdery mildew

Abstract

Peas growing in soil columns that allowed unrestricted downward growth and development of the root system and inoculated with Erysiphe pisi were subjected to 3, 4-day drying cycles. Water stress did not affect growth of the tap root but it did stimulate the growth of shallower lateral roots. Infection did not affect tap root growth in either well-watered or water-stressed plants. Infection severely reduced the growth of all lateral roots of well-watered plants and of those lateral roots of water-stressed plants which had reached the lower wetter regions of the soil. Infection of water-stressed plants only reduced the growth of lateral roots in the upper, drier regions of the soil after the third drying cycle. This loss reduced the root to shoot dry weight ratio of infected-stressed plants which had previously been equal to that of healthy-stressed plants and higher than that of unstressed plants whether the latter were healthy or infected. The decline in root weight was associated with the rapid death of older infected leaves on water-stressed plants and so the ratio of root dry weight to leaf area remained unaltered by infection. Solute and water potentials of the shoot apex of healthy and infected plants were lower in stressed than in unstressed plants but in infected plants they increased with time after inoculation. However, no change in turgor resulted in infected plants since solute and water potentials changed at similar rates. When plants were grown in culture solution and were water-stressed by addition of polyethylene glycol to the medium, growth of intact root systems was reduced by both mildew and stress, the effects being additive when they occurred together. The solute potential of tips of first lateral roots was lowered by water stress, but was raised by infection. Infection did not cause increased solute leakage from roots but reduced the ability of root tips to accumulate solutes in response to stress. Thus, when tips were excised from such roots and immersed in mannitol, extension was reduced in solutions of low osmotic potential. It is concluded that mildew leads to slower growth, or even death, of lateral roots in dry regions of the soil because it interferes with the supply of photosynthates needed for normal osmoregulation. Rapid growth of the tap root and changes in the balance between the absorbing and transpiring regions of the mildewed plant protect it from increased water stress in dry soil.